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How did you measure the power?

What is the power output?

Are you measureing the frequency, if so what is the range?

How stable is it?

Does the frequency change when you alter the supply voltage or put something near the antenna?

I've buit one of these circuits today as an experiment, I designed for around 1GHz and it worked too.

At 9V the frequency was 1034.5MHz when I went near the antenna it changed to 1036MHz, and whenI held the antenna it dropped to 933MHz.

The frequency continiously fluctuates by 1MHz or so.

The frequency changes with the power supply, from 1024MHz @ 5V to 1060Mhz @ 20V. Failing that you could connect it to a phase locked loop.

The only way to get round these problems is to use a regulated power supply for the oscillator, build it in a screened box and use a separate RF amplifier to drive the antenna.

I didn't attempt frequency modulation with this circuit - how can you use the frequency to carry information if it continuously varies?

Your circuit will probably be more stable as it only runs at around 100MHz, but these issues will exist to a certain extend.

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What I mean is, how to measure the power received by the receiver?

A standard receiver?? A spectrum analyzer? The RSSI output of the receiver (if it does have)? Receiver audio output (it won't work I thnk)???

Also... is the receiver got a reference antenna?

More is, how to measure the transmitter power??

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I don't have much frequency drift for my 453MHz LC transmitter with voltage variation......

I found that if I use normal variable capacitor to the LC tank, it will drift about 2 to 4 MHz..... but when I use a trim capacitor (I think with better accuracy and temperature variation ) the transmitter frequency is just fine with just 500KHz drift.... even my hands get close to it... it still works within my expected frequency range.

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I have a Communication Service Monitor in my U and am planning to use this equipment as my receiver. In fact, it can also be a spectrum analyser, frequency counter... and so on.

Regarding the tx power, again, am using another formula to manually calculate it. But frankly speaking, I am not sure whether this works... :-\

And how you guys measure the frequency accurately? What are you using?

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Hi Tasmania Kid,
You saw it on your 'scope. That crude and simple transmitter transmits AM as well as FM. It also transmits the 2nd harmonic of 100MHz (200MHz) and 3rd harmonic (300MHz), etc. Maybe the majority of its power is in its harmonics. So you can't simply determine its output power at 100MHz from its supply power.

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I'm using a frequency counter, it works up to 1.1GHz with a tresshold of 10mV and has an impedance 50ohm. I just stick a 75mm piece of tinned coper wire in the co-aill socket and place the transmitter within a couple meters of it. The frequency counter will tick over quite happily as long as the transmitter is within 4 meters of it.

Here's the schematic, I can't remember what transistor I used but it had a max Ic of 20mA and fT of 6GHz. L was an air core inductor with 5 turns of #32 magnet wire wound round a 1mm former with the tap after the first turn, to make the capacitor I just twisted the ends of the wire from the inductor together a few times.


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The 7001 transistors look like they need to disipate a reasonable amount of power, you could try a 2N5109. Also the effency might be improved by putting a 22pf (experiment with differant vlaues) capacitor across the inductor conected to the antenna, this will form a resonant circuit.

Or you could just connect a power amplifier to the antenna output of your simple 2 transistor circuit.

Here's a scematic:http://www.wenzel.com/pdffiles/class_c.pdf

And I am currently doing some research into class C power amplifiers, here's a very usefull link:

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Hi All,
I was planning to build such an F.M transmitter and got a lot of important information through the threads.thanx EL :).

Actually I was planning to connect such a transmitter to speaker output of my computer so that I can hear MP3 and other music from my computer on my radio FM band.
I think it would require an impedence matching stage at the i/p.but what x'tor configuration to use that has a low i/p impedence(8 ohms) and a high o/p impedence(in kohms)?

The second thing is how do you decide the range of x'mission?

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Hi Shekhar,
Which FM transmitter are you going to make?

To drive it from the speaker output of your computer you don't have to match the impedance and you also don't even have to terminate the amplifier with 8 ohms, unless it's a vacuum tube amplifier with an output transformer. Just use a 10K audio-taper volume control to reduce the speaker output to microphone level. Also remove the resistor at the input of the transmitter that supplies power to its electret mic.

The transmitters discussed here are really just toys and are not suitable for transmitting music:
1) Their radio frequency drifts with temperature change, supply voltage change (don't use a battery) and if you move near its antenna.
2) They don't have pre-emphasis, which is treble boost that all FM radio stations have and all FM radios have the opposite (treble cut) to reduce noise. The result will be that your radio will play the transmitter's music without any treble. It will sound like the treble tone control on your radio is turned all the way down.
3) They transmit in mono, not stereo.

