Diode devices

Di means two, and -ode is derived from electrode,* Thus, the word diode refers to a two-electrode device. Although in conventional low-frequency solid-state terminology, the word diode indicates only PN junction diodes in the microwave world, we see both PN diodes and others as well. Some diodes, for example, are multiregton devices, and still others have but a single semiconductor region. The tatter depend on bulk-effect phenomena to operate. This section examines some of the various...

A common test chassis lor projects

Part of the decision to build and test several direct-conversion receivers involved having a common chassis for all three designs. Although not very elegant, being made of scrap aluminum chassis and bottom plates from the junk box, it was at least low cost and effective. Figure 6-17 shows the receiver test bed front panel. It is Qtted with a Jackson Brothers calibrated dial with a 10,1 fast slow vernier drive. The 6.36-mm (0.25 in) shaft coupling on the vernier drive is used to turn either a...

Ground o

14-3 (A) Ac line EMI filter and (B) power cord on ferrite rod also reduces EMI by acting as a RF choke. coming from the offending device. It works much of die time because the principal source of the noise signal is radiation from the power line of the VCR or TV, The EMI mter should be installed as close as possible to the body of the'offending device. The filter in Fig. 14-3A uses LC elements to form a low-pass filter network that is placed in series with the ac power iine. Homebrew filters...

AC power line filtering

The ac power lines iire a source of conducted EMI and must be filtered extensively to make them really clean. Not only should the ac lines be filtered against RPI but also against lightning strikes at a distance. The lightning and other high-voltage transients can be handled per Fig. 15-10 with metal oxide varistors (MOV). These devices are made by various suppliers and can act like a pair of zener diodes back-to-back, In other words, they snip the high-voltage transient to a tower level,...

AM breakthrough

Stations in the AM broadcast band (540 to 1700 kHz), and in many regions of the world VLF broadcast band also, tend to be both relatively high-powered and very local to the receiver (thus very strong). As a result, many users of radio receivers for other bands are afflicted with the phenomena of AW breakthrough. This term refers to any of several phenomena, but the end result is AM interference to reception. Sometimes the problem is caused by simple overload of the front end of the receiver,...

Amplifier circuits

A simple fF amplifier is shown in Fig. tf-2. A simple AM band radio may have one such stage, while FM receivers, shortwave receivers, and other types of commulocations receiver may have two to four stag 's suel i as Fig. 8-2. This F amplifier is based oil the type of LC filter circuits discussed in Fig. 8-1 A. Transformer T, has a low-impedance tap on its secondary connected to the base of a transistor (Qi). Sin larly with Tly but in this case the primary winding is tapped for the collector of...

Applications of hybrids

TTie hybrid can be used for a variety of applications where either combining or splitting signals is required. in Fig. 12-2 there are two signal generators connected to opposite ports of a hybrid (ports 2 and 3). Power at port 2 from signal generator 1 is therefore canceled at port 3, and power from signal generator 2 (port 3) is canceled at port 2. Therefore, the signals from the two signal generators will not interfere with each other. in both cases, the power splits two we(ys, for example,...

Fpb

3- The cfij> aciuince of the air variable capacitor is determined hv how mur h the uitc-r t> Ur shaded by the stator plates (A) Minimum capacnance (H) uin mediate ( ip ici ancc (O mavmuun capacitance. 1, Minimum capacitance is found when the rotor plates are completely urimeshed with the stator plates and 2. Maximum capacitance is found when the rotor plates are completely meshed with the stator plates. figure 3-4 shows a typical single-section variable capacitor. The stator plates are...

Bandpass filters

A bandpass filter passes only those frequencies between lower and upper 3-d B cutoff frequencies and attenuates all others. One simple way to obtain a bandpass frequency response characteristic is to cascade low-pass and high-pass sections (Fig. 23-8). The LPF is designed with a cutoff frequency equal to the high cutoff frequency in the passband, and the HPF is designed to cutoff at the low point in the desired passband. The values are as calculated for the individual sections. 23-8 Bandpass...

Building signalgenerator and oscillator circuits

Signal-generator circuits produce RF sine-wave (or other waveform) output for purposes of testing, troubleshooting, and alignment. Although the subject of designing top-notch signal generator circuits can be quite deep, it is possible to reduce the possibilities to a small number for people whose needs are less stringent. This chapter looks at RF signal generators that can be used in a wide variety of applications on the electronic and radio workbenches. But first, take a look at some generic...

Cable TV noise

Many communities loclay are wired for cable TV. These systems transmit, a large number of TV and FM broadcast and special-service signals along a coaxial transmission line. They operate on Frequencies of 54 to 300 MHz in 36-ehanne) systems and 54 to 440 MHz in the 55-chaunel systems. Whenever a large number of signals get together in one system, intermodulation is a possibility and tlmt means signals outside of the official spectrum. The problem is that signals leak out of the cable TV system...

Capacitors and capacitance

Capacitors arc the other component used in radio tuning circuits. Like the inductor, the capacitor is an energy-storage device. Although the inductor stores electrical energy in a magnetic field, the capacitor shires energy in an electrical (or 'electrostatic) fie id. Electrical charge, is si or I iiuhe capacitor. But more about that shortly. The basic capacitor consists of a pair of metallic plates that face each other and are separakd by an insulating material, culled a dielechic. This...

Capacitors in ac circuits

When an electrical potential is applied across a capacitor, current will flow a.s charge is stored in the capacitor. As the charge in the capacitor Increases, the voltage across the capacitor plates rises until it equals the applied potential. At thi* point, the capacitor is fully charged, and no further current will flow. Figure 2-17 shows an analogy for the capacitor in an ac circuit. The actual circuii is shown in Fig, 2-17A, It consists of an ac source connected in parallel across the...

