[hpsdr] ISD Theory References?

[Philip Covington]

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

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