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...

B

2-10 (A) Solenoid wound inductor with transformer coupling liitk (B) inductor used as a uink circuit in radio transmitter (C) tapped inductor (D) tapped inductor on mobile antenna (E) roller inductor. The air-core coil shown in Fig. 2-IOE is a rotary inductor of the type found in some HF transmitters, antenna-tuning units, and other applications where continuous control of frequency is required. The inductor coil is mounted on a ceramic form that can be rotated using a shaft protruding from one...

Bnf

10-9 Balanced mixer with low-pass filter output for mixing two RF sources. (A) RF low-pass filter connected to balanced mixer, (B) physical implementation. A variation on the theme is shown in Fig. 10-8C. Although the output in Fig, 10 8A is taken from the emitter of the transistor, the variation in Fig. 10-8C uses a secondary winding over the tuning inductor (L ) to output the signal. Another variation useful for a wide variety of circuits is shown in Fig. 10-9A, and an actual example prototy...

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...

Breakdown voltage

The capacitor works by supporting an electrical field between two metal plates. This potential, however, can get too large When the electrical potential (i.e., the voltage) gets too large, free electrons in the dielectric material (there are a few, bul not too many, in any insulator) might flow. If a stream of electrons gets starlet , then the dielectric might break down and allow a current to pass between the plains. The capacitor is then shorted. The m ximum breakdown voltage o the capacitor...

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 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...

Common mode

In common mode conduction noise appears in multiple conductors flowing in the same direction. Hie filtering necessary for differential and common mode filtering is different. In differentia mode filtering it is necessary to place a filter m series with the hot lead, with suitable bypassing to ground. In the common mode case the same filtering must be applied to all affected leads. Figure 15-9 shows a filter that is suitable for both differential and common mode forms of EMI. Common mode and di...

Computer EMI

The case of computer EMI is very serious. Just place an AM radio anywhere close to a modern computer and you will hear lots of hash. In fact, with computer clock speeds reaching several hundred megahertz, the interference to FM radios can be tremendous. Figure 15-13 shows a method for connecting a digital connector pin that can cany EMI to a printed wiring board. The ferrite beads act like little RF chokes so will eliminate RFI in the VHF UHF region. Because the filtering is bidirectional, it...

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

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...

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...

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...

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...

FTr

f you make an RF JF transformer, then there will be two windings. Try to separate the primary and secondary windings if bot h are tuned. If one winding is not tuned, then simply wind it over the cold (i.e., ground) end of the timed winding no separation is needed. The Amidon coil forms are tight, but they do have sufficient space for very small disk ceramic capacitors inside. The 56-pF capacitors that I selected fit nicely inside the shielded can of the coil, so 1 elected to place it there....

Highpass filters

A high-pass filter (HF'F) attenuates signal frequencies below the -3-dB cutoff frequency and passes signal frequencies ahove the cutoff frequency. Single-section high-pass filters are shown in Figs. 23-6A and 23-6B. These are the inverse of the L.PF single-sect ion filters in that the capacitors are in series with the signal path, and the inductors are across the signal path. The version in Fig. 23-6A is the t-filter configuration, and that in Fig. 23-GB is the pi-filter. The values for the...

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...

Light dimmers

A lot of homes are equipped with dimmers instead of switches to control the lighting. These devices are based on silicon-ron trolled rectifiers (SORs) tliat cut the ac sine wave off over part of its cycle, producing a harmonic-rich sharp waveform. These devices can produce a sound described as frying eggs well into the shortwave spectrum. Diagnosis is simple turn off the light. If the noise stops when the light is turned off, then the dimmer is at fauji. Although it is possible to install LC...

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...

Lsi

5-23 (A) LM-386 audio amplifier integrated circuit (B) LM-3S6 standard circuit. provide two levels of gain. If capacitor C, is used, then the gain is X2G0, if the capacitor is deleted, the gain is X20. Normally, the gain will he set to X20 in receiver projects unless the design uses little gain ahead of the detector (which might, be the case in the simplest superhets or in direct-conversion designs). The LM-386 is relatively well-behaved in projects, which means that it takes only ordinary care...

Means of EMI transmission

EM is transmitted from the source to the victim system in I wo basic ways conduction and radiation. The difference is that the EMI (ravels along a wire in conduction and travels by air in radiation. In general (but not always), radiation (Fig. 15-1 A) occurs at high frequencies (> i() MHz) and conduction (Fig. 15-1B) occurs at low frequencies (< 30 MHz). In some cases, Lwrth radiation and conduction can occur. In those cases, either radiation occurs first and ihen the wave is conducted into...