Cascode pair amplifier

A cascode pair amplifier is shown in Fig 8-4. This amplifier uses two transistors (both JFETV) in an arrangement that puts Qi in the common source configuration and Oj in the common gate configuration. The two transistors are direct-coupled. Input and output tuning is accomplished by a pair of LC filters (L C and i C-). To keep this circuit from oscillating at the IF frequency a neutralization capacitor (Cj) is provided. This capacitor is connected from the output LC filter on Q2 to the input...

Coaxial cable transmission line coax

Perhaps the most common form of transmission line for shortwave and VHF UHF receivers is coaxial cable. Coax consists of two conductors arranged concentric to each other and is called coaxial because the two conductors share the same center axis (Fig, 1-12) The inner conductor will be a solid or stranded wire, and the other conductor forms a shield. For the coax types used on receivers, the shield will be a braided conductor, although some multistranded types are also sometimes seen. Coaxial...

Construction of RFIF transformers

The tuned RF IF transformers built for radio receivers are lypif diy w tutd on common cylindrical form and surrounded by a metal shield that can prevent interaction of the fields of coils that are in close proximity to each other Figure 2- WA shows the schematic for a typical RF IF transformer, and the sectioned view (Fig. 2-JUB) shows one form of construction This method of building transformers

Counters to EMI

There are two effective ways to counter EMI shielding and tittering (Fig. 15-3), The shielding is used to guai-d against radiation interference, while the filter is used to guard againsi conduction interfere ice. The filters have the advanlage of being bidirectional, so they also prevent inierference from flowing out of the system as well as prevent it from flowing in. PEAK VOLTAGE < kV) 15*3 Shielding and EMI filtering are suitable defenses.

Detector and demodulator circuits

The purpose of the detector or demodulator circuits is to recover the intelligence impressed on the radio carrier wave at the transmitter. This process is called demodulation and the circuits used to accomplish this are called demodulators. They are also called second detectors in superheterodyne receivers. In a superheterodyne receiver the detector or demodulator circuit is placed betweervthe IF amplifier and the audio amplifier (Fig. 27-1), This position is the same in AM, FM, pulse...

Dynamic performance

The dynamic performance specifications of a radio receiver are those that deal with how the receiver jjerforms in the presence of very strong signals either cochannel or adjacent channel. Until about the 1960s, dynamic performance was somewhat less important than static performance for most users. However, today the role of dynamic performance is probably more critical than static performance because of crowded band conditions. There are at least two reasons for this change in outlook (Dyer,...

Dynamic range problems

Dynamic range is basically the difference between the maximum and minimum signals that the receiver can accommodate and is usually expressed in decibels (dB). A description of dynamic range found in Hayward and DeMaw describe it as Pis the signal level associated with the third-order intercept point of the receiver A ds is the minimum discernible signal- Dynamic range is important in all bands, but there are cases where it is most important. An example is when receiving any frequency when the...

Filter applications

Figure 23-2 shows several different types of filter applications. In Fig. 23-2A, the filter is placed between the antenna and the antenna input of a receiver. Its function is to remove unwanted signals before they reach the front end. Whether a high-pass, low-pass, or bandpass Hlter is used depends on the local situation (i.e., the frequencies that you wish to eliminate). There are several good reasons for using a filter ahead of a receiver, even when the strong local signal is not within the...

Filtered Output

Ffi is the modulation index fc is the carrier frequency A is the peak amplitude < Ti(, , and a2 are constants- The terms In the brackets are the modulation and distortion products. The second-order terms are modulation and distortion, where the higher-order terms are distortion only. What falls out. of the equation is (hat (o keep distortion low, modulation index 00 must he low as well. Figure 27-5 shows what happens at the capacitor. The dotted lines represent the outpvit of the diode, which...

Filtering circuits against EMI

Electronic pollution is all around us. Radio and noise waves impinge us < ill of the time. Never has electromagnetic interference (EMI) boon so great as it is t xiay, < hie source defines EMI thusly * electromagnetic interference is a degradation in performance of an electronic system caused by an elociromagnetic disturbance At worst, EMI can cause a loss of human life, a , when it. interferes wuU au aircraft, or automobile electronic system, At best, it will pass uimolicod or will inlerfer...

Fixed capacitors

Several types of fixed capacitors are used in typical electronic < circuits. They are classified by dielectric type paper, mylar, ceramic, mica, polyester, and others. The construction of an old-fashioned paper capacitor is shown in Fig. 2-14. It consists of two strips of metal foil sandwiched on both sides of a strip of paraffin wax paper. The strip sandwich is then rolled up into a tight cylinder. This rolled-Lip cylinder is then packaged in either a hard plastic, bakelite, or paper-and-wax...

FM and PM demodulator circuits

Frequency modulation and phase modulation are examples of angle modulation. Figure 27-20 shows this action graphically. The audio signai causes the frequency (or phase) to shift plus and minus from the quiescent value, which exists when there is no modulation present. Frequency and phase modulation are different but similar enough to make the demodulation schemes the same. The difference between FM and PM is that the phase modulation needs no preemphasis curve t o the audio waveform (it does it...

Frequency on the Smith chart

A complex network may contain resistive, inductive reactance, and capacitive reactance components. Because the reactance component of such impedances is a function of frequency, the network or component tends to also be frequency -sensitive. You can use the Smith chart to plot the performance of such a network with respect to various frequencies. Consider the load impedance connected to a 50-11 transmission line in Fig, 26-11. In this case, the resistance is in series with a 2.2-pF capacitor,...

