[time-nuts] Software defined spectrum analyzer

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 12 02:02:43 UTC 2010


On 12/12/2010 02:31 AM, jimlux wrote:
> Don Latham wrote:
>> Hi Luis:I, too, am curious. But I haven't opened it yet. I suspect
>> something like an FPGA feeding a fast a/d somewhere in the 50-70 MHz
>> range. That is, a decomposition.
>> There may be a synthesized LO and mixer to get to the 50 MHz.
>> The info on the website says or used to say something about an sdr
>> modified to act as a spectrum analyzer.
>> One of the things I like about the device that played a part in my
>> purchase is the availability of a 10 MHz reference input. as a good
>> time-nut that shows some serious engineering attention to accuracy.
>> Don
>>
>
> I'm going to guess much simpler inside.. Essentially a programmable
> receiver like the Icom PCR1000, but with better bandwidth, running into
> a pair of 192kHz ADCs, for which easy USB interfacing is available.
>
> The question is really what does the 4GHz LO in the front look like?
> It's almost certainly some sort of VCO PLL scheme, probably with
> multiple VCOs. But, do they use a DDS? Probably not... They probably
> have a fast PLL that settles quickly so they step through in 100 kHz
> steps or so.

Two VCOs covering 1-2.2 GHz could through mixing and selecting between 
sum and difference output filters (i.e. lowpass and highpass) cover that 
without too much difficulty. Except for the highest end, there is a 
degree of freedom in setting the oscillators to their individual 
frequency which naturally could be used to avoid "bad" frequencies to 
leak into the IF for instance.

Cheers,
Magnsu



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