How / Why does radio "previewing" work ?

This is related to recent posts on Dewsters "digital theremin" thread related to EM radiation from theremins:

The question of EM radiated from a theremin .. What emits it, is it from the antennas, the inductors, or somewhere else - how bad is it...

Well - its a thing that has bothered me a bit with regard to monitoring or 'previewing' using an AM radio.. As has been discussed at TW a few times...

You see, I have never understood how this could work if the signal being detected was being transmitted from the antenna...

Ok, the fact that it works to any degree shows that EM is being transmitted... But what produces the audio? If this was the result of heterodyning the VFO frequency on the antenna with the radio's local oscillator, then the audio pitch from the radio would be different from the audio produced from the theremin, because the chances of both the radios LO and the theremins reference oscillator being exactly the same are, well, quite small ;-)

So ok - On the EW, the mixer is really crude - so both reference and VFO signals could be mixing and a AM signal (beat frequency) could be appearing on the antenna I suppose - But I doubt that this is whats happening, and it certainly wont be happening on any EW with a buffer, or on any other theremin where the oscillators are buffered or otherwise have no route by which AM could be appearing on the antenna.

But if the radio is just picking up the VFO from the antenna, and not mixing this somehow with the reference oscillator, and not beating this with its own local oscillator, well - we shouldnt hear audio from the radio !  should we ??

I suspect / hypothesise that the reason one can hear theremin audio on an AM radio is because this EM is being radiated by inductive components in the VFO and REFerence oscillator circuits, is mixing in the "aether" or in the radio circuitry, and is probably not coming from the antenna at all...

I have done almost no RF "probing" of this type at all - have never put a radio to my prototypes, and have always been quietly sceptical of this practice, mainly because I didnt trust that any 'results' would be meaningful, because there were "unknowns" involved with any radio I used for such "testing".

The biggest unknown probably being any possible interactions caused by the radios local oscillator (or oscillators?) (heterodyning or superhet?) As I dont feel  competent with radios and am not  particularly interested in them I simply avoided using them - particularly as I have test equipment I understand and trust  which does everything I need..

But as the whole EM matter is topic of the week, I thought it as good a time as any to float this... Usually I avoid mentioning anything EM related at TW ;-)

So my questions are these: 

Does radio previewing work?

Y ->  is the audio from such previewing the same frequency as the audio from the theremin ?

         N -> Ok, then nothing is "proved" - EM from the antenna could be mixing with radios LO.

         Y -> how does the reference oscillator (which is not connected to an "antenna") get recieved, or mixed, or whatever, to enable an AM radio to reproduce the difference frequency ?

Fred.

I should also add that if I remember correctly, last discussion on radio previewing. the tuned frequency of the radio at which theremin audio was detectable covered about 4 harmonics (as in, say the VFO was centred at 250kHz, audio was detectable when the radio was tuned to 250, 500, 750 and 1000kHz) ... Now that struck me as quite odd - because antenna waveforms I have seen tend to be quite sineusoidal - having strong EM up at the 4th harmonic just didnt seem 'right'..

However - signals, particularly the drive / excitation pulses, in oscillators (and through their inductors) can be quite rich harmonically - Also, if care is not taken in the decoupling of circuits, one could easily get strong radiations from tracks and whatever..

'Does Radio previewing work?' - Fred

In my limited experience of plonking a wee AM radio ontop of my theremins (Harrison 145 and EW) and listening to the output at the appropriate frequency ( and it's harmonics), previewing does seem to work: the noise in my ear is at the same pitch as the eventual sound from the amplifier connected to the theremin once I give it volume with the volume antenna.

Isn't there an audio mixer somewhere in the instrument that the outputted signal goes to before going to the amp? Maybe the signal is getting picked up from here?

Ok, maybe not...I'll awa back to making fiddles...

"Isn't there an audio mixer somewhere in the instrument that the outputted signal goes to before going to the amp? Maybe the signal is getting picked up from here?" - RoyP

Thanks for that info, Roy..

Yes, there is a mixed audio source - but if this is just audio then a radio wont pick it up....

