Sharp AM detector

[John K5PRO]

I dug this up in my old paper files. Seems like it could be useful, a synchronous detector that uses a narrow band filter for carrier instead of a PLL. Has anyone played with this for AM, better than envelope detector for fading effects? There may be some better chips than a CA3089 to do this.

[w3jn]

The Drake R-7 uses a similar AM detector arrangement, and as far as I know it's the only receiver that uses the limter/product detector method.  Two transistor amp/limiter fed to a MC1496 balanced modulator IC.

[John K5PRO]

Interesting. I love it, analog parts I can identify with, 2n3906 and MC1496. 

The R8 is different again, I suppose. I wonder how well the R7's detector worked?

[WQ9E]

John,

I don't know if the R7 is as effective as the sync detectors in the later R8 series but it does help with selective fading and what I like most is it doesn't have the issue of searching for lock during a net or round table.  I rarely use the sync mode in my R8 and R8A receivers because there is a short but definite time for it to lock when a new station starts transmitting and that ranges from annoying during a round table to useless during a net since you will often miss the first syllable which during a net means part of a call sign.  They do a great job with shortwave broadcast but I don't care for them for regular AM use.  The later iteration in the Drake designed E1XM shortwave portable is better but still has a little lockup delay.   I have never used the Sherwood unit and at the current pricing I probably never will.

The Drake R7 is a really nice receiver and performance wise it is the next generation R4C in terms of interference fighting and overall smoothness of operation.  I have mine paired with a TR-7A.  A couple of years ago I picked up a pair of Drake RR-3 receivers which are their commercial marine version of the R-7, it is very similar but is designed for rack mounting with a front mounted speaker, doesn't have the antenna splitter, and default band within a range is different.  It does have the AUX-7 standard with programmed modules for the marine emergency frequencies.

[N8ETQ]

   Hey John,

      I have, The CV-157 uses that method in ADDITION to
a PLL. The Local Carrier has to be pretty close in phase
relationships to the sidebands or you get "Goofy" Audio.
It almost sounds like a "Flanging" effect. (Am I aging
myself?)


      The CV-157 has the same drawbacks as his solid state
brothers except more so due to the motor tuned LO.  He was
designed for fixed frequency operation between ship and
shore stations where everyone is "Dead Nuts" on freq.
I think the carrier filter is only 50 or 60 cy. wide.

      You can read all about it here:

http://bama.edebris.com/manuals/military/cv157/


   Back in the day when Radio Merlin 1 (a BBC affiliate)
was rockin' out on 9560/9895 kc I used it quite a bit. Not so
much anymore. It's a beautiful machine and I still have him.

Maybe yet another 2015 Resolution Hi!

73

/Dan

I dug this up in my old paper files. Seems like it could be useful, a synchronous detector that uses a narrow band filter for carrier instead of a PLL. Has anyone played with this for AM, better than envelope detector for fading effects? There may be some better chips than a CA3089 to do this.

[nq5t]

John,

I don't know if the R7 is as effective as the sync detectors in the later R8 series but it does help with selective fading and what I like most is it doesn't have the issue of searching for lock during a net or round table. 

I owned an R-7 for quite a few years.  While it's certainly better than the standard detector arrangement in the TR-7 etc., it is not as good as the SE-3.  Sherwood sold a lot of SE-3's with IF converters for use in the R-7/a receivers.   The "AR" mode in the later SE-3's helped greatly with quick lock in roundtables at the expense of not having quite the excellent distortion characteristics of the standard loop time constant.

Another interesting approach to the issue of fading distortion was explored by Dallas Lankford using elliptic low pass filters (ELPAF).  There are several articles and some construction info on the Dallasfiles group on Yahoo.  I haven't tried it, but have been thinking about building a filter and seeing how it works in comparison.  Since it works at audio, there are no lock time issues, no 'not for this IF' issue, and you can easily tune around the signal passband to minimize interference.

https://groups.yahoo.com/neo/groups/thedallasfiles2/files/Fading%20Distortion/

The K3 also has a very good sync detector with auto tuning and selectable sideband.  It doesn't always lock well on AM signals close to the noise level in my experience, and lacks the low end fidelity of the SE-3, but works well in most normal situations.

Because the T-T Orion is a decent AM exciter, in the early days of the radio many of us tried, to no avail, to get T-T to include a sync detector for AM reception.  The designer of the 1.xx Orion firmware simply refused to consider including the feature, and it was left out by the designer(s) of the Orion's 2.xx firmware.  I don't know whether the last 3.xx firmware releases had a sync detector or not since my Orions were gone by then.

Grant NQ5T

[k7iou]

Dallas Lankford has written on the subject, enjoy.
https://web.archive.org/web/20101124132736/http://kongsfjord.no/dl/Audio/AM%20Synchronous%20Detector%20Experiences.pdf

https://web.archive.org/web/20101124124237/http://kongsfjord.no/dl/dl.htm#Audio%20processing

[k7iou]

One from July 93 QST
http://www.arrl.org/files/file/Technology/tis/info/pdf/9307028.pdf

[John K5PRO]

That one is interesting, in that it has both a PLL/synchronous, plus a quasi-synchronous detector. The QS could be improved by adding a narrow band filter before the NE604 limiter stage. If it only used this chip and the NE602 as a product dectector, could eliminate U1 in this circuit. I would put a wider IF filter before the input, then the narrow (XTAL) filter at 455 KHz between pin 14 and 12 of NE604 (614A these days). With some phase adjustment, it would do what the original posting from RF Design was doing using a CA3089 limiter.

[k7iou]

I was wondering if we could cure the fading affect at the antenna source vs at the receiver?
I heard of a RX antenna that lays on the ground in a t shape with a RX detector that changes polarity with the signal received. Has anyone seen this design? I thought it would be interesting to try vs the synchronous detector for the effects of fading.

[Steve - K4HX]

Yes. This is the main reason for diversity reception - using two antennas and two receivers. I've never heard of much success using only one antenna and one receiver.

I was wondering if we could cure the fading affect at the antenna source vs at the receiver?