[AMRadio] Trouble brewing ...

Donald Chester k4kyv at charter.net
Tue Dec 17 03:40:16 EST 2013

>>>>>I once had asynchronous detector in a simple Sony ICF 2010 receiver. It
was remarkably good on AM.

Bry Carling
Sent with my iPhone 6

> On Dec 16, 2013, at 4:55 PM, "Brett Gazdzinski" <b.gaz at comcast.net> wrote:
> That is why I love a sync detector that allows only one sideband
> It sounds just as good as normal with a lot less noise.
> Brett
> N2DTS >>>>>>

Actually, a  synchronous detector is basically designed to receive both
sidebands, but with the full advantage of a product detector as opposed to
an envelope  detector. If the receiver has variable selectivity, it may be
narrowed to the point that only one sideband is received, which may be
useful for dodging interference.

Some synchronous detector circuits have the internal capability of allowing
the reception of only one sideband, using the phasing method of sideband
rejection. But in this case it becomes nothing more than an old fashioned
"sideband slicer", not a true synchronous detector. The synchronous detector
functions by coherently demodulating both sidebands and vectorally adding
them at the detector output, so that they reinforce each other. 

The only advantage of a synchronous detector when used to receive only one
sideband of an AM signal, is that the PLL automatically locks the BFO onto
the carrier frequency, thus eliminating the frequency error of a manually
tuned BFO. Otherwise, this is nothing more than ordinary selectable sideband
reception; you could do the same thing using any receiver with a narrow i.f.
filter, BFO and product detector; just carefully zero-beat the carrier by
turning the main  tuning control, with the BFO set to the edge of the
pass-band. This cuts random noise in half, but reduces the signal by 6 dB.
If the interference consists  only of random noise, the net effect is a 3 dB
degradation of signal-to-noise ratio.  

Don k4kyv

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