Made in New Zealand: the Pacemaker Transistor Radio

By Dr Hugo Holden

called an 'additive mixer'. However, Made in New Zealand: the this is misleading because the mixing process involves multiplication of the Pacemaker transistor radio incoming signal with the oscillator signal, not addition. . . . plus a look at mixers, RF stages & image rejection The fundamental principle of the superheterodyne (or superhet) radio involves frequency conversion. This is done by converting the received signal frequency down to a lower frequency called the 'intermediate frequency' or IF. This is why mixers are sometimes also called 'converters'. In typical transistor radios, the IF is nominally 455kHz. The following IF amplifier stage is usually composed of two transistors and three IF transformers but some radios, such as the Pacemaker, only have one IF transistor and two IF transformers. Most of a transistor radio's signal gain and selectivity is in the IF amplifier. While the IF transformers are tuned to a centre frequency of 455kHz, their bandwidth is still wide enough to pass audio frequencies through to the detector. This bandwidth is typically between ±3kHz and ±5kHz. The preceding converter stage usually takes one of two forms: either A vital part of any AM superhet transistor a mixer transistor with a separate oscillator transistor feeding it or, radio is the mixer or mixer-oscillator and more commonly, a single-transistor this month we take a look at how these oscillator which also acts as a mixer. The latter is commonly referred to as circuits work. We also describe the New a 'mixer-oscillator'. Zealand-made Pacemaker Transportable When the received (ie, tuned) signal is 'mixed' with the oscillator signal, radio and compare its main features with sum and difference components of the Sony TR-72 described last month. the two signal frequencies appear in the mixer's output. So if a received signal frequency of 650kHz is mixed with an oscillator signal frequency Fr HE NZ-MADE Pacemaker is cred- medium-wave (ie, broadcast-band) ra- of 1105kHz, the sum and difference ited as being one of the world's first dios didn't have active RF stages right frequencies will be 1755kHz and commercial transistor radios to have a through into the 1970s. By contrast, 455kHz respectively. However, only tuned RF amplifier stage. In fact, it may shortwave radios often did. the 455kHz signal passes through the well have been the very first. But why was this and what are the IF amplifier due to the IF transformer Most transistor radios of the mid advantages of an active tuned RF stage? tuning. 1950s (for example the 1954 Regency Nearly all transistor radios employ Fig.1 shows the basic configuration TR1 and the 1956 Sony TR-72) did an "autodyne" mixer or "converter" of a superhet receiver and gives some not have an active tuned RF (radio- circuit. This is a combined mixer- example frequencies. In operation, frequency) front-end. Indeed, most oscillator circuit and is sometimes the oscillator frequency is set so that 82 SILICON CHIP siliconchip.com.au it always runs 455kHz (ie, the IF) higher than the incoming tuned signal SUM = 1105 KHz + 650 KHz =1755 KHz = Rejected frequency. That's done by simultane- DIFFERENCE =1105 KHz 650 KHz = 455KHz = Accepted = wanted sig nal ously tuning the antenna circuit and OSCILLATOR SIGNAL ITSELF =1105ICHz = Rejected the oscillator using a 2-gang variable DIFFERENCE =1560 KHz - 1105 KHz = 455 KHz = Accepted = unwanted IMAGE signal capacitor, so that the oscillator signal tracks 455kHz above the received 465 KHz frequencies. ANTENNA RANGE Of course, this tracking is never ab- 550KHz -1600 KHz F RESPONSE SPEAKER solutely perfect and there are tracking EXAMPLE: 650 KHz errors. However, with good design, Volume these errors are virtually zero at the ex- IMAGE = 1560 KHz DETECTOR • Mixer 55KHz F AMPL FIER AUDIO tremities of the band and in the centre. AMPLFIER As shown on Fig.1, there is another frequency known as the 'image fre- MIXER / OSCILLATOR UNIT quency' that could also be accepted by the IF stage. This image frequency [Os c Mato OSCILLATOR RANGE (or potential interfering radio station) 1005 KHz - 2055KHz will have a lower signal level than the wanted signal because the antenna coil *AMPLE: 1105KHz is tuned to the wanted signal. How- TUNING FILTER ever, if the image frequency signal is strong enough, it could break through. T As stated, the IF amplifier passes Fig.1: the basic configuration of a superheterodyne AM radio receiver. Also only 455kHz signals and rejects all shown are the various frequencies generated by the oscillator/mixer circuit other signals. The problem is that the when the set is tuned to 650kHz. Note that the oscillator is tuned so that it "image" frequency is 2 x 455kHz = always runs 455kHz higher than the tuned signal frequency. 