[hpsdr] QSD gain and noise.

[Phil Harman]

Hi Ahti,

Janus looks about  an ENOB of 20/21. Thanks for the mixer reference. It 
looks like with two of these plus a splitter and 1dB NF AF stage we are 
looking at an overall NF of  12dB from 160m to 6m without any preamp.  If we 
drive the H mode switches from an FPGA containing a CORDIC NCO then we 
remove the spurs caused by the DAC in the packaged AD9*** parts.

This looks like a very interesting receiver which should have excellent 
performance.  Pity there are so few experimenters about ;)

It would be interesting to apply the DAC approach to the QSD also to see if 
it can be optimised for each frequency.

73's Phil...VK6APH



----- Original Message ----- 
From: "Ahti Aintila" <oh2rz.sdr at gmail.com>
To: "Phil Harman" <phil at pharman.org>
Cc: <hpsdr at lists.hpsdr.org>
Sent: Sunday, September 23, 2007 5:43 PM
Subject: Re: [hpsdr] QSD gain and noise.


> 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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>
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