Frontend circuits

The front end of the radio receiver consists of the RF amplifier (if used) and the converter or mixer LO stages. The basis for the designs are the Signetics NE-6Q2 balanced-mixer integrated circuit. This device has limited dynamic range but is sufficient because it compensates with a better-than-average noise figure and sufficient conversion gain to make it possible to not use an RF amplifier in most projects. The pinouts for the NE-602 IC converter are shown in Fig. 5-19. The two inputs...

1

21-1 (A) PN diode, (B) PIN diode, and (C) PIN diode packages. sipate 400 mW. I used these diodes for the experiments performed to write this chapter because they are service shop replacement lines, and both ECG and NTE are widely distributed in local parts stores. An alternative that might be harder to come by is the MFN3404, which uses the TO-92 plastic package (Pig. 21-1C). Radio-frequency ac signals can pass through the PIN device and in fact see it under some circumstances as merely a...

Inductance and inductors

Inductance is the property of electrical circuits that oppose changes in the flow of current. Notice the word changes it is important As such, it is somewhat analogous to the concept of inertia in mechanics An inductor stores energy in a magnetic field (a fact that you will see is quite important)- hi ord r to understand the concept of inductance, you must understand ihree physical facts 1, When an electrical conductor moves relative to a magnetic, field, a current is generated (or induced) iti...

Info

A shorted stub can he built to produce almost any value of reactance. This can be used to make an impedance-matching device that cancels the reactive portion of a complex impedance. If you have an impedance of, for example, Z R + j'30 ft, you need to make a stub with a reactance of -> 30 ft to match it. TVo forms of matching stubs are shown in Figs. 19-1 OA and 19-1 OB. These stubs are connected exactly at the feedpoint of the complex load impedance, although they are sometimes placed...

J

5 10 15 20 25 Oc) collector current (MMampercs) 22-23 Noise figure va collector current. Now look at the field-effect transistor (FET), as used in microwave systems. The FET operates by changing the conductivity of a semiconductor channel by varying the electric field in the channel. TVo elementary types are found junction field -effect transistors (JFET) and metal-oxide semiconductor field-effect transistors (MOSFET), also sometimes called by the name insiUated-gate field-effect imnsis-tor...

LC Resonant tank circuits

When you use,iu inductor and a capacitor together in the same circuit, the combination forms an IX resonant circuit, which is sometimes called a fatik fit-nth or rvRonatit tank a rant. Those circuits can be used to i une a ndio receiver ,is well as in other applications. There are two b isic forms of LC resonant tank circuit scries (I-*ig. 2-20A) ivnei parallel (Fig. 2-20B). These circuits have nmch mi common md much Vhat makes thorn fundamentally different Irorn each other. The condition of...

Limedomain reflectometry on a budget

Doping out transmission line difficulties can be a tedious and difficult chore especially when the load end is not easily accessible. Although there are a number of different methods available, I want to discuss time-domain reflectometry Although the official, bouled-in-bond, 100-proof time-domain reHectometer (TDR) is a terribly expensive piece of equipment, some TDR methods can be used by any amateur who has access to an oscilloscope. Will the residts be as good as professional TDR equipment...

Mixer circuits in directconversion receivers

The principal element in any direct-conversion receiver (DCR) is the mixer. The mixer is a nonlinear circuit element that exhibits changes of impedance of cyclical excursions of the input signals. When mixing is linear, one signal will ride on the other (see Fig. 6-7A) ais an algebraic sum (i.e., the two waveforms are additive), but the product (i.e., multiplicative) frequencies are not. generated. In nonlinear mixing, the classic amplitude-modulated waveform (Fig, 6-7B) is produced when the...

More on bandpass filters

A bandpass filler is designed to pass all frequencies thai i re above a lower rutoff frequency and all frequencies that are below an upper cutoff frequency. Figures and 23-15 show two approaches to making a bandpass filter for the high-frequency (HF) shortwave bands. The cir cuit in Pig 23-14 is designed for a frequency I and of 2 to 33 MHz, so il will encompass the entire HF short wave band while eliminating interference from LF and MW AM band broadt asters as well as VHF stations. Notice that...

Noise

All radio reception is basically a game of signal-to-noise ratio (SNR). If the SNR is not advantageous, then the receiver ultimately fails. At low signal levels (Fig. 27-8) the SNR is poor and the noise dominates the output As the signal level increases, however, the noise level increases but at a slower rate than the signal output. The noise level comes to rest (see dotted line in Fig. 27-8) about .'1,7 dB over the no-carrier state but the signal level continues upward. 27-8 Noise vs output...

Noise and preselectors

The weakest signal that you car detect is determined mainly by the noise level in the receiver. Some noise arrives from outside sources, and other noise is generated inside the receiver. At the VHF UHF range, the internal noise is predominant, so it is common to use a low-noise preamplifier ahead of the receiver. This preamplifier will reduce the noise figure for the entire receiver. If you select a commercial ready-built or kit VHF preamplifier, such as the units sold by Hamtronics, Inc. (65...

Outer circle parameters

The standard Smith chart shown in Fig. 26-4C contains three concentric calibrated circles on the outer perimeter of the chart. Circle A has already been covered and it is the pure reactance circle. The other two circles define the wavelength distance (B) relative to either the load or generator end of the transmission line and either the transmission or reflection coefficient angle in degrees (C). 1 ILtTT It nni * fcl* P*IH 'it rtl U1 IMFCOAHCC 4f> ADMITTANCE COORDINATES There are two scales...

