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

Adrian rfnuts at arcor.de
Tue Nov 27 10:03:33 UTC 2012


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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