Audioguru's FM Transmitters

[audioguru]

Hi SolidHelix,
I can't post in instant messaging so here are schematics of my transmitter. Mod4 works much better than Mod3.

 

[mstechca]

thats alot of components for an FM transmitter, dont you think?

 

[Nigel Goodwin]

thats alot of components for an FM transmitter, dont you think?

No!.

The left part is a fairly standard 'bug' type design (with a few improvements), the rest consists of a voltage regulator to feed it (to help keep it more stable then most), and an extra output stage to give much higher output (and more range), it will also help make it more stable, as the aerial is no longer connected directly to the oscillator.

 

[TV-Engineer]

I was thinking that it's few components, compared to the Ramsey FM-10A I have sitting on my desk (faulty) right now.

I want to replace the Ramsey one with something that has less distortion and higher range, but can't find anything that uses a crystal to lock the frequency and has stereo inputs. Only ones I can find use 200 components including PIC chips for PLL frequency control. :cry:

I just wanna beam stereo around the house with low distortion and a good 40 to 50 meter range or so.

 

[audioguru]

Hi TV-Engineer,
Enter Micromitter in a Google search.
The micromitter is crystal controlled with a phase-lock-loop for FM channel selection. It is stereo with audio pre-emphasis and works pretty well. Its single IC does most of it so it is much simpler than a Ramsey transmitter.

 

[audioguru]

Hi MS,
Try my FM transmitter. Another Canadian made one and tested its far range. He saw the RF cops in their truck with antennas on top drive past the next day!
I told him that they were looking for me. :lol: :lol:

 

[TV-Engineer]

Thanks for the circuit AudioGuru. I can only find one place which sells the BH1417 and one place which sells a 7.6MHz crystal. Total price for both would be almost £50. Far too much...
:cry:

 

[Miles Prower]

Try my FM transmitter. Another Canadian made one and tested its far range. He saw the RF cops in their truck with antennas on top drive past the next day!
I told him that they were looking for me.

May be the "RF Cops" in Canada are more efficient? Here, in the USA, the FCC (Federal Communications Commission) has become a strictly a source for patronage (Colin Powel's son runs it, and WTF does he know about radio?) They spend all their time counting how many "bad words" Howard Stern uses, and look the other way concerning RF violations: fishing net bouys way out of their allocation and over powered (there's even a company in California selling deliberately non-compliant net bouys and they won't do a thing about it) kilowatt linears connected to CB transcievers, and the USA is the only developed country that lacks a long wave ham band.

Have you checked that FM transmitter for spurious emissions in the aircraft bands? Since the aviation VHF band is right above the commercial FM band, this could be a problem. The only times I've ever heard of the FCC doing anything about RF misdeeds involved deliberate/inadvertant interferance with aviation frequencies. The FAA is very serious about that.

Perhaps a BPF between the final and antenna would be a good idea?

 

[audioguru]

Hi Miles,
My transmitter has a BPF in its final RF stage, and its harmonics if any are way above 100MHz (200, 300, 400 etc). I don't think it would cause interference to aircraft's 118MHz unless an airplane is so close its radio is overloaded and I could hit the plane with a baseball bat!

Nobody has heard me swear or anything over the airwaves except once :lol: when I tripped.

 

[Miles Prower]

My transmitter has a BPF in its final RF stage...

Any LC tuner is a kind of a BPF, for sure. However, I had a follow-up BPF between the tuner and antenna in mind.

...its harmonics if any are way above 100MHz (200, 300, 400 etc).

There are other kinds of spurious signals other than harmonics of a carrier and its sidebands. These don't necessarily have any relation to the carrier frequency and/or the harmonics of the modulating signal. Such spurs could come within the aircraft VHF band. Have you checked this possibility? Furthermore, spurious response may not even originate from a circuit defect. Overmodulation can cause all sorts of unwanted radiation. This could have caused "splatter" that could have generated interferance. After all, that circuit of yours isn't just an FM modulator. It's an AM modulator as well.

