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In Google, enter Stereo Micromitter. The whole Silicon Chip magazine project is linked for free.
The project is crystal controlled and very stable. It has 14 station frequencies and one is selected with Dip switches.

I designed and built a simpler one recently. It sounds great and doesn't drift its RF frequency when it is in a metal box. It is too cold outside now to see how far it goes. Maybe I'll leave it inside and go driving while listening for it.

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Hi Dazza,
Its fairly high RF power of about 250mW is very illegal down under. It overloads cheap radios and fills their dial within about 30m. A good jammer! Your limit is only 10mW (only if you get caught). ;D
I used a tight layout on Veroboard and used ceramic decoupling caps at important locations. I found some neat little trimmer caps in a local shop, made for big watches I guess. They are only about 2.5mm in diameter, the same as their adjustment screw head.
The circuit will work fine with many other general purpose transistors such as 2N2222 or 2N4401. BC547 or BC549 transistors will also work but their E and C leads are reversed fom the 2Nxxxx ones.
I used that low-dropout regulator because I had some, it still works with only a 5.2V battery but needs the 100uF output cap for stability. A 78L05 regulator will work fine until the battery voltage falls to about 7.5V and doesn't need the big output cap.
I didn't show my low batt circuit: A bright 1.8V red LED in series with a 680 ohm resistor and a 5.1V 1N5231B zener diode. Fairly bright at 9V (3.5mA), dim at 7.5V (0.4mA) and off at 5.4V. The numbers don't add correctly because the 20mA zener is a lousy voltage regulator at low currents.

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Cool FM transmitter circuit audioguru!

Q1 pre-emphasis and amplifier, improves the sound quality.

Q2 hartly oscillator performs the frequency modulation.

Q3 Tuned RF amplifier, isolates the oscillator from the antenna.

LM2931A5.0 voltage regulator for the amplifier and oscillator stages to reduce frequency drift and noise on the power supply.

This is not too dissimilar to the one that I've been thinking of designing but you've saved me a transistor! I was planning to use a separate stage for pre-emphasis but you've integrated it into the audio amplifier stage.

Did you use some of the information from this discussion: http://www.electronics-lab.com/forum/index.php?board=13;action=display;threadid=2332 to help you design this circuit?

C14 looks a bit small, it will have an impedance of 72ohms @ 100MHz, is this intentional?

Did you do it to reduce the power output or as impedance matching?

I would recommend 470pf or even 1nf if I assume it's for DC blocking.

If you want impedance matching I would recommend an inductor to reduce the capacitance of an antenna shorter than 1/4 wavelength, but it wouldn't be needed if the antenna is 30" as you sugested.

How about harmonic output?

You could reduce this also by including a series LC tuned circuit in series with the antenna, and taking the RF output from a center tap in the coil might help too.

How did you measure the power output?

2N2222A or PN2222A have an Ft of 300MHz would be good alternatives for the transistors, I don't know about BC547 as its Ft is only 100MHz, 2N4401 would be ok to as it's Ft is 250MHz.

A PCB would be the best way to build this and surface mount components would be ideal.

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Thanks for the info audioguru ;),

I have made a couple of FM transmitters around the 10mW, and they really didn't work very well.

I think it's time to make some think with a little more grunt 8), and Yes that's right, it's only illegal if you get caught ;D.

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Thanks Alun,
I started investigating a circuit somebody posted that had its preamp transistor biased wrong and had no feedback. It was saturated with a new battery or when warm, and cutoff with a 7.5V battery or when cold. It also had horrible distortion with any battery voltage. Without pre-emphasis its output was way down at only a few KHz.

My design is based on Ron Elliot's project #54, even using his nice tidy coils, but mine are even tidier with a couple less turns to fit in a 0.4" grid.

It was a challenge to get a pre-emphasis boost of 15dB (North America) at 15KHz. At first I tried it with the preamp DC-coupled to the RF oscillator to keep the collector load as high as 10K for max gain without feedback. But the RF fed back and upset the bias of the preamp. Now with it cap coupled and with an effective collector load of about 8.2K, the boost is about 13dB and I can't hear its massive 2nd harmonic of the 15KHz.
I even tried a big emitter resistor for bias stability with a big cap across part of it for good low frequency gain. The 2nd harmonic distortion was horrible at 40Hz because the emitter of a transistor works like a rectifier. It is flat down to 40Hz now and sounds great with CDs and its microphone. Maybe 40Hz is too low if there is traffic or wind noise.
Aren't single transistor amplifiers terrible to get high gain and low distortion with a small supply voltage? I should have used Ron's TL071 opamp preamp.

I used a small C14 so the antenna and proximity to it doesn't affect the tuning of the output much and I figured that my whip antenna is about 70 ohms. The tuning of the output stage is fairly broad, nearly 10MHz or half the FM band. Tuned to 98MHz, its output is noticeably down at 88MHz and 108MHz but not cutoff.

I esimated its output to be about 250mW because the output stage is drawing about 55mA at 9V (495mW, 50% efficiency).

