Understanding the FM transmitter circuit [Help]

[oasis4355]

fm transmitter circuit

Hi,

I'm trying to understand this circuit, on the oscillator part.

https://electronics-diy.com/tx200.php

What's the function of L1? To my understanding it's to control the transmitting frequency. Since I want to transmit in 95Mhz,

and according to the formula: f=1/( 2*pi*srt[LC] )

where C=15pF

L=.....

Please correct if I'm wrong. If this is not the case, which parameter should i change to transmit in 95Mhz.

2nd question, for unlicense FM transmission, the max power that we can transmit is 1W, please correct me if i'm wrong.

Thanks in advance.

Bruce

Added after 13 minutes:

Another thing,

I'm trying to build an antenna for the circuit above, or should i just use a normal wire will do? like a dipole antenna

Bruce

 

[multanova51]

fm transmitter circuits

Hi Bruce

L1 is to adjust the oscillator freq. alright...
You are not correct in estimating the effective cap over L1 though:
What is parallel to the coil is:
- C6 = 15pF
- equiv cap of C7 and C8, approx 6.2 pF
- the input cap of the next stage, coupled with C11: in any case < 2pF
- output cap of transistor Q1, expect a few pF
- the stray cap of coil L1, the printed board etc
So, you end up with more than 15 pF, I estimate about 27pF.
You need therefore an inductance to resonate at 95 MHz which is about 102 nH

Your estimations on legal allowed RF power are not correct.
US FCC part 15 specifies about 11 nWerp while recently, ETSI allowed 50nWerp in their latest EN 301 357-1 standard.
Mind that the power specified is erp.... if you have some resonably tuned antenna you can expect a few dB loss only so 1W power is certainly way above 50 nano Watts, expect 0.5 to 0.7Werp.
The schematic mentions 200mW, I guess still way too optimistic because the DC power seems to be about 300mW only.

Also, ETSI and FCC regulations will require much more stability as you have with this free running oscillator.

But if it is for the hobby, no harm will happen probably unless you tune it out of the regular FM band and then some people of the Aviation safety might become very nervous. I guess most of the electronics experimenters have done this "sin", and I guess it will be forgiven later javascript:emoticon('')
Very Happy


rgds

 

[oasis4355]

bf199 transmitter

multanova.

Thanks for details, it's very informative.

About the transmission frequency, I'm wondering if I can transmit in ISM bands? Somewhere between 902-928Mhz.

Do i need another different circuit for this particular FM transmission? (as well as the reciever?). Or should i just change the parameters of the oscillator?

What about the antenna? Can i just use a dipole antenna (wire)? or should i go buy myself a better one?

Thanks in advance....

Bruce
(I've been cracking my head with all this questions, it's good to have it discuss with someone. EDA rocks.)

 

[multanova51]

understanding a fm transmitter

Bruce,

In principle you can use a one transistor circuit as well for 902-928MHz.
The circuit topology for the FM circuit does not allow to have low capacitance enough over the coil. You need a tuned circuit that is coupled to the transistor by a capacitor preferably.
You need a different topology, much smaller caps and inductances (around 10nH only) and a better transistor like BFR91 or one of the other BFS / BFR 's types with around 5 GHz Ft.
I have designed very different circuits at 902 MHz for wireless audio, it is mainly an issue of having a good pb layout or a good "dead-bug" construction. Things get unstable easily or just might refuse oscillating because of layout errors.

There is quite a bit of difference in stability though and attainable phase noise of the circuit at 902 MHz. If you want to use the circuit for narrow band voice you will need a synthesiser to stabilise the freq. both in TX AND in RX otherwise you will not be able to use it reliably. It drifts as you look to it...This makes things quite a bit more complex because a synth also calls for a microcontroller etc etc.
These circuits are not for beginners and you absolutely will need some meas equipment like a spectr analyser in order to get it right.


About the antenna for the FM band ... you best use one of the standard dipole antennas. Most antennas in the US are 300 ohm though, you need a lower impedance antenna which can be made by using just an open dipole iso a folded dipole. Or you can take just a lambda/4 wire.

