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Author Topic: AM squelch vs sideband-purposed syllabic types  (Read 3944 times)
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Patrick J. / KD5OEI
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« on: January 02, 2020, 11:44:12 PM »

I keep looking for AM squelch circuits that operate by detecting voice-range sound/noises based on syllabic timing. I find many 'SSB squelch' circuits that work by detecting audio frequency, but none seem to indicate that they will work with AM.

No interest in SDR options or software, just a small board I could make with a few devices to detect the timing and frequency bands like the SSB circuits do.

In noisy environments, especially mobile in modern vehicles with all kinds of electronic noises that are sometimes louder than the weak reception, the carrier squelch included on transceivers (IC702, Alinco DX-70, every CB ever made, etc.) seems almost useless.

It's very tiring to have the squelch off and listen to noise the whole time.

Has anyone tried using the SSB squelch circuits or something operating on the same principles for AM?
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« Reply #1 on: January 03, 2020, 04:37:17 AM »

I haven't found a good squelch for those conditions either. I back just off the RF gain. As noise blankers go, I like the one in the TS-440S which I have used mobile. Receiving AM signals on HF has become quite a challenge and it ain't gonna get better. It's a crap storm of RFI out there.
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« Reply #2 on: January 04, 2020, 07:25:19 AM »

Hello Pat,
                   I also wanted a great squelch but there were none around
which would do what I needed. That was 40 years ago.
I wanted a squelch which would have no upper signal strength, like if a neighbor next door connected a kilowatt SSB transmitter to the other end of my coax, it would still not open.
Like if a mega-watt level lightning bolt struck the house next door, it would not open. But there were no designs that I could find. So, I invented one.
Using common LM-324 OP Amps, four on one chip, I used the one thing that only a desired AM signal had going for it that none of the other modes had, I used time.

SSB speech and static crashes general have a slight pause between words as compared to your friendly AM signal.
So use the AM carrier itself and use a fast timer so that your AM carrier keeps the thing open but the other signals that are SSB or static crashes
keep resetting the system and keeping the relay closed. Yes, I used a small signal relay to switch the existing audio between a small dumey load and the speaker.
So AM, in a couple seconds depending where you set it, would switch the relay and let you hear fine but the SSB would never open up.
Worked fine and with an adjustable pot, you could set the length of time to "block" back to less than a second if the bands was quiet.
That was in 1980.
The article was in The Press Exchange
http://amfone.net/AMPX/PXContents.html

008 08/80 07 THE WA1MNQ SQUELCH ANTI SSB-CW-STATIC part 1, Michael Valente Sr. ,WA1MNQ

009 09/80 10 THE WA1MNQ SQUELCH ANTI SSB-CW-STATIC part 2, Michael Valente Sr., WA1MNQ

I am getting ready to move and with over 120 boxes packed I can not locate the article copies but maybe you can find it.

Mike WA1MNQ

 
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CW is just a narrower version of AM


« Reply #3 on: January 04, 2020, 01:41:56 PM »

"Tight Squelch? You say Tight? You don't know Tight. And I made it even tighter by putting the smallest knob on the control I could find and turning it fully clockwise!" Scotch Meter Man

Just throwing in some AGC based circuits from an old 2008 post.

http://amfone.net/Amforum/index.php?topic=16740.0
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DMOD
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« Reply #4 on: January 04, 2020, 06:32:52 PM »

Here is another form of AM squelch using vacuum tubes:


Phil - AC0OB

* AM Squelch.pdf (107.96 KB - downloaded 141 times.)
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« Reply #5 on: January 04, 2020, 09:31:11 PM »

Thanks gents for the help so far, looks like a common problem.

I think I'd like to look at one that detects the effects and artifacts of speech and includes timing, which is what Mike is describing in the AMPX articles he wrote.

