[hpsdr] New HPSDR card for VHF/UHF ?

[Grant Hodgson]

Shel

Many thanks for the link to Leif's site - I'd forgotten about that plot.  So
Leif measures -97dBc at 10kHz for a modified FT225;  approx -120 dBc is
available now using the uWSDR 2.3GHz LO divided by 16, so the LO should not be
a problem.

The issue of a high-performance IF sub-system may not be as bad as I had
initially guessed; your idea of using a MMIC after the mixer is a good one -
although it may be necessary to use one with a higher IP3 than the MAV-11, and
it may be that a simple low-pass filter is all that is required after it - the
1st IF is something that I haven't had chance to think about in any sort of
detail.

regards

Grant


On Thu, 9 Sep 2010 21:28:13 +0100, ct1izu wrote
> > My belief is that biggest problem with I/Q is probably the LO.  It would 
> > be
> > almost trivial to modify the uWSDR 2.3GHz PLL with the addition of a 
> > divide by
> > 16 prescaler to give 144MHz with fairly low phase noise - if -120dBc/Hz @
> > 10kHz is not low enough then I might be able to do some work on an even 
> > lower
> > noise version.
> 
> Looks to be about what a modded FT225 is, see some plots here 
> http://www.sm5bsz.com/txmod/ft225eng.htm
> 
> >
> > Downconvert to HF
> 
> > But in order to get the best out of the mixer it is necessary to terminate 
> > the
> > IF port in a good, broadband 50 ohm load - this is done either with a 
> > diplexer
> > or an attenuator (or both). Then a filter is required to remove the image
> > response at 266MHz - this might not be too difficult, the filter has to 
> > pass
> > 21-25MHz so a simple low-pass arrangement may suffice.
> 
> Active mixer termination also works well. My multiband h/b VHF tcvr 
> uses the level 17 TUF1H with a MAV11 terminator followed by 2 pole 
> Butterworth filter. I have used the dixplexer only, I modded my 
> IC202 to use the G3SEK FT221 front end which was the forerunner of 
> the MUTEK board with the level 7 MD108 mixer.
> 
> Shel CT1IZU




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