However, it is another possible area to look at.. as if there is a lot of modulated RF on the post-mixer audio anywhere in the theremin, this could be picked up by a AM radio I guess .. If this RF was leaking down audio or power leads that could also radiate EM.

Fred.

ps - this thread and these questions are deliberately open-ended.. I was reasonanably sure some form of radio previewing worked, from what has been said before at TW by you and others, and have been genuinely uncertain of the mechanism/s but thought it wasnt primarily from antenna radiated signal.

I have often commented on the high level of RF and the low quality of filtering on theremin audio outputs, but my main worry related to this HF causing problems when mixing with digital equipment (sound cards, effects etc)..

But your comment above just reminded me that this could be a strong contender for being a major EM radiation mechanism.. The focus has been on antennas and inductors at the front end, but actually this is an area where some care is usually taken with regard to EMI.. Could it be that the problems dont (directly) originate from that area, but 'get out' (and perhaps in) further up the circuit.... ?

(I think Dewster is probably right with his hypothesis that EMI gets in easiest through the volume antenna circuit)

***

"a mirror image audio sine wave rides on both outside edges of the variable oscillator RF wave" - RS

As you now know, this is the typical mixed waveform - its a RF sum frequency Amplitude Modulated by the difference frequency.. This is a signal AM radios will pick up easily.. Usually this signal is quickly rectified and filtered (or demodulated through some other means) and becomes "safe" audio with the RF components removed..

But a lot of theremins are poorly designed in this respect, and removal of the RF is not done well enough.. this RF is still Amplitude Modulated, and if this leaks out I think it could easily be the major source of "preview" radio signal.

Further to my last..

Purely hypothetical, but.. If the heterodyning is whats producing the AM RF signal radios can pick up..

Then it may be that digital theremin designs like livio's Theremino, where no deliberate mixing is done, will not radiate detectable EM the way that analogue theremins which have poorly designed filters do.

The above is purely hypothesis - just guessing at mechanisms at the moment and absolutely nothing has been substantiated in any way..

EM radiarion from antennas and inductors and now post-mixer circuitry are still, IMO, all on the table... And it may well be that there are different primary mechanisms for different theremin designs and/or constructions..

My biggest personal interest is whether the antennas are ever a major source of this radiated EM .. But the whole matter is important - particularly I think dor anyone wanting to produce theremins or related instruments on a commercial scale, where compliance with EMI directives is required.

I must re-state however that I am SURE that Electromagnetic Radiation plays absolutely no measurable or significant role in the actual operation of the theremin, which operates purely and exclusively on capacitance and electrical (NOT EM) fields.. However, that is not the subject of this thread, others think that EM plays a role in the sensing action of the theremin, I dont agree, but they are as entitled to their opinion as I am to mine - its for others to weigh up presented evidence for themselves.

The subject of this thread is not about theremin operation, it is about "side effects" of EM radiation which does occur from theremin circuits and/or antennas.

Fred.

You titled the thread: "How / Why does radio "previewing" work ?"

My answer did fit what you needed to hear so you called it spam and high jacking. Out of respect I removed it. It is better when we respect one another's space.

Christopher

Hi Christopher,

I have a copy of all the posts - got into the habit of doing that just in case.. I never accussed you of spamming.. but as I said, this thread is not about the "debate" on how theremins operate, its about radiated EM in an entirely different context...

You believe radiated EM is a big part of how theremins OPERATE and contributes significantly to such matters as sensitivity and linearity - I believe radiated EM plays no role in these functions.

I believe I am right, you believe you are right - you have presented your ideas on many threads, I have presented "my" (they are not actually "mine" - they are the ideas on which Lev Theremin based his design and which almost every commercial theremin developer and EE accepts) ideas on many threads..

THIS thread however is NOT about how theremins operate - it is about "side effects" of the radiated EM which DOES occur.

I am trying to respect your "space" - which is why I created a seperate thread for this topic rather than using the "livio playground" thread you started.

Thank you for respecting "my" space, or more importantly, the subject of this thread. And thank you for your useful contribution in detailing the waveform from the mixer (which I quoted) as that was entirely relevant to the subject of this thread.

Fred.