[time-nuts] Square to sine wave symmetrical conversion (part 2)

Bob Camp kb8tq at n1k.org
Sat Jul 25 12:29:27 EDT 2015


Hi

> On Jul 25, 2015, at 6:23 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> Hi,
> 
> On 07/25/2015 03:35 AM, Charles Steinmetz wrote:
>> skipp  wrote:
>> 
>>> [the 10MHz output is] not even close to being symmetrical.  The waveform
>>> on-portion (duty cycle) appears (surprising to me) to be much less
>>> than 20%
>>> 
>>> Now I'm under the assumption that proper rounding or conversion of the
>>> non
>>> symmetrical 10 MHz square to a sine wave will be a bit more involved.
>> 
>> That means there are significant even harmonics present, including the
>> second harmonic, which is a lot closer to the fundamental and,
>> therefore, harder to remove by filtering (a perfect square wave contains
>> only the fundamental and its odd harmonics).
> 
> If the symmetry error is minor, the second harmonic at 20 MHz will still be relatively small. However, it if turns out to be important, an LCR-serial link tuned to 20 MHz and with some resistance for sufficiently low Q can eat some of that energy up as it is placed between signal and ground.
> 

The problem with any “shunt to ground” approach on the output of a CMOS gate is that they (essentially) are voltage
output devices. Virtually all of their life is spent with the output shorted to either the supply line or to ground. If you hook
them up to an open circuit ( = a typical CMOS load) this does not create any idle current flow. If you hook them up to something
that looks like a load to ground or B+, there is current flow and they are hotter / less happy / more likely to burn out. 

With a gate driven filter, the idea is to have as few shunt elements as possible. You also want to isolate them from the
output with a “reciprocal” series element between them and the gate.

This may sound like a minor thing. Give it a try and see. You can easily get a 10:1 change in supply current on a gate
simply by flipping filter elements around. Yes, low Q can reduce that ratio a bit. Often you can only hit your harmonic 
numbers with a high(er) Q. 

Bob


> Cheers,
> Magnus
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



More information about the time-nuts mailing list