[hpsdr] Odyessey-Siren Rev B - 180 vs 90 degree conversion loss

[Ahti Aintila]

Hi Dan,

Finally, I think that I understood the essential difference between
the passive integrator and the active integrator. In the passive
integrator as your original Tayloe Detector is, you really get the
average voltage during the sampling times. In the active integrator
the resistor and capacitor are connected to the virtual ground
potential of the opamp. That way you get the integral of the sampled
charges in the capacitor. Over the big number of successive charge
pulses the voltage over the capacitor follows closely the modulated
envelope shape and amplitude.

Would that be an acceptable explanation?

73, Ahti OH2RZ

On 20/12/06, Ahti Aintila <oh2rz.sdr at gmail.com> wrote:
> Dan,
>
> Sorry the low clocking speed of my brains, I need more time to understand.
>
> You mention "each of the two detection caps" in the case of the 180
> degree switching. Actually I'm using four capacitors like the 90
> degree circuits. That should increase the detected voltage.
>
> Then another point. We cannot study the case during one RF cycle only.
> Our integrators have high time constants, so within one RF cycle the
> voltage of the integrating capacitor cannot change very much, but
> during the thousands of cycles of the modulated envelope the voltage
> can reach close to the peak value of the input signal, taking into
> account the loss in the source resistance and assuming high load
> impedance of the capacitors.
>
> To my layman's understanding, in the matched case the losses should be
> same for the 90 degree and 180 degree circuits. The main difference is
> that in the 90 degree case you have 4 switches charging the capacitors
> 90 degrees minus the rise and fall time of the switches, and in the
> 180 degree case the 4 switches carry the current 180 degrees minus the
> same rise and fall times.
>
> 73, Ahti OH2RZ
>
>
> On 20/12/06, Tayloe Dan-P26412 <Dan.Tayloe at motorola.com> wrote:
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > The detector integrates the fraction of the RF pulse onto the detection cap.
> >  Physically, this integration is done using the R/C low pass filter effect
> > of R, the system impedance, and C which is the detection cap.  The loss
> > appears across the system impedance and is not visible at the input to the
> > detector since the voltage drop across the system impedance has already
> > happened.  By selecting C we affect the roll off of the R/C low pass filter
> > and thus C is relative small for an SDR front end (0.01 uf – small RC time
> > constant = wider bandwidth, 10's of KHz wide), and relatively large (0.82 uf
> > – larger R/C time constant = narrower bandwidth; ~1 KHz wide) for a
> > traditional analog DC phasing receiver such as the NC2030.
> >
> >
> >
> > Integrating 180 degrees of the RF waveform onto each of two detection caps
> > mathematically gives square root of 2 times the peak or 0.707x the voltage,
> > a 3 db detection loss.  Evaluating the integration of a sine wave from 0 to
> > 180 degrees is a straightforward mathematical exercise.  On the other hand,
> > integrating over 90 degrees of the RF waveform on to each of four detection
> > caps gives 0.9x the peak RF voltage, a 1 db loss.  This is the same as
> > evaluating the integration of a sine wave from 45 to 135 degrees,
> > integration over the 90 degree section of a sine wave that covers the peak.
> > Thus, these losses do have a mathematical basis.
> >
> >
> >
> > The fact that folks look at the input to the detector and see that 1v pk-pk
> > RF waveform into the detector gives 1v pk-pk of detected base band audio are
> > missing the fact that voltage drop (loss) that has already happen across the
> > 50 ohm system impedance before the detector input. Thus it is easy to come
> > to the conclusion that the detector has basically no conversion loss and is
> > thus more of a "sample and hold" type process rather than an integrating
> > process.
> >
> >
> >
> > - Dan, N7VE
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