[hpsdr] ISD Theory References?

[Rob Frohne]

Hi Phil,

So if I understand right, the difference you make between the QSD and 
the ISD is more a circuit level thing than a functional block level 
thing?  I understand your analysis of the impedance issues.

Ahti, your circuit seems like it should give a lot better noise figure.  
If you take an SDR-1000, and do the same measurement with it, it seems 
to me you should get roughly a noise level of about -160dbm.  (I assume 
you can roughly hear a 1uV signal into 50 ohms with a 2.3 kHz bandwith 
to get this.)  I assume you have measured this?  Is this what you get? 

Phil,  I'm curious if you have actually built your preferred circuit.

Thanks for filling me in!

73,

Rob, KL7NA

Philip Covington wrote:
> On 8/31/06, Rob Frohne <frohro at wwc.edu> wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>>
>>
>> Hi,
>>
>> Pardon my ignorance; I haven't been party to all the discussion going on
>> with the ISD, and I'd like to be brought up to speed.  A Google search
>> for "Integrating Sampling Detector" reveals only one hit, the Pheonix
>> wiki page.  It seems that a number of people here on the list are
>> convinced that it is better than a QSD or commutating mixer.  It appears
>> that Ahti isn't completely happy with his experimental results.   Phil
>> has some simulation results on his blog page, but examining the
>> simulation requires Windows and I'm a Linux guy.  Gerald has a
>> description of the QSD in his first article:
>>
>> http://www.flex-radio.com/articles_files/SDRFMP1.pdf
>>
>> Referring to Figures 10 and 11 of that article.  Are we replacing the
>> leaky integrators (RC low pass filter )with an ideal integrator and
>> varying the time it is connected?   I'd like to see a block diagram.
>> Ahti's circuit sure looks a lot like the block diagram and simple
>> circuit described in Gerald's article (Figure's 10 and 11) to me.  So
>> what exactly are the differences between a QSD and an ISD?
>>
>> What are the theoretical advantages/disadvantages of an ISD over a
>> commutating mixer, or a QSD?
>>
>> Maybe if I understood all the above, I could make some progress on
>> understanding why Ahti's circuit isn't as good as expected, which is
>> needed to advance the Pheonix project, and I'm interested in that.  :-)
>>
>> 73 and thanks for bearing with me,
>>
>> Rob, KL7NA
>>
>> -- 
>> Rob Frohne, Ph.D., P.E.
>> E.F. Cross School of Engineering
>> Walla Walla College
>> 100 SW 4th Street
>> College Place, WA 99324
>> (509) 527-2075                   http://www.wwc.edu/~frohro
>
> This is the circuit that I recommend:
>
> <http://www.philcovington.com/HPSDR/TEMP_STUFF/isd_07282006-1.pdf>
>
> In the SDR-1000 QSD the instrumentation amp's input impedance is very
> high.  The impedance looking into the QSD is determined by the approx
> 90 ohm resistors in series with the secondary of the input transformer
> for OUT OF BAND signals.  This is because the  caps present a very low
> impedance to signals outside of the QSD bandwidth.  For signals within
> the bandwidth of the QSD the input impedance grows to a very large
> value the closer to DC you get.  This means any input filter is not
> terminated properly for IN BAND signals.  The addition of the 90 ohm
> resistors was an attempt to make the filters behave properly out of
> band.
>
> Since the switches in the above ISD circuit operate at virtual ground,
> the input impedance should be more constant for in and out of band
> signals.  The TBD resistors in series with the windings of the input
> transformer then determine the input impedance over a much greater
> range than the QSD.
>
> There also seems to be a large improvement in switch operation because
> you do not have a relatively large capacitor hanging off one side of
> the switch to ground.  Holding the impedance to ground constant on the
> switch output also appears to improve switch function.
>
> The particular SPDT switch is also faster.
>
> 73 de Phil N8VB

-- 
Rob Frohne, Ph.D., P.E.
E.F. Cross School of Engineering
Walla Walla College
100 SW 4th Street
College Place, WA 99324
(509) 527-2075			 http://www.wwc.edu/~frohro

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