910kHz above the tuned frequency, or 455kHz above the oscillator. As a result, the mixer/oscillator also con- performs an extraordinarily complex (1) the sum of the two initial frequen- verts it to 455kHz (ie, the difference role. Just how a mixer stage works is cies and (2) the difference of the two product) and so it is at risk of breaking often glossed over in many texts. How- frequencies. They both have half the through. ever, its function is critical in even the amplitude of the original waveforms. Fortunately, image frequencies above most basic AM transistor radio. Note: this result is also recognisable 790kHz are outside the AM broadcast In order to generate sum and dif- as the frequency spectrum of ampli- (MW) band and there are few (if any) ference frequencies of two periodic tude modulation (AM) itself, with a stations transmitting in the region waveforms such as sinewaves (or central carrier and a sideband on either from 1700-2500kHz to cause image cosine waves), the two signals must side with half the amplitude. Mixers problems. However, that's not the case in fact be multiplied together. It won't are in fact also modulators. on the shortwave bands where it's not work if the signals are simply added. In specialised mixer integrated uncommon for another station to be So how does a transistor mixer- circuits (such as the MC1496), the transmitting 910kHz above the tuned oscillator stage multiply two signals? multiplication of two signals is exactly frequency. First, let's consider two cosine waves as per the equation above. This IC is a One solution to the image problem with angular frequencies of wi and "voltage multiplier". However, in the is to have a tuned "preselector" or RF w2 radians per second (note w = 24, case of a single transistor mixer (or stage consisting of an extra transistor where f is the frequency). Multiply- mixer-oscillator) in a typical transistor and tuned circuit prior to the mixer. ing the angular frequency w by time radio, the situation is a little different. This stage helps to boost the wanted t yields the angle in radians. Thus, Fig.2 shows what happens when frequency and attenuate other frequen- the electrical or magnetic component two different signals, a and b, are cies further away (such as the image of a radio wave has the general form added and then amplified either by frequency). Y = A.cos(wt) where Y is the ampli- a linear amplifier in one example or Basically, it improves the selectiv- tude varying with time and A is the a "square law" amplifier in the other. ity and provides additional gain to peak amplitude, while the frequency As can be seen, linear amplification improve sensitivity (ie, to boost low- f = w 2r. results in simple scaled up amplitudes signal reception). It also increases the If we multiply the two normalised of signals a and b. On the other hand, signal level fed to the mixer, potential- angular components together, we get: if the amplification obeys a square law, ly helping to lower the noise generated cos(w/ t).cos(w2t) = 0.5cos((wl + then the product of signals a and b in the mixer itself. This is the design w2)() + 0.5cost(w/ - w2)11 appears as the term 2ab. approach in the Pacemaker radio. This trigonometric identity is avail- In the latter case, signals a and b able from many texts. So something have been multiplied by summation How the mixer works quite remarkable has happened. Multi- followed by square law amplification. The mixer/oscillator stage is usually plying the two waveforms has resulted Other signals also appear which are based on just a single transistor but it in two other components which are equal to the square of signal a and the siliconchip.com.au APRIL 2014 83

