AW: [sdiy] Dual Frequency Shifter update

[jhaible at debitel.net]

> The Raytheon RC4200 used the Log-Antilog scheme for multiplying as  
> the sum of logs. That is x * y = antilog((log x) + (log y)) 
> They have  
> to be DC biased for single quadrant. It had the lowest rated  
> distortion (0.1%) of the commercial multipliers (vs. 1% for AD633).

The circuit that makes a 4Q multiplier from a 4200 is quite complicated.
You need a lot of matched resistors, and a lot of trimming.
>From memory (I played around with a 4200 some years ago), you also
need to impedance-match the offset trim inputs to the signal inputs
because of the bipolar technolgy (bias currents).
0.1% distortion looks good, but it doesn't say anything about carrier
bleedthru, and the quadratic term of carrier bleedthru is a critical
parameter in clean audio 4Q multiplication. The AD633 is quite good
at this. Mind you, I don't know if the 4200 is worse (or better) than
a 633 in this - I just say as the 4200 isn't a 4Q mult per se, this 
parameter can hardly be specified for the _chip_; only for the complete
circuit.
Also, you must avoid running out of the 4200 allowed input range
by all means. When you overdrive a balanced modulator, it just clips.
When you overdrive a 4200, it oscillates. So you need extra headroom,
which will take away from your system performance.

BTW, the 4200 is used for the Ring Mod in the Korg MS-50.
Seems to work well there. But the demands are certainly lower than
for a FS application.


> What if you used a single log conversion BEFORE the dome filters,  
> then summers, then anti-log, then sum and differential 
> addition. Dome  
> filters should be DC linear, so logging before won't matter? 

I fear Magnus is right with what he replied.
I've made simulations to see the effects of rectifier ripple in
a companded FS system, and the problem is that unlike in ordinary
companded channels, the distortion from the compressor is not
partially cancelled in the expander, allowing short time constants.

Using nonlinear functions instead of a compander is equivalent to
a compander with zero time constants, I think.
So even though I cannot see how it could possibly work, I still
have a faint hope that there is some ingenious trick where everything
falls into place.
Tim ?!

JH.





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