Making A DIY Superheterodyne AM Radio Like Those You Buy

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A Home Made Superhet AM Radio Like Those You Might Buy In The Shops



(Well, almost)

Preface

Design Criteria

Design Description

Mediumwave AM Radio Schematic PDF



Oscillator Section

OK, so this is not a great oscillator circuit and could be replaced with a typical mixer-oscillator circuit out of a 1960s radio. However, it's very simple to understand, and I fancied separating out the mixer and oscillator functions for possible use in future superior radio designs. The transistor Q4 is biased up as a class A amplifier with some gain. The collector circuit provides feedback to the base via the tuned circuit. The two diodes D1 and D2 make a limiter to stabilise the amplitude across the frequency range. Q5 is a dedicated buffer for the output of the oscillator. This reduces the effect of subsequent circuit loads and helps keep the frequency stable.

Mixer

This is a differential amplifier comprising Q2 and Q3 with the two collectors connected together instead of going to individual loads to +BAT. The normal difference voltage output that you'd expect is therefore cancelled and you are left with the much smaller signal which is the effect of more current flowing in one transistor reducing the current flowing in the other, thus reducing its partner's gain and vice-versa. This is straight out of a Robert Penfold Babani Press radio book. The mixer output goes off to the intermediate frequency stages, and you can see that the mixer collectors go to +BAT through the primary of the first IFT. The output goes to a tap on the primary to avoid loading the resonant circuit too much. This preserves the Q and helps to keep the IF bandwidth narrow.

Intermediate Frequency Strip

The turns ratios for these home wound Intermediate Frequency Transformers, T1, T2, and T3, were determined by looking up the turns ratios used in TOKO brand transformers for the 1st 2nd and 3rd IFTs. The number of turns for the primaries were first determined by calculating the desired inductance to resonate with the parallel capacitor at the IF of 455kHz. fo=1/2pi*sqrtLC where C is about 220pF typically. Then using the tables supplied with the coil formers and cores the approximate number of turns can be found using the "Al" values. This value gives you a nominal inductance per 1000 turns, and the inductance is proportional to the square of the number of turns. So if the quoted Al for a certain former and core is Al=10uH/1000Turns, and if you wanted 5uH you would wind (5^2/10^2)*1000=250 turns. The Al values work pretty well and I didn't need to rewind after testing, though I may have changed the parallel capacitor value for T1. An RF signal generator was used to check the resonant frequency and to set the cores in about the right positions prior to wiring up.

IF Strip Biasing

An interesting fact reveals itself in the IF strip. The transistors do not need to be class A biased as you would normally do in a cascaded audio amplifier chain. Because the inductance of the IFTs stores energy, and we are dealing with an AM signal, you can bias the transistors pretty much in class C, only increasing the bias current when maximum gain is needed on weak signals. The signal at the collectors swings above and below +BAT thus also effectively doubling the possible signal swing range. This isn't so important for a 9V radio like this one, but there are very good MW radios out there with 3V and even 1.5V batteries where this must be an important design feature. The schematic shows the emitter resistors bypassed with a capacitor to maximise gain at radio frequency, while the 220R stabilises the bias current when it is flowing significantly under low signal conditions. The bias voltage itself comes from the network comprising the demodulating diode D3, the volume control R16, and R27 and R29. R28 and C15 remove the audio from the d.c. part of the demodulated signal. The network is arranged such that when there is no signal present the full bias voltage is applied to the IF transistors and you get about 2mA quiescent current and a fair gain. When a strong AM signal comes along it is rectified and the audio tapped off. But the rectified audio is also creates a negative bias which reduces the voltage on the electrolytic cap. The reduced bias voltage puts less bias current through the IF trannies and the gain is reduced. This primitive AGC action is similar to that used in the very old Roberts radios.

More interesting facts

IF strips like this are really quite non-linear depending mostly on the quality of your IFT windings. If you put in a perfectly sine modulated carrier then the shape of the AM signal at each IF stage depends on the bias current and the position of the core in the winding. Maximum linearity rarely happens at maximum resonance, which is where you tune it to in a primitive radio like this. I guess that the linearity can probably be improved by using nicely wound Litz style windings in the IFTs rather than just piling the turns on like I did. But, you won't hear anyone tell you that in a radio book and for certain you won't learn it on a university degree course. Despite the distortion added in the IF strip, depending on where the tuning slugs are and on the bias current set by AGC, this wireless sounds like any average AM radio.

