Is Synchronous AM detection all hype?

[ND9B]

I got all excited about synchronous detection after reading some articles on the subject. I just finished building the synchronous detector project in the April 1982 edition of Popular Electronics by Dave Hershberger, W9GR. I even went so far as to build an IF converter, so I could use it with my Kenwood R-599 receiver which has a final IF of 3.395 MHz.

It seems to be working flawlessly, the only problem is, it doesn't sound any better than the diode detector the receiver came with! I've listened to dozens of signals with varying amounts of fading and there is no audible difference other than slightly brighter audio with the synchronous detector. So far I have a breadboard with several hundred parts that doesn't perform any better than a single diode.

Is there some way of simulating a "damaged" envelope with my generator that could prove the concept? Is there something I'm overlooking? Has anyone actually heard an improvement with synchronous detection?

Bobby Dipole ND9B

[WQ9E]

With selective fading the sync detector provides much better detection BUT you don't often hear selective fading on fairly local (within 500 miles or so) ham signals.  Hooked up to a shortwave broadcast receiver with a station from Europe that has selective fading will provide a dramatic difference if your sync detector is working properly.  It doesn't make any difference for normal fading.

Another improvement you will note is if your receiver doesn't allow selectable sideband on AM but your sync setup does then the new detector will provide a great interference fighting tool by allowing you to avoid strong interference from either above or below (but not simultaneously) the desired station.  It also will help make understandable the signals radiated from some poorly adjusted modern rigs.

[k4kyv]

I'm not familiar with the Pop'tronics article, so don't know what kind of circuitry it uses.  If it is true synchronous detection and not the pseudo-synchronous detection that uses a limiter to wipe the amplitude modulation from the carrrier, which is then amplified and injected into the product detector to demodulate the sidebands, it should result in substantial improvement when copying weak signals near the noise floor.

My first experience with synchronous detection was the brute-force method of using tight coupling between BFO and the diode detector in a pre-WWII HRO, so that the BFO force-locked onto the AM carrier.  I actually discovered the phenomenon by accident one night when I noticed that the audio seemed to jump right out of the background noise when I zero-beated a weak AM signal with the BFO turned on, and the BFO locked onto the carrier of the AM signal that had been buried in summer static.

I use a Sherwood SE-3 with my 75A-4.  The audio is tremendously better than with the stock diode detector, but I hear much less difference when switching between the built-in envelope detector and synchronised product detector in the SE-3.  I think that is because the stock diode detector in the A-4 is so crappy; the SE-3 uses a low-distortion envelope detector rather than a simple diode.  It also makes a world of difference when monitoring slopbucket.  The SE-3 uses a high quality product detector whereas the stock Collins product detector is a piece of crap.  Before I got the SE-3, I hadn't realised how good (some) slopbucket signals could sound.

[w1vtp]

It would be good to know what kind of circuit the pop 'tronics used.  Another thought.  It would be good to know the rest of your setup.  Like Don, I use the SE3 and have found that it offers not only immunity to a lot of the distortion that comes with selective fading, it also makes the world of difference with AM signals that have a lot of positive modulation.  In fact, it is most remarkable to go between standard modulation detection and Sync AM detection. On these kind of transmitters.  I have heard distortion reports on such transmitters and was unaware of any problems until I switched to the standard detection mode - then the "distortion" showed up.

How about a general description of your hookup and a circuit of the SAM you built?

Al

[Opcom]

The SE-3, which uses a PLL, makes a huge difference in listenability on foreign SW stations what fade badly. There is no magic bullet that will make them perfect, it is an improvement.

[w3jn]

The Pop Tronix article is a true sync detector with a PLL.  That's the one that has selectable sideband, correct?

There is a huge difference with a sync detector, particularly with stations having a high modulation index and/or selective fading (where the carrier fades down but either sideband is still strong, thus simulating a DSB or SSB signal).  If you're testing it with close-in stations it may not indicate much of a difference.  Try it on the 41-meter band on some broadcast signals and I'm betting you'll hear the difference, if the thing is working right.

[ke7trp]

yep.  Just about useless for normal ham band use for day to day close in talking.

However,  If you are talking across the country on AM, it can make a drastic difference.  On 10 meters the other day, I had selective fading taking out entire words.  With the sync on, this was gone.  Talking Am to portugal, the sync helped out.

As you noted, the sound is a bit higher pitched with the synch on, This makes some stations jump out. I tried to get this on my video demonstation.