Measuring inductors and capacitors at RF frequencies

The measurement, of the values of inductors (L) and capacitors (C) at radio frequencies differs somewhat from the same measurements at low frequencies. Although similarities ejdst, the RF measurement is a bit more complicated. One of the reasons for this situation is that stray or distributed inductance and capacitance values of the test set-up will affect the results. Another reason is that capacitors and inductors are not ideal components but rather all capacitors have some inductance and all...

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...

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...

Problems with older equipment

Certain older amateur equipment exhibits a problem in frequency shift that ts reused by ihe use of ferriie cotes in the VTO atul ircoils. l-Vrrit-e cores age with time and beat and as a result have a different permeability than when the equipmeti' was first calibrated. I have seen th it problem even on tin1 legendary oltins perineal iility-tlined oscillator (PTO) used in their amateui siid commercial communications equipment. In some cases, a simple realignment will suffice to bring Lhc coil...

Pulsecounting detectors

Tram that varies with the audio modulation applied to the input signal, It is realized as audio in the low-pass filter consisting of R-i-G . Another circuit, shown in block form, used a zero-crossing detector and a limiter amplifier to eliminate the AM excursions that are noise to an FM PM signal. The output of the zero-crossing detector triggers a monostable multivibrator circuit- The output of the monostable is a pulse train that varies according to the modulating frequency applied. This is...

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...

References

ARRL Handbook for Radio Amateurs CD-ROM Version 1.0 (1996). Newington, CrT ARRL. Carr, J. J. (1998). Practical Antenna Handttook 3rd Edition. New York McGraw-Hill. Carr, J.J, (1997). Microwave and M wte, Co+wmumciUioiis iy> ch rvoiogy. Boston Newnes. Carr, J, J. (1996). Secrets of RF Circuit Design 2nd Edition. New York McGraw-Hill. Hagen, J. B, Ci996). Radio-Freqzie icy Electronics Circuits and Applications. Cambridge (UK) Cambridge Univ. Press. Hardy, J. (1979). High Frequency Circuit...

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...

Seriesresonant circuits

The series-resonant circuit (Pig. 2-20A), like other series circuits, is arranged so that the terminal current ( ) from the source (V) flows in both components equally. Z - V J82 + CA'l Xtf Z jRZ + (0 )2 Z - JR2 R 2-20 (A) Series inductor-capacitor (IX) ciriruir (B) parallel LC circuit (C) vector relationships The vector diagrams of Fig, 2-21A through 2-21C show the situation under three different conditions Figure 2-21A The inductive reactance is larger than the capacitive reactance, so the...

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...

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...

Tuned gain block TGB receivers

The TGB receiver (Fig. 13-4A) Is basically a variant of the TRF receiver, except that the tuning circuits are all up front, ahead of the gain. Two benefits are realized. First, the oscillation problem of conventional TRF receivers is avoided because the gain block is untuned. Second, the tuning up front eradicates much of the unwanted noise prior to it being amplified. The actual circuit is shown in Fig. 13-4B. The three amplifier stages consist of high-gain NPN transistors, such as the 2N4416....

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...

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,...

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...

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...

Superheterodynes

The superhet (Fig. 13-1) is the basic receiver design used in communications and broadcast receivers, it dates from the 1920s and is the most successful receiver design, in the superhet receiver, the incoming RF signal (at frequency F ) is filtered by a tuned RF-resonant circuit or a bandpass filter then applied to a mixer circuit. In most cases, the RF signal is amplified in an RF amplifier (Fig. 13-1), but that is not a necessary requirement. The mixer nonlinearly combines Fi with the signal...

Project

A radio direction-finding antenna can be used for a number of purposes, only one of which is finding the direction from which a radio signal arrives. Another use is in suppressing cochannel and adjacent-channel interference. This becomes possible when the desired station is in a direction close to right angles from the line between the receiver and the desired transmitter. Reduction of the signal strength of the interfering signal is possible because the loopstick antenna has nulls off both...

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....

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...