To see how to make really nicely wound double tuned IFTs, take apart an nice old 1960's British radio like a Hacker Herald.

Coils

An Aside - Oh, Here We Go

I always considered radio books which used ready made coils to be major cheats. Further to that, the Babani radio books always used to use Denco/Clacton/Maxi-Q coils who went out of business somewhere round the mid-(nineteen)eighties. Many amateurs will remember these coils I'm sure, being supplied in old fashioned 35mm aluminium film canisters which you were supposed to use for the screening can by drilling holes in the screw-on lid to accomodate a 9-pin valveholder, and a small hole in the main canister through which you could poke your screwdriver to tweak the brass screw. This would in turn move the ferrite core up and down. I still have a pile of these coils bought from a sale and three which I bought new at immense expense as a poverty stricken child to try to make an 80m band SSB SW radio receiver. It didn't work, but a direct conversion one that I made with a Denco coil did and this introduced me to my first experience of the frequency shifting effect of tuning through an SSB signal.

I still have the non-functional circuit, and here is a picture of the damn thing.

80m Band Radio Board With Denco Coils And Cans



The direct conversion receiver mentioned earlier ended up in junk box heaven, there to be slowly crushed by the pressure of continuing deposition.

The red coil is supposed to be a self oscillating mixer using a 2N3819 JFET. This thing would not oscillate, no matter how many wet fingers were poked around it, or how many times the coil connections were checked.

The coils are relatively enormous by modern standards, being made to go in valve equipment originally. They were made so that as a hobbyist, you could unplug the RF coil(s) and plug in another set to switch bands if you didn't want to use a wavechange switch. Their main advantage, as I see, is that they are very pretty colours. At the time, they cost what seemed like a lot of money. The sockets used would be 9-pin valveholder sockets.

Picture Of Spare Denco Coils And 80m Radio Circuit Underside

Bearing in mind that this was the 1980's, at the time, I was rather taken with this construction method of using plain matrix board and just soldering everything together underneath using the component legs and bits of wire. Note the classic layout error of doing the voltage rails in the same way that you'd see on the schematic diagram, putting one voltage rail at one side of the board, and the other voltage rail on the opposite side. A schematic is not a layout. The voltage rails should both run along one side and drop up and down to where they are needed. This closes-up many large current loops.



80m Amateur Band Receiver Board

These coils are nice, but they're not magnetically shielded types where there is an internal ferrite magnetic circuit surrounding the outside of the coil before it meets the external metal electrostatic screening can. Those magnetically shielded ones are the types used in the radio on this page. You might wonder if using the aluminium screening cans on these old coils caused the Maxi-Q to be rather less Maxi than the name suggests, being as they are, a big shorted turn albeit some substantially larger diameter than the actual coil. I'd have to measure it to have a properly justified whinge.  You could get around it by cutting a vertical slot in the screening cans. Not pretty, but functionally better.



Anyway...

No-one in the UK makes such coils or coil formers and cores any more. Toko make coils but it's generally very difficult to get small quantities, though Maplin stocked them for a while. Maplin are too busy stocking cheap Christmas lights nowadays to bother with such items. However a company called Amidon in the States do exactly this sort of thing. Ordering them on a credit card over the phone is easy, though you occasionally get charged import duty. The pitch of the connector pins is not 0.1 inch and the screening can legs are too fat for veroboard, so a bit of board mangling and soldering to the cans is called for. If I was being particularly brave I would've tried doing something like winding all the coils on chopped-up bits of biro, but the Amidon formers and cores seemed like a sufficient challenge.

AF Stage

This is very standard, using a simple voltage amplifier stage Q10 driving a class B output stage Q6 and Q11. The biasing is a bit fancy. It uses the transistor Q7 and the 1K adjustment trim. This effectively makes an adjustable voltage which is a multiple of the Vbe of Q7. Q7 is epoxied to the output stage transistors for thermal coupling. The output stage transistors are a bit weak to drive an 8 Ohm speaker very loudly but they manage a voltage swing of a few volts nonetheless. R20 should be adjusted so that about 5mA quiescent current flows in the output stage with no audio. The 100uF decoupling capacitor shown on the battery input is best located physically right across the collectors of Q6 and Q11 otherwise the audio stage can go unstable.

