Mike WU2D has an interesting series of videos on the simple detectors used in crystal radios. Mike's videos touch on a discussion that I had a few years ago with a member of the Vienna Wireless Association. I was describing how an envelope detector uses rectification to recover (demodulate) the audio from an AM signal. The club member dismissed this explanation and claimed that what really happens is that the diode simply mixes the carrier and the sidebands to produce audio. He claimed that the rectification "theory" was just a fable created to deal with people who really didn't know how mixers worked. See: https://soldersmoke.blogspot.com/2022/12/is-envelope-detection-fable-or-is-it.html
I was kind of disappointed by Mike's videos. He seemed to take an agonostic position on this issue. He framed it as "some people believe in the rectification explanation, and some people believe in the product detection explanation." As if both explanations are equally valid. They are not.
This morning, I asked AI about the two explanations. Here is what AI says:
Does a simple, single diode envelope detector function the same way as a product detector does?
AI Overview
No, a single diode envelope detector does not function the same way as a product detector because the envelope detector is a non-linear circuit that recovers the audio from a full amplitude-modulated (AM) signal by following its envelope, while a product detector is a linear mixer that recovers the audio from various modulated signals (including DSB-SC and SSB) by multiplying the modulated signal with a local oscillator. Product detectors are more versatile and offer better signal quality than simple envelope detectors, which are limited to demodulating DSB-AM signals with a full carrier.
I know that AI sometimes gets it wrong, but I think that on this one, it has it right.
I don't think it is necessary to include discussion of modulation percentages nor diode biasing to clearly explain what is going on.
Even if you are using a very weak signal and are completely in the square law region of the characteristic curve, you are still essentially dealing with a form of rectification: portions of the signal on the positive side of the curve will experience less attenuation than signals on the negative portion of the input curve.
When we use a crystal receiver, we are relying on the rectification done by the diode -- even if the rectification happens in the square law region. After the crystal there is some low pass filtering. The envelope of the AM signal remains and this is the audio signal that we listen to. That is why we call it -- correctly -- an envelope detector. And as the AI says, an envelope detector funcions differently than a product detector.
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I think you may have missed the point in Mike's videos. Two videos ago he said something along the lines of the product detection effect is dominate below a certain input level and rectification above that level.
ReplyDeleteIts easy enough to prove by just feeding a single sine wave into a single (silicon or Schottky) diode. Below a certain level there will be no output. Above that level you will get a dc level. I suggest silicon rather than germanium because the characteristics of germanium complicate things further.
Now do the same thing with modulated RF.
I think Mike choose his words carefully to avoid starting an on line fire fight.
I just know I will regret getting myself into this thread, but here goes...
ReplyDeleteRestricting ourselves to the envelope detector,
what if you, Bill and the Vienna Wireless club member are both correct, but in different domains?
In the Time Domain (i.e. as seen on an oscilloscope) your rectification explaination is valid and in the Frequency domain, his mixing explaination is valid.
Doesn't have to be an either-or situation.
73!
Mike: I don't think so. The VWS guy was saying that the diode in a crystal radio was just a mixer, and it was just mixing the carrier (which served at the LO) and the sidebands (which serve as the signal). You know, like we do with a product detector. I see it as being fundamentally different. The diode in a crystal radio just rectifies the AM signal. The diodes in a switching product detector are turned on and off by the LO, multiplying the RF by 1 an 0, or 1 and -1). Through this process we end up with sum and difference freqs, the hallmarks of mixing. The difference frequency is audio. To me, there is a fundamental difference between how the rectifying diode in the crystal radio works, and how the diodes in a diode ring product detector work. AI agrees. And I think Terman would also. 73 Bill
DeleteI was restricting my comment to the envelope detector, as in a single-diode unbalanced detector. I wasn't comparing the envelope detector with a product detector. Yes, indeed, different.
DeleteRe your VWA discussion above:
"I was describing how an envelope detector uses rectification to recover (demodulate) the audio from an AM signal. The club member dismissed this explanation and claimed that what really happens is that the diode simply mixes the carrier and the sidebands to produce audio". What I mentioned is that both explainations are valid, just in different domains.
Yes, Envelope Detectors and Product Detectors work differently.
Just two things that come to my mind,
ReplyDeleteFirst, most of the time it is not a really a rectification. E.g. with crystal radios amplitude is
often too small to reach the threshold. The guys who deal with this name it 'square law detection',
which describes what happens to a complex
signal that experiences a non-linear behavior.
Second, think of 'enhanced carrier detection'.
Injecting a strong carrier on the exact frequency
will increase loudness of weak signals
(still with a crystal radio), because the operating point is thus shifted from an only slightly non- linear to a much stronger one, close to on/off.
Both effects can be described as 'mixing',
though as stated before in the time domain
the rectification is more obvious.
73
Peter/DL3PB
Peter: But even in the square law zone it is more like rectification -- the positive portion of the signal is attenuated a lot less than the negative portion. 73 Bill
ReplyDeleteThe way I see it is to follow the mathematics. The diode characteristics are non-linear and therefore new frequencies are generated from the input signal - that's rather like a mixer, but lots of product signals are generated in practice. A realistic model for the diode is quite complicated, so simple analysis is quite problematic, but numerical simulations can help calculate the result. I had to do it once in a fit of energy at engineering school, but that convinced me that simpler approximations were helpful, if not perfect. So, model it as a rectifier if you like (in certain circumstances) or a mixer (in different circumstances) but don't expect either model to be perfect. If you need better than this, more complicated methods would need to be used - and then the complications arise, as noted earlier.
ReplyDeleteTo be honest, I can't see why this is a controversy.
Well, I just think it is important to understand how a circuit so central to radio actually works. And I don't think it is good enough to just point to the math and say, well, math says it should work. It is possible to really understand how diode switching mixers work: You can draw the output curves and SEE the sum and difference frequencies. We can do something similar with the envelope detectors, but this requires us to accept that these detectors work by rectifying the AM signal.
Delete