[AMRadio] Wanted: Receiver - New England (sync detector)


D. Chester 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 
other.

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.

Don k4kyv



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