Nasty radio-sounding noises at Zero Beat?

[i]If there is a 'muting' function, I feel a little P'd off - I have spent ages on getting my oscillators super-stable, so that there is no growl when exceeding the zero-beat distance![/i]

No need to be p'd off - when I wrote that response I was referring to this entry in the etherwave hotrod manual...

[i]C2 and C6 also provide weak coupling between the two pitch oscillators, which has the effect of synchronizing the pitch oscillators when their frequencies get very close together. This has the desirable effect of providing a stable 'zero beat', so that the instrument, once properly tuned, is silent when the player steps away from it.[/i]

Guess I wasn't quite as clear as I could have been.

Hi Gordon!
Lol! - never realised it was YOU! ;)
Yeah - That makes sense.. get the oscillators to lock so that the output frequency is zero..

And this adds an interesting angle.. in the 'zone' where the oscillators are close to locking (syncing) they are likely to produce more harmonics than normal (Audio VCO's use this technique to give interesting overtones in synths which have a sync control)..

My guess is that these 'near sync' harmonics are interacting with the volume oscillator, to produce the 'birdies'..

And the cure might be to reduce (or perhaps increase?) coupling between the pitch and reference oscillators..

To increase the pitch and reference oscillator coupling, a capacitor connected between Q1:C and Q3:C (probably something like 5pF or a 3 - 10 pF trimmer) - This will increase locking.

Reducing coupling is a bit trickier, but reducing C2 and C6 may do it.. reduce these to (say) 10pf.. but probably no smaller.. This should decrease locking.

I think, to get rid of birdies, you probably want to increase coupling.

Thanks FredM,
I think I'll try to capture the "birdies" for an .mp3, and then try RSTheremin's 'ferrite' fix and then report.

Good Luck!

teslatheremin

Hi tesla..

RSTheremins fix is really worth trying - If the problem is caused by harmonics on the pitch antenna (produced by the pitch oscillator syncing, or partially syncing to the reference oscillator) then a ferrite should attenuate these substantially.

I have been playing with some of my oscillator prototype boards since my last posting, and have certainly confirmed that there is a huge increase in harmonics at near-lock, and (surprisingly) even in full-lock... Both 2nd and 3rd harmonic levels are high enough that, when mixed with another oscillator (to simulate the volume oscillator) running at twice or 3x (+ a few kHz) the pitch oscillator harmonics + the 'volume' oscillator fundamental do mix (heterodyne) in the mixer to produce audio..

Must jst say that my real oscillators do not do this - My experiments were done on old prototypes with variable capacitors replacing antennas, so I could easily tune for worst case results.

It should also be possible to get rid of the problem by re-tuning either your volume oscillator (and its resonant tank) or by re-tuning your pitch and reference oscillators.. These should be tuned so that the frequency of the volume oscillator, when open, is far enough away from any reasonable harmonic from the pitch / reference oscillator.. But you will need accurate test gear, and a calculator, to set this up correctly.

Good Luck! ;)

I really appreciate the helpful suggestions and assistance for the issue.

It's clearly some kind of signal being inducted into the output, but I've not been able to trace it down definitively, yet.

But I have seen that Radio Shack does sell clip on ferrite cores for EFI isolation, so I'll give that at try.

Also, I'm wondering about the Etherwave Standard... When I power up my machine, it makes a VERY high pitched whine that quickly fades while what sounds like the radio-warbling accompanies it. Is this the circuit self-synchonizing with itself when it's first active? Does every Moog Theremin do this? There is no such burst of sound turning on or off my Wavefront-- so I thought I'd ask...

I'll let you guys know if the cores help reduce / eliminate the zero-beat radio-noises.

Thanks,
Rowsby