[hpsdr] IQSD

Tue Apr 17 10:21:43 PDT 2007

Thank you, Marco!
You saved my time and efforts in explaining these complicated matters.

In the mean time I simulated more. TINA gives pretty close 112.5 ohm
input impedance as it should be, when using your parameters for 180°
conduction angle. However, from the 90° simulation I calculate 87.9
ohm (instead of 50 ohm). That is something I don't understand. Maybe,
your 8 GHz sampling scope can shed some light to this black magic.

I attach my simulation files. Hopefully they are small enough to come
through. If not, I'll mail them privately to you.

73, Ahti OH2RZ

On 17/04/07, Marco IK1ODO -2 <ik1odo at spin-it.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> At 16.07 17/04/2007, Gerd, DJ8AY wrote:
> >Ahti and Marco,
> >
> >thanks for the info.
> >Up to now I do not know anything about TINA software.
> >I will try to find information on it.
>
> Texas Instruments web site,
> http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html
> It is an interesting spice-like simulator, and does transient and
> noise analysis.
>
>
> >I mainly wondered if you are using buffers between the switches and the
> >OPA1632.
>
> No, the focal point is the matching between the source impedance of
> the mixer output (220 Ohm in my last circuit) and the noise impedance
> of the opamp, to have a good energy transfer to the amplifier. I
> worked a lot on this point, and wrote an excel program to evaluate an
> opamp's noise figure in relation with the source impedance. I wanted
> to have a quiet (and simple) frontend, and the OPA1632 is the best
> choice that I found (found by Ahti, in effect!). It is cheap, very
> linear, has enough band and suits ideally the differential input of
> the professional audio interfaces. Any kind of buffer before the
> OPA1632 simply makes things more complex.
>
> >I also have some general questions on the QSD and hope to get your help:
> >
> >1. which criterias lead to the choice of the sampling capacitors (here 5n,
> >with SDR1000 15n ...) or what happens if you use a much smaller and a much
> >higher value?
>
> There is no sampling capacitor ;-) since it is not a sampling
> detector "à la Tayloe". The name QSD is misleading! The
> switch/diplexer group is a source with his Z that injects current in
> the virtual ground of the opamp. The integration is done by the 100pF
> cap on the opamp; the diplexer simply avoids that out of band energy
> (over 175kHz in this case, that is the crossover point of the
> diplexer) reaches the opamp. The 5nF capacitor is not a sampling
> capacitor in the usual terms. I wanted to have a wide RF bandwidth,
> to be able to receive +/- 96 kHz with a 192kHz sampling rate, so
> choosed a cross point about twice that. The integration capacitor is
> made small, again to be flat at the upper limit of the response (and it is).
>
>
> >2. which influence has the timing of the switching pulses (overlap, gap,
> >delay ...)
>
> Ah, this is a long discussion. My first mixer had 90° switch
> conduction angles, so no overlap. It worked very well up to 20MHz,
> with some degradation at 28 MHz. I could not test it at 50MHz (no
> fast dividers available). Then Ahti found that by simulating it in
> TINA the recovered voltage was greater using 180° conduction, like in
> the SoftRock mixer. I was in doubt, so I did my simulations, and saw
> a possible side advantage; using a wider conduction angle the real
> gate opening time is greater at high frequency (50MHz). Effectively
> the 180° mixer seems to work better at 50 MHz; at lower frequencies
> the difference is marginal. But it also has one less IC, so it is
> simpler. From an energy point of view, in the 90° version there is
> always a switch open; in the 180° version there are two switches
> open, and the Z is lower, but in any case all the available energy is
> transferred to the load (the integrator). With the 180° switch the
> integrator C has more time to charge, but the source V is lower, so
> at the end there is no much difference in output voltage. I may send
> you the TINA models to play with, I found it instructive.
>
> I'm still working on it, I have two things to try:
>
> - increasing the mixer input Z to 200 Ohm like my previous version,
> and the diplexer Z to 400 Ohm; I suspect that this could reduce the
> switching noise.
> - building a test jig to measure the switching delay and transients
> of the FST3125. I think we still have to understand it... now I have
> access to a 8GHz sampling 'scope, and I want to use it.
>
> I started working on this circuit in Sept. 2006. After seven months,
> I still have to understand it completely; it is only apparently
> simple, but difficult to optimize. Something may be done by
> simulation, something needs experimenting, and it takes a lot of time
> (I have a family and a QRL too, HI).
>
> 73 - Marco IK1ODO
>
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