Help understanding FM transmitter

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
8
I came across this FM transmitter circuit whilst searching for 27Mhz transmitter/oscillators :

Screen Shot 2019-09-17 at 11.17.42.png

I have seen several versions of this in my search. The documentation states it will transmit in the 88MHz to 100Mhz fm band.

I cannot see how the front end manages to modulate the transmitted frequency. To me it looks like the output of the audio amp (Q1) modulates the base of the Q2. Q2 is in common base so modulating the base will just shift the bias and change the gain. Changing the gain modulates the amplitude not the frequency.

I'm just starting to learn rf oscillators, they have always been a mystery to me so any help would be appreciated.

Thanks
Garry
 

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
8
? try phase-altering // the major key for these kind of circuits to work is relative placement of the components on the PCB board . . .
not the best example but matches your circuit https://cxem.net/radiomic/radiomic144.php
...
likely better layouts :


Thanks for the reply but I'm really trying to understand how the rf frequency is modulated by the audio section.
I'm not wanting to build this circuit just understand it.
 

DickCappels

Joined Aug 21, 2008
5,877
Yes, the audio signal does modulate the bias of both transistors, most likely that of Q2. The collector-base junction of Q2 is a revere-biased diode with the cathode (collector) connected to +9V while the anode (base) swings more or less positive and then back negative with respect to the nominal base voltage which is set by R6, and the current drawn from the base. The changing collector-base voltage caused by the audio signal on the base causes the collector-base capacitance to change as a function of the modulation voltage. As simple as that.
 

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
8
Yes, the audio signal does modulate the bias of both transistors, most likely that of Q2. The collector-base junction of Q2 is a revere-biased diode with the cathode (collector) connected to +9V while the anode (base) swings more or less positive and then back negative with respect to the nominal base voltage which is set by R6, and the current drawn from the base. The changing collector-base voltage caused by the audio signal on the base causes the collector-base capacitance to change as a function of the modulation voltage. As simple as that.

That is so neat ! But it also means that the carrier frequency is not just the tank circuit (L1 and C4) as I expected but also Cbc ( I've seen this referred to as Cpi in the small signal model) and that is related to the collector current. Calculating the resonant frequency is not as simple as I thought.

From my other readings C5 is the feed back capacitor, is there any 'rule of thumb' guides for this value? or can you point me to a simple guide that will explain how to calculate this value.

What I'm really trying to do is build a simple 27MHz transmitter and receiver with a range of 10meters. I'm only interested in sending a clear carrier frequency. But I want to understand what is going on in the circuits and not just copy circuits from the internet.

Thanks for the clear answer.

Garry
 

Sensacell

Joined Jun 19, 2012
2,438
These simple transmitter circuits output a sloppy mess - modulated both AM and FM - works, but not pretty.
 

DickCappels

Joined Aug 21, 2008
5,877
"From my other readings C5 is the feed back capacitor, is there any 'rule of thumb' guides for this value? or can you point me to a simple guide that will explain how to calculate this value."

No, I can't off-hand but I think this is an example of an unusual-looking Colpitts oscillator. That might help you.

The capacitance from collector to base of the Q1 and Q2 add to C4 since they are all in parallel. At frequencies like this, stray capacitance is a large factor.
For giggles: https://wiki.analog.com/university/courses/electronics/comms-lab-colpitts-osc

27 MHz? In about 1969 I set up a little transmitter based on a circuit like this in my apartment and tuned it up on Citizen's Band channel 11 (a popular calling channel). I managed to make contact with a local but the little transmitter drifted so much that the contact was soon lost. After that most of my experiments in that band were crystal controlled.
 

ci139

Joined Jul 11, 2016
702
Last edited:

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
8
it's a complex LRC network and i suspect the variable capacitor C2 introduced from voltage divider R4, Q1, R5 may also have it's part in it
there's a VCO that uses such effect (C3 is likely 10nF) . . . View attachment 186486
http://pdf.datasheetcatalog.com/datasheet/on_semiconductor/2N4264-D.PDF
___
i donno how it* suppose to work in real (* foster-seeley-detector-discriminator from fm-frequency-demodulation-detection-discrimination) but i got it to do smth. → View attachment 186493





Thanks for all the help guys, I think I have what I needed to know now. Dicks comments that his 27Mhz transmitter drifted a lot, and other posts that I have read also state this type of simple transmitter drifts, lead me to think I will need to use a crystal oscillator.

