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 Post subject: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 9:07 am 
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Location: Albany, NY
I have built a few tube based AM regenerative receivers and I was thinking about building a converter to convert broadcast FM down to the AM broadcast band so that my regenerative receivers could listen to it. The oscillator on the converter would be adjustable and so the regenerative receiver would be set to a fixed frequency. I've slope detected NBFM signals before with other equipment but those signals are much more narrow than broadcast FM signals and so I'm wondering if it is possible to slope detect broadcast FM signals on a regenerative receiver.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 12:55 pm 
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I could be wrong but I think your receiver would need to have a bandwidth of 150 kHz to resolve reasonably undistorted audio using slope detection. Armstrong's "wideband FM" system refers to the widely modulated RF frequency swing although many people mistakenly think it relates to audio frequency response.

Maybe you could think about building a super-regenerative FM detector which can receive FM broadcast frequencies and directly convert them to audio which then can, in turn, feed a small, on-board, AM transmitter. I have a Howard AM/FM table radio that uses a single 12AT7 tube as its FM detector. 100 MHz FM RF in...reasonably undistorted audio out (with careful tuning to find the sweet spot).

Dave


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 4:29 pm 
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928GTS wrote:
I was thinking about building a converter to convert broadcast FM down to the AM broadcast band ...

You would be better off finding an FM-to-AM converter made for '70s car radios.

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 5:54 pm 
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You'd need a bandwidth of +/- 100kHz in order for being able to accommodate the FM modulation depth, which means that the average Q of the tuned circuit of your receiver at AM broadcast band would need to be around 10. You would not use regeneration at all, and on top of that you'd probably also need to degenerate your LC tank with a shunt resistor, which would degrade the sensitivity of your receiver even more. It wouldn't work. Either you would have no sensitivity, or your audio would be distorted to square wave.

I'd rather use a shortwave regen (preferably in the upper band, e.g. 27MHz) and downconvert the FM there.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 8:15 pm 
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There is a possibility that the regenerative receiver will lock on and track the FM signal and because of the now changing regen frequency the fixed tuned regen front end will result in slope detection.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sat 27, 2021 8:32 pm 
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There are even designed for crystal radios using slope detectors for FM.

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Sun 28, 2021 3:22 am 
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There are two occasions where I have successfully slope detected FM transmissions.

Pre-war TV sets had AM sound. I was surprised how well they would slope detect the FM sound signal and apparently this was happening in the post war period where people still had AM sound TV's, but the transmissions had gone to FM for TV (in the USA at least).

The other is, I have found super-regenerative receivers will do it fairly well. I have a National 1-10 A and with the correct coil set to tune over the local FM radio band, it works better than I imagined it would, but the tuning is much more critical than for an AM station.

http://worldphaco.com/uploads/NATIONAL_ ... Radio..pdf

Like all these sorts of things though, there is no substitute for a proper FM detector. I have come to prefer the Ratio Detector over the years because of its excellent amplitude rejection and freedom from the requirement of an amplitude limiter driving it.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 1:09 am 
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I very much appreciate all of the insight. I had a feeling that the bandwidth of a broadcast FM signal might be troublesome to slope detect. As a test I took my SDR and tuned it to a local broadcast FM station, switched to AM detection, set the bandwidth to 10khz and attempted to sweep across the station looking for any reasonable detection. I was greeted with great deals of distorted audio. I then widened the bandwidth to around 180khz to simulate around what the width of an average FM filter would be and now slope detection was much easier with the resolved audio being much cleaner.

So, it seems like a super regenerative might be the way to go. I've spent some time looking online and I have found that schematics, experiments and plans relating to tube super regens in general, much less for FM broadcast use, are few and far in between. You can search for tube regenerative receivers and you'll find schematics and write ups all day long but not so much tube super regenerative receivers. It seems like the super regen is more found in the realm of transistor based circuits.

