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Old 10th Jul 2020, 8:28 am   #1
crackle
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Default AGC on FM

I was wondering, do FM radios, broadcast and Ham, have any form of AGC on receive, to either control the deviation or gain of the RF and IF stages.

Mike
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Old 10th Jul 2020, 9:20 am   #2
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Default Re: AGC on FM

On FM you'd normally have IF amplifiers which are driven in to limiting, so as to prevent any AM on the signal being demodulated in to audio, the FM detector will be sensitive to AM as well as FM.

Deviation is set at the transmit end, so I'd opine that the short answer is no, if we're thinking about AGC as applied to other forms of demod.

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Old 10th Jul 2020, 9:55 am   #3
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Default Re: AGC on FM

In an FM receiver, it is normal to rely on the limiting in the IF amplifier to handle the bulk of the variation in input levels from the antenna, but many receivers apply AGC from a relative signal strength indicator (RSSI) detector to the RF amplifier stage in order to expand the range between noise floor and overload.

The deviation, as said, is set at the transmitter and passes straight through the receiver to be seen at the discriminator. As the frequency gets shifted to IF, the modulation index changes, but not the deviation.

Almost all AM receivers rely on AGC to take out signal strength variations and to try to give a standardised amount of audio from a given percentage modulation from the transmitter. The AGC is derived usually from the AM detector and is applied to RF and IF amplifiers. A few specialist sets employ an amplifier in the AGC line, most sets use a -ve voltage created in the detector to adjust the bias and hence gain of the amplifiers.

In communications receivers, it used to be that the AGC didn't work well with morse and SSB transmissions, so it was common to switch off the AGC and to hand-control the RF/IF gain. This was as much an issue of detectors detecting the BFO in overly simple detector/demodulator schemes. Manual RF gain is still helpful when listening to Morse, though.

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Old 10th Jul 2020, 10:10 am   #4
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Default Re: AGC on FM

AGC was common on FM broadcast receivers and tuners, both valve and solid state.

One reason for it was to prevent front end overload under strong signal conditions. Similarly, it was also used to prevent overload in early IF stages ahead of the main selectivity. The idea was to ensure that the limiting took place after a reasonable amount of IF selectivity. If it happened too early, then selectivity would be impaired. Another reason was to provide for reasonably constant AF output on receivers with ratio detectors that did not have separate limiting stages. Yet another approach found at least in a small number of valve circuits was to use very fast AGC, that is unfiltered, in order to help with limiting. Effectively the IF stage in receipt of the fast AGC is modulated with inverse AM. It was used for example on the Quad FM tuner, from the second iteration of the Series B version.

The industry standard multifunctional FM IC of the 1970s, the RCA CA3089 and all of its many derivatives, included a section that provided RF AGC bias, with programmable delay in later versions, configured for use with dual-gate mosfet RF amplifiers. The same AGC was also sometimes used for a dual-gate mosfet IF pre-stage, e.g. Meridian 104.


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Old 10th Jul 2020, 10:34 am   #5
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Default Re: AGC on FM

I thought there had to be some sort of gain control to stop near signals overloading the RF stages.

Thanks
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Old 10th Jul 2020, 11:48 am   #6
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Default Re: AGC on FM

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Originally Posted by crackle View Post
I thought there had to be some sort of gain control to stop near signals overloading the RF stages.
But to do that properly, it needs to be controlled by a broadband enough detector to be able to sense the total power of all the signals tenderising those early stages. Most radios used a detector after the main selectivity so the front end gain tracked the needs of only the wanted signal.

Fine if the wanted signal was one of the big ones. Not effective for looking at a small signal with big ones around.

I'd give it 'close, but no cigar' as a piece of design

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Old 10th Jul 2020, 4:50 pm   #7
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Default Re: AGC on FM

Specifically with regard to CB radios which I know crackle has some interest in, those are often former AM-only designs with an FM demodulator like an AN240 added on to the end of what is still basically an AM receiver with the usual AGC feedback from the end of the IF stage back to the front end.

The question was actually about Ham and broadcast receivers - some Ham transceivers may also have had FM added on (for 10m) as a relative afterthought to what was essentially an AM / SSB design originally.
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Old 10th Jul 2020, 6:41 pm   #8
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Default Re: AGC on FM

Once you have enough AGC done to protect the front end stuff (common to both modes) you'd thing the FM side could look after itself with limiting.

But the issue I raised above of having a front end protected by a detector living after a narrower filter than the full bandwidth reaching that front end. It still has a very wide area of vulnerability to overload. True whether the AM detector is used on the linear path, or the RSSI output of a classic FM IF chip.

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Old 10th Jul 2020, 6:57 pm   #9
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Default Re: AGC on FM

AGC on FM receivers - yes, as noted upstream it has a place to prevent overload in the RF stages.

