Superheterodyne Radios Explained

The general public thinks there is one thing called a radio. Sure, they know there are radios that pick up different channels, but other than that, one radio is pretty much like the other. But if you are involved in electronics, you probably know there are lots of ways a radio can work internally. A crystal set is very different from an FM stereo, and that’s different still from a communications receiver. We’d say there are several common architectures for receivers and one of the most common is the superheterodyne. But what does that mean exactly? [Technology Connection] has a casual explanation video that discusses how a superhet works and why it is important. You can see the video, below.

Engineering has always been about building on abstractions. This is especially true now when you can get an IC or module that does most of what you want it to do. But even without those, you would hardly start an electronics project by mining copper wire, refining it, and drawing your own wire. You probably don’t make many of your own resistors and capacitors, neither do you start your design at the fundamental electronic equations. But there’s one abstraction we often forget about: architecture. If you are designing a receiver, you probably don’t try to solve the problem of radio reception; instead you pick an architecture that is proven and design to that.

There are other examples. Do you really work out a binary counter every time? Or how to make an op-amp amplify? You start with those building blocks. Of course, true innovation means you have to stop doing that and actually think of new and different (and possibly better) ways to do the same task. But most of the time you aren’t trying to innovate, you are trying to get the job done.

The video is pretty straightforward and doesn’t assume you have much radio background. However, it does manage to do some real demonstrations and it is worth a watch. There are many other receiver architectures, of course. Regenerative, superregenerative, homodyne (direct conversion), Hilbert, and Weaver are all types of receivers and there are doubtless more. The funny part is that many of the ideas we still use today came from one man: Edwin Armstrong.

17 thoughts on “Superheterodyne Radios Explained

  1. “….neither do you start your design at the fundamental electronic equations….”
    when the problem is not trivial, I often start at this point. A problem, an idea, a small sketch , basic physics, integrate maxell back and forth. Now you know the estimated working conditions, best/weakest signal intensity, necessary sample rate, power spectre of the driver…etc….etc
    After this I calculate the upper/lower conditions/specs and start searching digikey&friends. success rate ~80-90%, depends on the number of components that do not comply with the specification……I’M LOOKING AT YOU HOPERF!!!!! (for example)

  2. I early on didn’t know why, till I learned that the rubber meets the road in the IF stage(s). There was the all American 5, with 2 double tuned IF transformers. It’s transistor equivalent got cheapened to 2 single tuned transformers, just because of thin requirements for shirt pocket size. Those radios were a curse in our town with two locals 40 kHz apart. It wasn’t a problem on a car radio, they were 2 double tuned IF’s and front end RF stage tuned by the dial. Ham gear can go whole hog here with 3 or more stages of IF filtering. I recently got a Pioneer car radio for 5 bucks at a garage sale that let’s me get a station 60 miles away right next to a new local christian addition to our crowded FM dial. No other Pioneer I have or any other brand can do it.

    1. No no no! It is all wrong. Superhet does not use beats like that. The waveform from a sum (superposition) has a different phase than the sum and difference from a product (multiplier – where the phase inverts as the envelope goes through zero IIRC) and you don’t get the sum term, only the difference. The whole explanation using sound as an analogy is just physically incorrect. The explanation needs to be based on multiplying two cosines. Look at Single Side Band.

      (Note it is possible to “hear” hings that don’t exist due to non-linear response in your ear bones – like distorting a microphone.)

  3. I watched this video a week ago, so I might be mistaken, but I think he used a sum of waves in his example, which would never produce offset images needed for a superhet to work. The operation that offsets the frequency that interests us to the IF is multiplication.

    He demonstrated that addition does not work, but produced a wrong explanation of the why and kept going on. I’m sure this has been pointed out by others and perhaps a follow-up video is to be expected.

    1. Yup, absolutely right — and there are several comment on the YouTube video to this effect, that a non-linear element is needed to do the mixing.