Tuning up and Tracking

In practice you tune to a fixed frequency signal near the top end of the band and tweak the oscillator trimmer to get the desired top-end coverage and the antenna capacitor trimmer for best signal. Then you tune to a signal near the bottom end of the frequency range and tune the oscillator inductor for correct bottom end coverage and then the antenna inductor for best signal. If the fixed component values are about right to start with, this process should iterate to convergence on a fair compromise with at least good tracking at either end of the band.

Aerial

I could've used the traditional AM radio technique of putting the RF tuning coil directly onto the rod antenna. It was merely convenience of testing and building that caused me to use an RF coil on the board. Connecting up a standard ferrite rod of a few inches long with about 20 turns on it plugged into the aerial input will bring in all the decent strength stations. The simple AGC circuit means that you need to turn up the audio gain quite a lot for weaker stations. If you were getting enthusiastic you could make a much bigger square loop aerial of say three turns, half a metre on each side with a coupled resonant 'pre selector' winding, but then that's another project altogether.

Sounds

Aldershot Garrison Radio News In Jingle

I'm about three miles from Middle Hill in Aldershot where AGR broadcasts on a standard 1 Watt ERP restricted service AM license on 1287AM. It's a fairly weak signal and you can hear the noise, some of which is from the radio circuitry rather than base atmospherics. I can't resist sharing their cheesey voice-over jingles. Some day in the far distant future, people will realise that doing voice-overs in a hilarious Smashey and Nicey voice is not actually such a good idea unless you want to make your station sound comic.

Garrison Radio News Out

Close-Up Of The Completed Board Top View

Improvements

A 2026 Update Perspective

Yes, I'd probably get rid of that diode limiter in the oscillator. You'll see in other radio oscillators that the designer has arranged the oscillator supply and base bias so that as the output increases, the supply voltage and base bias drops. If you're going to buffer it, there's no need for a completely separate winding. The radio frequency side of the circuit is connected direct to the battery along with the audio amplifier. RF section positive rail should instead go through a resistor of some value and have a 100uF capacitor across it.

There are four transistors, SS9012, SS9013, SS9014, and SS9018. 9012 and 9013 are a 400mA complementary output pair which are much more suitable for driving a small speaker than BC549 and 559, which are running out of steam at a couple of Volts driving 8 Ohms. SS9018 is a nice VHF RF small signal transistor. If you need some authentic Fairchild SS9018s, I have some stock of the highest available gain band version of them (SS9018H) which I bought before they were made obsolete by Fairchild.  SS9014 is an NPN small signal rather like the BC549. In therory there's another one in the series, a PNP small signal equivalent of the BC559 that you might use in a PNP demodulator buffer stage.

Here's a recent video of the radio in use. I shown how a cardboard tuned loop antenna can be used to really help-out with a radio such as this, where the IF automatic gain control and ferrite rod antenna haven't been optimised very well.

DIY Superheterodyne AM Radio In Use

Amidon - Micrometals Shielded Coil Forms

I used "blue" cores to save turns on the windings. Here are some pictures of the higher frequency red equivalents that I never used, and a scan of the single page data sheet that came in the packet. I suspect that these are long since obsolete.

Amidon Shielded Coil Forms Detail

It's worth noting that the plated copper can is an electrostatic shield only, and that magnetic shielding and high overall core magnetic density is achieved by the square insert mating up with the circular ferrite plate on the base with the pins on. It's a closed magnetic circuit.

Amidon Associates Shielded Coil Forms Single Page Data Sheet

This single page would be supplied in the same packet as the coils that you'd ordered. I love these old data sheets.



There's a bit more to add, but that's enough superhet AM radio fun for now. I'll leave the headings as a reminder to me, and to pique the interest of the potential viewer:)

Air Spaced Variable Capacitor Superhet Tracking Adjustment

You fettle it by bending the outer rotor plates on the antenna section for maximum signal while tuning up the band.

Synchronous Demodulators

Are they any good for AM radio? Can you really get a theoretical 6dB improvement in S/N ratio? Some contentious comments may ensue, though at least I tried it, and I used this radio as the test bed.

Basic Synchronous AM Radio Demodulator PDF



Cue circuit description and contentious comments to be completed in the next exciting page update in another 23 years time.

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