Parallelresonant circuits

The parallel-resonant tank circuit (Fig, 2-24A1 is the inverse of the series-resonant circuit. The line current from the source splits and flows separately in the inductor and capacitor. The parallel-resonant circuit has its highest impedance at the resonant frequency and a low impedance at id other frequencies. Thus, the line currem from the source is minimum at the resonant frequency (Fig. 2-24B') and the voltage across the LC tank circuit is maximum. This fact is important in radio tuning...

Phase locked loop FMPM detectors

The phase locked loop (PLL) circuit can be used as an FM demodulator if its control voltage is monitored. Figure 27-27A shows the basic PLL circuit. It consists of a voltage-controlled oscillator (VCO), phase detector, iow-pass Filter, and an amplifier. The FM signal from the IF amplifier is applied to one port of the phase detector, and the output of the VCO is ooiinected to the other port. When the two frequencies are equal, there is no output from the circuit (or, the value is quiescent)....

Problems with RFIF transformers

The IF and RF transformers represent a high potential foi intermittent problems in radio receivers There are I wo ba-sic forms of procerus with the IF transformer in-termittetit operation ajid intermit rent noise. The beyT < nrc for a bad IF or RF irans-former is replacement, but be< ause old IF mid RF transformers art imi always available Unlay, more emphasis musi be place, on repair of i he transfnniKT Figure shows (he basic circuit f< > r n siugle-iuned IF transformer (others might...

Receiver circuits you can build

One of the joys of experimenting with electronic circuits is building a project that not only works well but is useful and provides a certain amount of either enjoyment or utility later on. Radio receivers fall into that category but are generally considered advanced projects that cannot be accomplished by the average experimenter. Many people erroneously believe that only kit-built receivers are candidate projects for them because of the alleged complexity of radio receivers. However, modem...

Selecting transistors

The criteria lor selecting bipolar or field-effect transistors in the microwave range depends on application. The importance of noise figure, for example, l> eeomes apparent in the front end of a microwave satellite receiver system. In other applications, power gain and output might be more important. Obviously, j ain and noise figure are both important especially in the front ends of receiver systems. For example, earth communications or receiver terminals rypicaily use a parabolic dish...

Semiconductor overview

Charge carrier mobility is a gauge of semiconductor material activity and is measured in units of emW-s. Electron mobility is typically more vigorous than hole mobility as Table 23-1 demonstrates. Notice particularly the spread for Group lll-A semiconductors. A bipolar transistor is one that uses both types of charge carriers in its operation. In other words, both electrons and holes are used for conduction meaning that both N-type and P-type materials are needed. Figure 22-20 shows the basic...

Skin effect

There are three reasons why ordinary lumped constant electronic omiponenls do not work well at microwave frequencies The ilrsl, mentioned earluT in i his rhap-Icr, is that component size and lead ler& rhs approximate nik roivavc wavelengths. Table 1-5, Old U.S. military microwave frequency bands (WWII-1 70) Table 1- . New U.S. military microwave frequency bands Post-19701 Band designation Frequency range Tabk 1-7. IEEE Industry standard frequency bawls 3-30 MHz MHz 300-1000 MHz 1000-2000 MHz...

Some actual measurements

Figure 24 1 shows two conditions that often show up on transmission lines open and short-circuited. It is rarely of practical difference which occurs because both need to be corrected at the antenna end. However, should it be important, you can- Tinie-dwwain rejtectometry on a budget not differentiate which is which by the VSWR reading- In both cases, the entire incident wave is reflected. The only difference is the location of the nodes and anti-nodes, which are out of phase with each other....

Special medical EMI problems

EMI problems exist in medical electronic devices such as electrocardiograph (ECG) or electroencephalograph (EEG) machines. These machines have to process signals on the order of a few microvolts ( xV) to about 1 millivolUjnV) in the presence of strong interfering signals. In addition to regular RFI problems, there are two additional problems. First, there is the problem of the defibrillator. This is a high-voltage (several kilovolts ) capacitive discharge device used to jump start the heart of...

Successive detection logarithmic amplifiers

Where signal level information is required, or where instantaneous outputs are required over a wide range of input signal levels, a logarithmic amplifier might be used. Linear amplifiers have a gain limit of about 100 dB, but only have head room of 3 to 6 dB in some circumstances. One solution to the problem is the logaritlimic amplifier. Radar receivers frequently use log amps in the IF amplifier stages. The successive detection method is used because it is difficult to produce high-gain...

The RF hybrid coupler

The hybrid coupler (Fig. 12-1) is an AF or RF device that will either (a) split a signal source into two directions or (b) combine two signal sources into a common path. The circuit symbol shown in Fig. 12-1 is essentially a signal path schematic. Consider the situation where an RF signal is applied to port 1, This signal is divided equally, flowing to both ports 2 and 3, Because the power is divided equally the hybrid is called a 3-dB divider, i.e., the power level at each adjacent port is...

The Smith chart

The mathematics of transmission lines, and certain other devices, becomes cumbersome at times, especially when dealing with complex impedances and nonstandard situations. In 1939, Philip H. Smith published a graphical device for solving these problems, followed in 1945 by an improved version of the chart. That graphic aid. somewhat modified over time, is still in constant use in microwave electronics and other fields where complex impedances and transmission line problems are found. The Smith...

Transient noise

A transient is any temporary (< 16.6 ms) overvoltage or overpower condition. Transients are either repeatabte or random in nature. An example of the repeatable type of transient is the discharge of an inductor or capacitor. Examples of random-type transients include electrostatic discharge (ESD), lightning, and the nuclear electromagnetic pulse. In the case of lightning, Fig. 15-2 shows the exposure of the U.S. electrical power system to lightning strikes. Clearly, high-voltage lines are...

