# Question about the frequency to voltage converter.

Posted: 3/3/2013 4:41:30 PM

Joined: 3/3/2013

hi everyone,

For building the circuit of frequency to voltage converter,

Is it possible for me to use the LM2917/LM2907 to build it?

I have built two identical circuit board for the volume antenna and

the pitch antenna, so this frequency to voltage converter will connect these two

circuit board.

thank you so much.

Posted: 3/3/2013 5:04:47 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

Hi Penpen,

For what function do you want a F2V ? You say you have a volume and pitch board, presumably you have a mixer outputting audio - What do you want to drive from the F2V?

If you are thinking about using it to convert theremin audio to a control voltage which tracks the audio, the LM2917/LM2907 are a bad choice - they simply do not have the accuracy or linearity required... but as to "it is possible" the simple answer is yes - that is what these ICs are designed to do.. But they are NOT good enough for controlling pitch (unless you are not intending to use the source frequency for any other purpose than generating the CV - This may not be as daft as it sounds!).

Also, if you are intending to use a F2V for pitch control, dont forget that the musical relationship is exponential.. A simple F2V will output a changing voltage proportional to frequency (a voltage increase proportional to Hz, as in V/Hz) but almost every voltage controlled synthesiser module accept a linear (as in, exponentiated) CV which increases at 1V per octave .. If you put a V/Hz CV into one of these the tracking will not work at all.

To generate a V/octave CV you need an exponential or log converter - The easiest fully described circuit to get 1V/Octave from theremin (or other monophonic simple waveform) is probably the one found in the Etherwave manual.

Also bear in mind that F2V's do not behave well for musical application if the F not higher than about 100Hz - Latency becomes so long that for practical purposes they are useless, even if they are capable of tracking, which most are not.

Fred.

Re-reading your posting I am a bit puzzled.. The F2V cannot "connect" these two boards - These boards connect such that the output of the volume board controlls a VCA which follows the mixer..

You would need to tahe your pitch signal for the F2V from before the VCA..

If you dont know what im talking about, read the Etherwave manual!

Posted: 3/3/2013 7:51:03 PM

From: Colmar, France

Joined: 12/31/2007

I think that since he built two identical boards for pitch and volume, both will output a heterodyned audio signal. In that case it makes perhaps sense to use a F/V converter to transform the volume "pitch" into a control voltage for a VCA which is still to come in his setup.

A F/V converter would in that case most probably be the better solution than what we find in the Theremax: a RC low pass filter followed by a single diode rectifier...

Posted: 3/3/2013 8:49:27 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

"I think that since he built two identical boards for pitch and volume, both will output a heterodyned audio signal. In that case it makes perhaps sense to use a F/V converter to transform the volume "pitch" into a control voltage for a VCA which is still to come in his setup.A F/V converter would in that case most probably be the better solution than what we find in the Theremax: a RC low pass filter followed by a single diode rectifier..." - Thierry

Complete agreement from me on this! ;-)

As I never use heterodyning volume circuits, this possibility (That penpen was looking to generate a volume CV from difference frequency) didnt occur to me,but it is rational, and as you say, the LM2917/LM2907 is likely to be a far better way to derive a CV than the Theremax RC->diode.

Fred.

Posted: 3/7/2013 9:30:56 AM

Joined: 3/3/2013

Hi Penpen,

For what function do you want a F2V ? You say you have a volume and pitch board, presumably you have a mixer outputting audio - What do you want to drive from the F2V?

If you are thinking about using it to convert theremin audio to a control voltage which tracks the audio, the LM2917/LM2907 are a bad choice - they simply do not have the accuracy or linearity required... but as to "it is possible" the simple answer is yes - that is what these ICs are designed to do.. But they are NOT good enough for controlling pitch (unless you are not intending to use the source frequency for any other purpose than generating the CV - This may not be as daft as it sounds!).

Also, if you are intending to use a F2V for pitch control, dont forget that the musical relationship is exponential.. A simple F2V will output a changing voltage proportional to frequency (a voltage increase proportional to Hz, as in V/Hz) but almost every voltage controlled synthesiser module accept a linear (as in, exponentiated) CV which increases at 1V per octave .. If you put a V/Hz CV into one of these the tracking will not work at all.

To generate a V/octave CV you need an exponential or log converter - The easiest fully described circuit to get 1V/Octave from theremin (or other monophonic simple waveform) is probably the one found in the Etherwave manual.

Also bear in mind that F2V's do not behave well for musical application if the F not higher than about 100Hz - Latency becomes so long that for practical purposes they are useless, even if they are capable of tracking, which most are not.

Fred.

Re-reading your posting I am a bit puzzled.. The F2V cannot "connect" these two boards - These boards connect such that the output of the volume board controlls a VCA which follows the mixer..

You would need to tahe your pitch signal for the F2V from before the VCA..

If you dont know what im talking about, read the Etherwave manual!