[imagine 50 US police squad-cars sitting in a parking-lot directly below their 'home' 2.5Kw transmitter - which was delivering several volts of RF to the receiver front-ends. You need some serious signal-level-management! I learned quite a bit from my year working in Texas.]

Plenty of FM receivers split the IF and sent one stream through hard clippers before detection, the other stream being sent through linear stages to drive a S-meter/Squelch circuit.
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Old 10th Jul 2020, 11:23 pm   #10
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Default Re: AGC on FM

Amusingly, some AM receivers used to have an added FM IF strip just so they could detect noise reduction on the FM 'audio' as an indication to open the squelch!

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Old 10th Jul 2020, 11:56 pm   #11
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Default Re: AGC on FM

Quote:
Originally Posted by Radio Wrangler View Post
But the issue I raised above of having a front end protected by a detector living after a narrower filter than the full bandwidth reaching that front end. It still has a very wide area of vulnerability to overload. True whether the AM detector is used on the linear path, or the RSSI output of a classic FM IF chip.
Wideband RF AGC was used in some FM broadcast receiving equipment from about the later 1970s. Some of the higher performance Ambit FM front ends, e.g. the EF5804, included a wideband RF AGC loop.

The National LM1865 FM IF sub-system IC of c.1984 effectively added an IF preamp to the CA3089 concept, with the block selectivity placed after the preamp. It generated both narrowband and wideband AGC, with adjustable thresholds to determine how much of each went to the RF stage. The data sheet devoted about 5 (out of 18) pages to operation and setting of the AGC system, including how to default to a conventional LM3089-type setup.

The Plessey SL1431/2 ICs of c.1980 were IF preamplifiers that also included wideband AGC generators for RF AGC. Whilst aimed at TV receiver applications, in which they both provided the desirable preamplification ahead of SAW filters and a different (better?) approach to RF AGC generation, I imagine that they were also suitable for FM broadcast and communications applications.


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Old 11th Jul 2020, 12:03 am   #12
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Default Re: AGC on FM

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Originally Posted by Radio Wrangler View Post
Once you have enough AGC done to protect the front end stuff (common to both modes) you'd thing the FM side could look after itself with limiting.
With block IF selectivity ahead of the main IF gain, no problem I’d think, and in the IC era it was done this way. E.g. the CA3089 et al had no AGC on its IF amplifier. But with distributed selectivity, as was the norm in the valve and early transistor eras, I think that some care was needed as limiting early in the IF chain resulted in reduced selectivity. RCA published something on this as part of its early FM IF IC literature.

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Old 11th Jul 2020, 1:54 am   #13
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Default Re: AGC on FM

The block selectivity makes no difference to strong signals in different channels. The selectivity passes only the channel that the set is tuned to and the AGC shifts to a voltage appropriate to the wanted channel. But if there is a very strong signal a few channels away it can still get passed by the RF selectivity and hit the RF amplifier and then mixer.

The AGC system only knows about the level inthe wanted channel, because it can only see the world through the IF filter. RF overload is something it cannot know about except in the case where the overloading signal is within the tuned channel.

Very high performance receivers need detectors throughout their structure so that gains/attenuation can be locally controlled to prevent local overload/intermod/blocking/crossmod problems.

You could visualise a group of many channels hitting the input port of a receiver. Several channels make it through the first stages of RF selectivity and get to hit the RF amplifier. The rest are attenuated and relatively smaller. The ones that make it get amplified. The next stage of RF selectivity winnows the group down a bit more, but still several channels will hit the mixer and pass through it to meet the block channel filter in the IF. From here only one channel should pass onwards. This gets demodulated to be listened to and detected to steer simple AGC.

The problem lies in the mixer being confronted with many more signals than the IF/detector ever sees. The receiver lacks pain sensors so it doesn't even know that its mixer or RF amp is hurting.

For a straight forward signal with not many others in its vicinity, the simple system works fine, but it comes apart when things get congested.

I'm a radio amateur, so to me the weaker signals are more probably from somewhere further and more exotic, so I naturally want to be able to receive weak signals close to strong ones.

I've spent the past fourteen years designing aviation comm radios and navigation radios. The aviation VHF bands are dead adjacent to FM broadcasts. Some humourous clown also put the instrument landing system channels right down at the bottom of the band closest to the broadcasters. A second humourous clown stuck big broadcast masts on hills close to approach routes for many airports. The radios have to be able to receive a -93dBm wanted signal while 200kHz away someone on Classic Gold Smooth Disco Beats is advertising incontinence products and sofas at +10dBm and a pilot is hoping the localiser will lead him through the fog down to the runway.