The reason for this extra need is that the mobile environment has greatly varying levels and types of background noise:
* City: leaky/corona arcing power lines.
* Highway: passing vehicles' electronic engine control systems making weird pulsating trash out to 30MHz+,
* Anywhere: my own vehicle's ignition noise which is greater or less depending on throttle position (cylinder pressure at firing), etc.. I don't want to fiddle with a computer-controlled vehicle's ignition that's for sure.
* Few or maybe just one place on a car/light truck to mount an antenna or two  - newer vehicles are very thin-skinned.
* Vehicles in general have a lot of ambient acoustic noise so adding to it by keeping the squelch open andf the volume high enough is very tiring. X2 for a van vs. car.

Such mobile-related noise, to my understanding and apparently as shown by the S meter, has a large effect on AGC voltage regardless of noise blankers and other noise blanking and noise limiting schemes, which would make it necessary to manually change the bias on a carrier operated squelching device while driving, which is what I want to minimize. On cheap radios it is done with the (carrier based) squelch control. So I am riding the squelch as it is now, or just setting it almost all the way up, which causes me to miss transmissions when the signal is getting weak.

This happens both on ham and CB radio, CB being the worst because of the cheap receiver, but also the CB is likely to be the guinea pig because t it has room inside and if I destroy it, only $45 is lost, compared against the Alinco DX-70 or IC702.

DMOD circuit is interesting but I have not understood it enough to know if the squelch floats with the AGC voltage or acts on audio.

Also, while I am not too offended by SSB while mobile, the idea of keeping squelch closed during carrierless signals is attractive. I am not put off by transistors or ICs at all in a mobile situation, we must all make sacrifices Smiley .
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« Reply #6 on: January 05, 2020, 03:03:15 AM »


...DMOD circuit is interesting but I have not understood it enough to know if the squelch floats with the AGC voltage or acts on audio...


Well, here is an updated and annotated schematic.

The squelch circuit threshold is set by the 500k pot. The more positive the cathode Vk the tighter the squelch.

White Noise has higher frequency components but is lower in amplitude so at a particular setting it can't break squelch.

Audio is higher in amplitude but lower in frequency content, so when it is present at the cathode, one sees more negative going content which allows the tube to conduct and audio current is seen at the plate and audio voltage is seen across R10.

This squelch system does not rely on AVC bias. AVC systems can be "falsed" with impulse noise so I never cared for using any AVC bias to modify the squelch system bias. AVC systems are usually too slow to do any controlling of squelch.



Phil - AC0OB
  

  

* AM Squelch.pdf (112.63 KB - downloaded 155 times.)
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« Reply #7 on: January 14, 2020, 04:39:52 PM »

If you look in my am rack, you can see a slim rack mount item, over the speaker.
This is a dsp voce extractor and a syllabic squelch. It works perfectly in am, the squelch can be operated as normal squelch activated by speech or can be set just to attenuate the channel noise of 30db when there is no speech. The squelch also activates a relay (normally open or Closed) , you can also activate the dsp noise filter that is really very effective in am, Removing chirp hiss and other noises from the speach (3 levels of filtering) It was used in governative agencies around the word, mine has ONU label.
I found one in a local hamfest but I have seen one or two of them on eBay . It’s a really good device.
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« Reply #8 on: January 21, 2020, 08:38:57 PM »

Many years ago, a Canadian company produced a device for marine radios
and  it was around $300 built, and around $50 as a kit.  
I can't find that one , but I'm looking


Don W4DNR
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« Reply #9 on: January 23, 2020, 12:11:56 AM »

Large things are OK for the shack, but my trouble is with the mobile radios. There is a CB I use for travel which has some room inside for a little board, and an HF rig, either the IC702 or Alinch DX-70 I would like to use.
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« Reply #10 on: January 27, 2020, 08:06:33 PM »

Back in the 70's and 80's when I was a marine radio tech I worked on Motorola Modar SSB rigs that had a voice only squelch that worked really well. The squelch was on a plug in PCB. That radio was based on the Motorola Micom commercial HF rig. Unfortunately I don't have a part number for that board but I'm sure the schematics for the radio are out there. 
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