Superheterodyne receivers

The superheterodyne radio receiver (Fig. 10) was invented in the early 1920s, but only a very few sources could supply them because of patent restrictions. Later, however, patents were pooled as the radio industry grew and eventually the patents expired. Hie superheterodyne design was so superior that within a decade it took over all but a very few radios and is still today the basic design of all AM and FM radio receivers.

The block diagram to a superheterodyne receiver is shown in Fig. 5-10. The basic idea of the superheterodyne is to convert all the RF carrier signals from the radio waves to a fixed frequency, where it can be amplified and otherwise processed. The stages of the basic superheterodyne receiver consists of a mixer, a local oscillator (LO), an intermediate frequency (IF) amplifier, a detector, and an AF amplifier. The latter two stages are also used in TRF radios and serve exactly the same function in superheterodynes. Better-quality superhet radios also include an RF amplifier.

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Superheterodyne Reciever Schematic

5*10 Block diagram to a superheterodyne receiver.

The RF amplifier boosts the weak signal from the antenna and provides the radio with some additional selectivity. It also prevents the signal from the LO from being coupled to the antenna, where it could be radiated into space. The RF amplifier is sometimes called the preselector in radio schematics.

The output of the RF amplifier is applied to the input of the mixer stage. Also the input to the mixer is the local oscillator (LO) signal. The two are mixed together in a nonlinear circuit; two produces at least four output frequencies: RF, LO, RF - LO (difference), and RF + LO (sum). Either the sum or difference frequency can be selected for the intermediate frequency (IF), but in the vast majority of antique or classic radios, it was the difference frequency that was used. For AM radios, the IF frequency is 455 or 460 kHz in home radios and 260 or 262.5 kHz in auto radios; FM

broadcast receivers arid most VHFAJHF monitors use 10,7 MHz as the IF. In many receivers, the LO and mixer are combined into a single stage called a converter. Another name for the mixer or converter i & first detector.

The IF amplifier provides the radio with the largest amount of signal gain and the tightest selectivity. It is the fact that the IF operates ott only one frequency (e.g, 455 kHz) which permits the very high gain to be achieved without oscillation and other difficulties.

The detector operates at the IF frequency and will demodulate the IF frequency to recover the audio. From the detector, it is passed on to the AF amplifier and the reproducer (speaker).

Using the NE-602/NE-612 chips

The Signetics NE-602Aand NE-612 are monolithic integrated circuits containing a double-balanced mixer (DBM), an oscillator, and an interna] voltage regulator in a single eight-pin package (Fig. 5-11). The DBM section operates to 500 MHz, and the internal oscillator section works to 200 MHz. The primary uses of the NE-602 are in HF and VHF receivers, frequency converters, and frequency translators. The device can also be used as a signal generator in many popular inductor-capacitor (LC) variable-frequency oscillator (VFO), piezoelectric-crystal (XTAL), or swept-frequency configurations. This chapter explores the various configurations for the dc power supply, the RF input, the local oscillator, and the output circuits. Certain applications of the device are also covered-

Superheterodyne Receiver Circuit
5-11 Internal circuit, of NE-603.

The NE-602 version of the device operates over a temperature range of 0 to +70°C, and the SA-602 operates over the extended temperature range of -40 to +85*0, The most common form of the device is probably the NE-602N, which is an eight-pin mim-DIP package. Eight-lead SO surface-mount ("D-suffix") packages are also available. In this chapter, the NE-602N is featured, although the circuits also work with the other packages and configurations. The improved NE-602AN and NE-602 AD are now available.