It all depends on what sort of environment you want or need to handle before your radio falls over.

Overload of an FM broadcast receiver isn't life threatening, just annoying.

In an environment like New York with lots of competing stations close by on skyscraper tops, getting the channel you want and only the channel you want demands something out of the ordinary in RF engineering.

I expect much of NZ is like the more remote parts of the UK. Relatively few channels in use and the prime neeed is for sensitivity and low noise.

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Old 11th Jul 2020, 1:22 pm   #14
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Default Re: AGC on FM

Very descriptive post David, clearly written by someone who's been there, done it, and had his share of front-end hurting while the rest of the world only sees his filtered, wanted, circuitry!

Last edited by kalee20; 11th Jul 2020 at 1:23 pm. Reason: Clarity
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Old 28th Aug 2020, 8:14 am   #15
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Default Re: AGC on FM

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Originally Posted by Radio Wrangler View Post
I expect much of NZ is like the more remote parts of the UK. Relatively few channels in use and the prime neeed is for sensitivity and low noise.
It varies somewhat. In the “big” cities, the FM band is reasonably well populated. I haven’t done a count locally, but we have an interesting situation in that just above RNZ Concert, which comes from the regional transmitter group at Mt. Te Aroha, some distance away, is a nondescript local station of the very highly compressed type, which comes from the Papamoa Hills transmitter, behind me just over my right shoulder as it were. The Beolit 707 portable requires very careful tuning before switching in the AFC in order to avoid audible interference. The NAD 412 doesn’t like it, either, with low-level birdies on Concert.

Returning to the main theme, in my earlier post I hadn’t intended to suggest that IF block selectivity would have any effect on the front end. Rather – and I didn’t do this clearly enough -my theme was that assuming a satisfactory front end, block selectivity would allow limiting to start at the immediately following stage, whereas with distributed selectivity – which was the norm in the valve era – it might be better to have most of the limiting later in the IF strip, to avoid compromising overall IF selectivity. Possibly that could require AGC on the early IF stage to prevent limiting. The limiting effect on selectivity was shown in this RCA item:

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Quote:
Originally Posted by Radio Wrangler View Post
Very high performance receivers need detectors throughout their structure so that gains/attenuation can be locally controlled to prevent local overload/intermod/blocking/crossmod problems.
It would seem that what might be called distributed AGC generation was quite rare in the valve era, I’d guess because of the difficulties of getting enough gain to develop adequate bias from the relatively small signals available from the front end or early in the IF strip. But that changed in the solid-state era, even for consumer-level equipment. The Revox A76 was an early “supertuner”, and it had an RF AGC loop that derived its input from just after the mixer, before the block selectivity. The B&O Beomaster 5000 had RF AGC derived from the 2nd (of 4) IF stages; it had distributed selectivity.

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Later, the Ambit EF5804 FM front end had two forms of RF AGC. First was a PIN-diode attenuator at the input, controlled by a wideband AGC loop fed from the mixer output, and second was a narrowband, from the IF processing IC, acting on gate 2 of both RF stage mosfets.

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Later still, the National LM1865 FM IF processing IC, mentioned above, provided another way of combining wide and narrow band RG AGCs.

Amongst simpler and fairly early solid-state tuners, the Leak Stereofetic had RF AGC derived from just after the IF pre-stage, and ahead of the 1st IF filter. The Sugden R21/R51 had RF AGC derived from after the 2nd (of 4) IF stages, which had distributed LC selectivity. The Rogers Ravensbourne and Ravensbrook, and the Quad FM3 had no AGC at all.

Looking back at the valve era, a casual look across a FM tuner few circuits on hand shows quite a diversity, even from a given maker.

The Pye HFT11 (same as the FenMan II RF section) had AGC on the RF, 1st IF and 2nd IF stages, derived from the limiter grid. On the other hand the following Mozart HFT108 had no AGC at all, neither did the HFT113.

The original Leak Troughline had RF AGC derived from the 2nd limiter grid. The Troughline II and 3 had RF AGC derived from the 1st limiter grid.

The Quad FM originally had RF AGC derived from the limiter grid, but was later changed to having “fast” AGC on the 1st IF stage also derived from the limiter grid, but unfiltered.

On its very early FM81 tuner, Chapman had AGC bias applied to the 2nd (final) IF stage suppressor grid, this bias being derived from the ratio detector. But for the FM81 Mk 2, it adopted RF AGC fed from the limiter (2nd IF stage) grid. Later than that it appeared to drop FM AGC altogether.

Lowther’s first FM tuner had AGC on the RF and mixer stages, derived from the limiter grid. But this unit, from 1953, could also receive VHF-AM, so that may have determined the AGC layout. Later FM only models did have AGC, but how it was arranged is unknown.

