[hpsdr] QSD gain and noise.

[Ahti Aintila]

Phil,

Here are a couple of links for different mixers:

http://groups.yahoo.com/group/softrock40/
Look in the Files section for "QSD Switching Duration..." from K5BCQ
(Kees). He is interested in 180 degree switching (like me) using fast
switches like SN74LVC1G3157. I think that the shorter switching times
and 180 degree nominal conduction time work favourably on higher
frequencies and gives better impedance match for bandpass filters.

For the improved H-mode mixer you'll find excellent information here:
http://www.xs4all.nl/~martein/pa3ake/hmode/hmode_assy.html

BTW, how many Effective Number Of Bits you can promise for Janus?

73, Ahti OH2RZ



On 23/09/2007, Phil Harman <phil at pharman.org> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>
>
> In reply to Ahti and Frank,
>
> Ahti, from memory the NF measurements you suggest are a little high but in
> the right ball park. The problem of connecting the Janus balanced inputs
> directly to the QSD caps is noise pick up since this is done at a high
> impedance level.  A little (low noise) gain after the QSD capacitors would
> allow Janus to be driven from a low impedance source and help with the noise
> pick up.
>
> Alternatively, we placed a header on the Janus board that would allow a
> little daugher board to be plugged directly in. The board could contain a
> QSD and the capacitors feed the balanced input over very short leads.
>
> It seems we still have a NF problem with the QSD  in that it rises with
> increasing frequency. The Flex5000 figures I have seen indicate that if we
> measure the NF on 20m then it increases by 10db on 10m and a futher 10dB by
> the time we get to 6m.
>
> I may be mistaken but I have yet to see a QSD that does not have this
> problem.  In which case we may want to investigate using say an H mode mixer
> followed by a 1dB effective NF audio stage.  This has the potential of not
> requiring a pre-amp on any of the HF bands. I recall seeing an H mode mixer
> that had I and Q outputs but can't seem to find the circuit. To optimise the
> H mode we need to change the bias point of the switches as we change
> frequency but an I2C driven DAC could do this easily.
>
> Some experimentation required!
>
> 73's Phil...VK6APH
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