Because the NE 602 contains both a mixer and a local oscillator, it can operate as a radio receiver "front-end" circuit. It features good noise rejection and reasonable third-order intemnodulation performance. The noise figure is typically 5 dB at a frequency of 45 MHz. The NE-602 has a third-order intercept point on the order of -15 dBm, referenced to a matched input, although it is recommended that a maximum signal level of —25 dBm (~3.16 mW) be observed. This signal level corresponds to about 12.6 rrtV into a 50-fl load, or 68 mV into the 1500-il input impedance of the NE-602, The NE-602 is capable of providing 0,2 |iV sensitivity in receiver circuits without external RF amplification. One criticism of the NE-602 is that it appears to sacrifice some dynamic range for high sensitivity—a problem said to be solved in the "A** series (e.g., NE-602AN and in the NE-612),

Frequency conversion or translation

The process of frequency conversion is called heterodyning. When two signals of different frequencies (Fx and F2) are mixed in a nonlinear circuit, a collection of different frequencies will appear in the output of the circuit. These are characterized nsFi, F2, and (nFi ± mFs)t where n and m are integers. In most practical situations, n and m are 1, so the total output spectrum will consist at least of Fit F^ + F>, and Fj - F2. Of course, if the two Input circuits contain harmonics, then additional products are found in the output. In superheterodyne radio receivers, either the sum or difference frequency is selected as the intermediate frequency (IF). In order to make the frequency conversion possible, a circuit needs a local oscillator and a mixer circuit (both of which are provided by the NE-602),

The local oscillator (LO) consists of a VHF NPN transistor with the base connected to pin 6 of the NE-602, and the emitter is connected to pin 7; the collector of the oscillator transistor is not available on an external pin. There is also an internal buffer amplifier, which connects the oscillator transistor to the DBM circuit. Any of the standard oscillator circuit configurations can be used with the internal oscillator, provided that access to the collector terminal is not required. Thus, Colpitis, Clapp, Hartley, Butler, and other oscillator circuits can be used with the NE-602 device, and the Pierce and Miller oscillator circuits are not,

The double-balanced mixer (DBM) circuit is shown in Fig. 5-12; it consists of a pair of cross-connected differential amplifiers (Qi/Qa with as a current source; similarly Q^/Qa with Qf, working as a current source). This configuration is called a Gilbert transconductance cell, The cross-coupled collectors form a push-pull output (pins 4 and 5) in which each output pin is connected to the V+ power-supply terminal through 1500-fl resistances. The input is also push-pull and is cross-coupled between the two halves of the cell. The local oscillator signal is injected into each cell-half at the base of one of the transistors.

Double Balanced Mixer And Oscillator
5^12 Transconductance cell double-balanced used in NE-602.

Because the mixer is "double-balanced" it has a key attribute that makes it ideal for use as a frequency converter or receiver front-end: suppression of the LO and RF input signals in the outputs. In the NE-602 chip, the output signals are F\ + F^, and Ft — neither LO nor RF signals appear in the output in any great amplitude. Although some harmonic products appear, many are also suppressed because of the DBM action.

dc power-supply connections on the NE-602

The V+ power-supply terminal of the NE-602 is pin 8, and the ground connection is pin 3; both must be used for the dc power connections. The dc power supply range is +4.5 to +8 Vdc, with a current drain ranging from 2,4 to 2.8 mA,

It is highly recommended that the V+ power-supply terminal (püi 8) he bypassed to ground wiih a capacitor of 0 01 to 01 n-F The capacitor should be mounted as close to the body of the NE-602 ¡is is practical; short leads are required in radio-frequency (RF) circuits.

Figure 5-13A shows the recommended power-supply configuration for situations where the supply voltage is +4.5 to +8 V. For best results, the supply voll rige should be voltage-regulated. Otherwise, the local oscillator frequency might not he stable, which leads to problems. A series resistor (= 100 to 180 ft) is placed between the V+ ¡mwer supply and the V+ terminal on the NE-602. If the power-supply voltage is raised to + 9 V, then increase the value of the series resistance an order of magnitude to 1000 to 150Ü ft (Fig, 5-13B).

If the dc power-supply voltage is either unstable, or is above +9 V, then it is highly recommended that a means of voltage regulation be provided. In Fig. 5-13C, u zener diode is used to regulate the NE-602 V+ voltage to 6.8 Vdc—even though the supply voltage ranges from +9 to +18 V (a situation found in automotive applications). An alternative voltage regulator circuit is shown in Fig, 5-13D. This circuit uses a three-terminal 10 voltage regulator to provide V+ voltage to the NE-602. Because the NE-602 is a very low current drain device, the lower power versions of the regulators (e.g., TSLxx) can be used. The low-power versions also permit the NE-602 to have its own regulated power supply—even though the rest of the radio receiver uses a common dc power supply Input voltages of +9 to more than +28 Vdc, depending on the regulator device selected, can be used for this purpose. The version of Fig. 5-13D uses a 78L09 to provide +9 V to the NE-602, although 78L05 and 78L06 can also be used to good effect.