Transmission line problems

Figure 26-tf shows a 50-ft transmission line connected to a complex load impedance, Zl, of 36 + j40 ii. The transmission line has a velocity factor (r) of 0,80, which means that the wave propagates along the line at 810 the speed of light (f 300,000,000 m s). The length of the transmission line is 28 cm. The generator (Vm) is operated at a frequency of 4,5 GHz and produces a power output of 1.5 W. See what you can glean from the Smith chart (Fig. 26-7). f 4.5 GHz 4.5 X 10 Hz Z,s 50 11 Zs 50 ft...

Types of oscillator circuits

The two major categories of oscillator circuits in electronics textbooks are relaxation oscillators and feedback oscillators. The relaxation oscillator uses some sort of voitage-breakdown device, such as a neon glow-lamp or imyunction transistor. The feedback oscillator (Fig. 10-1) uses an amplifier circuit and a feedback network to start and sustain oscillations on a particular frequency. Most oscillators that are useful for sine-wave signal generator circuits are of the feedback oscillator...

UHF and microwave diodes transistors and integrated circuits

Veiy few people needed to know anything about microwaves and microwave devices until now. For many decades, all service technicians and others only needed to know about devices that worked up to the highest UHF TV channel, which was just short of the microwave region. Even then, many local servicers would consider the entire UHF TV tuner as a black box component and replace it as a unit when it went bad. But today, with satellite TVRO units increasingly popular, two-way communications heading...

UHF and microwave RF transistors

Transistors were developed right after World War It and by 1955 were being used tn consumer products. Those early devices were limited to audio and Jow-RF frequencies, however. As a result, only solid-state audio products and AM-band radios were widely available in the 1950s. Development continued, however, and by 1963, solid-state FM broadcast and VHF communications receivers were on the market. Microwave applications, however, remained elusive. Early transistors were severely...

Units of capacitance

Capacitance (< is a measure of a capacitors ability hi store current or, more properly, electrical charge. The principal unit of capacitance is (he farad (F) (named after physicist Michael Faraday). One farad is the capacitance ihat will tfi> re one coulomb of electrical charge ( > .28 x 101S ekf.'Irons) ai an electrical potential nf 1 V, or, in math form 2-12 (A) Capacitors consist of a pair of conductors separated by a dielectric (insulator) (B) parallel plate capacitor. 0 electrical...

Use with receive antennas

Examples given above combine a receiver anil transnutter on a single antenna or antenna system. It is also possible to use the hybrid for antenna arrays intended for receivers. Antenna spac ed some distance A' apart will have different patterns and gains depending on the value of X and the relative phase of the currents in the two antennas. One can, therefore, connect the antennas to ports 2 and 3 and the receiver antenna input to port 1. A terminating resistor would be used at port 4. You can...

VHF problems

Figure 25-3A shows the block diagram of u F. 1 reviver Tim Lo (example in Fig- 2iV3B) is kept, on channel by a dc control vofhige Irorn the auioinai ic i'reqiienc control (AFC) output of die FM detector stag.'. The aciital mechanism of frequency control is ihe variable capacitance tvaracrori diode in (he oscillator circuit t)f in Fig. 35-3B). This type of diode is used in both U' luned circuits, is used in broadens 25-2 (A) Colpitis usdllai.or and (0) crystal i.'ttJpjits oscillator. receivers,...

Vidocassette recorder I VCR

The VCR is a magnificent entertainmnu product, and 1 own one. But I also own SWL and ham radio receivers, and I can always tell when a popular movie is on TV, Because lots of people videotape the movie (legalities notwithstanding), their VCRs are in operation for a couple of hours at a time. The VCR contains a number of radiation-producing circuits, including a 3-58-MHz color subcarrier oscillator On popular TV nights, I can hear a load of trash around that frequency, which is right in the...

Voltage and current in capacitor circuits

Consider the circuit in Fig- 2-18 an ac source connected in parallel with the capacitor, It is the nature of a capacitor to oppose these changes in the applied voltage (the inverse of the action of an inductor). As a result, the voltage lags behind the current by 90 degrees. These relationships are shown in terms of sine waves in Fig. 2-18B and in vector form in Fig. 2-18C. 2-18 (A) Ac ( ircult usuvg a capacitor (B) sine-wave relationship between V and , (C) vector represent atioiv Do you...

VSWR method

When a load impedance (_R + jX) is connected across an RF source the maximum power transfer occurs when the load impedance ( J and source im > od-ances are equal 2 , * If these impedances are not equal, then the voltage standing wai e ratio (VSWR) will indicate the degree of mismatch. We can use this phenomenon to measure values of inductance and capacitance using the scheme shown in Fig. 16-1 A. The instrumentation required includes a signal generator or other signal source and a VSWR meter...

Whats that mess coming from my receiver

AM-band, shortwave, and VHP monitor receivers oft,en produce output signals that are a terrible mess of weird squawks, squeals, and other assorted objectionable noises. Shortwave receivers in particular show a lot of weird signals, but the growing number of local electronic products are making the cacophony louder and messier than ever before. Fortunately, many of the interference problems that you'll see on your receiver can be fixed or at least reduced significantly in severity, Inierference...

Wiring boards

Radio-frequency projects are best constructed on printed circuit boards that are specially designed for RF circuits. But that ideal is not always possible. Indeed, for many hobbyists or students, it might be impossible, except for the occasional project built from a magazine article or from this book. This section presents a couple of alternatives to the use of printed circuit boards. Figure 1-4 shows the use of perforated circuit wiring board (commonly called perfboard). Electronic parts...

