[hpsdr] New HPSDR card for VHF/UHF

[Grant Hodgson]

I remembered to switch my brain on and I've now uploaded pdf block diagrams of
the two alternative options for a high performance 144MHz software-defined
receiver to:-

www.ghengineering.co.uk/VHF_HPSDR

Some more notes :-

i)  This is nothing more than a technical discussion - I can't commit to
designing an HPSDR project - my spare time goes on the uWSDR project(!)

ii) Either option can be made to work on the higher bands - BUT - the RF
band-pass filter takes on a totally different character as the frequency
rises.  The I/Q approach does image filtering in software on the baseband
signals, therefore the RF filter is there just to remove out-of-band signals.
 This can be made to work up to at least 24GHz without too much difficulty. 
With the HF option (which is another type of superhet) then the first RF
filter has to remove the first image response which is 42MHz lower than the
wanted signals.  At 144MHz this is easy, but is very difficult at 1296MHz and
almost impossible at higher frequencies unless waveguide is used.  So for
higher frequencies (220, 432, 903, 1296 etc.) option 2 starts to encounter the
problems of 'traditional' high-performance HF recieivers - stringent image
filtering and the need for a high performance mixer and 1st IF sub-system -
just the things that HF SDRs have got rid of!

iii)  For 144MHz there is no obvious (to me) answer as to which is 'best' for
reasons mentioned previously.  In terms of performance, it SHOULD be possible
to get similar levels of spurious-free dynamic range from either approach, and
it should also be possible to create an LO for the I/Q option such that the
dynamic range is not limited by LO phase noise.  The 'HF' option would require
some detailed design work in order to get a suitable 1st IF sub-system.

regards

Grant  G8UBN