FM crystal controlled micro-transmitter (RF Guru...)

[nina_p20]

Hello all,
I have to build a simple crystal controlled micro audio transmitter.
The range has to be arround 200 -300m and frequency about 110 -120MHz. I've tried to find such a schematic for almost a week - without results. All I found were non crystal transmitters, nothing new :cry:
I have a kit of a PLL device, but it's bigger than I need. I even tried to pay some so called "Pro", but the designs were too complicated....
I know that it can be done with a varactor + crystal, but the "overtone" is not clear to me, so , maybe there is a "RF " guru that may help me.
TIA,

 

[JimB]

At the risk of being a bit of a killjoy, what you are about to do is rather antisocial, not to mention illegal.
The frequency of 110 to 120 Mhz you mention is at the bottom end of the the aircraft band, lots of navigation beacons and such like. Who asked you to build a transmitter on such a frequency?

If you want a crystal controlled transmitter (FM) for VHF with a reasonable amount of deviation, you will either have to use a phase locked loop, or a low frequency crystal modulated using a varicap (varactor) diode, and several stages of frequency multiplication to get up to the desired output frequency.

JimB

edit note = "frequency multiplication" was incorrectly written as "frequency modulation"

 

[Roff]

At the risk of being a bit of a killjoy, what you are about to do is rather antisocial, not to mention illegal.
The frequency of 110 to 120 Mhz you mention is at the bottom end of the the aircraft band, lots of navigation beacons and such like. Who asked you to build a transmitter on such a frequency?

If you want a crystal controlled transmitter (FM) for VHF with a reasonable amount of deviation, you will either have to use a phase locked loop, or a low frequency crystal modulated using a varicap (varactor) diode, and several stages of frequency modulation to get up to the desired output frequency.

JimB

Jim, I thought that percent pullability was basically independent of crystal frequency, but **broken link removed** seems to imply that low frequency crystals can be pulled more (percentage-wise) than high frequency crystals. Do you know if this is true, or is Harry all wet in this case? If he is, it will be a first. It does seem to be consistent with your comments, assuming you meant to say multiplication instead of modulation.
In any case, he has a relatively simple crystal-controlled transmitter on this page. Nina_p20 has probably already seen it.

 

[audioguru]

I thought that percent pullability was basically independent of crystal frequency, but **broken link removed** seems to imply that low frequency crystals can be pulled more (percentage-wise) than high frequency crystals.

Harry says that a crystal oscillator can be pulled only a couple of kHz, so a low frequency crystal oscillator must be multiplied by about 72 times in order to have wideband modulation on the FM broadcast band.
Since it is impractical to use a low frequency crystal oscillator requiring many multiplication and filtering stages, his PLL FM modulation occurs to the error signal of the PLL not to its reference crystal oscillator. :lol:

 

[nina_p20]

At the risk of being a bit of a killjoy, what you are about to do is rather antisocial, not to mention illegal.

:lol: :lol:
Nope, you are wrong.
Most of the devices (non-crystal) sold all over as kits (and some of the schematics that appear at this forum...) deals with variable oscillators on this range, or worse, 88 - 108MHz, but this is not the point...
I've seen Harry Lythall's schematic, and read the theory some time ago, but, I need a much simpler (and smaller) device. I've buit such an oscillator a long(!!) time ago and it worked very well, just cannot remeber how I did it, and which crystal I've used...
Even buiding Harry Lythall's on a double sidded PCB with SMDs, it's still too large, so maybe....there is another possibility...
I'll be very thankful for your help :wink:
TIA

 

[bloki]

Crystal is used to acheive stable transmitting frequency.
It is possible to FM modulate crystal oscillator, but the deviation will be about 2kHz max. Greater deviation can be achieved by FM a LC oscillator and using crystal as reference for PLL. The last method to achieve suitable frequency deviation is to use phase modulation on crystal oscillator signal at 36 or more times lower frequency than transmitting one and use proper frequency multiplier as said before.
Another possibility is to use AM.

 

[nina_p20]

Thanx bloki,

It is possible to FM modulate crystal oscillator, but the deviation will be about 2kHz max. Greater deviation can be achieved by FM a LC oscillator and using crystal as reference for PLL

If so, I need a very simple (or better said) small PLL circuit...
From my Telephony learning (some 30y ago...) I need no more than +/- 3kHz deviation... but maybe I'm wrong.
Of course I may use higher frequencies, let's say 430M, but...:
A - I really don't know how to build such a device.
B - The worst (!) thing I need to buy a receiver for it, and they are very expensive. To build an UHF receiver is... "end of the road",
even all my life I worked in the electronics field... It's much easier to take a simple domestic FM receiver and do some changes...
If you have any apropiate links I'll be very thankful

 

[bloki]

For 110-120MHz range only commercial receivers are AM receivers I think.
To use OLD FM receiver it needs retuning and additional AF gain for +/-3kHz deviation. This receivers has sometimes AFC capabillity and transmitter maybe would not need crystal stability.

