Just built Etherwave doesn't work

My hypothesis is the resonant frequency of antenna circuit is away of oscillator's. Or you use off-grade coils.

I usually check the coil's SRF (or operating ability of the antenna circuit) by generator as shown below. The test coil is winded around the first section and has about 20 turns of isolated wire. The sensitivity of oscilloscope (V/div) is set on maximum. Changing the frequency of generator, I find the peak.


Of course you can just bring the probe close to the right part of the coil, but the desired signal can't be seen distinctly due to interference of 50 Hz.

 "the resonant frequency of antenna circuit is away" -- my words 

"away" = too low

If so on, you can try exclude not only the L10 as Thierry advised but L9+L10 and even L8+L9+L10 (That is a way to drastically increase the resonant frequency). Just to be sure you're on the right direction.

SUCCESS!!!

Finally I had a time to perform few experiments with L8 and L9 replacing by a short wire. Combination L7 + L8 (with removed L9) works fine. It's quite loud sound when hand is moved away from volume antenna and mute when hand in brought very close to it.

Thus my Etherwave is workable at last. Now I'm going to make a shelf(cabinet) for that and completely adjust the theremin.

Thank you guys for your help!

Also I had a problem of theremin's sound is some sawing. But that was resolved by connecting GND pins of U1 and U2 to power ground (ground of power cable/power outlet).

I also have a couple of additional questions:

1) According to my calculations the capacitance of volume antenna circuit should be about 18pF to reach resonant frequency about 560KHz. Why could it happen? It is ok or it's some fault?

2) The only antenna sockets I could find are made of brass. It's usual sockets for 3/8 water supply tube. The circuit board should be connected to these sockets (not to antennas directly). I think I will solder wires to the sockets using a high-active flux. But would it be ok to use brass sockets at all? Or I strongly need to find copper sockets?

Thanks.

A capacitance of 18pF is not realistic. A volume loop which is placed following "les règles de l'art" (sorry, can't translate this figure of speech, but it means following the rules of the art) sees normally an environmental capacitance of 7 to 8pF. Either your coils have ways too much inductance (did you check them with a LC meter?) or your actual setup adds too much environmental capacitance. I can't imagine any other explanation.

Correct setup means that the loop is installed horizontally in the air, about 1m or 3feet above the floor and the pcb at about 10cm or 4 inches horizontal distance from the loop (not more to not increase the wiring capacitance and not less to minimize the capacitive effect of the PCB onto the loop).

I can measure these 7 to 8pF on an open Etherwave not powered on but with the power supply connected, installed on a mic stand with the wire leading to the loop disconnected from the circuit board and connected to my LC meter which is positioned exactly above the circuit board while the other test lead goes to a ground point of the PCB.

The material which you use for the antenna fixings is not important as long as it conducts electrically. Copper, brass, nickel, silver, gold, platinum... you are free to choose! ;-)

Sorry for the delay.

I was checking every coil inductance before soldering on circuit board. Everything was within expected value +- 5%. Yesterday I also checked values of L7 and L8 mounted on the board. It was 2,55mH for L7 and 2,6mH for L8.

So nothing suspicious... That's why I wonder that combination L7+L8 (with removed L9 and L10) is workable on frequency ~560KHz. The only idea I have is capacitance due to some reason is much bigger than expected. I don't know...

Such as I'm going to build one or two Etherwaves more, I would like to ask additional questions for possible future cases.

1) Do I correctly understand that frequencies of pitch oscillators are strongly depended on LC filters' (C1+L5+L12 and C5+L6+L13) resonant frequencies? And we have the only way to shift frequency of one of pitch oscillators - just correct C1 or C5. Or we can change the frequency by some another way?

2) The same question for volume oscillator. Can we change its frequency without replacing or adjusting L11 and L14?

Thanks.

Uber said: "Such as I'm going to build one or two Etherwaves more, I would like to ask additional questions for possible future cases."

I always had a rule in my electronic projects that I only build what "cannot" be bought off the shelf, that is what makes DIY rewarding. If the EtherWave pcb stuffed and tested costs on $96 (57 pounds) may I ask what your future goal is by make more of the same thing. You should try other models to compare performance.

Are you in the UK? Sometimes their language trips up my understanding of what is written, it may be the accent.

Christopher

"Do I correctly understand that frequencies of pitch oscillators are strongly depended on LC filters' (C1+L5+L12 and C5+L6+L13) resonant frequencies? And we have the only way to shift frequency of one of pitch oscillators - just correct C1 or C5. Or we can change the frequency by some another way?"  - Ubermann

There is actually a double resonance going on and they interact in complex ways.  The first (and IMO the main) resonance is a series LC formed by the antenna and L1+L2+L3+L4.  The second resonance is a parallel LC formed by C1 and L5+L12. This double resonance seems to have a moderate linearizing effect on the pitch field near the antenna if you tune it just right (but IMO it's not worth all the trouble).