signal. In fact, the transfer curve can be represented by a polynomial of the LINEAR vs SQUARE LAW AMPLIFIER with a gain of G: form: ax + bx2 + cx3 + dx4 . etc. It turns out that "cubing" a sinewave, for example, results in a third >- Y = Gx7 = G(a+b)2= G( a2 + 2ab + b2) harmonic, ie, three times the fre- rn quency. This means that there can be in \I= Gx =Ga +Gb second, third, fourth, fifth etc harmonics of both the oscillator signal and the tuned frequency at the mixer's output. "Added" signals a and b However, these signals are rejected by end up getting multiplied the IF amplifier which only amplifies 455kHz ± about 5kHz. A transistor operating at a low bias level has a voltage amplification re- Input Signal x = (a+b) sponse curve which is non-linear. In Fig.2: here's what happens when signals 'a' and 'b' are added and then fact, it's very similar to the blue curve amplified either by a linear (red) amplifier or a "square law" (blue) shown in Fig.2. This isn't obvious from amplifier. Note that if the amplification obeys a square law, then the most transistor data sheets because product of signals 'a' and 'b' appears as the term "2ab". the transistor's base-emitter voltage usually isn't plotted against collector square of signal b. If we represent these So squaring a cosine (or sine) wave current. signals as cosine waveforms instead, doubles its frequency and one easy Instead, more often than not, the we find out what happens when two method of frequency doubling is to base current is plotted against collec- added waveforms are squared: pass a sinewave through a squarer tor current which looks more linear (a + b)2 = fros(w/t) + cos(w2t)]2 circuit. As a result, the output of a and the slope at any point is the small = / + 0.5cos(2w/t) + 0.5cos(2w2t) + simple transistor mixer stage consists signal current gain. cos((w/ + w2)t] + cos[(wl - w2)t] of a "cocktail" of different signals, as A transistor's base-emitter junction Again the sum and difference of the follows: has a response (or function) that's very two waveforms has appeared but this (1) the received radio station frequen- similar to a simple diode. The collec- time their amplitude hasn't halved. cy; tor current is converted to a voltage The "1" represents a DC component. (2) the oscillator frequency; by the impedance in the collector load In addition, there are components (3) the sum of the oscillator and tuned circuit. So a transistor operating at low which are twice the frequency (ie, station frequencies; bias levels, is an approximate square second harmonics) of the original (4) the difference between the oscil- law device when used as a voltage cosine waveforms. lator and tuned station frequencies; amplifier. (5) twice the tuned station frequency; So too are field effect transistors and 1.5 Base-emitter voi.tage versus (6) twice the oscillator frequency; and an MPF102 junction FET, for example, collector current (7) the transistor's DC bias. also makes a good mixer or mixer/os- I 1 , In addition, for anything other than cillator (and these better approximate perfect square law amplification, there a square law function). -VCE z' V y. = 25°C will be other frequencies or harmonics Fig.3 (from a Philips manual) shows i in the mixer's output current or output the base-emitter voltage versus the

