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

[Tayloe Dan-P26412]

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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