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

[Ahti Aintila]

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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