X

2'9 (A) Resistor-inductor (RL) circuit (B) vector relationships. Ait air-core inductor actually lias no core so it might also be called a careless coil Although it ran be argued thar the performance of an inductor differs between air and vacuum cores, the degree of difference is so negligible as to fall into the dec-imal dusf categoiy. Three different forms of air-core inductor can be recognized. If the length ( > ) of a cylindrical coil is greater than, or equal to, I he diameter (rf J then...

Vectors for RF circuits

A vector (Fig, 18-1 A) is a graphical device thai, is used to define the magnitude and direction (both are needed) of a quantity or physical phenomena. The teiigth of the arrow defines the magnitude of the quantity, while the direction in which it is pointing defines the direction of action of the quantity being represented Vectors can be used in combination with each other. For example, in Fig. 18-1B we see a pair of displacement vectors that define a starting position (Pi) and a final...

Hum

Hum is caused by the alternating current (ac) power lines and may be radiated into the DCR through the antenna circuit, radiated into the wiring of the set, caused by ground loops, or communicated to the DCR circuits as ripple in the dc power supply. In the first two instances, the hum will have a frequency of 50 Hz (Europe) or 60 Hz (North America), which are the ac power-line frequencies in the latter two cases, it will be 100 Hz (Europe) or 120 Hz (North America) if full-wave rectified dc 1...

Interpreting radio receiver specifications

Radio receivers are a key element in radio communications and broadcasting systems. This chapter presents some of the more important receiver specification parameters. It will help you understand receiver spec sheets and lab test results, A radio receiver must perform two basic functions It must respond to, detect, and demodulate desired signals It must not respond to, detect, or be adversely affected by undesired signals. Both functions are necessary, and weakness in either makes a receiver a...

Low audio output

One of the frequent complaints about DCRs is that the audio level is too low for comfortable listening. The mixer output level is very low, so a DCR typically requires L1 27 turns, 28 wire on Amidon T-50-2 core L2 33 tums, 28 wire on Amidon T-50-2 eore 6-5 Two filters for eliminating AM broadcast band interference. a large amount of gain in the audio amplifier chain to produce even minimal levels of power to earphones or loudspeakers. Additional gain and a reasonable power amplifier can be...

Combining inductors in series and in parallel

When inductors are connected together in a circuit, their induetances combine similar to the resistances of several resistors in parallel or in series. For inductors In which their respective magnetic fields do not interact, the following equations are used 2-5 Inductors in an antique radio set. In the special case of the two inductors in parallel If the magnetic fields of the inductors in the circuit interact, the total inductance becomes somewhat more complicated to express. For the simple...

Building IF amplifiers

Most of the gain and selectivity of a superheterodyne radio receiver are in the intermediate frequency (IF) amplifier. The IF amplifier is, therefore, a high-gain, narrow -bandwidth amplifier. Typically, IF power gams run in the 60- to 120-dB range, depending on the receiver design. It usually has far narrower bandwidth than the RF amplifier For example, 2,8 KHz for an SSB receiver and 500 Hz for a CW receiver. The purpose of the IF amplifier is to provide gain and selectivity to the receiver,...

Cyq

7-11 (A) Untuned (broadband) coupling transformer and (B) winding the transformer on a toroid. Take special note of those two transformers. These transformers are known generally as wideband transmission-line transformers and can be wound on either toroid or binocular ferrite or powdered iron cores. For the project at hand, because of the low frequencies involved, 1 selected a type BN-43-202 binocular core. The type-43 material used in this core is a good selection for the frequency range...

A hypothetical radio receiver

Figure 9-1 shows the block dis gram of a simple communications receiver. We will use this hypothetical receiver as the basic generic framework for evaluating receiver performance. The design in Fig. 9-1 is called a superheterodyne receiver at d is representative of a large class of radio receivers it covers the vast majority of receivers on the market. Other designs, such as the tuned radio frequency (TRF) and direct-conversion receivers (DCR), are simply not in widespread commercial use today....

Receiver improvement strategies

There are several strategies that can improve the performance of a receiver. Some of them are available for after-market use with existing receivers and others are only practical when designing a new receiver. One method for preventing dynamic effects is to reduce the signal level applied to the input of the receiver at all frequencies. A front-end attenuator will help here. Indeed, many modern receivers have a switchable attenuator built in to the front-end circuitry. This attenuator will...

B

2-24 (A) Parallel-resonant LC circuit (B) and Z vs frequency response. Vnftnm- tj't'i entrant in t Hj'Ui ikn' V'. ruiis 45 Choosing component values for LC-resonant tank circuits In some cases, one . f the two basic ty e of resonant., tank cirruil (either series< r parallel) is clearly preferred over tin1 other. hu in other eases oil her rot lit I ho used if il is used correo ly. Por example, in a trap (.i.e., a circuit that pre ems a particular frequency from passing) a parallel-res* nam...

Lowpass highpass bandpass and notch

FUters are frequency-selective circuits that pass some frequencies and reject others. Fillers are available in several different flavors low-pass, high-pass, bandpass, and notch. All of these filters are classified according to the frequencies that they pass (or reject). The breakpoint between the accept band and the reject band is usually taken to be the frequency at which the passband response falls off -3 dB. The four different types are characterized below. Low-pass filters pass all...