One circuit (shown starting at 3:02 in the video) that did pique my interest was this one:

https://www.youtube.com/watch?v=B8W_IsCJ0C0

I say that because I often hear of people using tubes like the 6C4, 6ES8, 12AT7, 6DJ8, etc for FM super regen duty but generally at higher (100vdc+) voltages. And yet, here is a 12BH7 being used at 12vdc. Does this circuit look like it has merit? Do you see any things that jump out at you as being less than optimal?


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 1:59 am 
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The Fremodyne was a simple tube superregen. Here's one version:
https://www.cool386.com/fremodyne/fremo1.html


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 2:39 am 
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Online doesn't necessarily reflect the past.

Superregens were fading long before the internet, more so tube versions, so random schematics may not be posted.

But hobby electronic magazines had lots of superregens, some for ham use, others for "police band", but they weren't uncommon for the FM broadcast band.

The search function at that American Radio History site is awful (too many results) but Elementary Electronics in the sixties, Electronics Illustrated in that early sixties, and Popular Electronics till about 1968 had various projects on the topic.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 3:07 am 
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I have seen slope detection simple receivers for FM, usually some sort of direct detection device, no IF or superhet, for BC FM. I would assume for it to work well the tuned circuit would need to be rather high Q factor? I myself got a car FM converter, installed a jack for a 12VDC wall wart, and made it with a thin short coax output cable to the radio, with spade lugs for the ant. strip. Just ran a long piece of wire into the antenna jack on it, hard wired in. I think I got a 10 foot wire. It works OK on FM and when you turn off the conv., it bypasses the ant. directly to the radio, which performs well on AM with 8-12 feet of wire. I may have put some regulator or filtering in the converter for the wall wart output, I forget. The converter has an adjustment internally for a clear spot in the AM band. The sound quality is much superior to regular AM, sounds excellent on my Zenith. It is a Spark O Matic converter. An unfortunate name for electronics. Used to be dollar garage sale devices, now the classic car nuts have driven up prices to retail or more on ebay.

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 3:15 am 
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Check out an overview: https://www.electronics-notes.com/artic ... inator.php

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 3:17 am 
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But, on a hopeful note, see this: http://electronbunker.ca/eb/FMCrystalSet.html

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Mar Wed 31, 2021 1:34 pm 
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Here is the service information for my Howard 474 AM/FM table radio which uses the 1-tube Fremodyne technique for FM.

http://www.nostalgiaair.org/pagesbymode ... 009293.pdf

Dave


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Apr Thu 01, 2021 1:33 pm 
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mblack wrote:
Online doesn't necessarily reflect the past.

Superregens were fading long before the internet, more so tube versions, so random schematics may not be posted.



One thing is that super-regens never in fact went away, the use of them actually grew. Due to the simplicity of the design, they remain ubiquitous in home and car automation systems as the receivers for the remote controls.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Apr Thu 01, 2021 5:13 pm 
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I'm talking hobby circles. People find disembodied schematics "on the web", and generally they will be useful (except for those tunnel diode circuits). So my statement explains why they might not be found.


Superregens were there in the sixties in hobby magazines, but general searches don't seem to show the scanned magazines. So you have to find them more directly.

In 1971, the ARRL Handbook had a brief passage about them, not at all explaining them. And a 2M AM transceiver using one, from a few years earlier. Superregens were gone from the hobby magazines from that point on, except in collections like "101 Projects for under Ten Dollars" that accumulated older articles.


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Aug Sat 07, 2021 11:54 am 
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Hi 928GTS,

I've built a simple FM regenerative receiver that uses slope detection. It does work!

https://nandustips.blogspot.com/2019/03 ... radio.html

Regards,

Nandu.

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Last edited by vu2nan on Aug Sat 07, 2021 5:36 pm, edited 1 time in total.

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Aug Sat 07, 2021 4:37 pm 
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mblack wrote:
I'm talking hobby circles. People find disembodied schematics "on the web", and generally they will be useful (except for those tunnel diode circuits). So my statement explains why they might not be found....
Yes, I'll admit the search function is awkard in the World radio history site.

