[Premium-Rx] Comparing AM Synch Detection to AM Product Detection

[Bob Milne]

Hi Ahmet,

Other than the fact that you can't exactly synchronize the receiver's
BFO with the oncoming carrier, I often find product detection
preferable to synchronous detection (especially on AM voice signals).

Because the internal BFO is usually at least 20 dB or so down on the
filter skirt, the SSB filter acts as a kind of "shelving filter"
boosting the frequency response until the edge of the pass band is
reached. This seems to add an extra degree of crispness that makes
voice signals easier to understand.

Conversely, a synchronous detector doesn't operate well once you tune
your receiver so the incoming carrier is beyond the edge of the filter
pass band. In fact, most synchronous detectors will lose lock if you
try and tune the incoming carrier 20 dB down on the filter skirt.
That's the same reason raising the low-frequency cutoff point with a
"brick wall" DSP filter doesn't produce the same clarity as listening
with a product detector and SSB filter.

Also, if you're using a product detector you will NEVER lose lock no
matter how badly the signal fades.

If you want to listen to, say, music on a good quality AM signal using
a wider IF filter bandwidth, then the synchronous detector is better.
You can get good low-frequency response without the swishing effect of
a slightly mismatched BFO/product detector combination. And, of
course, more highs.

In my humble opinion, both detection schemes have their
place--depending on the incoming signal quality and what you're
listening to. But if I had to choose just one, I'd go with the product
detector on a very stable premium receiver with 1-Hz tuning
resolution. Now, if only the majority of premium rigs came with noise
blankers!!!

Regards....
....Bob

On Fri, 20 May 2005 00:59:48 +0200, you wrote:

>
>Hello everybody.
>
>I am wondering how Synchronized AM Detection would compare to regular Product Detection
>of AM signals (detecting it as SSB signal).  The only difference in the Synch detection is
>basically RF and LO at the Product Detector are in Phase.  Does this offer any
>added benefit such as improved Audio quality, better fighting Fading,... etc ?
>
>The reason of this question is that I am comparing the two methods and I am not sure
>if there really is any difference as far as my ears can distinguish. The Sync detection being 
>a more complicated circuitry and requires better design, is it worth the effort ?
>
>I am not too sure but I think the mathematical formula for both methods comes down to same 
>after the multliplication.
>
>Regards to all.
>Ahmet