Re: voltage to resistance convertor?

"Larry Brasfield" <donotspam_larry_brasfield@xxxxxxxxxxx> wrote in message news:hjT7e.10$AI4.494@xxxxxxxxxxxxxxxxxx
> "Andrew Howard" <ask.me@xxxxxxxxxxxxx> wrote in message
> news:heH7e.11799$5F3.5931@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> [Brasfield wrote:]
>>> Right now, your requirement appears to be:
>>> 1. Accept an input voltage, 'Vx', ranging between 0 V and 9 V,
>>> on one port, consisting of pins VxIn and VxRef.
>>> 2. Provide a "resistance", 'Ro', on another port, consisting of
>>> pins RoA and RoB.
>>> 3. If current is passed thru Ro, the voltage across it should
>>> be Vx * 2500/9, within some unknown tolerance. There is
>>> no presently known limit on what that current might be.
>>> 4. If a voltage is applied across Ro, the current passing thru
>>> it should be 9/(2500 * Vx), within some unknown tolerance.
>>> There is no presently known limit on what that voltage might be.
>>> 5. The accuracy implied by items 3 and 4 applies only over the
>>> frequency range 0.2 Hz to 20 Hz. Outside that range, any
>>> voltage or current result is acceptable.
>>> 6. The connection, if any, between the Vx port and Ro port, is
>>> unspecified. They might be required to be isolated from each
>>> other for an unbounded voltage difference. Or it may be fine
>>> for either RoA or RoB to be connected to VxRef.
>>>
>>> I state the above to clarify what is missing. For any of the
>>> unknown bounds and limits, smaller will be easier and if
>>> it can be unipolar rather than having to be bipolar, the
>>> design may be simplified. Not having to isolate the ports
>>> will greatly simplify the problem.
>>>
>>>> In case you are wondering, it is for a Guitar FX pedal I am semi-designing.
>>>
>>> What sort of circuit is going to use the synthesized
>>> resistance? Can it accept a voltage or current instead?
>>> There may be an easier way to solve the real problem
>>> than synthesizing a resistance.
>>
>>
>> This is the circuit that needs the variable resistance. It is the basis for a simple phaser. Apparently it works by using the
>> fact that the emiiter is 180 degrees out of phase with the collector (or something). The varying resistance has been labelled R1.
>> VCC
>> + (9V)
>> |
>> |-----'
>> .-. .-.
>> 47K| | | |2K2
>> | | | |
>> '-' '-' 1uF
>> | | +[/
>> 1uF | '----[|------'
>> \] | |/ [\ | |
>> IN o------|]---o---| BC548 | | 1uF
>> /]+ | |> .-. | \]
>> | .----' | |<-----|]----o OUT
>> o | | | | |2K5 /]+
>> | .-. .-. | '-'R1
>> === 22K| | | | | | o
>> GND | | | |2K2'---' |
>> '-' '-' ===
>> |-----' GND
>> |
>> ===
>> GND
>>
>>
>> (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)
>
> I concur with Roger on the misconnection between
> the top end of the pot and its wiper.
>
>> I have decided that I would prefer a sine wave, if that makes any difference to anything.
>
> Sorry, but I cannot understand what you mean by that.
> Are you saying you want the resistance versus control
> voltage function to be a sinusoid? Or that you intend
> to use sinusoids as the control voltage?
>
>> Both the phaser circuit and the sine wave generator will be using the same power supply.
>
> That pretty much resolves the isolation issue. The
> control voltage can be assumed to share the same
> ground with your "phaser".
>
>> I am fairly new to this, so I am unsure about what you mean by having an isolate resistance, or the difference between
>> unipolar and bi polar. I am guessing that unipolar means that it doesn't go below 0V (changing DC?) and that bipolar means
>> otherwise (AC?). If so, wouldn't it be possible to bias an AC waveform into DC, or am I getting confused.
>
> You've roughly got the bipolar/unipolar distinction, except
> bipolar means can have either of two signs and unipolar
> means can have only one sign or be zero. A bipolar signal
> could have DC content or not, and it is meaningful to speak
> of the AC content of a unipolar signal. The AC/DC concept
> and the unipolar/bipolar concept are not the same.
>
>> I have not yet figured out what I would be using to generate the sine wave, so if anyone has any suggestions, that would be
>> great.
>
> There are many ways to do that. Digitally synthesized
> sine generators are pretty cheap these days.
>
>> Also, I am not sure how to figure out the current needed, and I have never dealt with this type of circuit before, so I can't
>> really guess. It would probably be in the low mA range though.
>
> Yes. But I am not inclined to help go down the synthesized
> resistance route. (I concur with Roger on that as well.)
>
> It would be easier to build a voltage controlled "phaser"
> than to synthesize a voltage controlled resistance to put
> into that circuit you drew.
>
> For example, by differencing the output of two multipliers,
> one fed with (Vmax - Vcontrol) and the other fed with
> (Vcontrol), (where Vmax is the fullscale multiplier input),
> you would get the effect you are after. You can get a
> good multiplier in an 8 pin package.

(Sheepishly), the same function can be accomplished with
a single 4-quadrant multiplier. One input is the audio to be
subjected to "phaser" manipulation, the other input is biased
to be +/- fullscale at the extremes of the control input range,
and the audio output is the multiplier output. With suitable
biasing, it could all be done with a single supply and an 8-pin
multiplier such as the AD835. See
http://www.analog.com/en/prod/0,,773_862_AD835%2C00.html

>> Thanks for the help


You're welcome.

--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


.

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