Units of measure

Input signal levels, when specified as a voltage, are typically stated in either microvolts (pnV) or nanovolts (nV) the volt is simply too large a unit for practical use on radio receivers. Signal input voltage (or sometimes power level) is often used as part of the sensitivity specification or as a test condition for measuring certain other performance parameters. TVvo forms of signal voltage that are used for input voltage specification source voltage (Vemf) and potential difference ( d), as...

The pulse source

Almost any source of square waves in the vicinity of 1 MHz can be used for ihe pulse generator. If you have one of those function generators thai oui pul pulses h> 1 MHz or more, then use it. Be careful, however, il the output impedance is 600 l (as some are). For those a you might want to fashion a GOO- to 50-fl transformer or a simple resistor pad. Alternatively, you can build your own pulse sourre. Many different forms ofTTL oscillators can be used as a pulse source, Figure 24-2A showK the...

Inductor circuit symbols

Figure 2-6 shows various circuit symbols used in schematic diagrams to represent inductors. Figures 2-6A and 2-6B represent alternate, but equivalent, forms of lhe same thing i.e., a fixed value, air-core inductor (coil in the vernacular). The other forms of inductor symbol shown in Fig. 2-6 are based on Fig, 2-6A but are just as valid if the open-loop form of Fig- 2-6B is used instead. The form shown in Fig, 2-6C is a tapped fixed-value air-core inductor. By providing a tap on the coil,...

PIN diodes and their uses

Most modem radio transceivers (i.e., frafisnutter rect'iuer) use relayless switching to go back and forth between the receive and transmit states. In many cases, this switching is done with PIN diodes. Similarly, IF filters or front-end bandpass fillers are selected with a front-panel switch that handles only direct current- How Again, PIN diodes. These interesting little components allow us to do switching at RF, IF, and audio frequencies without routing the signals themselves all over the...

Frequency shift problems

Resonant circuits in modern electronic equipment might be LC-tuned by a combination of inductance and capacitance or tun d by & piezoelectric resonator element (crystal). In either event, the cause of a sudden unwanted shift of operating frequency is usually some form of mechanical srauma somewhere in rhe circuit. In other words, some component is either broken or has an intermittent connection. Figure 25-1 shows a partial circuit of an oscillator. The resonance t f this c-irtmi is...

Considerations for good DCR designs

It probably does not surprise many readers that there are some principles of good design that result In superior DCR performance. Some of these principles were discussed by Campbell10 and others.11 Even relatively simple DCR designs, including those based on the Signetics NE-602 integrated circuit double-balanced modulator12 and the I> opular LM-386 audio amplifier, have proven to be very sensitive and free of hum and jnicrophonics, even though that combination is not without critics....

Radio receivers theory and projects

One of the things that fascinated people about radio from its very earliest days is that signals arrive seemingly by magic through the air from long distances, t was very quickly discovered that radio signals are electromagnetic waves exactly like light and infrared, except for frequency and wavelength. Radiowaves have a much lower frequency than light therefore, the wavelengths are much longer. The wavelengths of radio signals range from around 25,000 m at the low-VLF raivge down to...

R1

Where V is Vj and R is Rt or is V- and R is R depe tiding on which voltage drop Is being measured. We can use the voltage divider concept to find either inductance or capacitance by replacing R with the unknown reactance, Consider first the inductive case. In Fig. 16-2B resistor has been replaced by an inductor (LThe resistor Ri is the inductor series resistance. If we measure the voltage drop across (i.e., in Fig. 16-2B) then we can calculate the inductance from As can be noted in Eq. (16-5)...

Installing coaxial connectors

One of the mysteries faced by newcomers to the radio hobbies is the small matter of installing coaxial connectors. These connectors are used to electrically and mechanically fasten the coaxial cable transmission tine from the antenna to the receiver There are two basic forms of coaxial connector, both of which are shown in Fig. 1 -13 (along with an alligator clip and a banana-tip plug for size comparison)- The larger connector is the PL-259 UHF coimector, which is probably the most-common form...

Signal generator for the AM and shortwave bands

The same general type of circuit can be used for the AM band (530 to 1610 kHz) or for the shortwave bands (1610 kHz to 30 MHz). The version shown in Fig- 10-7 uses a standard transformer that has a 217-(mH inductance in the primary winding, Tliis particular example uses a Toko coil from Digi-Key (Part No. TK-I903) in conjunction with a 365-pF broadcast variable capacitor to cover the AM broadcast band. For other frequencies or if F and L are known but C is unknown or if F and C are known but L...

Bandwidth of RFIF transformers

Figure 2-31A shows a parallel-resouam RF IF transformer and Fig- shows the usual construciion in which the two coils and arc wound at distance d apart on a common cylindrical form. The bandwidth of the KF'IF transformer is the difference between the frequencies where the signal voltage across the otil put winding falls off 6 dB from the value at the resonara frequency (F(i), us shown in Fig. 2-31C- If F arid F> are 6-dB (also called the 3-< iB jiohit when signal power is measured instead of...

Basic theory of operation

The DCR is similar to the superheterodyne in underlying concept The receiver radio frequency (RF) signal is translated in frequency by nonlinear mixing with a local oscillator (LO) signal (heterodyning). Figure 6-J shows the basic block diagram for the front end of both types of receiver. The mixer is a nonlinear element that combines the two signals, Frf andFbo. The output of the mixer contains a number of different frequencies that obey the relationship Frp is the frequency of the received...