Drift problems

The other frequency changing problem is old-fashioned drift. Unfortunately, most electronic components exhibit some temperature sensitivity. This sensitivity is usually measured in terms of a temperature coefficient, which specifies a certain shift of value in pans per million (ppm) per degree Celsius of temperature shift. The temperature coefficient (TV) can be either positive or negative. A positive temperature coefficient (FT,-) indicates the value increase with increases in temperature. A...

Microwave ovens

Most American homes are equipped with microwave ovens. These devices use a magnetron tube to produce several hundred watts of microwave power on a frequency of approximately 2450 MHz. The high voltage applied to the maggie is typically pulsating dc i.e., it is rectified ac, but not rippie-filtered (as are true dc power supplies)). This pulsating dc causes Tiash in radio receivers. Although better-quality microwave ovens are equipped with EM filters, many are not. However, most manufacturers or...

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...

Varactor applications

Varactors are electronically variable capacitors. In other words, they exhibit a variable capacitance that is a function of a reverse bias potential. This phenomenon leads to several common applications in wl ch capacitance is a consideration. Figure 3-14 shows a typical varactor-tuned LC tank circuit. The link-coupled inductor (I2) is used to input RF to the tank when the circuit is used for RF amplifiers (etc.), The principal LC tank circuit consists of the main inductor (Li) and a...

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,...

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...

Solving frequency drift problems

Few circuit problems arc less welcomed thitn frequency drift. This problem is chat acterized by the radio transmitter or receiver changing frequency not under the influence of Tire operator. Actually, two different problems are seemingly related drift and shift. Drift is a gradual change of frequency, usually as a function of temperature. Shift., on the other hand, is an abrupt cltange nf frequency The causes of these two related phenomena are different, but. they are often confused with each...

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....

Circuit Board

15-13 Computer connector fitted viith ferrite beads. The principal offender with respect to noise from computer systems is the monitor, This is because of two factors. First, the deflection circuits tend to operate in frequency ranges (under 40 kHz) that are below many other systems and they have lots of harmonics- Second, those deflection circuits tend to be high-power. The answer to the probiem is to place shielding around the circuits and a common mode choke in the signal line. EMI...

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

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....

Phaseshifted hybrids

The hybrids discussed thus far split the power half to each adjacent port but the signals at those ports are in-phase with each other. That is, there is a 0 phase shift over the paths from the input to the two output ports. There are, however, two forms of phase-shifted hybrids. The form shown in Fig. 12-6A is a 0-l80 hybrid. The sig- 11-5 Combining two antennas in T R switch, nal ovet the path from port 1 to port 2 is not phase-shifted (0 ) while that between pott 1 and port 3 is phase-shifted...

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...

Heat problems

Because the root of many drift problems is the tenliberal ure coefficient of capacitors and inductors, it seems obvious thiit lemperaimv needs to be com trolled in radio equipment. In the past, several otherwise we I -regarded pieces of equipment suffered drift because of the tremendous heat inside the cabinet. Ventilation tnd a blmver might help in some eases In other cases, using a little uisuhiion in and around tic offending oscillator is also helpful. In certain erysial oscillator c ire...

I

Used in the low-HF region (< 10 MHz), 0 001 t.F can be used up to 100 MHz, and 100 pF above 100 MHz. The RF choke (RFC ) should be 2.5 mH in the low-HF region, 1 mH from about 10 to 30 MHz. 100 piH from 30 to 100 MHz, and IO41H above 100 MHz. These values are not critical and are given only as guidelines. Although it might be a bit t ricky to get a 1 - mH RFC to operate well at 100 MHz, there is no really hard boundary for these bands. 20-19 Postamplifiers (A) NPN and (B) MAR x. Doubly...

Ab

1*8 SMelded RF construction (A) dosed box showing dc connections made via coaxial capacitors (B) box opened RF construction (e.g., layout, grounding, and shielding) and found that by following the rules, one can be as successful building RF stuff as at lower frequencies. One problem that has always been something of a hassle, however, is the shielding that is required. You could leam layout and grounding, but shielding usually required a better box than 1 had. Most of the low-cost aluminum...

Dynamic range

Reciprocal Mixing

The dynamic range of a radio receiver in the range from the minimum discernible signal to the maximum allowable signal (measured in decibels, dB). Although this simplistic definition is conceptually easy to understand, in the concrete its a ttJe more complex. SeveraJ definitions of dynamic range are used (Dyer, 1993). One definition of dynamic range is that it is the input signal difference between the sensitivity figure (e.g., 0.5 puV for 10 dB S + N N) and the level that drives the receiver...