Jason varied by model, and the Jasonkit and Jason models with the same designation did not always have the same circuitry. Inter alia one could find examples of RF AGC only fed from the 1st or 2nd limiter grid and 1st IF stage AGC only fed the same way.

The RCA New Orthophonic FM tuner had RF AGC fed from the 1st limiter grid.

Quote:
Originally Posted by crackle View Post
I was wondering, do FM radios, broadcast and Ham, have any form of AGC on receive, to either control the deviation or gain of the RF and IF stages.
For the FM broadcast receiver case, I think the answer is that AGC, to the RF and/or IF stages, to control gain, was quite often a feature. However, as there was not the same very strong need as in the AM case, designers had considerable freedom as to whether or not they included AGC, and if so, whereto it was applied and wherefrom it was derived.

I am not sure about the deviation aspect. AFC was often used to keep the received signal in the centre of the IF channel, and here one also found variations, such as mild AFC to keep the receiver on-tune after initial manual tuning, strong AFC to provide a form of automatic fine tuning, and variable AFC whose effect could be adjusted to the conditions. Keeping the signal in the centre of the IF channel also kept it on the most linear part of the demodulator slope, and also facilitated symmetrical limiting. Limiting an asymmetrical signal produces spurious phase modulation, as I understand it.

On deviation control itself, I think that there were some efforts made to reduce the received deviation, say from ±75 to ±15 kHz, by applying very strong AFC feedback, so allowing the use of a much narrower band IF bandwidth. (Philips may have done some work on this?) But I don’t think that it came to anything as far as FM broadcast reception was concerned.


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Old 28th Aug 2020, 8:19 am   #16
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Default Re: AGC on FM

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Originally Posted by Radio Wrangler View Post
Amusingly, some AM receivers used to have an added FM IF strip just so they could detect noise reduction on the FM 'audio' as an indication to open the squelch!

David
Codan 7004 professional HF SSB coms receivers used a similar technique.
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Old 28th Aug 2020, 12:10 pm   #17
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Default Re: AGC on FM

There arises the question of just what is a high performance FM tuner/radio?

One could be engineered for very low noise in the RF sections, along with plenty of gain to do the best job possible on low level signals from a long way away. Ones which do this have good reputations amongst certain users.

Another could be engineered with a front end which stays linear with high levels of input signals and avoids intermod/crossmod etc in an area congested with many strong signals.

Inevitably, you can't get the best of both these worlds at once.

You might need both at once if your musical/news interests drive you to want a distant station while you live in a city positively throbbing with many broadcasters all trying to outdo each other, all optimodded to sit at peak deviation full-time.

So there is an art in compromising between the extreme designs aimed at the two situations.

There is a website 'Tuner Information Center' which can be quite interesting. There are information and opinions from a wide variety of people. Some go on about the sound and employ the full gamut of audiophile adjectives and parameters, others are more objective and involve careful testing. There's plenty of stuff and you have to take your pick. It's a bit fixated on 'Shootouts' which as you might not agree with the criteria which one won is judged on might not be for you, also the criteria are not reliably the same on several different shootouts, so them being used to put things in pecking orders is a bit daft, but people love to put things in orders (How many websites on the '100 greatest kazoo riffs' etc?) The snag is that there are many different pecking orders, one for each different parameter, so the problem swings round like a weather cock to how do you compromise between the different pecking orders.

Anyway, the website makes very interesting reading, but you'll have to think about what you read.

Another tool for the man in an area with a lot of stations available is the use of a beam antenna mounted on a rotator.

The antenna gives you more signal without the noise figure disadvantage of a preamp. Higher gain beams tend to have narrower beamwidths and so you not only gain more signal from your wanted direction, you also get better suppression of the others. This really is a win-win situation. If one site radiates many signals, the levels will be roughly similar, so you only get into the need for high dynamic range thing when you get interested in a more distant station, hopefully on a different bearing.

The UK had it's hyper-regulated BBC only FM band where all the stations came off the same mast, so we never got bitten by the dynamic range problem. People in poor signal areas were in poor signal areas for all channels at once. They needed sensitive, low noise receivers and there was nothing except for passing PMR transceivers to hit them. So the UK viewpoint came from the direction of sensitivity, not overload.

The US was a lot more of a free for all. Beams and rotators were used there.

No one size fits all....

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Old 3rd Sep 2020, 9:25 pm   #18
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Default Re: AGC on FM

Multimode receivers really do need switchable time constants for CW/SSB and AM/FM.
I once owned a FT-690R2 which had a compromise single agc setting. You would tune
in a weak FM signal with a reduced noise level, then a second later the S meter would
drop and the noise level increased. A switch (or even AGC off) would have been better.
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