A FERRITE ROD & MIXER CURRENTS Fig.4: a typical COIL , 1-transistor self-oscillating . _ QSC COIL mixer stage. 0.5 This is similar to the one used in the Sony TR- ..... i STATION 72 and is often TUNING / referred to as • . _ , an `autodyne ._ 4... converter'. 0

``"Square Law" IF Fig.3: base-emitter voltage versus TRANSFORMER _„."7PLACEMENT the collector current for a typical OPTIONS PNP germanium transistor. Note that the lower section of each curve is non-linear and has an approximate `square law' characteristic. 84 SILICON CHIP siliconchip.com.au 4k7 10 OSC Neut ca V/C3 10k 270 1k Ferrite V/C2 RF Neut 100k Rod 0.28v

0.033 lin — 0.047 "S1 uF uF -0.38v VIC1 100k 1FT2 4k7 0.033 0.022uF ? 2N247 uF red Vd Tone grn 30k 30k 2k 2k2 SuF MOM 470 AGC -0.25v 0.033uF 0.033uF O.033uF .033uF • T (no sig nal DC voltages)

270 'WV' 270k ON / OFF 10 270k TFK = 2.5uF 2k2 + 25v OC602 10k 8uF 6V TFK 25V • Batt. OC602 270 270

32uF 50 uF 47k 0.005 uF= 150k +T3V 180, 5w, WW •

Fig.5: the circuit of the NZ-made Pacemaker transistor radio. It uses seven PNP transistors, with X1 functioning as an RF amplifier. X2 is the mixer/oscillator, X3 the IF amplifier and X4-X5 form the audio amplifier stage. collector current for a PNP germanium collector in the Pacemaker radio. in series with the oscillator section to transistor. As can be seen, the lower There are many other single-tran- reduce its value by the correct amount. section of each curve is non-linear sistor self-oscillating mixer configura- Referring back to Fig.4, RI and R2 and has a square law characteristic, tions. Often, the feedback that's neces- are the transistor's bias resistors and especially for base-emitter voltages of sary to sustain oscillation is taken from these set its DC operating conditions. less than 0.5V. the oscillator coil and applied to the The tuned RF signal from the small transistor's emitter instead of its base. coupling coil on the ferrite rod is fed Typical mixer However, they all have the same func- to the transistor's base circuit, while Fig.4 shows a typical one-transistor tion and it's necessary for the transistor the oscillator feedback signal is fed to self-oscillating mixer stage. This is the to be lightly biased and operating in the base via Cl (which helps maintain basic configuration used in the Sony its non-linear region. oscillation). TR-72 (except that the TR-72 uses an The variable capacitor (V/C) tunes Note that the coupling coil on the NPN transistor) and is often referred to the ferrite rod antenna circuit and is ferrite rod has a relatively small num- as an autodyne converter'. The mixer ganged to a second variable capacitor ber of turns feeding the transistor's in the Pacemaker radio described later which tunes the oscillator coil. In most base circuit. This ensures that the main has a slightly different configuration. transistor radios (eg, the Sony TR-72), tuned winding on the ferrite rod is The mixer's output, containing all the tuning-gang section used in the not heavily loaded. Capacitor C2 and the signal components, is fed to the oscillator is smaller that the antenna resistor R3 help the transistor maintain primary of the first IF transformer. section. This is done to ensure that a stable DC bias condition. As with the other IF transformers in the oscillator frequency tracks the Finally, the polarity of the oscillator the radio, this is tuned to a centre tuned frequency, so that they remain coil windings is such that the feedback frequency of 455kHz. close to 455kHz apart with minimal is positive to sustain oscillation. Cap- The first IF transformer's primary tracking errors. acitor C3 bypasses any radio frequen- winding is simply placed in series In some radios (such as the Pace- cies on the supply line. with the mixer circuit's output. It's in maker) though, the tuning gang sec- series with mixer transistor's emitter tions are identical. As a result, an extra Pacemaker circuit details in the TR-72 and in series with the capacitor, called a `padder', is placed I was unable to locate the original siliconchip.com.au APRIL 2014 85 The view inside This differs from the mixer circuit of the Pacemaker the Sony TR-72 which uses out-of- Transportable. phase feedback to the transistor's base. Most of the parts are obscured by IF amplifier its large metal chassis although As mentioned, the Pacemaker has the ferrite one less IF amplifier stage and one less rod antenna IF transformer than typical transistor is visible, as radios of the time. However, the result- are the 3-gang ing loss of gain and selectivity in the tuning capacitor, IF section is compensated for by the the battery and gain provided by the tuned RF stage the driver and based on transistor X1. audio output In fact, the net overall gain is similar transformers. to that in a standard radio. However, due to the selectivity of the RF stage, the image rejection is substantially improved and there is probably also less mixer noise. The 455kHz IF signal from mixer manufacturer's schematic, so it was using a fixed 10pF capacitor to ensure X2 is fed to IF transformer IFT1 and laboriously traced out from the radio stability. This is necessary because then to neutralised IF amplifier stage itself which is probably a 1960 version. the tuned circuits in the base and