NE-602 input circuits

The RF input port of the NE-602 uses pins 1 and 2 to form a balanced input. As is often the case in differential amplifier RF mixers, the RF input signals are applied to the base terminals of the two current sources (Q5 and Q6 in Fig, 5-12). The input impedance of the NE-602 is 1500 ft, shunted by 3 pFat lower frequencies, although in the VHF region, the impedance drops to about 1000 ft.

Several different RF Input configurations are shown in Fig. 5-14; both single-ended (unbalanced) and differentia] (balanced) input circuits can be used with the NE-602, In Fig. 5-14A, a capacitor-coupled, untuned, unbalanced input scheme is shown. The signal is applied to pin 1 (although pin 2 could have been used instead) through a capacitor, CV t hat has a low impedamre at the operating frequency. The signal level should be less than -25 dBm, or about 68 mV rms (180 mV peak to peak). Whichever input is used, the alternate input, is unused and should be bypassed to ground through a low-value capacitor (0.001 to 0.1 (jtF, depending on the frequency).

A wideband transformer coupled RF input circuit is shown in Fig, 5-14B. In this configuration, a wideband RF transformer is connected so that the secondary is applied across pins 1 and 2 of the NE-G02, with the primary of the t ransformer connected to the signal source or anteima. The turns ratio of the transformer tan be used to transform the source impedance to 1500 ft (the NE-602 input impedance). Either conventional or toroid-core transformers can be used for TV As to the previous circuit, one input is bypassed to ground through a low-reactance capacitor.

C3 3 jaF

5-13 Dc power supply t omvectiorvs for the NE-6G2.

ci X

input input

Raynet Frequencies
input
CS J_ 0.047 nF
Capacitor Uses And Function

5-14 Input circuits for the NB-603.

Tuned RF input circuits are shown in Figs. 5-14C-14E and 5-15. Each of these circuits performs two functions: (a) it selects the desired RF frequency while rejecting others and (b) it matches the 1.5-kil input impedance of the NE-602 to the source or antenna system impedance (e.g., 50 ft). The circuit shown in Fig. 5-14D uses an inductor (L,) and capacitor (C,) tuned to the input frequency, as do the other circuits, but the impedance matching function is done by tapping the inductor; a dc-blocking capacitor is used between the antenna connection and the coii. A third capacitor, C^, is used to bypass one of the inputs (pin 2) to ground.

Another version of the circuit is shown in Fig. 5-14E. it is similar in concept to the previous circuit but uses a tapped capacitor voltage divider (CVCi) for the impedance-matching function. Resonance with the inductor is established by the combination of Ci, the main tuning capacitor, in parallel with the series combination of C2 and C&

The previous two circuits are designed for use when the source or antenna system impedance is less than the 1.541 input impedance of the NE-602. The circuit of Fig. 5-14F can be used in all three situations, input impedance lower than, higher than, or equal to the NE-602 input impedance—depending on the ratio of the number of turns in the primary winding (L3) to the number of turns in the secondary winding (¿0* The situation shown schematically in Fig. 5-14F is for the case where the source impedance is less than the input impedance of the NE-602.

The secondary of the RF transformer (¿0 resonates with a capacitance made up of C] (main tuning), C2 (trimmer tuning or bandspread), and a fixed capacitor, C3. An advantage of this circuit is that the frame of the main tuning capacitor is grounded. This feature is an advantage because most tuning capacitors are designed for grounded-frame operation, so construction is easier. In addition, most of the variable-frequency oscillator circuits used with the NE-602 also use a grounded-frame capacitor The input circuit of Fig. 5-14F can therefore use a single dual-section capacitor for single-knob tuning of both RF input and local oscillator-

Figure 5-15 shows a tuned-input circuit that relies, at least in part, on a voltage variable-capacitance CVaractor") diode for the tuning function. The total tuning capacitance that resonates transformer secondary L2 is the parallel combination of Ci (trimmer), C2 (a fixed capacitor), and the junction capacitance of varactor diode, Dl, The value of capacitor C3 is normally set to be large compared with the diode capacitance so that it will have little effect on the total capacitance of series combination CyCDi, In other cases, however, the capacitance of is set close to the capacitance of the diode, so it becomes part of the resonant circuit capacitance.