You can indeed shift the second resonance by adjusting or trimming C1, but adjusting the inductor is probably the more stable route as adjustable capacitors can be finicky (not that I have much direct experience here).

Like RS Theremin recommends, you might want to try other designs.  With its double resonance the EW is a strange and complex beast to build and own.  I'd recommend going higher in frequency and using only a single resonant LC.  Higher frequencies will give you more heterodyning sensitivity which you can trim back with tank capacitance.  Personally I'd look for oscillators that don't require a transformer but work with a simple tank.

This is the best article I have found on the matter of series/parallel coils used with theremins -

https://dl.dropboxusercontent.com/u/1548721/Theremin/Theremin_Ant_Linea.pdf

(IMO the above article by Thierry should be compulsory reading for anyone engaged in debate about the function of the series inductor - Ok, you may still want to disagree with the conclusions, but IMO it gives a clear picture of the thinking behind the topology, and outstanding insight into its operation in the EW circuit)

Fred.

Ps - Absolutely not wanting to knock Dewsters comments in any way, and in many situations I agree that simpler topologies are better -

But If you already have EW oscillators built and running, I strongly suggest that you dont try to modify these to get them to connect directly to the antenna without the right series antenna inductance - I dont believe this is what Dewster was suggesting, but just want to give this caution in case you think it was!

With the small oscillator (tank) inductance and the large tank capacitance, the EW oscillators will ONLY operate an any satisfactory way if there is the correct inductance between it and the antenna - To get it to operate directly connected to the antenna you will need to reduce the tank capacitance massively (down to perhaps 200pf or less) and hugely increase the inductor (to perhaps 1mH) and deal with other concequences such as the increased resistance of the inductor (lower Q).. Attempting such a modification would be utter folly unless you are well equipped both with test gear and with the required knowledge - and if you have the required knowledge you probably wouldnt modify the EW oscillator to do this, you'd use a different oscillator circuit.

 There is another big advantage to staying with the unmodified EW circuit, particularly if you dont have the time or competency to develop your own design - and that is that there is at least one supremely competent EW engineer (yeah, theres only one )-: here on TW who can guide you if you have problems in ways that you will not get for any other theremin.. Ok, IMO he's not always right, and sometimes talks utter nonsense (even about the EW) - But everyone at TW talks nonsense some of the time - its part of the reason were all here! 

;-)

"But If you already have EW oscillators built and running, I strongly suggest that you dont try to modify these to get them to connect directly to the antenna without the right series antenna inductance - I dont believe this is what Dewster was suggesting, but just want to give this caution in case you think it was!"  - FredM

Yes, that's what I meant, thanks for clarifying that Fred.  If you've got the EW circuit working and you are happy with it, by all means keep using it.  But if you want to experiment in other directions, I'd maybe try one of the single FET oscillator topologies.

Guys,

The theremin I'm building now is not for me but for my old friend. He is musician and singer. Some time ago he asked me to build a hand-made theremin for him. Browsing Internet I found a scheme for Etherwave (dated 1996; scheme 1 from my first post of the thread). I'm not an electronic engineer and I was needed to have some complete scheme for the device.

A lot of theremin schemes I found require self-made coils with few electrically separated wounds on one core, and some specific transformers. I didn't want to deal with self-made coils and tranformers at that time. Etherwave is one of most populated theremins in the world so that means the scheme is definitely workable and has good sound characteristics. Also it has so unusual amplifier type (based on transconductance amplifier) which is good to amplify audio signals (as I understood from Internet). And also Etherwave uses completed factory-made coils. So I decided to build it.

I understand that there can be a lot of schemes for theremin and many of them are probably have some better characteristics. I just want to build etherwaves for my old friend and for me. I also understand that completed factory-made PCB should be definitely better and more stable but I would like to have a hand-made theremin by myself. I'm not crazy to build a lot of Etherwaves.:) It's senseless and too hard.

Actually I didn't mean to make some redesign of any of oscillators. I meant to correct any value (capacitance, resistance,...) to shift its frequency. To make an oscillator's redesign is an idea but of course it needs a deep investigation of the area to find an appropriate solution.

Dewster said: "I'd recommend going higher in frequency and using only a single resonant LC.  Higher frequencies will give you more heterodyning sensitivity which you can trim back with tank capacitance."

Could you suggest me a good heterodyne theremin scheme using the higher frequency for future building.

My accent is a normal effect. I live in Russia. And actually I wondered that it looks like "British style accent".:)