Once again thanks for the help, I'll go and try and find the crystals and gone back when I need more help.

Garry
 

Audioguru

Joined Dec 20, 2007
11,251
The FM transmitter circuit shown in this thread did not work years ago on another website so built it to confirm its design errors and I corrected them:
1)The audio transmitter is saturated when the battery is new and is cutoff when the battery voltage has dropped a little so I biased it better and used a 5V low dropout voltage regulator for it.
2) The radio frequency changed as the battery voltage ran down so I connected the RF oscillator transistor to the 5V regulator.
3) The radio frequency changed when something moved towards or away from the antenna so I added a buffer transistor between the oscillator and the antenna.
4) The received sounds were muffled like you stereo with its treble tone control turned down all the way so I added Pre-emphasis like all FM radio stations have.
My corrected circuit works well, sounds perfect and has a range that is about 2km across a large river valley to a hifi tuner. It is probably illegal to use without a licence.
 

Attachments

Thread Starter

GarryO'Keeffe

Joined Jan 28, 2015
8
The FM transmitter circuit shown in this thread did not work years ago on another website so built it to confirm its design errors and I corrected them:
1)The audio transmitter is saturated when the battery is new and is cutoff when the battery voltage has dropped a little so I biased it better and used a 5V low dropout voltage regulator for it.
2) The radio frequency changed as the battery voltage ran down so I connected the RF oscillator transistor to the 5V regulator.
3) The radio frequency changed when something moved towards or away from the antenna so I added a buffer transistor between the oscillator and the antenna.
4) The received sounds were muffled like you stereo with its treble tone control turned down all the way so I added Pre-emphasis like all FM radio stations have.
My corrected circuit works well, sounds perfect and has a range that is about 2km across a large river valley to a hifi tuner. It is probably illegal to use without a licence.


Thanks for posting the updated circuit, what you said makes complete sense. Also thanks for posting the photo as this gives me an idea of what the inductors should look like. I have not yet tried to wind an inductor and to see one that has an actual value really helps.

As I said earlier I wasn't going to make this circuit as I really want a 27Mhz tx, but I might have a go.

Thanks
 

ci139

Joined Jul 11, 2016
702
connected the RF oscillator transistor to the 5V regulator
gave the up vote for stable variant of X-tal-less transmitter

//// i tested some FM bug-s --

the 1-st had large diameter coil and was assembled in "open-air" 3D wire-mesh , fed from non-stabilized supply , with simple wire antenna
-- got it near stable -- drifted some 50kHz per minute

the last 1 had regulated supply , coils fixed on pcb , tuned antenna (with autonomous resonator)
-- it was seemingly stable at least the ~1h period of testing ...
... there was no intentional pre-emphasis but the sound was better than with some low cost speakers attached to AUX LINE OUT ??
_____________
about transmitter iC-s ::
http://www.circuitstoday.com/stereo-fm-transmitter-using-ba1404
https://www.radiolocman.com/shem/schematics.html?di=55292 (BH1417)
https://www.maximintegrated.com/en/design/technical-documents/app-notes/6/688.html (MAX2606)
https://hackaday.io/project/55963-tiny-fm-transmitter (MAX2606)
 
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Audioguru

Joined Dec 20, 2007
11,251
Did you notice that my coils have enamel insulation? I "stole" a short length of the wire from a crossover coil in a two-way speaker.
 
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