Transmitter

[Ken O]

HI all,

I was looking at some RF circuits. I have a question concerning the
capacitor c4 in the folowing diagram:
http://www.uoguelph.ca/~antoon/circ/tx15trak.htm
Apparently this capacitor keeps the carrier oscillation ( C3 L1 ) from
decreasing , or by keeping its voltage steady. Theoretically we would not
need that. But with heat loss we do, I do not understand is HOW the C4
capacitor do that, that is keeping a constant voltage for the oscillator.

thanks

ken

 

[Tom Biasi]

HI all,

I was looking at some RF circuits. I have a question concerning the
capacitor c4 in the folowing diagram:
http://www.uoguelph.ca/~antoon/circ/tx15trak.htm
Apparently this capacitor keeps the carrier oscillation ( C3 L1 ) from
decreasing , or by keeping its voltage steady. Theoretically we would not
need that. But with heat loss we do, I do not understand is HOW the C4
capacitor do that, that is keeping a constant voltage for the oscillator.

thanks

ken

The circuit is not very conventional. He spikes the tank with a LED pulser
chip.
Why don't you direct your questions to Tony?
Post on his forum regarding this circuit.
Here:
http://www.uoguelph.ca/~antoon/Forum/Forum.html

Tom

 

[Andrew Holme]

HI all,

I was looking at some RF circuits. I have a question concerning the
capacitor c4 in the folowing diagram:
http://www.uoguelph.ca/~antoon/circ/tx15trak.htm
Apparently this capacitor keeps the carrier oscillation ( C3 L1 ) from
decreasing , or by keeping its voltage steady. Theoretically we would not
need that. But with heat loss we do, I do not understand is HOW the C4
capacitor do that, that is keeping a constant voltage for the oscillator.

Normally, single transistor VHF transmitters have an emitter resistor.
Here's an example http://www.reconnsworld.com/transmit/sens_fmt.gif

The transistor operates in common base mode, with the 4.7pF capacitor
providing feedback from collector (output) to emitter (input).

Although that tracker circuit shows an earth symbol connected to the
emitter, the "common" point is really the junction of C2, C3 and L1. Think
of everything between this point and the emitter as a battery and resistor
in series and you have the standard circuit.

 

[Ken O]

Normally, single transistor VHF transmitters have an emitter resistor.
Here's an example http://www.reconnsworld.com/transmit/sens_fmt.gif

The transistor operates in common base mode, with the 4.7pF capacitor
providing feedback from collector (output) to emitter (input).

Although that tracker circuit shows an earth symbol connected to the
emitter, the "common" point is really the junction of C2, C3 and L1.
Think of everything between this point and the emitter as a battery and
resistor in series and you have the standard circuit.

My question was more with the use of C4. If I remove c4, the circuit
should
still operate. But I read on a web page that this capacitor is needed
in
real life because there is loss. this capacitor help keeping the
voltage to
the tank circuit. I just like to know how it keeps the voltage of L1
and C3.
The way I see it, C3 and L1 oscillate at about 90MHz all the time, then
the
beep (output of the IC1) turns on the base slowing the oscillation .
Therefore, c4 theoretically is not needed , right?

ken

 

[Rich Grise]

My question was more with the use of C4. If I remove c4, the circuit
should
still operate. But I read on a web page that this capacitor is needed in
real life because there is loss. this capacitor help keeping the voltage
to
the tank circuit. I just like to know how it keeps the voltage of L1 and
C3.
The way I see it, C3 and L1 oscillate at about 90MHz all the time, then
the
beep (output of the IC1) turns on the base slowing the oscillation .
Therefore, c4 theoretically is not needed , right?

Wrong. It is an integral part of the oscillator. If you remove it, the
best you will get is some ringing (a damped sine wave) when your timer
gives it its "ON" pulse.

Try googling for "oscillator circuit" or so.

Good Luck!
Rich

 

[Andrew Holme]

My question was more with the use of C4. If I remove c4, the circuit
should
still operate. But I read on a web page that this capacitor is needed in
real life because there is loss. this capacitor help keeping the voltage
to
the tank circuit. I just like to know how it keeps the voltage of L1 and
C3.

The transistor is an amplifier. Some energy is fed from the tuned circuit,
via C4, to the emitter, which is the amplifier's input. The amplifier
supplies just enough energy back into the tuned circuit to replenish what is
taken/lost, thereby sustsaining the oscillation. If you remove C4, you
break the feedback loop, and oscillation ceases.

The way I see it, C3 and L1 oscillate at about 90MHz all the time, then
the beep (output of the IC1) turns on the base slowing the oscillation .
Therefore, c4 theoretically is not needed , right?

The output of IC1 powers the oscillator. When there is no power, there is
no oscillation.

 

[Andrew Holme]

The transistor is an amplifier. Some energy is fed from the tuned
circuit, via C4, to the emitter, which is the amplifier's input. The
amplifier supplies just enough energy back into the tuned circuit to
replenish what is taken/lost, thereby sustsaining the oscillation. If you
remove C4, you break the feedback loop, and oscillation ceases.


The output of IC1 powers the oscillator. When there is no power, there is
no oscillation.

OK, maybe I take that last sentence back, having looked at the LM3909
datasheet. If there was sufficient voltage between pins 6 and 4 to keep the
oscillator going all the time - and I don't know if there is - then it would
oscillate "all the time" and be both amplitude and frequency modulated by
the flasher. But, either way, C4 is required.