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Made in New Zealand: the Pacemaker Transistor Radio

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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 trans- formers 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 am- plifier. 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 usu- ally 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 sine- wave, 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 harmon- ics 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.
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