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

[Support HpW-Works.com]

Hi Bruce,

>> The equivalent phase noise (measured in dBc/Hz) is essentially independent of
>> the sample size.

The dBc/Hz normalization is based on this (also the sample rate). While the sample
rate & sample size = FFT Bin size (resolution or filter band width) is used to get
the required correction factor of the spectrum to get the dBc / (1Hz) Y scaling
back.

>> So increasing the sample size isnt particularly useful for reducing the phase
>> noise floor.

By theory, yes... but we use a sound card with a lot of flicker noise on the lower
end, also we have the 10...20Hz low freq. cutoff due the usage of a servo / single
5V power. Also the raising noise below 100Hz (ADC serve & noise on the ADC power /
input circuit) limits the performance.

In my simple test increase of the sample size reduced the noise floor in better way
than just using sample size with 1-2K and large averaging cycles.

Keep in mind, Bin resolution is sample rate / sample size:

Example: 

- 32khz / 32678 = about 1Hz
- 32khz / 1024  = about 31 Hz


>> However with a sound card plus a mixer a somewhat lower number of samples
>> should suffice since there is no carrier.


Ideally the input circuit & ADC of the 3562A would be nice but with much larger RAM
buffer and ASIO interface O:)

Hanspeter

>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf
>> Of Bruce Griffiths
>> Sent: Dienstag, 27. November 2012 12:31
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Best phase detector / mixer for 100MHz?
>> 
>> Support HpW-Works.com wrote:
>> > Bruce,
>> >
>> >
>> >>> There's no evidence of a cross power spectrum function in this suite.
>> >>> One needs to be able to average at least 10,000 cross power spectra
>> >>> for some applications.
>> >>>
>> > In the PSD (power density)&  PSP (power spectrum) there are cross
>> > power and cross power complex average implemented (selectable using the
>> spectrum channel mixer)!
>> > Additional to this you may apply / add averaging of the resulting
>> > spectrum or use additional peak hold.
>> >
>> > Average of 10'000 cross points is a large count and often seen on 1-4k
>> sample size.
>> > Better in my opinion is to use a higher sample size 32k-64k and then
>> > less averaging is required.
>> >
>> The equivalent phase noise (measured in dBc/Hz) is essentially independent of
>> the sample size.
>> So increasing the sample size isnt particularly useful for reducing the phase
>> noise floor.
>> The increased frequency resolution achieved by increasing the sample size is
>> only useful for measuring spurs.
>> In the direct digital method of measuring phase noise a few terasamples (a few
>> gigasamples at baseband) need to be processed to achieve a sufficiently low
>> instrument noise floor.
>> However with a sound card plus a mixer a somewhat lower number of samples
>> should suffice since there is no carrier.
>> > Just download the evaluation version with fully feature set.
>> >
>> > HpW
>> >
>> >
>> >
>> Bruce
>> 
>> 
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