There is an FM stereo transmitter IC that is used in many car MP3 to FM adapters. It costs only $2.90US. It uses a crystal and has a Phase-Locked-Loop for rock-solid frequency stability. It must have a powerful radio output because in most circuits it uses an output attenuator. However, it is suface mount only, it is reported to have a fairly high distortion and its audio frequencies above about 10KHz are reduced. I have attached a magazine's circuit that uses it.


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I agree these transmitters are not good enough for music but they're more than sufficient for voice communication.

Frequency stability isn't too bad if you isolate the oscillator from the antenna with an RF amplifier (see the 4 transistor transmiter) and run the oscillator from a voltage regulator. Temperature is not too much of a problem if you use a decent quality capacitor in the LC tank circuit.

The circuit you posted is the best option for a good quality transmitter but for where the quality is unimportant this level of complexity isn't required.

Another thing, is the attenuator network required or can you omit it to get more power?

You could always connect it to an RF amplifier to get more power too.

I don't know exactly, but by looking at the components used to looks like you need something with a high ft ( >500MHz) a resonable power disipation (>1W) and an Ic > 300mA.

Try a 2SC1324.

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  • 2 weeks later...

Hi Ras,
The Micromitter is designed for Australia's 10mW RF power limit and uses their and Europe's 50us pre-emphasis standard. Check to see what your country's standards are. Here in North America both are different (100mW and 75us).

I don't want to discuss politics and I don't know your country, but some countries have a different radio-frequency band for their FM broadcast band to stop their people from hearing broadcasts from the rest of the world.

The project's RF frequencies are only odd-numbered ones: 88.1, 88.3, 107.5, 107.7 etc. Your country and its digital radios might have only even-numbered frequencies: 88.2, 107.6 etc. You better check.
The Micromitter is designed for 88MHz to 108MHz, but its switches tune it to 7 frequencies at each end:
"However, unlike previous FM transmitters published in SILICON CHIP, this new design is not continuously variable over the FM broadcast band. Instead, a 4-way DIP switch is used to select one of 14 preset frequencies. These are available in two ranges covering from 87.7-88.9MHz and 106.7-107.9MHz in 0.2MHz steps."

Go to Google on the web and enter Micromitter. You will get a link to the entire article for free.

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Silly me for not searching this website, try this circuit:

I very much doubt the output will reach 4W - you could always run it of a lower voltage to reduce the power.

You might need a pre-amp on the input though:

A 2N2222A will do for this pre-amp.


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Are you aware there are dedicated chips out there for transmit and receive circuits? They are somewhat expensive, but since you are connected with a University, you might be able to get samples.
www.rfdigital.com has quite a few choices. I personally like the Lynx Technologies RXM-433 and TXM-433 devices. These are 433 Mhz and have simple connection to micro. Other frequencies also on that site.
My point is simply that if your test is to measure this device and not to build it from scratch...and if you can get a sample...then why burden your school project with the basics? Just my opinion, but that would be like the Space Shuttle Engineers spending time building their own transistors.


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Hi Alun,
Your "preamp" won't have enough gain, only 4.5 if you're lucky.

The value I calculated if the Hfe is 100 was 4.47, and it's good enough to use with an condenser microphone. If the microphone output is 500mVp-p a gain of 4.5 would be fine.
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Hi Alun,
Electret microphones and phantom-powered condenser mics have about the same low-level output as dynamic coil and magnet mics, about 14mV p-p at a distance of 10cm with a normal speaking voice.

Since the FM transmitter circuits need an audio input of about 400mV p-p, my mic preamp has a gain of 28.5 before pre-emphasis.

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Hi, in regards to the circuit in the following link, I fix my inductor at 0.22uH then my variable cap at 3-18pF. So supposedly I should get a frequency of 88-108MHz right?

But the tx frequency which I measured in the receiver is 50+MHz, y is this so??

I tried to use my radio as the receiver and I talked to the mic, the radio receives my voice but this is the harmonic of the tx frequency isn't it?
In addition, without changing the capacitance in the tank circuit but tuning the radio to different stations (say 103-105MHz), it still can receive my signal. Why? I mean, if I transmit 103MHz, the radio should receive at 103MHz only, is this true?

What is the purpose of connecting an inductor in series with the antenna?


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