A varactor diode is tuned by varying the reverse bias voltage applied to the diode. Tuning voltage V% is set by a resistor voltage divider consisting of R2t and Ri The main tuning potentiometer (J?0 can be a single-turn model, but for best resolution of the tuning control use a 10- or 15-turn potentiometer. The fine-tuning potentiometer can be a panel-mounted model for use as a bandspread control or a trimmer model for U3e as a fine adjustment of the tuning circuit (a function also shared by trimmer capacitor, C{).

Circuit Repair
R3 1 kii

S-15 Vatactor-tuned input circuit for NE-602.

The voltage used for the tuning circuit (K0 must be well-regulated, or the tuning will shift with variations of the voltage. Some designers use a separate three-terminal IC regulator for VM but that is not strictly necessary. A more common situation is to use a single low-power 9-V three-terminal IC voltage regulator for both the NE-602 and the tuning network. However, it will only work when the diode needs no more than +9 Vdc for correct tuning of the desired frequency range. Unfortunately, many varactor diodes require a voltage range of about +1 to +37 V to cover the entire range of available capacitance.

In due course, the oscillator circuits will also show a version of the Fig. 5-15 circuit that is tuned by a sawtooth waveform (for swept frequency operation) or a digital-to-analog converter (for computer-controlled frequency selection).

NE-602 output circuits

The NE-602 output circuit consists of the cross-coupled collectors of the two halves of the Gilbert transconductance cell (Fig. 5-12) and are available on pins 4 and 5, In general, it doesnt matter which of these pins is used for the output; in single-ended output configurations, only one terminal is used and the alternate out put terminal is ignored. Each output terminal is connected internally to the NE-602 to tlirough separate 1,5-kft resistors.

Figure 5-16A shows the wideband, high-impedance (1.5 kil) output configuration. Either pin 4 or 5 (or both) can be used. A capacitor is used to provide dc blocking. This capacitor should have a low reactance at the frequency of operation, so values between 0.001 and 0.1 p-F are generally selected.

Transformer output coupling is shown in Fig. 5-16B, In this circuit, the primary of a transformer is connected between pins 4 and 5 of the NE-002. For frequency converter or translator applications, the t ransformer could be a broadband RF transformer wound on either a conventional slug-tuned form or a loroid form. For direct-conversion autodyne receivers, the transformer would be an audio transformer. The standard h\ transformers used for audio coupling can be used. These transformers are sometimes marked as impedance ratio rather than turns ratio (e.g., 600 £1.600 il, or 1.5K:1.5K).

Frequency converters and translators are the same thing, except, that the converter terminology generally refers to a stage in a superhet receiver and trcmstator is more generic. For these circuits, the broadband transformer will work, but it is probably better to use a tuned RF/1F transformer for the out put of the NE-602. The resonant circuit will reject all but the desired frequency product; e.g., the sum or difference IF frequency. Figure 5-16C shows a common form of resonant output circuit for the NE-602- The tuned primary of the transformer is connected across pins 4 and 5 of the NE-602, and & secondary winding (which could be tuned or untuned) is used to couple the signal to the following stages.

A single-ended RF tuned transformer output network for the NE-602 is shown in Fig. 5-16D, In this coupling scheme, the output terminal of the IC is coupled to the V+ dc power-supply rail through a tuned transformer. Perhaps a better solution to the single-ended problem is the circuit of Fig, 5-16E. In this circuit, the transformer primary is tapped for a low impedance and the tap is connected to the NE-602 output terminal through a dc-blocking capacitor. These transformers are easily available in either 455-kHz or 10.7-MHz versions and can also be made relatively easily.