Thank you for your answer. Is you saying that i can skip the F2V and just build

a VCA to connect the volume and pitch circuit board ? but the output of the volume board is

a frequency output so how can it control the volatge of the VCA in order to

control the volume? And would you mind giving me some more advice about build the

VCA since i am not so good at that.

thank you so much

Posted: 3/7/2013 3:19:43 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

Real Sorry, Penpen..

It is my misunderstanding of what you were trying to do which has caused the confusion.

Thierry understood you correctly, and his answer is a good one.

If you have a varying frequency coming from your volume antenna board, then this needs to be converted to a voltage so that it can drive a VCA - The IC's you are looking at, if correctly used, should acomplish the frequency to voltage conversion.

Fred.

Posted: 3/7/2013 5:04:27 PM

Joined: 3/3/2013

so if i don't use the heterodyning volume circuits, is that i can use your

### recommendation for the VCA circuit ?

thanks again

Posted: 3/8/2013 9:21:42 AM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

Hi Penpen,

As I understand it, you have made two heterodyning boards - It seems to me that you are best staying with your way of doing things (converting the difference frequency from one board into a control voltage for a VCA).. This is a perfectly acceptable solution used in many theremin designs.

An alternative is the way done in the EW and SC theremins, where there is one variable oscillator for the volume circuit (as in, no volume reference oscillator and no heterodyning in the volume circuit), and this drives a tuned circuit of some sort, which gives a varying output voltage proportional to the volume oscillator frequency.

My scheme is to take the pitch reference oscillator and (usually, although this is not essential) multiply this with a PLL to some higher related frequency (usually twice the reference frequency) - This frequency does not vary - The volume antenna circuit comprises a tuned circuit the frequency of which varies as a function of hand capacitance .. I feed the (multiplied) reference into this, and the output from this filter (amplitude / phase variation) is used to generate a CV.

But the above is just "my" scheme - There is nothing wrong with the other ways of doing things - I prefer "my" scheme because there is never any problem regarding oscillator interactions or ghost tones or the like, and also because it allows me to have as many "volume" circuits as I want so I can construct multi-dimensional "volume" 'pads'. (which can be used for all sorts of functions like panning or changing tone or selecting formants or controlling external synth modules etc)

In fact, my latest theremin designs and prototypes only have one fixed oscillator which drives pitch and volume resonators - I get voltage out for everything, shape the linearity with analogue circuits, and have voltage controlled circuits (including voltage controlled heterodyning "theremins" - which can be any theremin circuit) tracking the CV outputs .. So, for me, well - my head is in a somewhat different "mode" from most theremin designers at the present time ;-) ... For simple theremins, you are probably best staying in "known" territory - for the simple reason that you can get help with conventional circuits here - follow my "path" and I may be the only person who can help you - And I may not be arround for long - For one thing, the Ankou has been coming too close to me recently for comfort ;-)

Even if the Ankou doesnt visit, I am not sure how long I will bother spending hours on the theremin anymore - most people only want something that makes noises and are not bothered about playability - they can buy what they want for well under \$100 ready built or in kit form - its simply not worth the time and effort and frustration of helping people to fix their own constructions which are usually inferior to a kit they can buy for a \$44 .

At this time I am not ready to release schematics - Frankly, All I get from the wanna-be world theremin expert here at TW and his followers is a bucket of sh*t thrown at me whenever I share anything here at TW, and nobody here has the guts to back me up when I take any stand on any matter - even though some email me with support.. I have decided its just not worth the effort to share anything here or be an active contributor anymore - I will partake in conversations like this and help individuals where I can, but no more "big disclosures" - TW doesnt deserve these (and most people arent interested anyway).

Email me privately (my email is in my avatar.. its:  fred [the character for {at} ] fundes [the character for {dot} ] co [the character for {dot} ] uk ) and I will put you on my mailing list and give links to my publications when I create them.. (I may even give you a basic schematic if you ask nicely ;-) But TW has lost me from a perspective of me posting anything "new"* here.

If you do email me, please give as much information about yourself as you are comfortable doing - Your TW name, Your real name, Nationality, home language,  age, expierience, test equipment you own, projects and plans etc - this helps me to have a better idea of what may be useful to you.

ps - the invitation to email me is open to all who are genuinely interested in my ideas for their own use..

Fred.

* "There is nothing new under the electronic sun" - Thierry

ps -

Also, my designs tend to be "comprehensive" than most - for example, my "volume" circuits have employed magnetic amplifiers / saturable reactors to facilitate automatic tuning, and had temperature sensing included for stability - It is impractical for someone to build one-offs of these circuits - the saturable reactors are not easy to make and you need more test equipment to get them right than most people have ... Simplifying the designs for hobbyists is possible (one could replace the reactors with other means of tuning - something I may decide to do anyway), but takes more time than I am willing to give.... I have been designing for production, not for one-offs.