fred said:
I have found an fm radio transmitter circuit diagram,
can't remember where it was so I put it here:
http://www.geocities.com/x_file_space/fmt-1.gif
This circuit looks too simple to be any good, but just
out of interest I was wondering how the fm modulation
is occuring. Obviously the tuned circuit will determine
the main frequency, but how varying the base of Q2
varies the frequecy this I don't see, any ideas?
Here is my long, boring guess about how this oscillator works:
http://www.meridianelectronics.ca/circ/fmt1.htm
I don't think its a colpitts oscillator, at least as I understand them. A
colpitts oscillator will use two series caps in the tank as a phase shifting
device, to allow the phase of the excitation waveform to vary by 180 degrees
from the feedback waveform. This is done so it can be fed back into an
inverting transistor amplifier, such as a common emitter amplifier.
For this one, however, I think that the way to analyze this is to see that
Q2 is configured as a common base amplifier, which is NOT an inverting
amplifier. Also, the capacitors that influence the frequency are all in
parallel, not in series.
For a common base amplifier, the signal is applied to the emitter, and the
output is at the collector, in phase (mostly) with the input. (There is a
slight phase shift through the transistor, which I believe will limit the
max output frequency.)
The gain of the amplifier increases with the impedance of the tank, and the
tank impedance increases as the frequency aproaches the resonant frequency.
The capacitor C5, in parallel with the capacitance between the collector and
the emitter in the transistor, form the feedback of the system. They modify
the voltage at the emitter, which is then amplified back to the collector,
forming a feedback loop.
The gain of the amplifier is maximum when the impedance of the tank is
maximum. That will happen when the tank is in resonance, ie, when the
frequency is equal to
1
f0 = ----------------
2.pi.sqrt(L1.Ct)
where Ct is the total capacitance in the tank... Thus, the system will
oscillate at frequencies near the point where there is maximum impedance,
ie, the tank resonance frequency.
Ok, that allows it to oscillate, and sets the carrier frequency. Now lets
talk about modulation.
Since the inductance of the tank is known, varying the capacitance of the
tank will vary the carrier frequency, thus modulating the output.
The capacitance of the tank is due to the parallel combination (ie, the sum)
of C4, C5, and the capacitance between the collector and emitter of the
transistor. Thats C4+C5+Cce. The last term, Cce, is a series term of Cbe,
which is proportional to Ic, as Kevin Aylward mentioned in his message. Ic
is modified by the base voltage. Consequently, the total capacitance of the
tank, Ct, is influenced by the bias voltage of the transistor, which is how
the output signal is modulated.
Unfortunately, the capacitance of the transistor is hard to predict.
Consequently, the variable cap is needed to tune the system to a particular
frequency. I believe Cce will be influenced by temperature as well, so the
system will require tuning at different temperatures, and probably as it
warms up as well.
Additionally, I believe the antenna will also contribute slightly to the
capacitance of the tank in this transmitter. However, I don't have any way
to predict how much of an effect that will have.
Comments requested...
Regards,
Bob Monsen