Re: Sherwood SE-3 Sync Detector mod for ham use

[AB2EZ]

Ian et. al.

I agree...

As an observation... a trend I see in my own shack is as follows (not necessarily typical):

1. Several vintage receivers and transmitters that I truly enjoy owning and using

2. Auxiliary equipment, used in conjunction with the vintage equipment, that is state of the art (at least in the context of things that I can afford):

a. Analog power meters, modulation monitors, etc. =>  (over the next few years) a single directional rf coupler ("rf pickup") followed by an A/D converter directly sampling the RF outputs of the directional coupler, followed by software-defined modules that accurately compute and display (simultaneously as windows on a large LCD monitor + optional supplementary LCD displays): forward and reflected power, modulation indexices when I'm transmitting AM, various measures of the fidelity of my transmitted signal's modulated envelope with respect to the input audio signal to the transmitter, carrier frequency, rf envelope as a time waveform, etc; and also provide an off-air monitor output to my headphones.

b. A general purpose software-defined receiver that can connect to the i.f. output of any of my vintage or modern receivers... and which deliver that i.f. signal to an A/D converter (eventually the same A/D converter used in a.) ... which will demodulate it using whatever demodulation method I want, and which will also compute and display the spectrum of the i.f. signal, the demodulated envelope of the i.f. signal, etc.

Stu

[Bacon, WA3WDR]

The  carrier sync can be done in the PC with I and Q from a fixed BFO outside, but the soundcard is usually AC coupled, and this will put about a 40 Hz wide notch somewhere in the passband.  With external carrier sync, the hole is centered on the carrier or hypothetical carrier.  DC coupling can remove that hole, but it is tricky because an offset error will put a heterodyne in its place.

Ultimately, processor-controlled carrier sync is probably the way to go.  With todays soundcard technology, a small external processor on an external sync IQ unit might be the way to go.

Once a procesor is on the outside unit, it starts to make more sense to do A to D there too, and skip the soundcard altogether.

[WA1GFZ]

HPSDR does A/D external from low frequency I/Q inputs. The input frequency is usually 5 to 10 KHz. then sends data over USB to the computer.
Newest SDR A/D right off the preselector. TAPR is supposed to be demonstrating Mercury at Dayton. Perseus samples at 80 MHz while QSR1 and Mercury will have the faster 125 MHz device. An affordable 160 MHz A/D is just hitting the street.
The local oscillator may go away once the dynamic range can be matched with the A/D. The preselector will be the gate keeper.