|R1 - 47K
R2 - 22K
R3 - 39K
R4 - 100
R5 - 27K
C1, C2, C4, C5, C9 - 1n
C3 - 4.7uF
C7 - 6.8pF
C8 - 75pF
C10 - 100nF
C11 - 2pF
C12 - 10pF
C13 - 100uF
Q2 - BF199
L1 - 3.5 turns / 5mm dia. variable
coil (65nH - 80nH).
L2 - Ferrite bead 1 turn or use 1uH
Power Consumption: 35mA
RF Power: 200mW
TX200 - 200mW FM Transmitter
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! ;)
to use PLL to digitally tune the frequency of the TX200 transmitter?
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.
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.
TX200 a Stereo Transmitter!
The schematic and instructions
for building this PLL module can be found at
After building it and doing some testing with
it I was amazed by how well this Phase Locked
Loop circuit performs. It's stable, compact,
very easy to build and connecting it to TX200
transmitter was a piece of cake!
You can easily turn TX200 into
a high quality stereo transmitter by connecting
this stereo encoder module that you can either
it yourself or buy
it as a kit. Stereo encoder kit
includes all the hard to find components such
as NJM2035 IC and 38KHz crystal.
Where to get the Parts
If you are building the above FM Transmitter and have trouble finding components such as variable coils, BF199 transistors or ferrite beads we are distributing these components in our Electronics-DIY store.
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