A Simple Short-Wave Superheterodyne

Radio Record, Volume V, Issue 5, 14 August 1931, Page 16

A Simple Short-Wave Superheterodyne

'By

Cathode

ITH the release to the licensees of the Radio Corporation of America of the patents involved in the superheterodyne principle of reception, there has taken place a most remarkable re: vival of interest in this type of receiver. Those manufacturers who had previousliy had some experience with superheterodynes and who. could consequently enter on theix production with » minimum of delay have found themselves in-the fortunate position of being able to cope with the sudden ¢demand. Some manufacturers, indeed, are coneentrating upon superheterodyne — receivers to the exclusion of the very fine tr.f. receivers, with which they built their reputation. Many authorities consider that except for small low-priced receivers, the doom of the t.1.f. is sealed. We must admit that we would hardly like to go so far as this ourselves. : We intend to describe in these pages the design and construction ‘of twa superheterodyne receivers of widely different type. The first, intended for short-wave reception, is battery-operated, and is an exainple of the simplest possible type of superhet. receiver. The second receiver to be described is of a much more elaborate type. It is entirely a.c. operated, provision being made for the excitation of a dynamic speaker field. Three band-pass filters are incorporated in the intermediate

er, yet without introducing high-note loss. tuuplifier, giving an order of selectivity far greater than any commercial receivA radio-frequency stage is used ahead of the screen-grid first detector so as to permit of an outdoor aerial being used if desired, although the available amplification of over ten million times makes. it unnecessary to use anything but a small indoor aerial. In short, it is a receiver whose owner could confidently place it alongside any set commercially available in New Zealand, secure in the knowledge that it will more than favourably compare with any opponent. One last word--it works splendidly on short-wave. .

Features of the Superhet. BEFORE dealing with the details of the design and construction of the simpler superhet., a few notes on the unique features of this type of receiver will render it easier to understand why manufacturers have pounced upon it immediately the patents became available. Briefly, the essential feature distinguishing a superheterodyne is the conversion ‘of the incoming signal firequency to one of lower frequency, but carrying the original modulation. This lower frequency is then amplified and detected just as if it were an incoming long-wave signal. Practically all superhets. employ substantially the same means of changing the frequency. In addition to the incoming signal frequency, an oscillating ‘valve is employed to generate oscillations of a frequency differing someWhat from that being received. The two frequencies are. then applied to the grid of the first detector valve. It.can be shown mathematically that, .as a result of’ the beating of the two frequencies, the.input to the first detector is the sum of the two applied frequencies, but modulated at a frequency of half their difference. The effect of the first detector, of course, is to convert the modulation into a new wave form of twice the modulation frequency, just as an ordinary detector valve does in the case of modulation at audio frequencies. In the output of the first detector, then, there is present, among other things, a frequency equal to the difference between the incoming and generated oscillations, and this new frequency carries the original modulation transmitted by the broadcasting sta. tion. , It may, therefore, as previously explained, be amplified, detected, and made to operate a loudspeaker just like any other signal, At first sight it may appear that we have gone to a-lot of trouble without accomplishing very much. [In the output of the first detector we still have modulated high-frequency oscillations just like those collected from our aerial

except that they ave of lower frequency, | But how much easier to handle is this lower frequency. Whereas amplificution of the incoming frequeney would be fraught with difficulties, the lower (but still super-audible) frequency can. be amplified with the greatest ease. Moreover, since by adjusting the fre-\ quency of the local oscillator we can © vary the "lower" frequency (or, as we ' will now call it, the "intermediate frequency") to what we will, we can tune the amplifier once and for all and make the frequency suit the amplifier instead of vice-versa. Thus the necessity for constantly adjusting the several tuning condensers of a multi-stage amplifier is gone. The intermediate tuning is fixed, the tuning of the local oscillator being, varied instead. Selectivity Problems Vanish. LAStry, the superheterodyne has the inherent advantage of. outstanding selectivity. This is due to the fact that a difference of frequency appears unchanged after the first detector, and is a larger percentage of the small intermetliate frequency than of the: large original frequency; and selectivity depends on percentage differences. Thus, take a 300-metre signal ma¢ lated at 1000 cycles. Because of Spee / side bands, this acts as if it were a mixture of three currents, at 999 kilocycles, 1000 kilocycles, and 1001 kiloeycles respectively. If. we set to beat with it a current of 900 kilocycles we shall have in the output (among others) currents of 99 kilocycles, 100 kilocycles, and 101 kilocycles. Thus the modulation is preserved unchanged. Now, suppose an interfering station on 297 metres (1010ke.) This is 1 per. cent away from the first, and is hard to tune out. Our beating current of 900 kilocycles will set up in the detector output a current of 110 kilocycles, which is not 1 per cent. off * but 10 per cent. off that which we are trying to get. In fact, the separation (in percentage) of the two stations is multiplied by the ratio of the intermediate to the incoming frequency. r

The action of the supersonic prin‘ciple itself, then, is to multiply the . separation between stations. To take advantage of this, we must still have a selective intermediate amplifier, But if it is too selective, it will cut the higher -notes which form ‘the extreme ’edges of the band of wanted signals. Thus, taking the ¢ase mentioned above -3800 metres-but modulated at 5000 frequency and "converted’’ to 100 kilocycles by the first detector, we find that the amplifier should have the same overall performance for all frequencies: between 95 kilocycles and 105 kilocyeles, but should not work outside these limits. We want a resonance curve like Fig. 1. Actually, such -a.‘curve can be approximated only. Later, in connection with the more advanced design to be presented, the matter of obtaining the ' required selectivity without cutting side-bands will be considered in some d&tail.

