Bruce Griffiths schrieb:
Richard (Rick) Karlquist wrote:
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
http://tf.boulder.nist.gov/general/pdf/2556.pdf
Where all the high level mixers measured have a higher phase noise
than lower level mixers/phase detectors like the ZRPD1.
Bruce
I can't confirm their results, at least not when plugged into a HP
11848A which loads the PD with 50 Ohms.
Too bad they are unclear about the applied power.
(1) Is it 'LO Power = maximum mixer power = 11 dBm', that is +11 dBm
into both ports?
(2) Is it +17 dBm due to 'mixers with a nominal LO rating of +7 dBm and
maximum rating of +17 dBm'?
Or, is it 50 mW as specified by Minicircuits, which would be the maximum
mixer power as by (1)?
With +16...+17 dBm signals I'm getting a noise floor of -175 dBc/Hz
which is quite poor but understandable as in order to drive it at
nominal input power, I have to put 6 dB attenuators between the
oscillators and the phase detector. With only 3 dB attenuation, I'm
getting -178 dBc/Hz. I wasn't sure if the 50 mW spec refers to the total
RF power into the ZRPD-1 or if that is the max. power into each input,
so I didn't try +17 dBm. I would have expected to see -180...-181 dBc/Hz.
At that input level the 11848A internal PD gets me -179 dBc/Hz (no
attenuators required).
A WJ M9GC was 1 dB worse than the internal PD (also w/o attenuators).
Surprisingly I've got the best results (-181) with a +23 dBm mixer (w/o
attenuators of course).
I did not have +23 dBm signals at hands, but I don't see that the ZRPD-1
can compete with the 11848A PD or an external +23 dBm mixer at input
levels of +17 dBm and above.
Adrian
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