<div dir="ltr">Hi all,<div><br></div><div>@David: thanks for the quick and detailed solution! I'm glad you took into account the linearization as well, as I'll need that too. One thing in your solution caught my eye, and that is that you're summing signals in front of the 2164's input. I know you can sum signals on the input of the I-V convertor, but did not know it could be done even earlier, before the 2164. Are you sure it would not cause interactions between the summed signals (i.e. can the input of the 2164 also be considered virtual ground)?</div><div><br></div><div>@cheater00 & Matthew Skala: good point about the definition of N-quadrant multipliers. Here's how I understand it: </div><div> </div><div>- A 4-quadrant multiplier is capable of taking into account the magnitude and sign (+ or -) of both (biplolar) inputs and flip the output according to both signs (as in normal math multiplications). This would be the case for a ring modulator</div><div> </div><div>- A 2-quadrant multiplier can take into account the magnitude of both inputs, but only the sign of 1 input (the second input should be unipolar and cannot sign-flip the output). This would be the case for a normal audio VCA.</div><div><br></div><div>- A 1-quadrant multiplier can take into account only the magnitude of both inputs, and not deal with either of the signs to cause sign-flipping of the output. In the common case this requires two unipolar signals, but I feel my case of a bipolar wave of which the amplitude is decreased up to it's lower bound also fits this... what do you think?</div><div><br></div><div>Rutger</div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Op di 18 jan. 2022 om 02:48 schreef David G Dixon <<a href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</a>>:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>
<div>
<div dir="ltr" align="left"><span>Hi Rutger,</span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><span>It is really easy to do
what you want to do.</span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><span>If I understand you
correctly, for an input signal of 10Vpp centred on 0V, you basically want the
output signal to grow from a baseline of -5V, so that when the VCA is off,
the output is -5V DC, at 50% gain, the output is 5Vpp from -5V to 0V, and
at 100% the output is 10Vpp from -5V to +5V.</span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><span>Let's assume that you
have built the standard linearized VCA circuit and that you have good 5V and -5V
reference voltages available. Presuming that your input resistor into the
amplifying 2164 VCA is 30k, and the feedback resistor on the I-V converting
opamp is also 30k, then all you have to do is this: Sum -5V into the
2164 VCA, in parallel with the input signal, through a 30k resistor, and
sum +5V into the I-V converter also through a 30k resistor. The +5V
into the I-V converter will apply a constant bias of -5V to the output
signal. The -5V into the VCA will counteract this bias in proportion to
the VCA's gain, such that, then the VCA is off, the bias is -5V, and when
the VCA is at unity gain, the bias is 0V.</span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><span>Easy peasy,
lemon squeezy. Schematic attached.</span></div>
<div dir="ltr" align="left"><span></span> </div>
<div dir="ltr" align="left"><span>Cheers,</span></div>
<div dir="ltr" align="left"><span>Dave
Dixon </span><br></div>
<div dir="ltr" lang="en-us" align="left">
<hr>
<font size="2" face="Tahoma"><b>From:</b> Synth-diy
[mailto:<a href="mailto:synth-diy-bounces@synth-diy.org" target="_blank">synth-diy-bounces@synth-diy.org</a>] <b>On Behalf Of </b>Rutger Vlek via
Synth-diy<br><b>Sent:</b> Monday, January 17, 2022 1:04 PM<br><b>To:</b> SDIY
List<br><b>Subject:</b> [sdiy] 1-quadrant multiplier with
2164<br></font><br></div>
<div></div>
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<div dir="auto">Dear all,
<div dir="auto"><br></div>
<div dir="auto">I think i need a bit of wisdom from the 2164 guru's around here.
The case is as follows:
<div dir="auto">I'm in need of a 1-quadrant multiplier (VCA) that works on a
bipolar signal. The input would be a 10Vpp triangle wave centered around 0V. The
output should be the same with the VCA fully open, but... when closing I would
like to reduce the level with respect to the -5V power bound, so that a VCA
half-open would result in a triangle of half the amplitude, sitting between -5V
and 0V and a fully closed VCA would give -5V DC.</div>
<div dir="auto"><br></div>
<div dir="auto">Of course I could level-shift both input and output of a 2164
bases VCA, but I feel it could be simpler. Would it work to connect a -5V signal
to the ground pin of a 2164? And would that also require a shifted control
voltage?</div>
<div dir="auto"><br></div>
<div dir="auto">Regards,</div>
<div dir="auto"><br></div>
<div dir="auto">Rutger</div></div></div></div>
</blockquote></div>