[time-nuts] Best phase detector / mixer for 100MHz?

Richard (Rick) Karlquist richard at karlquist.com
Fri Nov 23 18:27:38 UTC 2012



On 11/23/2012 9:38 AM, Bruce Griffiths wrote:
>
> NIST have shown (at least at 10MHz) that the high level mixers they
> tested are noisier than the ZRPD1.
>
> Bruce

Do you have a citation to where they said that?
What you quoted doesn't make sense, at least, out of context.

We need to clarify phase detector sensitivity specs.
For conventional (IE 50 ohm) phase detectors, it is
apples vs apples to just go by the volts per radian number.
However, mixers like the ZRPD1 artificially triple the
voltage sensitivity by operating at 500 ohms, and using
transformers to connect to 50 ohm equipment.  Doing this
doesn't increase the possible signal to noise ratio.
Consider this thought experiment.  Build your best 500 ohm
phase detector and postamp.  Now replace with a 50 ohm
phase detector and connect 3 postamps in parallel.  It is
a wash.  Of course, you don't have to actually do this.
You can simply use an op amp like the LT1028 with very
low noise voltage.

To actually put a 500 ohm detector on a par with a 50 ohm
detector, the 500 ohm detector would need to use 3 diodes
in series compared to one in the 50 ohm case.  With only
one diode per arm, the maximum drive power utilization is
considerably lower.

Rick Karlquist N6RK




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