 

[audioguru]

Rohm make some FM transmitter ICs. Their latest one uses a crystal oscillator with PLL. It is fairly small with 22 pins and is surface mount. Many of the MP3 to car radio transmitters on the market use it.
It is in a project by Silicon Chip magazine. Go to Google and enter Stereo Micromitter in the search box.

The project includes all the stuff for it to transmit in stereo, and with pre-emphasis (treble boost) that perhaps you don't need. It also includes an RF attenuator to reduce its range that you will probably leave out.
RF frequency selection in the 88MHz to 108MHz FM broadcast band is with up to 8 DIP-switches, 4 frequencies near the bottom of the band and 4 more near the top.

I saw a small MP3 to car radio transmitter made by Maxell (cassette tapes and CDRs) with only 4 frequency selections, its size was just big enough for it to hold 2 AAA cells and it was selling for about 20 bucks. :lol:

 

[nina_p20]

As I said, the main problem is the transmitter. Any device built so far is instabile, and cannot be used as a "body transnitter". Even if you go near them they deviate
So, maybe I'll get a "magic circuit"....

 

[JimB]

At the risk of being a bit of a killjoy, what you are about to do is rather antisocial, not to mention illegal.

:lol: :lol:
Nope, you are wrong.

OK have it your way.

You may be in the USA where the FCC part 15 (?) regulations allow all sorts of things to radiate at low power, but you did not fill in your location so it is impossible to tell.

JimB

 

[Nigel Goodwin]

As I said, the main problem is the transmitter. Any device built so far is instabile, and cannot be used as a "body transnitter". Even if you go near them they deviate
So, maybe I'll get a "magic circuit"....

What's wrong with the circuit Audioguru posted above?, it uses a crystal controlled PLL and should be nice and stable!.

Also, it's advisable to build it (or any other transmitter) in a fully screened metal box, and mount the aerial on an external socket.

 

[nina_p20]

@audioguru,
I have an almost similar circuit, using MC145168 IC.
About the same Xtal.... it's too big, I need a much smaller one... I know it's possible as I've seen it working in UK several years ago, but haven't bought it (stupidity)... Several...I think 5 transistors, no IC, worked like a charm... Body transmitter, about 1.5 x 4 cm....
How can they do it is not clear... I don't remember the name of the firm, but it has ready made devices only, but I got a privilege as they show me their lab...
Thanx a lot...

 

[nina_p20]

@Nigel Goodwin
well, nothing wrong with the circuit.... but is BIG ... :lol:
I've done a search at the forum, have seen it before... but here the size counts !!! :lol:

 

[JimB]

Jim, I thought that percent pullability was basically independent of crystal frequency, but **broken link removed** seems to imply that low frequency crystals can be pulled more (percentage-wise) than high frequency crystals. Do you know if this is true, or is Harry all wet in this case? If he is, it will be a first. It does seem to be consistent with your comments, assuming you meant to say multiplication instead of modulation.

Ron

Yes, I did mean multiplication, not modulation and I have edited my original post.

In terms of "pulling" crystals, as a very poor general rule, it is only possible to pull a crystal 1 or 2 khz, there are lots of exceptions to this though.
So as a percentage of the crystal frequency, yes a lower frequency crystal will move further than a higher frequency crystal.

In a VHF/UHF crystal controlled transmitter, it is usual to have a multiplication ratio of at least 12 to give a reasonable amount of deviation at the final frequency.
However if you are building a broadcast type transmitter with 75khz deviation, it is usual as audioguru infers, to have a much higher multiplication ratio.

JimB

 

[audioguru]

When I worked for the audio equipment manufacturer TOA, their original 72MHz then later UHF performers' mic/transmitters used tiny crystals.
Digikey and Newarkinone have "millions" of crystals in all shapes and sizes. :lol:

 

[Nigel Goodwin]

@Nigel Goodwin
well, nothing wrong with the circuit.... but is BIG ... :lol:
I've done a search at the forum, have seen it before... but here the size counts !!! :lol:

It's pretty small for what it does!, and the chip is only 22 pin and surface mount, I think your requirements are probably unachievable if you're wanting to get it any smaller?.

 

[nina_p20]

How about:
https://hem.passagen.se/communication/buggtx.html
Can it be converted to work at 110-120MHz?

 

[bloki]

Circuit looks OK and can be modified for frequency you want. Problem can be to find appropriate crystal. Output power is only 10mW and at 120MHz will be lower. I think it is not enough for 300m distance.

 

[Nigel Goodwin]

How about:
https://hem.passagen.se/communication/buggtx.html
Can it be converted to work at 110-120MHz?

Looks fine, but you are aware that it's only NBFM (Narrow Band FM), the other design was WBFM (Wide Band FM).

As the article says, it's output power is dependent on the emitter resistor of the final stage - but NBFM goes a LOT further than WBFM - with a suitable receiver over 300m should be easy to obtain.

BTW, I also agree with JimB, the frequencies you mention are certainly illegal in most parts of the world!.