X3. This in turn feeds IF transformer Fig.5 shows the circuit details. Apart collector sections of X1 operate at the IFT2 which then feeds the resulting IF from the RF stage, it's similar in many same frequency and would otherwise signal to the detector diode. respects to Sony's TR-72 but one obvi- exchange energy with each other via The detector diode carried no type ous difference is that the Pacemaker the transistor's Miller capacitance, number but is probably an 0A90 or Transportable uses PNP germanium causing the stage to burst into oscil- similar. The detector has been ar- transistors while the TR-72 uses NPNs. lation. As stated, the tuning capacitor ranged with bias control. The diode's It also uses one less transistor in the IF (V/C) has three identical sections and cathode is at the AGC voltage, which is stage, although both are still 7-transis- looks the same as those commonly about -0.38V, and this tends to exces- tor radios. used in valve radios of the time. sively forward bias the diode, even in The RF stage (or pre-selector) is The mixer-oscillator stage is based the absence of signal. To counter this, based on transistor X1 (a 2N247), an RF on transistor X2. This has in-phase another negative voltage (developed coil and one gang of the 3-gang tuning feedback from the oscillator coil to its across a 151d2 trimpot) is applied to capacitor. This RF stage is neutralised emitter in order to sustain oscillation. the anode. In my set, this was about -0.28V, Most of the smaller parts are which leaves a residual 100mV of mounted on three small circuit forward bias. This value was prob- boards and are wired together ably the factory setting, since a small via eyelet connectors. amount of detector forward bias can help demodulate weak signals. Automatic gain control AGC is applied to transistors X1, X2 and X3. Note that less AGC range is applied to mixer-oscillator X2, as it is sourced from a divider comprising 1001d2 and 2M52 resistors. High AGC levels can deactivate an oscillator and some so designers consider it unwise to apply AGC to this stage. In this circuit though, the 2MS2 resistor biases X2 so that it stays in oscillation even though some AGC is applied. Audio amplifier stages The audio amplifier also has some interesting features. Input stage X4 is a grounded-emitter amplifier and is DC stabilised by collector-to-base feedback via a 1MQ resistor. Direct coupling from X4's collector is then 86 SILICON CHIP siliconchipcorn.au The Pacemaker Transportable is housed in an attractive timber case with the on/off/volume and tuning controls mounted at the top. Note that the tuning dial has 'North Is' and 'South Is' sections and carries NZ station call- signs. used to establish the bias on driver frequency and negative DC feedback most of the parts are covered by a stage X5. X5's emitter current in turn from the collector of X6 (ie, via the metal chassis and are protected from sets the bias current for output transis- 2701d/resistor). This contributes to the the owner's prying fingers. However, tors X6 & X7 (both OC72s), depending DC stability but this effect is limited the ferrite rod assembly and the audio on the voltage developed across a 180Q due to the relatively low resistance driver and output transformers are 5W wirewound adjustable resistor. of the output transformer's primary visible, along with the elliptical Rola This arrangement is very unusual. windings. It could also possibly be loudspeaker and the 3-gang variable It means that the output stage's bias some sort of anti motor-boating net- tuning capacitor. and current stability is controlled by work, as its AC frequency response is The radio runs from a 6V lantern input transistor X4's Vbe (base-emitter such that it's only active below about battery (also visible). This is extremely voltage) characteristic, which is very 100-200Hz. long-lasting in this application since dependent on temperature. As it heats Once again, this arrangement is very the current drain is only about 10mA. up, X4's Vbe drops and the transistor unusual. There is no conventional Another photo shows the chassis as- turns on harder. This lowers the volt- audio AC negative feedback in the sembly after it has been removed from age on X4's collector and in turn lowers Pacemaker, such as seen around the the cabinet. Most of the smaller parts the base bias voltage applied to X5. audio stages of radios such as the Sony are mounted on small boards and are As a result, X5's emitter voltage TR-72. Normally, feedback is derived wired via eyelet connectors. drops and this tends to 'throttle back' from the speaker itself and fed to the Finally, as with the Sony TR-72, the the output transistors. So it would driver transistor's emitter circuit. Pacemaker Transportable is housed appear the designers have used audio in an attractive timber case with the input transistor X4 as a "temperature Assembly method controls mounted at the top. It also has compensation device" for the output One of the accompanying photos a metal carrying handle. All in all, it stage. shows the view inside this unique is a well-made radio with excellent There is also a small amount of low- radio. There's really little to see since performance. SC

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