[time-nuts] power spectrum of hard limiter output

ehydra ehydra at arcor.de
Mon Feb 14 18:32:43 UTC 2011


I rethinked the problem after reading several papers.


OK. We have an if-strip. To avoid spectral regrowth in later stages 
because of saturation, the input must be <momentary> amplitude 
controlled. Pulses and bursts at the input will saturate somewhere in 
the stages. An common AGC is always design-limited by solving for speed 
AND compression at the same time.

Now, looking at the spectrum of a click - the same as a pulse. For 
example cw transmitted by amateur radio morse. How to avoid the annoying 
band splitter? One possibility is to shape the amplitude response in 
time for the pulse by tanh function. This will 'compress' the spectrum.

A differential transistor stage realizes a tanh function.

A CMOS inverter is in effect the same. One of the transistors of the 
differential stage is 'inverted'.

So I think a CMOS inverter already realizes an realistic and simple 
pulse shaper for pulses in an if-strip.


I tested different ideas with schottky-diodes somewhere in the if-strip. 
In the end, I must say that leaving such out of the design is simply the 
best.


Any comments?


- Henry


ehydra schrieb:
> Hi Magnus -
> 
> Well. I made homework for sure. Endless viewed websites what others had 
> done.
> We all seek for the singularity of beauty in our art. At the moment this 
> is a CD4007 or similar without AGC. Cheap, effective. Not oversized. I 
> came from TCA440. One of the not many receiver chips where the IF is at 
> one output reachable. Then I found I must increase carrier frequency 
> beyound 30MHz. So the integrated mixer entered useless. I used an 
> external device. Next I found that the integrated IF-strip is not one of 
> the quiets... Remembering several statements about using CMOS inverters 
> in s.e.d. for analog reasons, the way was clear...
> 
> Implementing an AGC makes the design much more complicated. Can 
> oscillate etc.
> 
> Going without AGC seems a reasonable way if the modulation is one of the 
> phase modulation systems where amplitude is not very important.
> 
> Then look how big is the dynamic range of a simple CMOS inverter.
> 
> Meanwhile I found two examples made by others. So the way looks not to 
> idiotic ;-)
> 
> - Henry
> 
> 
> Magnus Danielson schrieb:
>>> I find it more and more curious a working concept not to find in the
>>> I-net. I'm looking several weeks. Maybe wrong keywords.
>>
>> Well, have you considered what an AGC might do for you?
>> It has been the traditional way of reducing the effect of signal level 
>> on phase-detector gain and hence on the loop gain. The hard-limiter 
>> text is to be seen in this context where the SNR steers the 
>> compression factor, i.e. change of loop gain.
>>
>> Cheers,
>> Magnus
> 
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-- 
ehydra.dyndns.info



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