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

[nina_p20]

@Nigel Goodwin,
After almost 30y I've worked at the Ministry of Communications in my country, I think (well...I've checked) the "legality" of the frequencies I'm asking. The law is very simple: Any device can work at those frequencies, if it's output is less than 25mW. About CB for example (AM) they limited it to 100 mW and 88-108 (as most "non pro bugs" are transmitting) - not permitted at all. We have - as much as I know for certain - at last about 20 FM pirate stations that transmit over this range 88-108MHz, and noone cares that their output is more than 100W, and their harmonies are doing real trouble to airports.
I want to build a really small device, less powerful than permitted. The devices like the one I'm trying to build are sold all over the world. I've bought more powerful devices from all Europe and US. Crystal and Non-crystal controlled devices. You can find them all over the net, sold as a stand alone or with Air-Band receivers. I just wanted to clarify once for all this point.
About the technical help: I really hope that someone that really knows the RF field, may help me with this project. I can spend money and buy a ready made device and just copy it, but I really want to understand what am I doing, why should I use a "x" crystal and not a "y" one. Which crystal is used for "z" overtone.
This was not my field at my work, I've worked on OTDRs, TDRs, electronic Meggers and so on, but I hope I could understand and build this device.
BTW, the operating the circuit with BH1417F at several mW is legal?... buiding a nice RF amp to it can really make a lot fun to neighbours :lol: :lol:
So, please, if you can help and have the good will, I'll be vey thankful

 

[nina_p20]

@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.

I think than even 200m is enough, but as Nigel Goodwin mentioned earlier it works on Narrow Band FM, and only at about 50MHz, I have to do some modifications, and here I need your and any other group's member's help please. I've learned oscillators about 30 -35 years ago, and haven't use the theory in practice from then. About VCOs and other devices it's ok, but about crystal controlled devices....zero.
TIA for your help

 

[Nigel Goodwin]

@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.

I think than even 200m is enough, but as Nigel Goodwin mentioned earlier it works on Narrow Band FM, and only at about 50MHz, I have to do some modifications, and here I need your and any other group's member's help please. I've learned oscillators about 30 -35 years ago, and haven't use the theory in practice from then. About VCOs and other devices it's ok, but about crystal controlled devices....zero.
TIA for your help

It's already being explained endlessly!, FM modulating a crystal depends on how far the crystal can be pulled, which is why you generally use a PLL circuit (as posted above) for crystal controlled FM.

You may be able to get the deviation you want by using a far lower frequency and many more multiplier stages?, but obviously everything gets larger and more complicated the more stages you add.

 

[Nigel Goodwin]

@Nigel Goodwin,
After almost 30y I've worked at the Ministry of Communications in my country, I think (well...I've checked) the "legality" of the frequencies I'm asking.

If you had your location filled in perhaps we might have some idea what country it was? - presumably somewhere in the 'third world' if such sloppy regulations are in force?.

A quick reverse IP check gives your location as Israel, and suggests Tel Aviv?, I wouldn't consider Israel as 'third world' (not by a LONG way!) nor expect them to have such sloppy regulations? - however, I've no personal knowledge of the situation there, although I do have a couple of friends there.

 

[nina_p20]

Revealed ... :lol: :lol: (about 120km from T.A.... )
I'm sorry I haven't edit my profile, gave no much importance....
The point is I'm stucked... I'm losing my hopes.... Almost getting up...
Still cannot understand how "spy-shops" or detective agencies have 2x3 cm modules, at the same frequencies...
As I can see, some of the members here are realy "PRO"s, and if they cannot help me, boy, I'm in a real trouble....

 

[Nigel Goodwin]

Still cannot understand how "spy-shops" or detective agencies have 2x3 cm modules, at the same frequencies...

To make them small they probably use all surface mount components, also the crystal controlled ones are probably NBFM, and NOT WBFM, which makes it simpler and smaller. There's no requirement for WBFM, and NBFM gives greater range with lower power - as long as you have a suitable receiver (the spy shops sell matching receivers).

However, the problem with a small transmitter is the size of the power supply for it?, not much point having a tiny transmitter if you need a huge battery for it!.

I'm presuming from your previous posts that you're not experienced in radio construction?, you should be aware of the difficulties constructing RF circuits, particularly at VHF frequencies (and above), and also the alignment required for such devices, do you have test equipment for aligning them?.

 

[nina_p20]

Thank you for responding,
I've built many RF circuits and yes, I'm aware about the problems.
I've used them a very long time ago, but, even being older, I may remember how to use them - I hope so...
I've never built any circuit with surface mount components, but with a good loupe I can handle it...
I have several pieces of measuring equipment, as a counter up to 5GHz, an Oscilloscope up to 500MHz (dual), Field strenght meters, several Flukes (the most newer I suppose), SWR meters, Scanners and several Weller stations, including a desoldering one... Not much, but I think i can handle some RF jobs...
So, your opinion is, as I've understood, is to give up
The second posibility is to try to build a NBFM Receiver....as those are very expensive and almost cannot be found here...
The circuit published here earlier is bigger than I can use, and it's stereo so it's a waste of place on the PCB... Maybe there is another smaller IC that may do the job...
I appreciate very much that you try to help me

 

[Nigel Goodwin]

Thank you for responding,
I've built many RF circuits and yes, I'm aware about the problems.
I've used them a very long time ago, but, even being older, I may remember how to use them - I hope so...
I've never built any circuit with surface mount components, but with a good loupe I can handle it...