Still another single-ended tuned output circuit is shown in Fig. 5-16F In this circuit, one of the outputs is grounded for RF frequencies through a capacitor. Tuning is a function of the inductance of Lj and the combined-series capacitance of Cj, Cj, and C3, By tapping the capacitance of the resonant circuit at the junction of Gr-Cs* it is possible to match a lower impedance (e.g., 50 ft) to the 1.5-kft output impedance of the NE-602.

The single-ended output network of Fig. 5-16G uses a Jow-pass filter as the frequency-selective element. This type of circuit can be used for applications, such as a heterodyne signal generator in which the local oscillator frequency of the NE-G02 is heterodyned with the signal from another source applied to the RF input pms of the IC. The difference frequency is selected at the output when the low-pass filter is designed so that its cul -off frequency is betwf-en the sum and difference frequencies.

In Fig, 5-I6H, an IF filter is used To select the desired output frequency. These filters are available in a variety of different frequencies and configurations, including the CoUins mechanical filters that were once used extensively hi high-grade communications receivers (262-, 455-, and 500-kHz center frequencies). Current high-

(Use either ouput.)

(Use either ouput.)

4

Ul

jflf-RF

U1

NE-603

ouput

NE-602

5 Pfl

ci ^

4 1

U1

U1

NE-602

NE-602

5

T1 j RF out

Low-pass filter j^j

Low-pass filter j^j

Pcb Layout Crown 602

5-16 Output circuits for NB-602.

grade communications receivers typically used crystal IF filters centered on 8.83,9, or 10,7 MHz or 455 kHz (with bandwidths of 100 Hz to 30 kHz). Even some broadcast radio receivers use IF filters. Such filters are made of piezoceramic material and are usually centered on 260 or 262.5 kHz (AM auto radios), 455 or 460 kHz (other AM radios), or 10,7 MHz (FM radios). The lower-frequency versions are typically made with 4-, 6-, or 12-kHz bandwidths, and the 10.7-MHz versions have bandwidth® of ISO to 300 kHz (200 kHz is most common).

In the circuit, Fig. 5-16H it is assumed that the low-cost ceramic AM or FM filters are used (for other types, compatible resistances or capacitances are needed to make the filter work property). The input side of the filter (FL1) in Fig. 5-16H is connected to the NE-602 through a 470-ft resistor and an optional dc blocking capacitor (CO. The output of the filter is terminated in a 3.9-kft resistor. The difference IF frequency resulting from the conversion process appears at this point.

One of the delights of the NE-602 chip is that it contains an internal oscillator circuit that is already coupled to the internal double-balanced mixer. The base and emitter connections to the oscillator transistor inside the NE-602 are available through pins 6 and 7, respectively. The internal oscillator can be operated at frequencies up to 200 MHa. The internal mixer works to 500 MHz. If higher oscillator frequencies are needed, then use an external local oscillator. An external signal can be coupled to the NE-602 through pin 6, but must be limited to less than about -13.8 dBrn, or 250 mV, across 1500 ft.

The next section shows some of the practical local oscillator (LO) circuits that can be successfully used with the NE-602, including one that allows digital or computer control of the frequency. Oscillator circuits are covered in greater detail in Chapter 13.

NE-602 local oscillator circuits

There are two general methods for controlling the frequency of the LO in any oscillator circuit: inductor-capacitor (LC) resonant tank circuits or piezoelectric crystal resonators. Both forms are considered, but first, the crystal oscillators.

Figure 5~17A shows the basic Colpitts crystal oscillator. It will operate with fundamental-mode crystals on frequencies up to about 20 MHz. The feedback network consists of a capacitor voltage divider (Cj/Cs), The values of these capacitors are critical and should be approximately:

VfjiHz

The values predicted by these equations are approximate, but work well under circumstances where external stray capacitance does not dominate the total. However, the practical truth is that capacitors come in standard values and these might not be exactly the values required by Eqs. (5 3) and (5-4).

When the capacitor values are correct, the oscillation will be consistent. If you pull the crystal out, then reinsert it, the oscillation will restart immediately- Alternatively, if the power is turned oif and back on again, the oscillator will always restart.

Superheterodyne Receiver Circuit
LI

XTAL 1

25 to 50 MHz 3rd overtone

XTAL 1

25 to 50 MHz 3rd overtone

0.001 jiF

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