[hpsdr] Mercury SSB Phase Noise Testing

[Larry Gadallah]

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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