As long as you don't use the very smallest components it's not bad, just use a good magnifying glass, a pair of tweezers, and a steady hand - I use a head mounted binocular magnifier, which sits on a band around your head and can be pulled down over your eyes. This maintains your stereo vision, and leaves both hands free - it helps a GREAT deal.

I have several pieces of measuring equipment, as a counter up to 5GHz, an Oscilloscope up to 500MHz (dual), Field strenght meters, several Flukes (the most newer I suppose), SWR meters, Scanners and several Weller stations, including a desoldering one... Not much, but I think i can handle some RF jobs...

OK, that looks fairly good, I would perhaps add a grid-dip meter (or more usual these days gate-dip meter) to those.

So, your opinion is, as I've understood, is to give up

No, just to be aware of the reasons for the lack of ready made projects out there.

The second posibility is to try to build a NBFM Receiver....as those are very expensive and almost cannot be found here...

Basically you need a dual-conversion superhet, usually they have the first IF at 10.7MHz (as usual for VHF FM) and the second at 455KHz (as usual for AM radios). As you're demodulating at a much lower frequency you get much higher audio levels out, and the 455KHz IF filters give you a nice narrow bandwidth.

There are specific IC's designed to do this exact job, they are often used in amateur radio 2M radio receivers.

In fact I've got a kit I built somewhere, for a 2M receiver, it was freely tuneable using a VCO, but it would be simple to make it crystal controlled. If I recall correctly?, it was a Ramsey kit.

A quick google found this **broken link removed** which unfortunately doesn't give any details?.

The same google also found http://www.pcs-electronics.com/en/guide.php?sub=schematics, which you may find helpful?.

I've also found the link for the Ramsey kit . It also (rather nicely) provides the manual for the kit at https://www.electro-tech-online.com/custompdfs/2005/08/FR146.pdf.

 

[nina_p20]

Thank you again Nigel Goodwin,
I think I have somwhere an old GDO, I think it's a HeathKit (as I remember)... :lol:
The option of ramseyelectronics looks good, not too expensive... but I'm trying to use the transmitter on 110-120MHz, and not sure it may be modificated. (I had a very bad experience trying once, a long time ago to change the freq. of a TDA7000 - never succeeded ) I'll try to order one tomorrow, maybe this will be the "cure" :lol:
I'm really disappointed there is no other, simpler sollution...
Thanx a lot again for the links, and if you'll suddenly get a good idea for stabilizing a nasty bug, I'll be very glad if you'll drop a line...

 

[bloki]

For NBFM transmitter you can probably use MC2833. It's SMD and contains MIC amplifier, FM oscillator with external crystal and two transistors which can be used as multipliers. Output power is about +10dBm.

 

[Nigel Goodwin]

Thank you again Nigel Goodwin,
I think I have somwhere an old GDO, I think it's a HeathKit (as I remember)... :lol:

Heathkit made some really great gear, it's a pity you can't still buy their kits?, but obviously lower prices on commercial equipment priced them out of the market :cry:

The option of ramseyelectronics looks good, not too expensive... but I'm trying to use the transmitter on 110-120MHz, and not sure it may be modificated. (I had a very bad experience trying once, a long time ago to change the freq. of a TDA7000 - never succeeded ) I'll try to order one tomorrow, maybe this will be the "cure" :lol:

If I recall correctly?, the Ramsey kit gives suggestions on modifying it?, it's only the front end and local oscillator tuning that require altering slightly - and as supplied it will most probably already cover the bottom end of that frequency range!.

 

[nina_p20]

@ bloki 8)
Thanx a lot for the information !!!
I couldn't even imagine there is an IC for this purpose !!!
I'm trying to find a supplier to get a sample... maybe this will be the answer ....
Thanx again !!!

 

[nina_p20]

@ Nigel Goodwin,
I've ordered Ramsey's VHF Mini-Receiver , and I think I'll get it in a month or so...
So, I'll have time to "play" with the ICs that you and bloky (MC2833) suggested, even I prefer to use discrete components...Transistors were enough for me..., but as I cannot find such a design, I have no choice...
I hopped it will be much simpler....but it's like a surgery, never know what will be the result...
Thank you very much

 

[bloki]

You can download manual for VHF Mini-Receiver FR-146 here:
https://www.electro-tech-online.com/custompdfs/2005/08/FR146-1.pdf
Manual does not contain schematics only block diagram and PCB population.
Receiver is originally designed for 136-175MHz band. It can be retuned for 100-120MHz. First local oscillator is simple varicap VCO with added AFC. Tuning to a transmitter can be a bit dificult because the frequency vernier is simple potentiometer and resolution is bad.