It was fun, I haven't worked with RF for about 40 years. For the last week I had FM RF all over my workshop. On my 'scope, my DMM and even on me!
I am lucky that I haven't been visited by the RF cops again. I got caught with a badly sparking fan that I made when I was a teenager. I liked its ozone but they had many complaints of TV interference. ;D

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  • 2 weeks later...

Hi audioguru,

I'm planning to use your circuit with a few modifications of course.

Firstly I plan to use Ron's op-amp based input stage as I hope it will give better sound quality.

And I plan to take the output from a tap in the coil or by winding another coil in the middle so it will act as a transfromer. I don't know much about VHF transformers but I'm going for step down so I think about 3 to 5 turns in the middle will do. I've attached a schematic of the proposed changes to the output stage please let me know what you think.


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Hi Alun,
It is a lot of trouble just for a toy transmitter, isn't it?

I think you're underestimating these simple circuits, good design and construction is the key not complexity and I don't think this circuit is too complex.

But you have good ideas.


We should also at least make its RF frequency crystal-controlled and the desired frequency selected with a frequency-synthesizer or PLL?
He, he. ;D ;D

You could even make it computer controlled. :D

But check out this for complex:
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Hi Alun,
Both Ron and Harry have some nice designs. I especially like Harry's low distortion and very-low-AM varactor-diode modulator.

Have you considered what happens when the transmitter is over-modulated? The simple AM-FM oscillator like mine is easily over-modulated and makes a huge frequency deviation, and maybe even gets completely cutoff. The tuner on my Sony Walkman has AFC and it actually pushes my over-modulated transmitter away! ;D

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I did think about over modulation as this would be a good explanation of why the signal covers such a large bandwidth, in theory this would be infinite if the deviation wasn't restricted. If over modulation is a problem it would cause distortion, if your design's deviation is 1Mhz and the receiver is expecting just 75Khz,then signal would be clipped causing bad distortion. You could solve this problem quite easily be reducing the gain of the audio amplifier.

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Hi Sasi,
It would have much more output power and melt the little 2N3904 at the output, and the 2N3866A would also get hot and help drain the battery quicker.

Using either a 2N3866A or a Mosfet would damp the output tuned circuit so they would just cause a very high supply current. Maybe if the output was unbiased so it operated in class-C. Then the output tuned circuit will be able to swing much higher than the supply voltage, but the harmonics would be very strong.

Which schematic is your pcb for?

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Hello Sasi,
If you want higher power use the 4 watt transmitter circuit on this website.

Iv'e thought of another improvement to the circuit.

The diode allows C13 to discharge properly as before the PN junction of Q3 acted as a rectifier and C13 just got charged up more and more each cycle, the diode allows it to discharge on negative cycles. With this modification in place R8 could be removed and C13 should be reduced to 10 or 4.7pf. The base now recieves a 50% duty cycle so it will operate in class C. The diode must be a decent quality VHF rectifier. Also use the tapped inductor or transformer (see previous page of replies) if you can be bothered to wind another coil.

Pllease let me know what you think.


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Hi Sasi,
Oh! It is my circuit.
When I saw a great big round electrolytic capacitor on your pcb for C4, I thought it would bypass R5 completely at all audio frequencies, causing high gain and distortion, without pre-emphasis. Adding a through hole above the "8" will fix the pcb by allowing for a metallized plastic-film cap for C4. Keep holes for both types of caps. Then applications of the transmitter that require a lot of audio gain but don't care about sound quality can use an electrolytic cap for C4.

You guys are RF power hungry. I pity your neighbours who will get lots of TV interference from the boosted RF harmonics. Don't blame me when the RF cops come to your places for a visit. ;D

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Hi Sasi,
That 60W RF amp in our projects section doesn't even say what frequency it amplifies. Since it mentions QRP which is for ham or amateur radio, and it has a very high-value tuning cap and a coil with a core, it is probably for short-wave, certainly not for the FM broadcast band.  ???

Have you seen the tiny size of the MAX2206? 1mm by 1mm! But it has only 4 connections while your circuit shows an IC with 6 connections. Wrong IC.  ::)

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You got the correct IC, I got the wrong one!

Those guys at Maxim must have tin ears. The high-value cap at pin 3 will cut high audio frequencies, and the circuit doesn't have pre-emphasis capacitors across R3 and R4.
An FM radio receiving a transmission from that transmitter will probably sound worse than an AM radio, especially in North America with our severe de-emphasis!  :'(

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Hi Sasi,
If you could find a cheap cable TV amplifier that isn't overloaded all the time like most of them, an antenna could be connected to its output.

In the olden days, my cable had about 78 channels and the cable company gave just barely enough signal for one TV set. When you added a splitter for a second TV set, it cut the signal level in half for both of them, looking weak. You could order a splitter from the cable company, pay for it over and over every month as a fee, then all they would do is turn up the level a little for their splitter.
So I bought a cheap cable TV amplifier. It was overloaded on nearly every channel! I read up about them and discovered that with a few channels they are fine, but with many channels the signals intermodulate each other. Since the picture of TV is AM, it made a mess with my 78 channels. I changed its single microwave transistor and biased it strongly (it got fairly hot) so that it worked pretty well. It would make a good transmitter for you.  ;D

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