FM transmitter (mod4)

[Chippie]

can anyone tell me hoe to do a technical paper.......
what s expected from engineering students,...
and what brings the winning strategy...

In a nutshell a technical paper should cover the following imho:

1.Index
2 Executive Summary
3 Introduction
4 Main body of text
5 Calculations if used
6 Conclusion
7 Appendix
8 Links to extrnally referenced material

The executive summary at the front basically contains the same as the main body butis written in shortform so as not to bore the pants off the reader...If he wants to know more, read the rest of the document...


That's what I was taught, mebbe others have a different view

 

[sreejiths]

any 1 of u check its range??????
need any license???????????

 

[EmmKay]

Trouble shooting after build

Hi all,
I have built the FMTx Mod4..

and after finishing it I encountered a couple of issues..

1. as I could not get the low dropout V regulator I used an LM78L05.. That gets heated in less than 50 secs and shuts down. So now I am replacing the V regulator with a regular LM317T circuit (set for 5V). Hopefully that will cure the heating issue. (done while wrting this email..) working perfectly

2. I constructed a simple peaking circuit and attached it to the antenna output .. I am not getting an reading on the DVM from the circuit.

Here are differnt voltage readings that I have taken along the way..

Supply regulated and filtered 9.60V
Q3-Vc = 9.60V
Q3-Vb = 3.6V
Q3-Ve = 8.1mV


Q2-Vc = 5.05V
Q2-Vb = 4.19V
Q2-Ve = 4.17V

Q1-Vc = .860V
Q1-Vb = .211V
Q1-Ve = .209

Mic+ = 2.03V

How do I tune the circuit .. what sequence would I apply. The circuit is made on a pcb to spec with no component alternates at all...

The board size is 3 cm by 1.5 cm

I do not have a scope or a field strength meter so I need guidance in resolving the problems. I am reasonably versed in electronics but RF is something that I have been thrown into and have been reading up on. So please pardon any of my simple idiotic seeming statements and questions.

Any help would be greatly appreciated.

Cheers all

EmmKay

p.s. total current draw is 172.6 ma for the Tx including the 317T 5V regulator...

 

[mbarazeen]

i think the coils are not so accurate, may be you can try half of the turns, with a frrite rod inside to tune the Q2. 1st you better test it with Q1&Q2 only and tune it on FM, once you see a strong signal you solder the Q3 and check again, this oscilator is not so stable and you may get the frequency shifted when connected to an antenna or to a wire.

also try C2053 for Q3 it may give you some good results. if you need stable signal you better buy a PLL tiny module on ebay( i bought few recently and costs $5.3) with stereo option and use two stage power amp like Q3. it gives so stable out and frequency could be set by dip switches.

razeen

 

[audioguru]

Hi all,
I have built the FMTx Mod4..

and after finishing it I encountered a couple of issues..

1. as I could not get the low dropout V regulator I used an LM78L05.. That gets heated in less than 50 secs and shuts down.

All semiconductor manufacturers make many low dropout 5V regulators. The 78L05 will stop regulating when the battery voltage drops to about 7V which will happen soon.

Your circuit has a short that is causing the heat. My circuit draws only about 10mA from the regulator and the regulator heats with only 9V - 5V) x 10mA= 0.04W which is nothing. It is cold, not warm.

So now I am replacing the V regulator with a regular LM317T circuit (set for 5V). Hopefully that will cure the heating issue. (done while wrting this email..) working perfectly

Since the 78L05 got hot but the LM317 did not then maybe the pins on the 78L05 were connected backwards.
The LM317 also will stop regulating when the battery voltage drops to about 7V.

2. I constructed a simple peaking circuit and attached it to the antenna output .. I am not getting an reading on the DVM from the circuit.

I don't know what is a peaking circuit. I made a "field strength meter" with an antenna and a detector diode for tuning my output.

Here are differnt voltage readings that I have taken along the way..

Supply regulated and filtered 9.60V
Q3-Vc = 9.60V
Q3-Vb = 3.6V
Q3-Ve = 8.1mV

Wrong. The base to emitter voltage of a 2n3904 transistor is about 0.7V, not 3.6V. Maybe its pins are connected backwards.

Q2-Vc = 5.05V
Q2-Vb = 4.19V
Q2-Ve = 4.17V

Wrong. It is saturated. I don't think it is oscillating.

Q1-Vc = .860V
Q1-Vb = .211V
Q1-Ve = .209

Wrong. The collector voltage should be about 2.0V. The emitter voltage should be about 0.044V and the base voltage should be about 0.61V.

Mic+ = 2.03V

That is fine.

How do I tune the circuit .. what sequence would I apply. The circuit is made on a pcb to spec with no component alternates at all...

Too many voltages are wrong in your circuit so it does not work.

My circuit works perfectly and its current from the 9V battery is about 53mA.

