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Parts List:
R1 - 47K
R2 - 22K
R3 - 39K
R4 - 100
R5 - 27K
C1, C2, C4, C5, C9 - 1n
C3 - 4.7uF
C6 - 15pF
C7 - 6.8pF
C8 - 75pF
C10 - 100nF
C11 - 2pF
C12 - 10pF
C13 - 100uF
Q1, Q2 - BF199
L1 - 3.5 turns / 5mm dia. variable coil (65nH - 80nH).
L2 - Ferrite bead 1 turn or use 1uH inductor.
Technical Specifications:
Supply Voltage: 9V
Power Consumption: 35mA
RF Power: 200mW
About TX200 - 200mW FM Transmitter
Here is the latest and greatly improved TX200 VFO/VCO FM
transmitter. The most versatile transmitter to date that can be
turned into high fidelity stereo PLL based 200mW FM transmitter. It
is a perfect circuit for transmitting your music around the house
and yard. TX200 uses only two coils; one in the oscillator and the
other one in the 200mW VHF amplifier so it should be fairly easy for
anyone to build. It also includes built-in pre-emphasis and C5 for
enhanced sound quality. While assembling the transmitter care must
be taken to make sure that C1 is directly connected to L1 and C9 to
L2. These caps eliminate the distortions form the DC supply and
improve the sound quality greatly. 9V voltage supply is also very
important because it provides the exact amount of current to Q1 to
produce loud and clear sound quality. I hope that you'll have as
much fun as I had while building this transmitter. Enjoy! ;)
Power Meter
TX200 comes with built-in LED based power meter. This is a very
helpful tool that will tell you if your transmitter's oscillator is
working properly. If RF signal is transmitted the LED will
illuminate. Besides that, it will also give you a quick visual way
to check how much power is being transmitted. I highly recommend
that you have the transmitter and power meter on the same PCB. If
you like to experiment a lot, you will appreciate this inexpensive
but externally helpful addition.
How to use PLL to digitally tune the frequency of the TX200
transmitter?
If you want to use Phase Locked Loop (PLL) to digitally tune the
frequency of TX200 then you will need to make small changes to the
transmitter's oscillator. Theoretically, change original variable
frequency oscillator (VFO) to voltage controlled oscillator (VCO).
PLL uses voltage to change the capacitance of the varicap diodes /
frequency of the oscillator so two BB105 varicap diodes or any
15-30pF varicaps will be perfect for this.
You vary the capacitance of the varicap diode(s) by applying voltage
supply between 1 - 8V. If you apply 1V the capacitance will be
around 34pF, and if you apply 8-30V the capacitance will be 12pF
(minimum). Now, you may wonder why do we need to start from 1V? The
reason is that if you apply voltages below 1V the capacitance may go
as high as 200pF and we simply can't use that. 100K resistor will be
used to lower the current of the voltage, because varicaps require
only 0.2mA of current to operate!
With this knowledge we replace C6 - 15pF capacitor with two varicaps
and one resistor. That much is enough to be able to use 1 - 8V
voltage to change the oscillator's frequency. The second change that
we will need to make is to use 2pF capacitor and connect it to the
Q2 transistor's collector. PLL will use this to sample the frequency
and compare it with digitally selected frequency. PLL will then
compare those frequencies and if they are not the same PLL with vary
the voltage that is applied to the varicaps, until those two
frequencies are the same.
Before you even connect PLL circuit to the TX200 transmitter you
will need to test if the oscillator is working properly. We can tune
through entire FM band (88-108MHz) by applying the voltage to the
varicaps. To do so, connect "PLL - Voltage Control" PIN to the 10K
potentiometer as shown below. With this setup you will be able to
use potentiometer to change the frequency of the transmitter. If you
can't go through the entire FM band or if your frequency is out
completely then you will need to change the inductance of the L1
coil. |
