[hpsdr] An high dynamic QSD front-end

[Ahti Aintila]

Hi Marco and all,

After some studies and simulations I have a pleasure to congratulate
Marco for the excellent QSD mixer configuration. However, as Marco
refers to my ISD experiments with OPA1632 and also mentions 90 degree
sampling time better than the 180 degree sampling, I want to add some
comment.

Marco is completely right (as well as Dan Tayloe) in the case of
"passive" integration, that is used in the original "Tayloe detector"
and many other configurations after that.

In the case of my "active" integrator version (ISD), where the
integrating capacitor is in the feedback loop of an operational
amplifier, the opposite is true. That is because the current to be
integrated flows against to the virtual ground potential (and and also
the current flow time is longer), hence the current is not influenced
by the accumulated voltage level of the capacitor. In the Tayloe mixer
the current is related to the difference between the signal voltage
and the accumulated capacitor voltage.

I made a couple of simple simulations using my ancient Pspice  Student
version. Please, note that the simulation results may be compared only
passive against passive and active against active like lemons to
lemons and oranges to oranges. I did the simulation using only one
channel (I or Q, but not both at the same time) The clock frequency
used was 10 MHz and the signal 10.01 MHz. Two amplifiers were used for
ISD to get the differential outputs, that is the reason for two
sine-wave signals. The QSD was run without amplifiers, so only one
output trace. See the pictures:
<http://kotisivu.dnainternet.net/ahti/sdr-1000/QSD_ISD_180_90.zip> .

By the way, my earlier real measurements in last August gave same kind
of results, but I did not record them because the 180 degree active
circuit looked as an obvious choice for me.

73 and Prosperous Integration for the New Year,
Ahti OH2RZ

On 06/01/07, Marco IK1ODO <IK1ODO at spin-it.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi all,
>
> as a few members of this list already know, I did a lot of work about
> expanding the dynamic range of a QSD mixer.
> Some results of my work are in http://www.spin-it.com/sdr/IK1ODO_SDR1.html .
> Possibly this may be a good starting point for Phoenix; I am willing
> to cooperate, if possible.
>
> Please have a look at the page and the other two, with more
> measuraments and tests. This may not be the ultimate SDR, but I think
> that the results are quite interesting, and it's simple to implement.
>
> 73 - Marco IK1ODO
>
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