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Mic preamp and Tx on MAX260x

inklen

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Hi!

As some of you may know I'm new to electronics but I'm trying to learn and understand...

I've made couple of mic preamps and they all finally work. Now, I found multiple articles/videos on FM tramsmitter made with VCO (MAX260x). So I started to read about RF, FM modulation (and other types of modulation), LNA, PLL's, etc. Too much information, I do not understand all that yet (will be reading about it probably for the whole year).

I guess at this point I understand how FM transmitter/receiver works when it comes to VCO. So I want to try to build one. MAX 260x has this tune pin and people usually connect electret mic to it, so the sound is transmitted ("modulated"?) on top of the carrier frequency. The tune pin has its range of voltages and FM has this... what it's called... standard frequency deviation of 40 kHz!? Does this mean I can't feed the output from mic preamp to this tune pin? I think the signal swing will be too high for the tune.
And if I need to catch quieter sounds how would I do that? Compression?

Anyway, these are my current thoughts.
I have to say books on RF are NOT easy to follow.
 

barry

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Not sure what you mean by "standard frequency deviation" I don't see that in the data sheet for the MAX2606/9, but it's probably the VCO center frequency error which has nothing to do with the tuning range, which according to the data sheet is 14%/V.

If you think your signal swing is too high, then lower it!

The whole point of FM is that frequency deviation is proportional to amplitude. If you apply "quieter sounds" then your frequency will deviate less from the center point than for "louder sounds"; that's the way it's supposed to work. If you add compression then you're losing dynamic range.

I'd suggest you just get a chip and play with it. Actually, get several, because you'll probably damage one; at least I know I would damage one. Maxim has what they call an "eval kit" for this chip, but it looks a little pricey to me for what it actually is (just a crummy little board with the chip and a few passives.)
 

inklen

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What I don't really understand is this - suppose I generate a deviation of 3 MHz (not sure the terms are correct). So my loudest sounds create that huge modulation. Would a regular FM tuner produce such sound? I guess not.

The idea is to be able to "hear" both - quiet and loud sounds, which means wide dynamic range. In case of MAX260x it means I'll have a wide signal swing for tune pin...

ok, I will definitely play with it. The parts are on hand and the PCB is ready.
 

barry

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I just looked it up; now I know what you meant about standard frequency deviation. +/- 75Khz is the modulation range. So, your maximum signal level would correspond to 75KHz away from the center frequency. This is for the standard FM broadcast band.
 

inklen

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Yes, this is it. And that's why I ask basically - if I want to have a mic preamp then its should be limited so, that this standard freq deviation would not exceed 75 kHz. What if I need more amplification of the mic input?

Anyway, I should try it and see. But I don't understand this part even in theory. Do people use other kind of circuits in such cases (without VCO)?
 

vfone

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Looking to the datasheet tuning plot of the MAX2606 I see that has about 18MHz/V tuning gain.
To decrease the tuning gain of the VCO (MHz/V) you can use a resistive divider on tuning pin nr.3.
 

barry

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Maybe this is not the right part for doing fm broadcast, since it has such a wide band. You’d have to have very low voltage to get 75 KHz range.
 

inklen

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Yes, that's what I thought too. But now the question is - what is the right way of doing this?...
--- Updated ---

Looking to the datasheet tuning plot of the MAX2606 I see that has about 18MHz/V tuning gain.
To decrease the tuning gain of the VCO (MHz/V) you can use a resistive divider on tuning pin nr.3.
Or I guess I can just tune the output of the preamp, right?
 

barry

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Looking to the datasheet tuning plot of the MAX2606 I see that has about 18MHz/V tuning gain.
To decrease the tuning gain of the VCO (MHz/V) you can use a resistive divider on tuning pin nr.3.
Terrible idea. The maximum voltage for 75KHz excursion voltage would be 4mv. Accuracy and noise performance would be pretty awful.
 

inklen

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Yeah, I'm going to finish a book first as RF is a black magic to me at the moment.
--- Updated ---

I just found out there are chip antennas! Never heard of it.
 

vfone

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Chip antenna is no such amazing but as it is a "wine bottle antenna" :)

 

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