Review the normal voltage readings of NPN transistors.
Your Q3 has a base to emitter voltage that is more that 5 times what is possible, your Q2 is saturated instead of oscillating and your Q1 is cutoff instead of amplifying.
Audioguru

 

[EmmKay]

update

here is an update..

No shorts in the actual board. but the lastest batch of resistors I got from the shops were Category C from china.. hence el cheapo resistors. This meant that what colour coding is on the resistor, the resistor might not be of that value. it turns out that the 220R resistor was in actual 23R while 160K 47K resistors were infact 4.7R and the 160K resistor was 330K after being checked ..

Anyway. problem fixed and I am off to a flying start. got a recption on the radio and on my home made peaking/FSM device I managed to get about 3V on the antenna output.

I have brought down the transmitting range from 98 to 92MHx by tunning the caps. Need to bring it down further below 88 or take it above 104... am reading up on how to do that!.

Thank you again audioguru for such a brilliant piece of work. It has explained RF to me in a way I would never have been able to learn.

next step is to use something like IC567 to send encoded signal across fm and use the same ICC to decode on the receiver end.

Cheers all

Emmkay

 

[audioguru]

I am glad that you got your transmitter to work.
Add a 10pf ceramic capacitor in parallel with each coil and re-tune the trimmer capacitors to a lower frequency.
Nothing should touch the antenna since the capacitance of the output tuned circuit and its frequency will change.

 

[EmmKay]

Got that .. I just got back from a drive testing the transmitter.. on the beach front I managed to get the signal on the car radio over a distance of 4.5KM in line of sight.. but it covered most of the bands from 88 - 92.. need to narrow down the bandwidth. I will add the caps as suggested and post an update!..

Q.. if the coil is close to or touching a 100uF polar capacitor does it affect the LC circuit..?????

on my board the distance between the coil and a smoothing cap is barely .25 mm or less..

Thank you again!..

I have finished the LM567 side of the circuit and it is generating a 4KHz signal. Now I have to figure out if I connect it before or after the 330nf Coupling cap after the audio amp. I am guessing that the first stage of the circuit will be complete removed. i.e. upto and including C3.

cheers

EmmKay

 

[audioguru]

My FM transmitter severely overloaded my cheap Sony Walkman radio so the signal was all across its dial. At a distance and with the Local-distant switch on the radio set to local then it appeared only at one sharp frequency.

My FM transmitter has its coils spaced 0.2" away from the circuit board and no parts within 0.3" away from the coils.

Yes, you do not need the parts that feed C3. C3 is important to block DC from your tone source.

 

[EmmKay]

UpDate

Couple of things that I came up against while doing this project.
1. never take components at face value always check them with whatever means necessary.

2. never take your handi multimeter for granted, it serves a bigger purpose in the grand scheme of things.

3. Finally when deling with RF printed PCB's are always the way to go if you are a biginner like me, there are two many tings to check when things don't work, so make sure your circuit board is elimnated in the first instance.

Onwards and Upwards..

After the success of this exercise, I am planning to make 5 more beacon transmitters to be installed at our farm.

Some questions that came out from my exercise.

1. Can the tuning process be made any simpler. For now I connected my FSM to each output of the stage and tuned it for max. after that, I connected the default audio output to sync the TX with the FM Rx. And after that I removed the deualt audio stage and replaced it with a LM567 circuit to produce 2.75KHz input for the FM Tx.

This whole process went on for days (in my mind) but in reality took about 3 hours on the whole.

2. What would be the affect on the circuit if the 9V rail was upped to 12V leaving 5V rail as is. An another stage identical to the Q3 stage was added to the circuit after Q3. I have seen circuits that employ upto 3 stages all identical after stage 2 but am yet to comprehend the actual nature of these additions.

3. last but not least. as I forget to write it down, what voltage should I be feeding to C3 for the optimal performance of Q2 stage.

Cheers

EmmKay

 

[audioguru]

1. Can the tuning process be made any simpler? For now I connected my FSM to each output of the stage and tuned it for max.

You should not connect your Field-Strength-Meter to the circuit because its capacitance changes the tuned frequency. Have the FSM at least 1m away from the circuit and have the circuit at least 2cm away from anything. The circuit can be spaced inside a metal box and then connect the box to 0V.

2. What would be the affect on the circuit if the 9V rail was upped to 12V leaving 5V rail as is?

I think Q3 would get too hot.

An another stage identical to the Q3 stage was added to the circuit after Q3?

I think it would just make interference on many frequencies (its harmonics).

3. last but not least. as I forget to write it down, what voltage should I be feeding to C3 for the optimal performance of Q2 stage.

I didn't measure the signal level. It is pretty low for an FM radio to produce the same output as an FM radio station playing loudly.

 

[EmmKay]

hmmmm

Just when I thought I was all done, something cropped up!.

