Equivalencies for Pop. Electronics Theremin

I would say that a simple oscilloscope and a frequency counter are basic requirements when building/servicing theremins.

Since we have to deal with relatively low RF frequencies, a 5 MHz bandwith oscilloscope is sufficient. I use this one which can be found for a few dollars on ebay: UT81b (http://www.uni-trend.com/UT81B.html). For precision frequency measurement I use this frequency counter (http://cgi.ebay.com/FC-1-Portable-Frequency-Counter-10Hz-2-6GHz-VX-6R-/370419401839?pt=LH_DefaultDomain_0).

You can use a battery-powered AM radio to see if the oscillators are working.

Place the radio near the Theremin while it is powered up and tune the radio for a dead spot. Hard to adequately describe it in words, but when the radio is tuned to the same frequency as the Theremin's oscillators, there will be no sound or static on the radio's speaker. This is because the Theremin's RF signal is thousands of times stronger than what is being broadcast over the air.

Don

I agree with Thierry on this - get an oscilloscope!
If you are getting into electronics, the two essential pieces of kit you need are a good digital multimeter and as good a 'scope as you can afford.

Some multimeters give extra ranges which can be useful - some incorperate a basic frequency counter (you need one with frequency span up to 1Mhz - some come with range up to 15Mhz, but better to lose higher frequencies and gain resolution if this is the trade-off) - about $50 should get a reasonable combined multimeter / frequency counter.

As Thierry says - you dont NEED high bandwidth on a 'scope to see basic Theremin waveforms.. But .. with anything less than 20MHz you are stuck if there are any nasty HF signals causing problems.. things like unwanted oscillations on opamps and voltage regulators just will not be seen. What you select depends on how much electronics you plan to do - If you are intending seriously getting into electronics, then get the best kit you can right at the start - not only is this a wise investment (it costs a lot more to replace equipment which is inadequate than it does to pay a bit more initially) but it will enable you to learn more quickly.. You dont learn much from a board you cannot get to work because you cannot see nasty HF spikes on the supply rails! - You learn by seeing these spikes and correcting the problem by fitting adequate decoupling capacitors.

Also - The latest test kit is not always the best value for money - There are loads of good analogue 'scopes on the 2nd user market.. Scopes like the Kikusui COS6100A (100MHz dual channel chunky analogue) can be found for as low as £50, and these absolutely kick the s*** out of new digital 'scopes costing ten times that price.

I normally would also have taken an "analogue" 2nd hand o'scope. But since I travel often with my workshop in order to be present at different theremin gatherings and festivals, I preferred the portable variant which has the multimeter included. It also shows the frequency but the precision is not as good. That's why I have the additional small freq. counter.

Good recommendation on 'scopes, Fred! Analog scopes are best for audio work. DSOs (digital storage oscilloscopes) are great for capturing a waveform to transfer to your computer.

You have to be careful with DSOs with regards to "aliasing". If you don't understand what that term means, DSOs are definitely not for you! i.e. DSOs lie! if you are not careful.

Don

[i]"..I preferred the portable variant which has the multimeter included.." Thierry [/i]

yes - this is a major problem with older equipment.. it is generally much more larger, heavier, and less portable. My portable kit consists of a Pico ADC-212 DSO (http://www.picotech.com/212_6.html) , UT60A Multimeter (http://www.uni-trend.com/UT60A.html), and a laptop PC to which these connect - This gives me 'scope, frequency counter, spectrum analyser,logging and multimeter functions etc..

The UT60A is a fantastic multimeter for its price, and usable as a basic frequency counter.. Also extremely useful is its direct display of duty cycle on incoming waveforms.. at about £40 it is a good buy - particularly if you want to check drift in a circuit, where its ability to connect to a PC and have its data logged is great (just wish it was USB not RS232).

The 'scope is expensive - but to get good enough Y resolution for basic spectrum analysis, one really needs better than 12 bit resolution.. The ADC-212's 12 bits is only just usable for non-critical analysis.. and the majority of cheaper PC DSO's only have 8 bit Y resolution making them useless for serious waveform analysis.

