Soviet East German Radio

[Nowhere-Man]

It has been heavy going on the FM theory.. According to my books, a modulating signal alters the frequency of the carrier wave. On the negative signal amplitudes, the carrier frequency deviates less but more on the positives. I then got to a point where detection was discussed by converting the frequency spikes into amplitude detection through the detector valve. At that point I got a bit mentally wacked and closed the book till next time.

[pwdrive]

It's best to try and keep it to it's basics, with FM:

1) The amplitude of the carrier doesn't vary with the audio signal.
2) The amplitude of the audio signal causes the carrier's frequency to vary either side of it's center frequency (deviation)
3) The frequency of the audio signal causes a varying rate of change of the carrier's frequency.

In other words, at the receiving end, the amplitude of the audio is determined by the amount of deviation of the carrier from its center frequency and the frequency of the audio is determined by the rate of change of the carrier's frequency.

This old time video might be worth a watch:

https://www.youtube.com/watch?v=gfz1FbIOMbs

Hope this helps.

Lawrence.

[ppppenguin]

Lawrence is right to keep it simple. And at that basic level it really is as simple as he says. But if you go in deeper the maths for calculating the sidebands for FM gets quite nasty But you don't need any of that to get a basic understanding of how to send and receive FM.

[Terry]

On the negative signal amplitudes, the carrier frequency deviates less but more on the positives. 

If that is what your book says, it is very confusing because it suggests that the deviation is not equal for positive and negative modulating signals (assuming a sine wave for simplicity). This is quite simply not true, nor is there any rule dictating in which direction the carrier frequency moves for positive and negative modulating signals, which is an alternative meaning that may be derived from that statement.

I would suggest that you ignore that book completely because the only thing it is achieving is to completely confuse you.

As both Lawrence and Jeffrey have said, keep it simple.

The information contained in Lawrence's post is all you need to understand.

[pwdrive]

"On the negative signal amplitudes, the carrier frequency deviates less but more on the positives."

I think it's just the way NM wrote it and maybe not the book, I think by "less" and "more" he's meaning a lower frequency and a higher frequency, I reckon the toughest bit for him will be trying to figure out how exactly the FM ratio detector works in the receiver theory wise, most of use have learned that but I suspect that most of us forgot wot's wot the following day.

Lawrence.

[Mark Hennessy]

As I've said before, many text books are written by people who are more interested in showing the world how clever they are, rather than creating a tool for people to learn from.

It's so frustrating for us - we can see the "rabbit hole" that NM is in, but while he's determined to stick with these archaic texts, he's trapped. Despite everyone's efforts to help him see the light.

NM, you need to take a step back here. Learning how to learn is an important skill, and it appears that this is the problem here. A big part of that is learning how to decide what is worth concentrating on, and what can be put to one side. As a beginner, that's difficult. It can be difficult even if you're not a beginner (trust me). When I went to university, almost none of the staff there were willing to help me with that - I genuinely believe that was either because they didn't know how, or simply didn't want to. The thing is, I already knew more than some of them did. There's no quality more destructive in a teacher than insecurity or jealousy.

Here, you've got all these people trying to help you avoid all these problems. We have no ego or side. We just want you to help yourself. Please listen to us and give us some sort of indication that you've heard. Our patience is not limitless.

The Art of Electronics contains a good overview of FM modulation. Easy to read and understand, and easy on the maths. I have gone further in the past, and can categorically state that there's no need to unless you're doing specialist things. Fixing radios barely needs the basics, frankly. To put it another way, understanding the theory to a level beyond what Lawrence explained will not help you fix radios. In other words, it's a distraction. I'd go so far as to say "a waste of time". By all means come back to it later, but sort the basics first. Learn how an FM demodulator works.

[pwdrive]

To help NM put all this into context with his Saalburg 5170, in the text books it will show a tertiary winding as part of a ratio detector circuit and also an electrolytic capacitor connected across the output of the two diodes, it will say that the tertiary winding is tightly coupled to the last IF's primary winding and that the audio signal is derived from it, you can see these two components in the receivers schematic, L23 is the tertiary winding, it fits the bill as described in the books because it's tightly coupled to the transformers primary as indicated by the common tuning slug coupling it to the primary winding (L25) you can also see that the audio output on FM is taken from that via a simple filter network and ends up at switch contact 17.

The electrolytic capacitor is easy to see and it also fits the bill per the text books as it's connected across the output of the two diodes, the capacitor is C51 in the receivers schematic.

Given a decent signal the audio output at the switch contact 17 should be nice and clean, the voltage across C51 will be -ve DC with respect to chassis when a signal is received, the voltage typically can be between -ve 20 and -ve 30 volts in some receivers with a very strong signal and will be less as the signal strength decreases, that -ve voltage is used for the AGC and also as the control grid grid voltage for the magic eye tuning indicator.

So, from a troubleshooting point of view what we are expecting that's connected with the ratio detector is a good quality audio output and a functioning AGC voltage....just like an AM detector.

Lawrence.

[Refugee]

What NM has done is to dive into the complicated science and he has taken everything as equal on something like a single stage amplifier.
When a theoretical scientist shows all the calculations he or she will show everything including things what we can easily ignore.
Take the voltage between the base and emitter on a transistor and add the effect temperature has on it.
We usually see a resistive biasing arrangement with ordinary resistors with 5% tolerance and right away you have picked up an error that is greater than the change due to temperature. If you are not making a lab grade thermometer it can be ignored.
You have to go through the whole lot and cycle the maths adjusting one figure at a time to see how much the result varies and then it will reveal what we can ignore and what is important.
Many of the calculations were made before the transistors were sealed into cans and this means that some of the figures were based on the dimensions of the bit of silicon that has now been hidden never to be seen again.

[Crackle]

And how does the stereo signal get modulated on the carrier and how is that demodulated?

I find it difficult enough to comprehend how a stereo record works, let alone stereo FM reception received on a single frequency.  
But I dont let myself get too bogged down in these scientific principles, they are not essential for me to know, I only need to understand that "it happens" and which components are important in the process. My lack of academic knowledge in this respect doesn't stop me getting on with trying to repair a radio, although FM always does seem a little more complicated.
The thing to remember with FM and any VHF radio for that matter is dont move the RF components around too much. Replace components with like for like and keep them in the same relative position to each other.

It always helps if you have a service manual in English, like a "Trader" sheet, which goes some way to briefly explain the circuit and its working.

I feel NM could benefit from "learning" on some UK radios with decent service information, rather than radios which have next to no information in English so he has to resort to looking up "examples" in a text book and expecting the equipment he is comparing the example to, to be exactly the same. There are so many cheap readily available mains and transistor radios made in the UK which he can gain experience on.
When I go the the BVWS auctions they are practically giving them away for £5 sometimes.


Mike

[Mark Hennessy]

I feel NM could benefit from "learning" on some UK radios with decent service information, rather than radios which have next to no information in English so he has to resort to looking up "examples" in a text book and expecting the equipment he is comparing the example to, to be exactly the same.

I think that's a really good suggestion, Mike