I have persisted with it and have found intersting articla.

If you PM Gregg (egg) he appears to be a whiz at navigating the WRH site and oten posts snips of articles within minutes of a need to know posting...

The Heath-kit Six-er, the two-er were FWIR super-regenerative receivers.

The 20's era "Autoplex" was also a super but you wouldn't get that up to 100mhz, be difficult to get it to 2mhz :)

Rca made a 5 meter "radiotelephone" that used a 19 dry battery operation using a modulated oscillator and a supergen receiver.

Frank Jones published a ham handbook 5 meters and up there is at least one supergen project in there.

Attachment:
5-meters and up.jpg
5-meters and up.jpg [ 102.99 KiB | Viewed 775 times ]


If tubes aren't you cup of tea, sign on to the Radioboard. There you will find many receiver experimenters mostly solid-state....

http://theradioboard.com/rb/search.php? ... d=newposts

chas

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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Aug Thu 12, 2021 12:18 pm 
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Location: Boston,Ma
Hey All.

I have tried the “big signal “
approach ie FM loop or TV rabbit ears, staring at one site 2 miles ,
also in strong area of another site.

No matter what you do, this type of AM slope is mostly mushy sounding. But at times if you messed around with things, sometimes you could clean some or a lot of the mush out.

So , the take away is that AM crystal sets on the FM broadcast band are only “OK”,
but not great . Quick and easy fun though .

Cavities etc probably a better choice, but not a beginner’s
project.

Maybe passive audio circuits could clean up more mush , ie any required specs for band pass, etc.

Also, some audio amps ( ie HF crystal set amp, etc) here are prone to sometimes strong FM broadcasts usually mixed into an inter modulated mess of 2 or 3 locals.It can be eliminated by coupling changes , etc.It varies, sometimes it’s around and sometimes not so much.

AM mix variations extreme here-after much rain or
snow melt mix mess at several points in X band.3 or 4 strong local AM from lower in the band all smash up on 1690,1710 and others.

When things dry it goes away.Like clock work.

K


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 Post subject: Re: Slope detection of broadcast FM?
PostPosted: Aug Thu 12, 2021 2:51 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1561
Chas wrote:

If you PM Gregg (egg) he appears to be a whiz at navigating the WRH site and often posts snips of articles within minutes of a need to know posting...



One thing that surprises me is the assumption that someone who is a whiz can find interesting construction projects on the net, the ones that are there of course. The reality is that many of the interesting articles and construction projects still remain in textbooks that have not been scanned into a hosting service yet.

I recognized this issue many years ago and decided to create my own library of technical books. While it was expensive, space occupying and time consuming, I have access to many construction projects and designs that are not on the net.....yet. Probably, they are not because people don't want to scan the vintage books in, like me, because it damages their bindings.

A particularly good UHF regenerative receiver that slope detects FM well, was a design that used one 6CW4 Nuvistor and two germanium transistors. I built my version of it. I used rugged pre coated die cast enclosures, a rubber lined battery compartment, for the 6V lantern battery and the 45V B battery. I dumped the usual self threading screws for the back and fitted 1/8" bsw Heli-coils for the threads and used 1/8" CS stainless steel bsw screws so the backs can be taken off thousands of times with no thread wear issues. I also used corrosion resistant Titanium screws and had engraved label plates made that are fixed with miniature Hex head brass screws. The idea was to make the super-regenerative transceiver units look good and be long lasting. They are interesting in that they achieve a 30 mile line of sight communication with just the 3 active components. (photos attached). Not shown in the photo are the two Telephone handsets that go with them.


Attachments:
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UHF2.jpg
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UHF3.jpg
UHF3.jpg [ 713.92 KiB | Viewed 674 times ]


Last edited by ACORNVALVE on Aug Fri 13, 2021 12:29 am, edited 1 time in total.
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