[hpsdr] New HPSDR card for VHF/UHF ?

[Grant Hodgson]

More numbers, opinions, speculation, dreams...

The FT225RD with Mutek front end has a (measured) Input IP3 of approx. 0dBm. 
That's the IIP3 of the whole receiver system, and must not be confused with
figures for just part of the system such as mixers, transverters etc.

The FT225RD is a good benchmark for any new 144MHz designs - for many, many
years it was widely regarded as the best 144MHz transceiver, some still
consider it so.

I did some system calculations for a design of a 2m direct-conversion variant
of the microwave SDR (uWSDR) project which I presented at the RSGB VHF
convention in 2009.  This indicates that a system IIP3 of +15dBm is achievable
with a system NF of 2dB.  That's ~100dB spurious-free dynamic range, or ~150dB
blocking dynamic range.  

The limiting factor would then be phase noise; -120dBc/Hz at 10kHz offset
isn't too difficult for a PLL but I'm not sure if that is good enough and I'm
not sure how it compares to the FT225RD, so a better LO may be needed in order
to achieve best-in-class performance.  But PLL technology marches on and it
may be possible to beat this figure now without too much difficulty.  There's
also the possibility of a very low spur and phase noise DDS solution using the
AD9912.

The system calculations were based on the LT5517 I/Q Downconverter with an
SPF5043Z LNA and the same ADC as used in Janus - AKM5394a.  I/Q upconverters
and PA MMICs giving ~100mW are now readily available for 144MHz.

A lot of the initial work has been done for a 144MHz front-end for the uWSDR
project and it wouldn't take a huge amount of effort to produce a VHF or even
UHF version for the Atlas bus with class-leading performance especially given
that Janus is available.  I've done a lot of the ground work for such a system
and would be happy to share it with anyone who could take it further whilst I
try and get the original microwave side of the project kick-started again.

I'll try and get the Powerpoint presentation uploaded to the 'net if there is
any interest, although it's very similar to the one I gave at Microwave Update
in 2007 which is on the website :-

uwsdr.net

regards

Grant G8UBN





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