Fading signals
 

Has anyone gotten a good oscilloscope look at the type of signals
that distort by cancellation of part of the carrier, i.e. the
signals that synchronous detectors detect much better than other
types? I'm wondering about the characteristics that make these
signals so hard to understand when they "fade" into that condition.

--
All relevant people are pertinent.
All rude people are impertinent.
Therefore, no rude people are relevant.
-- Solomon W. Golomb

Fading signals
 

clifto wrote:
Has anyone gotten a good oscilloscope look at the type of signals
that distort by cancellation of part of the carrier, i.e. the
signals that synchronous detectors detect much better than other
types? I'm wondering about the characteristics that make these
signals so hard to understand when they "fade" into that condition.

If you are talking about AM reception (and I think you are): I highly
recommend that you have some fun with an ISB (independent sideband)
receiver, with USB and LSB going into different ears. It's fun. Not
sure exactly what you learn :-).

I would also recommend that you look at spectra +/- a few kHz rather
than just waveforms. You can clearly see selective fading rolling up
and down under some conditions and hear the resulting distortions.

I too am generally interested in homebrew synchronous AM detectors. I
am not convinced that they are better than diversity antennas or ISB
listening (of course not everybody has diversity antennas or ISB
receivers.)

Tim.

Fading signals
 

"clifto" wrote in message
...
Has anyone gotten a good oscilloscope look at the type of signals
that distort by cancellation of part of the carrier, i.e. the
signals that synchronous detectors detect much better than other
types? I'm wondering about the characteristics that make these
signals so hard to understand when they "fade" into that condition.

--
All relevant people are pertinent.
All rude people are impertinent.
Therefore, no rude people are relevant.
-- Solomon W. Golomb

Not sure just what you mean, but AM needs a carrier to work. In an envelope
detector, the carrier is needed to have the correct waveform so that the
audio can be extracted. If there is no carrier, or too little, the waveform
is not what is needed for the detector to work. Listen on a sideband
receiver and you don't need the carrier and only the fade occurs with no
distortion.
Just look at the waveforms for AM and Single sideband in tutorials which are
all over the web and it is obvious. If the carrier goes down, you don't get
that waveform. I couldn't find any good papers on the web showing the
waveforms of AM and what happens as you reduce the carrier, but,...
Here are some references that may help you.
http://www.st-andrews.ac.uk/~www_pa/...rt9/page2.html
http://www.radio-electronics.com/inf...t/sync_det.php
http://www.radio-electronics.com/inf...e_am_demod.php
Compare the waveforms in those with what is here.
See Fig 1 in this paper. http://www.scott-inc.com/ham/ssb_im.pdf It is
what you see when the carrier fades compared to the classical waveform you
see for AM above. Note the nice sine wave is gone.


73, Steve, K9DCI

Fading signals
 

Steve Nosko wrote:
"clifto" wrote...
Has anyone gotten a good oscilloscope look at the type of signals
that distort by cancellation of part of the carrier, i.e. the
signals that synchronous detectors detect much better than other
types? I'm wondering about the characteristics that make these
signals so hard to understand when they "fade" into that condition.

http://www.radio-electronics.com/inf...e_am_demod.php

I think this one best illustrated what I've read sparse information
about, that multipath will sometimes cancel part or all of the
carrier.

--
All relevant people are pertinent.
All rude people are impertinent.
Therefore, no rude people are relevant.
-- Solomon W. Golomb