[hpsdr] Mercury SSB Phase Noise Testing

[Ronald Cox]

I am setting up a HP 10544A Crystal Oscillator at 10 MHz, the specs on  
it are -115dBc/Hz at 10 Hz offset (max) and -130 dBc/Hz at 10 KHz  
offset (max). That should work better?

Ron, W9KFB
Indianapolis

On Oct 26, 2009, at 4:22 PM, Larry Gadallah wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> John Miles wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> Typically the phase noise of any SDR will simply be that of its  
>> DDS, which
>> can be extrapolated from the DDS's spec sheet for the clock and  
>> output
>> frequencies being used.  You can't measure it with an 8656B, as the
>> generator is far noisier than any DDS.
>>
> Just to be clear, there is no DDS in the Mercury. Mercury is a  
> direct sampling SDR, where the RF signal is directly converted to  
> digital data by the LTC2208 ADC. The selection of a specific signal  
> from the 0-54 Mhz spectrum happens later in the digital domain  
> processing by using a DDC (digital down-converter) module, which  
> executes in the FPGA. This results in a lower bandwidth signal (192  
> kHz?) that is then sent to the PC over the USB and further processed  
> and demodulated in the PC. The clocking of the ADC in Mercury is  
> handled by a low-noise, low-jitter oscillator that drives the ADC at  
> roughly 130 million samples/second (122.88 Mhz to be exact), but  
> this is a fixed frequency. The only variable frequency components in  
> Mercury are purely software.
>
> Contrast this to the direct conversion approach used by Flexradio,  
> Soft-Rock etc. where a DDS (or a crystal oscillator) is indeed used  
> to down-convert the signal(s) of interest to baseband (0 Hz IF) and  
> this signal is then sampled and the baseband samples are sent to the  
> PC for processing (USB or via a Soundcard, depending on where the  
> sampling is done) at somewhere between 44 and 192 Khz.
>
> The interesting difference between these approaches is that since  
> the entire HF spectrum can be sampled at once with the direct  
> sampling approach, the selection of signals of interest is done by  
> code contained within the Mercury FPGA. This means that it is  
> possible to receive multiple signals simultaneously, limited only by  
> code space in the FPGA. For example, Alex, VE3NEA's Skimmer Server  
> uses the QS1R receiver (very similar in architecture to the Mercury)  
> to simultaneously receive and decode all of the CW portions of 7 HF  
> bands at once with a single QS1R SDR receiver!
>> The best approach would be to simply use the receiver and FFT  
>> software to
>> look at a single tone from a clean crystal oscillator.  The result  
>> is the
>> phase noise response, less 10*log(BW) where BW is the bandwidth per  
>> bin,
>> plus a dB or two for the window function's noise response.
>>
> This approach would work well, particularly if the phase noise  
> characteristic of the crystal oscillator were known in advance and  
> could be subtracted from the measured results.
>
> Cheers,
>
> -- 
> Larry Gadallah, VE6VQ/W7         lgadallah AT gmail DOT com
> PGP Sig: B5F9 C4A8 8517 82AC 16B6  02B6 0645 69F0 1F29 A512
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