MOO iH foe lowband VHF

7-Z JFET active preselector circuit. source bias is supplied by the voltage drop across resistor R*2, and the drain load is supplied by a series combination of a resistor and a radio frequency choke (RFCl), The RFC should be 1000 p,H (1 rnH) at the AM broadcast band and HF (shortwave), and 100 jlH in the low-VHF region (> 30 MHz). At VLF frequencies (below the AM broadcast band), use 2,5 mH for RFCl and increase all 0.01-jlF capacitor values to 0J All capacitors are either disk ceramic or one...

AM envelope detectors

An amplitude modulation (AM) signal consists of a slow audio signal which revolves around an average radio frequency (RF) carrier signal. It is essentially a multiplication or mixing process in which the RF carrier and AF signals are both output, along with the (RF AF) and (RF + AF) signals. Because of the selectivity of the transmitter circuits, only the RF carrier and the sum and difference signals appear in the output. The AF signal is suppressed. The sum signal (RF 4- AF) is known as the...

Special variable capacitors

In the preceding sections, the standard forms of variable capacitor were covered. These capacitors are largely used for tuning radio receivers, oscillators, signal generators, and other variable-frequency LC oscillators. This section covers some special forms of variable capacitor The split-stator capacitor Is one in which two variable capacitors are mounted on the same shaft. The split-stator capacitor normally uses a pair of identical capacitors, each the same value, turned by the same shaft....

Common mode and differential currents

Noise currents can flow in two modes differential and common mode. These are defined as follows. 15-i Simple L-section filters (A) internal capacitor input type and (B) external inductor input type. 15'7 Several situation* and the filters to implement tliem. 15'7 Several situation* and the filters to implement tliem.

Discriminator circuits

One of the classic FM discriminator circuits is shown in Fig. 27-21. This circuit uses a special transformer that has two secondary windings. One secondary winding is turied slightly above the IF frequency while the other is tuned the same amount below the IF frequency. The two frequencies are spaced slightly more than the expected transmitter swing. Their outputs are combined in a differentia pair of diodes (Py and The outputs of the diodes are connected to load resistors R and Normally, when...

The Smith chart in lossy circuits

Thus far, you have dealt with situations in which loss is either zero (i.e., ideal transmission tines) or so small as to be negligible. In situations where there is appreciable loss in the circuit or line, however, you see a slightly modified situation. The VSWR circle, in that case, is actually a spiral, rather than a circle- Figure 26-10 shows a typical slttiation, Assume that the transmission line is 0.60A long and is connected to a normalized load impedance of Z 1.2 + j 1.2. An MT UBTKt UM...

Prafll

4-7 (A) Flat mounting (B) on-end mounting (C) secured mounting (use nylon machine screws) (D) mounting high-power or high-voltage toroidal inductors or transformer s (E) suspending toroid inductors on a dowel (F) mounting method for a 'single-turn primary transformer in RF watt-meters or VSWR meters. are passed through holes in the board to solder pads underneath. The method of Fig, 4 7B places the toroid at right angles to the board, but it still uses the leads soldered to copper pads on the...

HFVHF buffer amplifier

A buffer amplifier is used to isolate the output of the oscillator from the load, or circuits that follow. Variations in the load of an oscillator can pull the frequency in* correctly so the buffer is used to prevent that problem. Figure 10-5 shows the circuit for a buffer amplifier that can be used in the low-frequency (LF)f high-frequency (HF), and the lower end of the VHF ranges- The amplifier device w the buffer amplifier is a 40673 dual-gate MOSFET transistor (or NTE-222, which is a...

JFET preselector circuits

Varicap Schematic Diagram

Figure 7-2 shows the most basic form of JFET preselector. This circuit will work into the low-VHF region. This circuit is in the common source configuration, so the input signal is applied to the gate and the output signal is taken from the drain. The Figure 7-2 shows the most basic form of JFET preselector. This circuit will work into the low-VHF region. This circuit is in the common source configuration, so the input signal is applied to the gate and the output signal is taken from the drain....

Cl000 Lowlii fain

A type 7809 three-terminal voltage regulator JC will provide 9-Vdc regulated output from a + 12-V (or higher) dc power supply. The use of a voltage-regulated power supply, as described, will also help one problem with this design. The tuning is a function of applied voltage, so it will necessarily change when the battery voltage drops. The receiver in Figs, 5-27 and 5-28 is a superheterodyne model, meaning that it converts the input RF frequency to a lower intermediate...

D

5-26 IF amplifier project (A) completed project (B) input end (C) output end (D) input amplifier stage and (E) output amplifier circuit. when the ATTN is high (i.e., 2.4 V < V < 5.2 V ), then K is energized and inserts the -20-dB attenuator in the path. The IF amplifier of Figs. 5-25 and 5-26 provides about 80 dB of gain at 455 kHz.

Actual circuit details

The actual RF circuit is shown in Fig. 7-12 it is derived from a similar circuit found in Doug DeMaw's excellent book WIFB's QRP Notebook (ARRL, 225 Main Street, Newington, CT 061U). The active amplifier devices are JFETs that are intended for service from dc to VHF. The device selected can be the ever-popular MPF-102 or its replacement equivalent from the SK, ECG, or NTE lines of devices. Also useful is the 2N4416 device. The particular device that I used was the NTE-45I JFET transistor. This...

Tuned radio frequency TRF receivers

The TRF receiver (Fig. 13-3) uses a cascade chain of tuned RF amplifiers 04 Ai, andji3) to amplify the radio signal. The TRF was the first really sensitive design in the early 1920s and was eclipsed by the superhet in popular commercial receivers, But in the VLF range, the TRF is stilt popular especially among homebrewers. A problem with the TRF receiver is the possibility of unwanted oscillations, wliich are common in tuned triode devices, such as MPN bipolar transistors. Peter Taylor's column...