Conflicting Requirements. S to ‘the actual frequency to be chosen for the intermediate amplifier, there are conflicting requirements which lead to a compromise. In order to obtain maximum selectivity, the frequency should be low; but this is not very important for the total selectivity is limited, as just pointed out; the limit can easily be reached with quite a high intermediate frequency. The frequency should, however, be considerably lower than the lowest frequency to be received. On the othér hand, if the frequency is too low, there are increasing difficulties due.to the LI, being too near the modulation fre-

quencies, Practically 30 kilocycles is ‘getting near the limit. The second detector and audio system operate precisely as in ‘A more normal receiver-except, perhaps, that the input to the second detector is larger. Designing the Shortwave ‘Super. [TN entering upon the process of design of the first and simpler set, every effort was made to make its circuit and construction free from elaboration and expense. It was realised that many constructors would at once see the advisability of experimenting with this receiver before embarking on the construction of its more elaborate fellow. A large part of the apparatus used may be rescued from the junk-box, or from firms which specialise in cheap second-hand and disposal parts. . As the receiver is intended primarily for shortwave reception, it was ¢con-

sidered permissible to dispense with a separate oscillator valve and make the first detector perform this service. The objection to this practice on broadcast wavelengths is that the tuned circuit controlling the frequency of the local oscillations is too far off tune so far as the incoming signals are concerned. If a low intermediate frequency is chosen, however, the tuned circuit need only be a comparatively small percentage off tune for the very high frequencies being received, The gain in ‘simplicity is considerable. So far, then, we have the cireuit shown in fig. 2. This is obviously identieal with an ordinary shortwave receiving circuit except that no control

of regeneration is provided, the reaction coil being arranged so as to . cause the valve to oscillate steadily at all settings of the tuning condenser, It will probably occur to readers possessing shortwave receivers of the regenerative type, without our point-: ing it out, that they can quite well use the detector portion of their existing receivers for: their experiments with supersonic reception, and thus save the necessity for building up this part of the apparatus specially. Intermediate Amplifier. HH next part of the receiver to be’ considered is the intermediate amplifier. Again with simplicity as our object, we suggest tuned plate coup-. ling, using straightforward single coils. of 20 to 40 turns tuned with semivariable condensers of the ‘"Formodenser" type. A two-stage amplifier will be ample,

preferably using screen-grid valves; ordinary three-electrode valves may be used, however, if they are stabilised by series grid resistances. So far, then, we have something like fig. 3. The Second Detector. "THE next step is to add the second detector and a single audio amplifier

to the rig. If it is desired to receive Morse it will be necessary to provide reaction on the second detector. so that it may be made to oscillate. Normally, however, this would not be done and the complete circuit diagram would be similar to that shown in skeleton form in fig. 4, The choke and by-pass condenser in the plate circuit of the second detector are very necessary to prevent I.F. from feeding into the audio amplifier, where it "would probably be amplified and cause instability. The choke must be a radio-frequency one of very high inductance (ie, having a great number of turns), and if a choke of this type is ‘not available one may be improvised by winding 1500 or 2000 turns of very fine wire on a slotted former of the type previously described jn-this journal. The detailed specifications and layout of this receiver will be given in the next article.

¥t is much more satisfactory to do the job at the source as there are no doubt others likewise troubled by the noise. At the beginning of this article we mentioned the various sources of interference. If you know of any of these in the neighbourhood immediately suspect them. Noise from these can be easily stifled by the simple expedient of chokes and condensers, The condensers should be of one to four microflarads depending on the intensity of the noise, and of sufficient voltage test. Remember that in the case of an a.c. line the working voltage is just double that of the rated voltage. Thus a 240 main really has a voltage of 480, and thus the working voltage of the condenser must be able to accommodate this with safety. A 500 working would just do, but there is no margin of safety, so it should be increased to 760 or 1000 working. This means that the test must be 1000 or 2000.

The choke must be capable of passing all the current supplied or taken by the apparatus, and thus its wire must be carefully selected. No, 18 d.c.c. is suitable for anything up to 2 amps, 16 to 4 amps, and so on, The coil indicated in the diagram is made by winding wire on a fibre, bakelite or paraffin-treated wood, or cardboard spool having a core diameter of 3-8ths of an inch, an outside diameter of idin., and a winding space of 7-16ths of an inch wide. If enamelled wire is used, it is best. to wind on in layers with insulating paper between the layers. Spools wound with cottoneovered wire should be treated with shellac or insulating varnish, and then baked. The number of turns is not

critical, it being sufficient to wind the form full of wire of the proper size. A typical choke for a load of 2 amperes or less would be wound with approximately 560 turns of 18 wire d.c.c. wire. When heavier wire is used the spool dimensions should be increased. A simpler choke can be made by winding a solenoid coil with wire of the requi-

site gauge. About 150 turns should be wound on a former with a three-inch diameter. Both chokes are air core and iron must not be introduced into them. Further Causes. TRAMWAY and telephone noises also call for attention. The former can be very persistent, and only by the close co-operation of the tramways authorities can a really effective eure be brought about. Careful bonding of the rails and the installation of filters on the trams themselves are necessary, and though not expensive there is sometimes some reluctance toe comply. Tramway interference is sometimes picked up by telephone lines and ecarriled by "wired’’ wireless into the homes. where the h.f. current has only a short way to jump to the set. Tram

interference can be minimised by following the general lines previously laid down for aerials and earths. Noise coming from the telephone can be rectified by the P. and T. Department. In conclusion, do not be too ready to blame the power transformer ontside your gate for noise. There are few pieces of electrical apparatus so innocent.