[sdiy] Dual Frequency Shifter update

[Grant Richter]

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).

I always wanted to try using RC4200s since fooling around with the  
Bode and Buchla FS units gave me the impression that the multipliers  
are the weak point for sideband suppression (perhaps more audible  
than dome filter errors, because the sideband bleed through has  
harmonics produce by the multiplier errors?).

Is is possible you could use summers instead of multipliers using a  
sum of logs trick on a SYSTEM basis?
That could save parts, and possibly improve performance.

Let's see, you would need the log of all 4 quadratures, but then  
summing is the same as multiplying if you take the anti-logs  
afterwards. That is what you would get if you used RC4200s for the  
multipliers.

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? It would  
replace the multipliers with linear summers, possibly improving  
sideband suppression and S/N ratio.

I have not it given any rigorous thought. Just thought I would toss  
it out for possible discussion.

On Jul 18, 2006, at 1:48 PM, JH. wrote:

>>> (The whole thing is very unfinished - companders don't work well
>>> on fast transients, for instance). You heard the upshift/downshift
>>
>> That's why I built a log amp - compresses the signal with no  
>> delay. Use the
> same type log
>> amp on the otherside in a feedback path and you get expansion.  
>> Together
> should be close to
>> a compander pair.
>
>
> A log amp!
>
> That requires a very precisely unity gain in the channel in order  
> not to
> cause any
> distortion, doesn't it?
>
> Where did you put the encoder? One encoder before the dome filter,  
> or two
> encoders at the multiplier inputs?
>
> Tim, I'd love to see this!
>
> JH.