      Having built my own transistor superheterodyne receiver (https://www.youtube.com/watch?v=s5XYp0EaCTM), I realized that one problem often glossed over in introductory explanations is that the IF amplifier strip, with so much gain at one frequency, has an annoying tendency to oscillate that must be suppressed! It took me several iterations to solve this; I ended up using common-base amplifiers (to eliminate the Miller effect) and low-L, high-C tanks (to reduce the unwanted influence of parasitic, capacitive output-to-input coupling). Only then could I get a stable, 3-stage IF strip with I guess around 60 dB of amplification.

      I’m still working on perfecting the AGC and the AF amplifier. One of these days when the project is “finished” I hope to do a more complete write-up on my blog.

      1. I think the [Technology Connections] dude will probably not leave it at that and I’m sure he’ll come up with a great way of correcting this mistake and use it as a pretext for going even deeper into the radio world.

        I like your radio construction. Unexpectedly, this old school kind of point-to-point on solid ground plane construction looks fresh in this age of fancy arduinos with multicolour solder masks. What do you use as the base plate? It looks like copper foil. Perhaps vertical walls could help minimising crosstalk between modules? Looking forward to read about your finished project here.

        1. Yup, my ground plane is simply copper foil stuck on some thick cardboard. Very cheap and easy to make (everything can be cut with scissors). For more permanent constructions and greater physical stability, I use thin wood as the supporting base.

          One thing I like about this radio (and that is made possible by the superheterodyne architecture) is that it uses only a tiny ferrite rod antenna designed for MW frequencies 0.5-2 MHz, yet I am using it out-of-spec at higher shortwave frequencies where there must be greater signal losses (due I think to hysteresis in the ferrite). Nevertheless, due to the massive (and filtered) 60 dB amplification — possible thanks to the fixed-frequency IF — I can still extract plenty of usable signal. Long live the superheterodyne!

    1. What’s that supposed to mean?

      The Wadley loop was a means to an end. It didn’t improve operation, it did away with endless crystals. Before it, there were very expensive receivers that did the same thing, and the first Racal (1957) that used the scheme was very expensive. Within a decade the HRO-500 came along, not too different circuitry, but using a real synthesizer to do away with all those crystals.

      The Wadley loop was a neat thing, but it added an extra mixer in the signal path, a stage prone to overload, but the Wadley loop needed extensive shielding to avoid lots of spurious signals.

      At best it was a short term solution.

      The Barlow Wadley portable shortwave receiver in the early seventies was aleap forward, because that level of receiver hadn’t been seen in such a small package, and though it was hundreds of dollars, it was cheap for that level if receiver.

      But even then it was on the cusp of digital synthesizers, more like concurrent, if you had the money for lab quality receivers. Within a decade ham transceivers would have digital synthesizers for tuning, 1KHz or less per step, and the legendary Sony 2001 was available.

      None of this changed the concept of the superhet, but changing the variable oscillator to a synthesizer allowed stable operation at higher frequencies, which meant up conversion became common. That allowed for simpler front end tuning. It moved IF selectivity closer to the front end, good for preventing overload.

      The Wadley loop had nothing to do with car radios, unless it was a full coverage shortwave receiver.

      Michael

      1. I didn’t mention the Wadley Loop circuit for your benefit, you obviously knew all about it (but not how to understand basic English). I mentioned it as a clever and relatively low cost solution to the ever present problem of local oscillator drift in consumer grade receivers for short wave use. True, it didn’t last very long because electronics changed too quickly but at the time.

  4. FYI, a lot of older car radios used a variable inductor for tuning, as opposed to a variable capacitor. I was told the variable inductor was more thermally stable. (Cars operating in a wide temperature range as opposed to household radios)
    And cars, with their large metal mass provide an excellent counter poise for the antenna, therefore better reception of weak (i.e. distant) signals)
    I have an old Wards Airline 5 tube table radio that does use a variable inductor for tuning.

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