If all is well, then, obviously the Canadian "RF Cops" are actually doing their jobs. May be you should consider relocating to the USA? It's incredible what the FCC will let you get away with. :roll:

So long as your name isn't "Howard Stern", that is. :lol:

 

[audioguru]

My FM dial is full of FM stations. I was using the frequency of a low-power foreign-language station on the other side of my city. I guess someone could have heard my interference and complained.

I can easily overmodulate my transmitter when I get excited so I try to remain calm. I have never heard "splatter" before so now I know the problems it causes to nearby RF frequencies.

I am only about 10km from a major international airport but the only time that low-flying planes occur is when they test their radar. I tried but can't reach them with my baseball bat.
They fly very fast. Maybe the doppler shift would cause them to hear my 100MHz on their 118MHz receiver. Then they would be out of my range in a split second.

It all started when someone posted a simple 2-transistor FM transmitter. I looked at it and asked how does it keep operating when its battery drops a volt. So I made it to prove it and developed this better one. :lol: :lol:

 

[Miles Prower]

OK, I guess that settles the question. Time to move south, audioguru: those Canadian "RF Cops" are too efficient.

I can't imagine the FCC being so diligent that they'd be out there in force, in one day, looking for an illegal milliwatt transmitter running off a 9V battery just because someone complained about interferance on a local broadcast. :shock: (Really, how bad could it have been anyway?)

Takes a helluvalot more than that to light a fire under the FCC's ####. :roll:

 

[audioguru]

I think it was the last day of the month and the RF cops had to meet their quota. :lol:

 

[monkeytree]

Audioguru i made your transmitter (but stupidly forgot to ask what frequency it was)and i need it to be between 80-110 i have spent two days trying to find it on different radios but it wasnt really there

 

[audioguru]

Hi Monkeytree,
The oscillator's trimmer capacitor in my FM transmitter circuits tunes them over more than the FM broadcast band of 88MHz to 108MHz. I must use a plastic screwdriver to tune it because a metal one causes its frequency to drift so much I think the oscillator is even stopped. Adjusting the trimmer very slowly I hear a "pop" from my radio as the frequency sweeps past. Then I fine-tune it on the radio because the trimmer adjustment is very sensitive.

Did you use a tight layout of the parts and make coils like in my picture?
Did you substitute any parts or values?
How much current is it drawing from the battery?

 

[monkeytree]

A1 yes close together
A2 c6 and c13 to 5.2 to 30pf
Lm2931A5.0 to 78Lo5
c12 to 33pf
(and c4 is 100nf
A3 (problem? :roll the current is too unstable to get even an average reading
(but that might be my rubbish multimeter :roll: )

 

[audioguru]

Hi Monkeytree,
Your substituted parts should be fine except the 78L05 might operate poorly when the battery voltage drops below 8V. My low-dropout regulator works fine when the battery voltage drops to 5.6V.
Did you build the FM transmitter on a pcb?

 

[monkeytree]

i am using 6 AA batteries in a holder giving out 9.75V so that 'should'
be okay?
and yes i did do it on a pcb.

 

[Antey]

I can't post in instant messaging so here are schematics of my transmitter. Mod4 works much better than Mod3.

As I am very interested in this particular topic - so I tried to simulate the upper (mod 3?) schematic, and I got something like this [see below]... any idea what may be wrong ? Either in ciruct or simulation/simulator (SIMetrix) ?

And what's the purpose of Q3 anyway ?

Frequency : 98.3 with C4 @ 8.2 pF
Interesting obserwation : regardles of V1 the oscillations are attenuated. Either when it gives single pulse (between 10 and 20 us) or works continiously. What's going on ?

 

[Nigel Goodwin]

And what's the purpose of Q3 anyway ?

Q3 is an RF amplifier, it produces much greater range, and makes it more stable by giving a degree of isolation between the oscillator and aerial.

I consider simulators a waste of time, particularly for this type of circuit, where it's not likely to even get close.