If you want to experiment in UHF then there are various possibilities by using IC's intended for garage door openers and car-keys. Most of them use either 315 (US) or 433 MHz (EU). For the EU band one can also get 868 MHz versions. They use a quartz at a low freq. and this is multiplied by 16 or 32 in order to come to the UHF band.
Then you only need to apply modulation to the quartz.
Of course you need to find a receiver as well.


Hope you can do something with this. Do'nt know exactly what you want to do though....

Of course you need to stay in the foreseen radio bands at UHF as well. The fact that they are "ISM" and licence-free does not mean you can use anything there legally.
For your experiment again it is a minor "sin", if you want to bring products on the market commercially, you need to tackle the issue differently.

Rgds

 

[E-design]

transistor quartz transmitter

I also think the 200mW is overrated from a BF199 as mentioned before. Maybe around 70mW at the fundamental. Total device dissipation is only around 350mW. You often find on the web people claiming 300, 400mW with a transistor that sometimes have a hard time to provide 50mW at 100MHz. I don't know how they measure the output or if they even bother to measure.

 

[oasis4355]

bf 199 fm transmitter

Hi again,

Actually, I'm trying to do this for my final year project in my university. And it needs to be analog, or I could just use some IC...

Here's a question that pop into my mind lately...

Assuming e-design said was correct, if the transmission power is only 50mW, could i relate this to the Friss transmission formula?

And how do we cope with the multipath interference? Or this receiver that i plan to build will correct this problem?

h**p://www.users.bigpond.com/cool386/6tr/srrx.html

Talking about interference, if i'm transmitting a frequency same with other FM stations, will the stronger signal dominates? If so, how to make my transmitter to dominates?


Bruce

 

[multanova51]

bf199 circuit

Bruce

Yes the power will be less as correctly pointed out by e-design.
You cannot get this out of a BF199. You can get more perhaps by choosing another transistor.

Assuming 50mW you can indeed use the Friis formula to estimate fieldstrength at a certain distance.

I cannot understand why you should consider a superreg recvr for this purpose!!!!
It is not very well suited to detect wideband FM. Furthermore IF selectivity is not so good :-(
The simplicity of the circuit might be attractive but in reality there is quite a bit more complexity as expected at first glance...

In any case you need to be away from strong nearby regular FM transmitters otherwise they will interfere. In tech literature you also might find the term "protection ratio". Former CCIR and now ITU-R have defined protection ratio curves for a normal FM broadcast receiver.
The curves are used to do the planning of offical FM transmitters geographically mainly but they can also be used for FM radio revr designers to determine how much IF selectivity will be needed. If an interferer is on the same frequency you will have the "capture" effect: a signal a few dB stronger can "capture" the wanted signal easily. However if they are stereo, the effect is extended very much longer and you need a signal 20..30 dB stronger before you have completely noise-free capturing. For stereo, the same is true for the protection curves. There is quite a bit of difference between mono and stereo.

I recommend you Google this or take any textbook on FM radio and do some additional reading.

I am surprised you are considering such a project for final year at university.
In this case you definitely will have to explain WHY you have done te choices like this. I think the WHY's in the architecture considerations are THE main thing and building the thing will be of second order.
Not to say that this project is unsuitable for a final year. You can, on a transmitter alone, already achieve this. For example designing a "state of the art" stereo transmitter alone that would meet latest ETSI specs and 50nW only is challenging enough.

Sorry for my long story..

rgds

 

[oasis4355]

bfr91 project italy

Thanks Multanova

Again, it's very informative. Currentlly i'm doing some reading on the "protection curve" like you have mentioned.

Like you have said, superegenerative reciever might be not that good for selectivity, i wonder where can i find circuits for a superheterodyne reciever (in FM band)?

Bruce

 

[sssaaa]

50nw fm transmitter

HI
i want to know that what is the actual output in mWatt of this circuit which Mr.aoasis4355
want to make(tx200).

 

[E-design]

garage door transmitter specification erp watts

The only sure way of knowing the actual output is to build a prototype and measure. Doing a simulation will give us some idea of the maximum output. Output may also vary by quality of the actual transistor or the manufacturer. Not all BF199's will perform the same from various manufacturers.