|[AMRadio] Wanted: Receiver - New England (sync detector)|
k4kyv at charter.net
Mon Feb 18 11:08:16 EST 2008
> When conditions on poor, sometimes you can use the CW/SSB position as a
> poor man's synchronous detector. So don't rule out a receiver that has a
> decent BFO in it. It can be an aid in copying weak signals on AM.
> Pete, wa2cwa
That works if there is enough coupling between BFO and 2nd detector to
force-lock the BFO onto the AM carrier. I discovered the phenomenon with a
pre-WW2 HRO. The old HRO has notoriously poor BFO coupling because of a
tiny coupling capacitor of only a few pf that is nothing more than a piece
of bakelite about a third the size of a postage stamp, sandwiched between
two metal plates the same size, so I padded it by bridging a 100 pf mica cap
across the whole thing. That actually overcoupled the BFO, but gave me
enough injection that I could comfortably copy SSB, and it handled strong
CW signals much better. I discovered one evening when I was trying to copy
a weak AM station with the BFO on, under heavy summer QRN conditions, that
the BFO appearded lock onto the original carrier whenever I tuned it exactly
to zero-beat, and the audio from the modulation seemed to jump right out of
the background noise at me as soon as lock occurred. I could tune the
receiver maybe plus or minus 50 Hz before pulling the BFO out of lock, so
it required one hand on the tuning dial or BFO knob if the receiver or the
station had the least bit of frequency drift.
If the BFO won't lock on to the carrier, you are better off narrowing up the
selectivity and copying the AM signal as SSB. You still have two sidebands
to select from, and there is often less QRM on one sideband than on the
A great advantage of the phase locked BFO in a synchronous detector is that
the noise and QRM you hear are products only of the carrier (BFO) and each
component of the sidebands + noise. With a standard envelope detector, such
as a diode type, you hear products of the carrier beating with the noise
noise and QRM in addition to the sidebands, but also every component of the
noise/QRM beating against every component of the sidebands, and every
component of noise/QRM within the passband beating against every other
component of noise/QRM. These additional intermodulation products add a lot
to the rubbish you hear in the headphones or speaker.
Much of the "advantage" of SSB over AM is due to the superior s/n ratio of a
product detector over the diode detector at the receiver, as described
above. With the sync detector SSB loses that advantage (or AM loses that
disadvantage), and AM becomes much more competitive with SSB.
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