wireless transmitter

[audioguru2]

Hi Alun,
The RF output power of the Stereo Micromitter is attenuated to meet Australia's 10mW limit. Rhom's datasheet gives no spec's but says they will be provided by request (and a payment?).

 

[tazmania_kid]

help.....i couldn't get both 2N5109 and 2SC1324 in the market... :'(

 

[audioguru2]

C'mon up here!
Our market has everything.

 

[rasOfir]

i m about to build the SiliconChip transmitter, just 1 question :
i live in Israel, is there any changes i should make in the circuit in order to operate it here (israel) or should i build it as-it-is ?

 

[audioguru2]

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.

 

[Alun]

Silly me for not searching this website, try this circuit:
http://www.electronics-lab.com/projects/rf/021/index.html

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.

 

[audioguru2]

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

I improved and reported my test of the preamp section of an FM transmitter here:

View attachment 36609

 

[MP1]

tazmania_kid,
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.

MP

 

[Alun]

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.

 

[audioguru2]

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.

 

[tazmania_kid]

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?

http://www.aaroncake.net/circuits/fmtrans.htm

 

[audioguru2]

Hi Tazmania,
1) 0.22uH is too high when you add about 10pF worth of wiring capacitance to its tuned circuit. Reduce the number of turns in its coil or spread-out the turns. Most toy FM transmitters use 0.1uH.
2) The transmitter has harmonics because it has only one tuned circuit. "Real" transmitters have many tuned circuits to attenuate harmonics.
3) Many people complain about picking-up their transmitter at many point on the dial of a cheap clock radio. I think that the radio's input stage is overloaded by the transmitter. Cheap radios don't have a tuned circuit ahead of its input RF amplifier like "real" radios have.
4) The inductor in series with its antenna is supposed to be part of an impedance matching circuit when you have coaxial cable to a low impedance antenna. If you have only a piece of wire for an antenna, the transmitter will probably work better without the extra coil.

 

[tazmania_kid]

I didn't use an air core coil, instead I used a normal inductor which I bought from supplier. Is this possible?

 

[audioguru2]

Hi Tazmania,
The project instructs you how to simply make a suitable coil. Why did you buy an inductor with the wrong value?

 

[tazmania_kid]

As am new to circuit design, I thought, with coil value at 0.22uH, a 3-18pF capacitance change would in theory give you a frequency range of 80-196MHz. Too bad, it's not as easy as I think

 

[audioguru2]

Too bad, it's not as easy as I think

Yeah, you must think of everything at these very high frequencies. Since stray capacitance has doubled the "theory" capacitance, buy or make a smaller inductor to match.

 

[tazmania_kid]

ok, I will try to make a coil myself and see what's the output

 

[Alun]

The coil in series cancels out the capacitance of an antenna that's shorter than 1/4 wavelength.

You could reduce the harmonic output by adding a LC tuned circuit in parallel and series with the antenna this will help to filter out the harmonics.

 

[tazmania_kid]

Hi, anyone can explain to me the differences between the attached circuits in terms of construction and their performance?

The reason is: understanding more on these circuits will help me proceed with better and more complicated tx circuits.

Circuit 1:
http://www.geocities.com/tomzi.geo/4w_fm/4W_transmitter.htm

Circuit 2:
http://www.boondog.com//tutorials/rfTransmitter/rfTransmitter.htm

Circuit 3:
http://www.aaroncake.net/circuits/fmtrans.htm

 

[audioguru2]

Hi Tas,
It's obvious that the 1st complicated 4W transmitter (same as the one on our site) will flatten a battery in no time and probably put you in jail! It has a strong signal by using two RF amplifier stages, and proper LC impedance matching circuits.

The 2nd circuit is a cheap little toy with a range of a whopping 25 feet!

The 3rd circuit isn't much better than the 2nd but has a mic preamp for greater sensitivity.