I strongly advise against PC DSO's .. They look like the perfect solution - the ability to put the waveform onto a large monitor, the ability to capture and manipulate this data with the PC.. But, in reality, unless one pays a lot of money you will only get 8 bit Y resolution.. and this is not good enough! - I rarely use my ADC-212 in the lab.. only use it for quick spectrum analysis.. for serious spectrum analysis I use an ancient HP333A distortion analyser and manually sweep the spectrum.

[i]" DSOs lie! if you are not careful." - Don[/i]

They do! .. Its a bit like simulation - you need to know what you expect to see, so that you can tell when the DSO is lying, and correct the settings. With analogue, you see whats there - or, more accurately, you never see something which isnt there!

Fred.

Finally it appeared that one of my transistors was not working, possibly during the board-to-board transfer process...

Anyway, before finding this out, I've bought the recommended "equivalent" BJTs. Not tried to replace the FETs yet, though, it appears that the local electronics stores don't have the recommended types.

You could almost say it works now, although I haven't yet managed to tune it at least semi-decently. Part my fault though, I am pretty sure I need to get a cabinet for it before expecting anything similar to proper tuning?

However, the volume antenna does not seem to do much and it's too hard to identify whether it does anything at all - it might just be perceiving "ghost" variations in volume because I'm expecting them to happen...
Is there a way to tell whether this is due to improper tuning of the coils or due to a fault in the electronics?

[i]"I am pretty sure I need to get a cabinet for it before expecting anything similar to proper tuning?"[/i]

Yes - you will need to do some re-tuning when you have a cabinet.. BUT..

Dont expect this Theremin to be good in terms of tuning or linearity.. There is nothing to linearize the response.. with plate antennas (as per article) one can just about get away with this.. but with a rod pitch antenna it wont be good.



[i]"However, the volume antenna does not seem to do much and it's too hard to identify whether it does anything at all - it might just be perceiving "ghost" variations in volume because I'm expecting them to happen...
Is there a way to tell whether this is due to improper tuning of the coils or due to a fault in the electronics?"[/i]

Tuning of the volume oscillator is critical - if the osc frequency does not cover the frequency which the filter is tuned at, there will be little or no change to the DC level on Q6:G.

1.) First check that the volume oscillator is running, and responding to hand position.. Without a 'scope or frequency counter all you can really do is to check the DC on Q5:C.. this is the only low impedence point which a multimeter will not load.

2.) If you get changing DC (as you move hand near volume antenna) on Q5:C, then at least you know the oscillatior / filter are doing something..

I have not read the article, but presumably some expected voltages are given...

3.) If you get changing DC, and if the DC levels are correct for biasing the FET (Q6) Then this fet should act as a variable resistance S-D.. Hmmm.. I would try connecting a 1M resistor between Q6:D and 0V.. Really dont like the idea of a floating drain on a FET.. And Dont like the idea of the audio being shunted to the +V rail, for that matter.. Even with the decoupling, at the very least there should be a 100R resistor between Q6:S and the +V supply,and a 100n in parrallel with a 10u capacitor between Q6:S and 0V.

As I said earlier - this is, as far as I can see, a truly horrible VCA, and I would not be too surprised if it caused you bother!

I have one question.. I do not understand what motivates people to select, out of the dozens of Theremin designs freely available, this particular Theremin (or, for that matter, the Glasgow Theremin).. It really puzzles me!

Fred.

Well, for me the motivation was that this was the simplest two-antenna theremin I could find.
After looking at it better, the EM doesn't seem to be that monstrous a project though, will have to try it.

I am going to try your test routine right now!

By the way, I compared the schematics of this apparatus ant the EM theremin the other day and noticed the huge inductances connected to the antennas of the EM. As far as I understand, they have little to do with oscillator frequencies. What are they there for? Do inductive antennas help linearize the playing field? Would that help in any way if I did something similar with this one?

Coils in series with the antenna form a second resonant circuit with the antanna's own capacitance. If this resonant frequency is just a bit below the oscillator's frequency, there will be better linearity and a more important frequency deviation when you add your hand capacitance. So the oscillator's resonant circuit (L/C ratio) and the linearization coils have to be calculated to match in a special way. You can't simply take a "normal" oscillator and add coils.