Stations that try to act like a broad cast station by pumping up the bass to sound big and full, Will sound like complete garbage to a distant station.  In this situation, my sync detector helps ALOT as it seems to cut bass and boost midrange.

Flipping it on, allows me to copy these idiots with the big booming bass, little to no compression and no talk power. 

The more you use it, the more you will learn when to flip it on.

C

[Pete, WA2CWA]

Last Fall during the 10 meter openings, the sync detector helped greatly in pulling out some of the weak stations running converted CB sets or during high levels of QSB. When working New Zealand, the sync detector helped tremendously in making the station copyable on the Flex (no shame here).

[k4kyv]

The biggest disadvantage I find with using the SE-3 in sync mode to copy AM on the ham bands, is that in most roundtables the stations are not zero-beat with each other, and whenever it's another station's turn to transmit it takes 2-3 seconds or more for the PLL to find the new carrier frequency and lock on, unless I manually re-tune.  If someone makes a two-second comment off-frequency or tries to break into the QSO, I often miss him entirely.  In a fast break-in AM QSO, forget it.  I switch to envelope detector mode on the SE-3.

Pete, I scoured the flea market several times at Dayton, but didn't see you or your ManualMan vendor spot at the usual place.  Were you there this year?  I thought I had covered the entire outdoor area thoroughly by early Friday afternoon and scanned it several more times by the end of the day Saturday, but if I overlooked your spot, I wonder what else I missed.

[KM1H]

April 1982 Popular Electronics
http://user.netonecom.net/~swordman/Radio/ShortWaveRadio.htm

Also includes the July 82 correction

Carl

[ND9B]

The proof of the pudding is in the oscilloscope. I was trying to figure out how to come up with a distorted envelope to test the synchronous detector, and it occurred to me to just mis-tune the receiver. This worked like a charm making a truly crappy envelope. And low and behold, the receiver's diode detector fell on it's face, with the synchronous detector doing a great job. The top trace is the output of the IF. The bottom trace is the detected audio. The signal was from my RF generator, 1.880 MHz modulated at 100%. The article from PE can be found at http://www.epanorama.net/sff/Radio/Receivers/Synchronous%20AM%20Radio%20Detector.pdf. There is a correction in the next month's edition.


Synchronous Detector


Diode Detector

Bobby Dipole ND9B

[Pete, WA2CWA]

Pete, I scoured the flea market several times at Dayton, but didn't see you or your ManualMan vendor spot at the usual place.  Were you there this year?  I thought I had covered the entire outdoor area thoroughly by early Friday afternoon and scanned it several more times by the end of the day Saturday, but if I overlooked your spot, I wonder what else I missed.

Every time I saw you coming, I covered my table and hid behind my car.

[KM1H]

Is there a PC board available for the PE version?

[W2VW]

Pete, I scoured the flea market several times at Dayton, but didn't see you or your ManualMan vendor spot at the usual place.  Were you there this year?  I thought I had covered the entire outdoor area thoroughly by early Friday afternoon and scanned it several more times by the end of the day Saturday, but if I overlooked your spot, I wonder what else I missed.

Every time I saw you coming, I covered my table and hid behind my car.

Isn't it easier just to change the space number?

[ND9B]

Is there a PC board available for the PE version?

Unfortunately no. But, FarCircuits has the PC board for the project in the 1994 Fall edition of Communications Quarterly. I'm going to build one of these also.

Bobby Dipole ND9B

[ND9B]

The biggest disadvantage I find with using the SE-3 in sync mode to copy AM on the ham bands, is that in most roundtables the stations are not zero-beat with each other, and whenever it's another station's turn to transmit it takes 2-3 seconds or more for the PLL to find the new carrier frequency and lock on, unless I manually re-tune.  If someone makes a two-second comment off-frequency or tries to break into the QSO, I often miss him entirely.  In a fast break-in AM QSO, forget it.  I switch to envelope detector mode on the SE-3.

W9GR's design in PE automatically switches to envelope detection when the carrier PLL loses lock. I haven't tried it in a roundtable yet, but there shouldn't be any "holes" in the audio.

Bobby Dipole ND9B

[ke7trp]

ND9B. Thanks for posting those O scope pictures. That was really neat and I enjoyed them man.

