Bert,
On 07/10/2014 04:55 PM, [email protected] wrote:
As part of the FE 405 B project a separate output circuit is in the works.
The universal controller and auxiliary board are the same as used in the
FE5680A GPSDO but because of the very low ADEV a separate circuit board that
divides by three and has also two ground isolated transformer outputs is in
the works.The question is what is the best sine to square wave converter
with the lowest ADEV contribution. I am looking at Bruce's circuit using the
ADCMP600. Any other ideas?
Do look at the Wenzel clockshaper [1], look at the TADD-2 [2] schematic.
It's a PNP long-tail pair. The strategy is to provide modest gain. A
known strategy to reduce 1/f noise and to some degree thermal
differences is naturally feedback, as you will find in the NIST papers.
Once you have the slew-rate up, going in for the kill with a straight
comparator should give you all the nice output slew-rate you can wish for.
Thus, this is not all that different to the mixer-setup you have done.
I have modified my TADD-2 such that one of the output channels is fed
from the input circuit, and this provides me with a squared up version.
For a counter such as DTS-2070C, the difference is significant, which
helps to show the potential of this simple design.
I think the basic approach can be improved, and how far one has to go
depends on how "clean" source you have. You end up with interesting
measurement problems.
An indirect way to measure the goodness of a squarer is to insert some
known sine disturbance at say 30 or 40 dB below the signal. A straight
comparator won't work very well. Be careful with selectivity of LC, as
it is a nice way to become temperature dependent, so low-Q solutions is
needed.
Cheers,
Magnus
[1] http://www.wenzel.com/documents/waveform.html
[2] https://www.tapr.org/kits_tadd-2.html
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