Frequency modulation (analog).

[xles]

Long time viewer, first time poster.

First, a little background: I somewhat recently decided that I'd like to get into analogue circuit design, and also audio. I pondered for a while; what would let me learn this, and get me a nice toy out of it. Then it dawned on me, a modular synthesizer.
Now, this circuit was originally something I wanted for something completely different, but figured that I'd might as well weave it in there and make a module for it.

Coming from a purely digital background, working mostly with micros, analogue circuits are strange and mysterious to me. Most of them seem to me operated by means of love and Gummi Magic. But after searching the web for a while, reading up on modular synthesis design, I came across an annoyingly simple way for frequency modulation; Feed the source to be modulated to the input of a VCO (It still annoys me that I didn't see this sooner).

However, this didn't help me in my search for what I needed, because what I need is this: A way to modulate one wave onto another, that is to say, I need to be able to externally provide both the source and carrier waveforms. And since most (all I've seen anyway) VCO designs are self-oscillating, I'm once again back to square one.

So... Does anyone have any experience in analogue circuit design and know what the hell I'm on about?

Cheers.

Edit: By the looks of the front page, this is topic #1000, go me! ^^

[NiHaoMike]

The most common way is to use a PLL.

[xles]

The most common way is to use a PLL.

Oh?
Care to elaborate? Assuming of course that PLL is indeed what I think it is; A phase locked loop...

[NiHaoMike]

http://en.wikipedia.org/wiki/PLL

[DJPhil]

You're going to probably find an avalanche of info online, but I can suggest a good resource to begin.

The Computer Music Tutorial - Curtis Roads

It's a bit dated, but it's comparable to The Art of Electronics for electronic music. It's monstrously huge. If you don't want to buy it you can probably find it in a library (a university is your best bet). If you manage to eat the whole thing it'll take you from the point in history where electricity entered music all the way to digital synthesis, covering a great deal of analog on the way. I cannot recommend it highly enough.

I know that doesn't speak to your specific question, but I hope that helps.

[xles]

You're going to probably find an avalanche of info online, but I can suggest a good resource to begin.

The Computer Music Tutorial - Curtis Roads

It's a bit dated, but it's comparable to The Art of Electronics for electronic music. It's monstrously huge. If you don't want to buy it you can probably find it in a library (a university is your best bet). If you manage to eat the whole thing it'll take you from the point in history where electricity entered music all the way to digital synthesis, covering a great deal of analog on the way. I cannot recommend it highly enough.

I know that doesn't speak to your specific question, but I hope that helps.

That sounds like a cool book to check out, I checked the library database and the closest copy is at the 'state music library' down town (I love the internet, such an amazing resource). It should be noted though that I'm not in it for the music, I figure I'm too dumb for it. I tried some tracking back in the 90s on my Amiga 500+, but after a weeks worth of rhythm-less bloops I gave up.

As far as the original question goes, it's more of an annoyance than anything. The damned thing has been bugging me for months. At least now I've gotten far enough to have the vague concepts of what I want to do rather than an ambiguous thought starting with the words "what if...". Just need to find a way to do it practically.

[qno]

What kind of carrier frequency are you looking for?

In the old days i use to make an oscillator with an LC ciruit. Then placing a varicap diode in parallel with the C of the resonant circuit (decoupled by an other C) Bias the varicap with a few volts and put the modulation signal on it thru a capacitor and you have frequency modulation.

[xles]

What kind of carrier frequency are you looking for?

In the old days i use to make an oscillator with an LC ciruit. Then placing a varicap diode in parallel with the C of the resonant circuit (decoupled by an other C) Bias the varicap with a few volts and put the modulation signal on it thru a capacitor and you have frequency modulation.

That's just the thing, I'd like to build a circuit where I can provide the carrier as a source signal independently of the actual modulating circuit. Be it generated by another VCO, an audio source or a DC voltage.

[Zad]

Phase modulate the carrier? If the modulating is sufficient, a phase modulated carrier is not dissimilar to a frequency modulated one.

[TechGuy]

That's just the thing, I'd like to build a circuit where I can provide the carrier as a source signal independently of the actual modulating circuit. Be it generated by another VCO, an audio source or a DC voltage.

As long as your modulation frequency is evenly dividable, a PLL such as the 4046 should work. You adjust the VCO to your carrier frequency, and use a frequency divider to reduce the modulation frequency. The PLL keeps the two frequencies locked together.

Perhaps this appnote will be useful:
Using 60Hz Power Line Frequency as an Accurate Real Time Clock Timebase
http://www.intersilsemi.com/data/an/an1342.pdf

[xles]

... As long as your modulation frequency is evenly dividable ...

Well, as I said, I wont know what the modulation frequency might be, it might be an AM signal, or 0Hz (fixed voltage), and no one likes division by zero...
I'll take a look at that appnote though, looks interesting.

[DJPhil]

Perhaps this appnote will be useful:
Using 60Hz Power Line Frequency as an Accurate Real Time Clock Timebase
http://www.intersilsemi.com/data/an/an1342.pdf

I'd just been reading about this. I had a theory that the long term drift was kept to a minimum by the power company because the meters they bill households by are (used to be) mechanical and depend on this for accurate billing. I imagine there's regulation to keep them from drifting too far. I think I'll look into it more, my curiosity's demanding answers.

Besides, everyone's clock radios would be off and civilization as we know it would cease!

[scrat]

... As long as your modulation frequency is evenly dividable ...

Well, as I said, I wont know what the modulation frequency might be, it might be an AM signal, or 0Hz (fixed voltage), and no one likes division by zero...
I'll take a look at that appnote though, looks interesting.

Inside the 74HC4046 or 74HC9046 there is a VCO, which is usually needed for the PLL. You can either use it in a PLL in a narrow bandwidth regulation (AC coupling the modulating signal into the VCO as a disturbing signal in the regulation loop) or just open the loop and give a command to the VCO (which is quite linear). Both don't require particular ratios between the two frequency values.
The two approaches have two different behaviours: in the first the carrier is frequency stable (as long as you give it a stable reference e.g. from a crystal) but accepts only DC free modulator signals, while in the second carrier frequency depends on the tolerance of the IC, the VCO input voltage and external passive components values.
My advice is to take a look at the 74HC4046 datasheet (NXP), it contains a big amount of information, examples included.
Somewhere I have the schematic of a very simple FM modulator (it was indeed a transmitter) which used a 100 MHz crystal and a transistor, while the modulation was made by means of a varicap, which voltage caused the crystal frequency to be pulled).

[Zad]

A varicap on the oscillator is the standard way of doing FM modulation. AM modulation is even easier, as it can be done at pretty much any stage after the oscillator. DC scaling is no problem, analogue multiplier mixers do it all the time.

[Zero999]

You could just use the VCO section of the 74HC4046, if frequency stability isn't important.

It's also possible to make a VCO with the CD4007 - one of the transistors gets uses as a variable resistor and the others form the oscillator but I can't find the schematic.

You could also try using a varactor with the CD4060, if you need a low frequency VCO.

[qno]

If you want a separate carrier and a modulated frequency you neet a multiplier.

Say you want a 20 Mhz signal modulated you can build a 10 Mhz reference oscillator and a 10 MHz VCO.
Put both in a multiplier and filter out the 20 Mhz signal.