Using a simple LM567 in oscillator mode and generating 1.2Khz signal (I found that anything below 1KHz is not easily processed on the receiver end), connecting the output to the input of C3, doesn't seem to work!. if I go back to the audio stage and feed a tone through the mic all works fine. but generating a tone using a 555 or 567 doesn't seem to work.

12V input at Q3 stage didn't heat up any of the components but the overloading was extended to the complete fm range from 87 to 108.. so I guess that is a no no as it would have been affecting above and below frequencies that I have no way of measuring!.

Have dropped the idea of another stage similar to Q3 as it seems to be an over kill and extra work/tuning. The Mod4 Tx performs exceptionally well without it!.

The voice transmission over 1.5 km does have a lot of hissing in it. but a simple tic toc sound from a speaker placed infront of the mic can be heard clearly over 4.5Km without any hissing noise. The radio I am using is a Sony Table top multiband radio with an FM range of 70-120MHz. Connecting an led to the headphone output seems to translate the tones into flashes nicely as well.


Would appreciate any advice at this stage.

Cheers

EmmKay

p.s. I just checked my first ever post on another thread. I didn't mean to replicate the questions. I came across the other thread first and after posting my question there waited for a day and it hadn't appeared so while looking for more threads related to Mod4 TX came across this thread and started posting here. It was never intended to be a discomfort to anyone.

 

[audioguru]

Hi EmmKay,
The signal level at C3 is supposed to be low. You were blasting a very high level tone to it which probably caused the RF oscillator Q2 to stop. Attenuate your tone signal with two resistors to feed into C3.

 

[EmmKay]

temp effect on Mod4

@audioguru. You were correct in your last comment. after going through the varios mod4 posts I came across your thread for input from the ipod for mod4TX. I implemented that change in the design and it worked perfectly.

After running the TX for a number of days I have come across some anamolies.

During the night there is no drift in the reception frequency, but during the day I have to adjust the Rx a number of times to get clear reception.

Could it be that due to temp changes the air core inductor changes its properties and causes a frequency drift..??

Night time temp is around 25-27 degrees celsius while during the day it varies from 27 to a max 47 degrees celsius.

is there a way of making certain that the TX frequency is locked once it is set to the desired point.....

cheers.

Emmkay.

 

[audioguru]

If you use good quality NPO ceramic capacitors and good quality trimmer capacitors then the frequency will not change much for normal temperatures.
But you have a temperature as high as 47 degrees C??? Do you live inside an oven???

Modern FM transmitter ICs that are made for MP3 to car radio transmitters use a frequency synthesizer with quartz crystal and phase-locked loop circuit for excellent frequency stability.

 

[EmmKay]

I am installing the Tx and Rx at my property boundry in a city called MULTAN in pakistan. It is one of the hotest places in Pakistan. Summer temp can easily go to 52 degrees C. but nights are a bit cooler. only by a margin.

As the boundry is about 1.5 km away from the house, instead of running cables for the signals, I am using the fm tx and Rx to get the status of the
boundry.

I have 6 transmitters installed on various corners. No trees, no bushes just aarid land. so I get a clear line o sight. No fm apart from 103 (local radio). so a lot of flexibilty re tunning.

I ended up using 3 Fm radios tuned to different frequencies to get the signal and demodulate. All works well at the moment apart from some of the issues I mentioned. the FM's were the old type with manual tuning.. the others i got were the digital type and they didn't fare well in the temp. (so I am reading up on making FM Rcv'rs)

Would love to know if there is a way of having one receiver at home for all Tx's

cheers.

 

[audioguru]

An FM radio has a "capture ratio". If two signals have the same RF frequency then only the strongest one is received and the weaker one is not detected. A good FM radio circuit has a low capture ratio where the stronger signal must be only a little stronger than the other for the other signal not to interfere.

That is why airplanes and air traffic controllers use AM so that a "Mayday" call can break in and be heard.
You also want all your transmitters to be heard.

 

[EmmKay]

so your suggestion is that ideally I should be looking at AM instead of FM....?????
or there is something else in RF that I might consider....

..** This might be a silly idea but no harm in asking!!! is it possible to build a Rx that would scan pre programmed FM frequencies and locks on each one for a duration of perhaps 1-2 sec. so that the received signal can be processed by the back end!!!!!

 

[audioguru]

If you can buy modern western parts then you can make anything.

 

[EmmKay]

Can get most of the parts here, and what I can't get I can always get from UK or USA so that is not a problem.

Perhaps the answer about the choice of AM being more suitable could be addressed..??

I think I will hold on to the idea of a single multi channel receiver for a while until I have finished my reading of what RF is and after being satisfied with what I have learned I will pursue thaat particular option.

Thank you audioguru for all the help that you have provided. Much appreciated.

cheers

Emmkay