My comments in-line below:


On Wed, 21 Feb 2007 20:53:41 -0600, craigm wrote
in :

Larry Dighera wrote:


This is a first draft. Critique, corrections, and suggestions for
improvement and inadvertently omitted information welcome.





AM Synchronous Detector Review: Sony ICF-2010 vs RL Drake R8B

Two terrific SWL receivers with comparable performance up to a point.

Operating technique differs between these two synchronous detectors.
Because a synchronous detector is phase-locked on the station's
carrier frequency, it is able to overcome phase distortion introduced
in the incoming signal by dynamics and irregularities in the Earth's
ionosphere and magnetosphere.

The distortion is caused by selective fading where the carrier is reduced in
amplitude resulting in the equivalent of an over modulated signal. It is
the effective overmodulation that results in a distorted signal when using
a peak detector.

There is a reasonable definition for 'selective fading' he

http://en.wikipedia.org/wiki/Selective_fading
frequency selective fading is a radio propagation anomaly caused
by partial cancellation of a radio signal by itself — the signal
arrives at the receiver by two different paths, and at least one
of the paths is changing (lengthening or shortening). This
typically happens in the early evening or early morning as the
various layers in the ionosphere move, separate, and combine. The
two paths can both be skywave or one be groundwave.


And there is an in-depth discussion of 'selective fading' he

http://www.hard-core-dx.com/nordicdx...al/fading.html
There are two primary causes of signal fading on shortwave
multipath cancellation and polarization rotation.
...
Signals demodulated by a sync detector are not subject to
selective fading of the carrier as long as there is enough carrier
present to keep the detector locked. Long time constants on the
carrier tracking, phase locked loop allow the synchronous detector
to "fly wheel" through short carrier fades without losing lock.


[A mathematical treatise on fading is available he
http://www.mwrf.com/Articles/ArticleID/7964/7964.html ]


So, to incorporate the concept of 'selective fading' into my original
sentence, it might be re-worded thusly:

Because a synchronous detector is phase-locked on the station's
carrier frequency, it is able to overcome audio distortion
introduced in the incoming signal by dynamics and irregularities
in the Earth's ionosphere and magnetosphere, as well as multi-path
selective fading.


The restoration of the signal's phase
relationship results in a significant increase in readability.

Rather than 'restoring the phase', you are switching from a peak detector to
a product detector which does not rely upon receiving a carrier to
demodulate the signal.

That statement seems to contradict the operation of the synchronous
detector described in the www.hard-core-dx.com article citation above.
A product detector has two inputs, one being the
received signal, the other being a locally generated frequency (I choose
not to say carrier). In a sync detector, the locally generated frequency is
phase locked to what remains of the received signal's carrier. (Note, by
using SSB you are doing the same thing, EXCEPT the locally generated
frequency is not locked to the incoming signal. Thus fine tuning is needed
when using SSB. I don't call this ECSS, because the you do not Exhault the
Carrier.)


There is an elementary discussion of detector types he
http://en.wikipedia.org/wiki/Detector_(radio)

Although the 'peak detector is not mentioned in the above article, the
'envelope detector' is, and it seems to be synonymous.

The above article defines a 'product detector' thusly:

Product detector
A product detector is a type of demodulator used for AM and SSB
signals. Rather than converting the envelope of the signal into
the decoded waveform like an envelope detector, the product
detector takes the product of the modulated signal and a local
oscillator, hence the name. This can be accomplished by
heterodyning. The received signal is mixed, in some type of
nonlinear device, with a signal from the local oscillator, to
produce an intermediate frequency, referred to as the beat
frequency, from which the modulating signal is detected and
recovered.


So, rewriting my original sentence:

The restoration of the signal's phase relationship results in a
significant increase in readability.

to encompass the notion of product detection might look like this:

The restoration of the incoming signal's original carrier
amplitude results in a significant increase in readability.

Is that a correct statement?


Both receivers' synchronous detectors are enabled by pressing the
respective 'Sync' button. The difference in their operation occurs
when adjusting the other signal enhancing function of this remarkable
AM detector.

One of the primary tools employed by the radio operator is the
selection of bandwidth appropriate to the current reception
conditions. Narrowing the bandwidth is effective in removing two
additional types of signal degrading effects: atmospheric noise, and
adjacent channel splatter and heterodyne. The AM synchronous
detector provides the means to continue the exploit of this bandwidth
narrowing philosophy significantly by providing the means to further
restrict the detection envelope to only a single sideband of the
inherently double-sideband AM signal

Without the sync detector one can still narrow the bandwidth to include one
sideband and the carrier.

I suppose that's true, although I hadn't appreciated it until you
pointed it out.

So, this sentence:

The AM synchronous detector provides the means to continue the
exploit of this bandwidth narrowing philosophy significantly by
providing the means to further restrict the detection envelope to
only a single sideband of the inherently double-sideband AM signal

could be re-written to remove the exclusivity implied like this:

The AM synchronous detector provides an additional means to
continue the exploit of this bandwidth narrowing philosophy
significantly by providing an additional means to further narrow
the detection envelope to only a single sideband of the inherently
double-sideband AM signal

Better?

snip

Hope this helps

craigm

Craig,

I sincerely appreciate your help in pointing out inaccuracies and
educating me about the technical aspects of the use of synchronous
detectors. If you have further critique, or issues with my proposed
changes, please continue.