C

[Opcom]

The biggest disadvantage I find with using the SE-3 in sync mode to copy AM on the ham bands, is that in most roundtables the stations are not zero-beat with each other, and whenever it's another station's turn to transmit it takes 2-3 seconds or more for the PLL to find the new carrier frequency and lock on, unless I manually re-tune.  If someone makes a two-second comment off-frequency or tries to break into the QSO, I often miss him entirely.  In a fast break-in AM QSO, forget it.  I switch to envelope detector mode on the SE-3.

There is a new feature that makes lock-in faster for ham use. There is a switch on the front for it.

[Opcom]

SOME INFO, THE CORRECTION:

Look here too:
http://user.netonecom.net/~swordman/Radio/ShortWaveRadio.htm

 That Popular Electronics article is very informative. It explains a lot about the principles of AM and detection, and what causes the problems with fading and distortion. It explains very well the theory of how this synchronous detector works, how to build it and how to set it up and align it. It works with a fairly wide range of signal levels and includes the phasing circuitry for selectable sideband operation as well. In fact, selectable sideband circuitry is optional and can be left out. The primary active parts of the circuit are an MV2115 Varactor diode, three 2N3904 transistors, an LM318N op amp, 3 TL074CN quad op amps, a TL072 dual op amp, and two CD4053BCN multiplexers. The phase detector itself is a 74C932 but that is described as possibly hard to find and instructions show how to substitute a CD4046 PLL. There's also a zero-center tuning meter. A switch selects two different loop time constants, to make it easier to tune in the signal and then to keep the detector locked even when the signal fades very deeply. All in all, I thought it was a pretty impressive article, and very comprehensive. Especially for a mass-market hobby magazine.

> NOTE: The article has a published correction in the July '82 issue of
> "Popular Electronics" on P. 6

in Fig. 4, pin3 of IC8 should be connected to pin 11 of IC2
not pin10.
and an unpublished correction where the "ENV"
> (i.e. Envelope Detector) position of switch S1B should be ground (GND) not
> N.C. as shown

[k4kyv]

The biggest disadvantage I find with using the SE-3 in sync mode to copy AM on the ham bands, is that in most roundtables the stations are not zero-beat with each other, and whenever it's another station's turn to transmit it takes 2-3 seconds or more for the PLL to find the new carrier frequency and lock on, unless I manually re-tune.  If someone makes a two-second comment off-frequency or tries to break into the QSO, I often miss him entirely.  In a fast break-in AM QSO, forget it.  I switch to envelope detector mode on the SE-3.

There is a new feature that makes lock-in faster for ham use. There is a switch on the front for it.

That is the "AR" mode.  It still takes one or two seconds to lock on to the carrier.  With AR mode off, you have to manually re-zero the receiver every time an off-frequency station transmits.  AR mode works OK for old-buzzard round tables, but is still too slow for fast break-in QSOs.

[Steve - K4HX]

The PE design is one of the better thought out designs I've seen. As Opcom mentioned, it has two time constants for the PLL loop. It also has a built-in envelope detector that is in use until the sync detector locks up. At that point, the output audio is soft-switched from the envelope detector to the sync detectors. This arrangement should avoid the annoying heterodyne Don mentioned when all stations in a roundtable are no on the same frequency.

[K5UJ]

It's all about S/N ratio.  better with sync. detect on weak sigs.   In my case I need all the gadgets I can get since I don't have room for a beverage.  I never completely realized the usefulness of a good S/N ratio until I started operating AM.  It applies to all modes but is important even for casual operating on AM.

I also found the scope traces really interesting and add my thanks to Clark's.

[k4kyv]

April 1982 Popular Electronics
http://user.netonecom.net/~swordman/Radio/ShortWaveRadio.htm

Also includes the July 82 correction

It's posted as a DjVu file, which requires a free browser plug-in available from http://www.lizardtech.com/ in order to view it.

How do you download the plug-in?  I checked the web site and  they list a dozen plug-ins, but I see no reference to anything about DjVu.  http://www.lizardtech.com/downloads/plugins.php

[Pete, WA2CWA]

April 1982 Popular Electronics
http://user.netonecom.net/~swordman/Radio/ShortWaveRadio.htm

Also includes the July 82 correction

It's posted as a DjVu file, which requires a free browser plug-in available from http://www.lizardtech.com/ in order to view it.

How do you download the plug-in?  I checked the web site and  they list a dozen plug-ins, but I see no reference to anything about DjVu.  http://www.lizardtech.com/downloads/plugins.php

Here's the PDF version

[Rob K2CU]

Download Irfanview. it let's you view most every type of image file, and in some cases convert them. It is also used with several of the radio image programs, like Easypal  and Windrm.