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Specifications:
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Supply
Voltage: 4 - 6V
Transmission Frequency: 87.7 - 88.9MHz,
106.7 - 107.9MHz (200kHz steps)
Output RF Power: 20mW
Audio Frequency: 20 - 15KHz
Separation: 40dB
Power Consumption: 30mA |
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About
BH1417 Stereo PLL FM Transmitter
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This is the
latest BH1417 FM Transmitter design from RHOM that
includes a lot of features in one small package.
It comes with pre-emphasis, limiter so that the
music can be transmitted at the same audio level,
stereo encoder for stereo transmission, low pass
filter that blocks any audio signals above 15KHz
to prevent any RF interference, PLL circuit that
provides rock solid frequency transmission (no more
frequency drift), FM oscillator and RF output buffer.
There are 14 possible transmission frequencies with
200KHz increments that users can select with a 4-DIP
switch. Lower band frequencies start from 88.7 up
to 89.9 MHz, and upper band frequencies start from
107.7 up to 108.9 MHz.
BH1417 can be supplied with 4 - 6 voltage and consumes
only around 30mA, providing 20mW output RF power.
BH1417 provides 40dB channel separation which is
pretty good, although older BA1404 FM Transmitter
chip provides slightly better 45dB channel separation.
BH1417 is only available in SOP22 IC case so this
may be an inconvenience for some folks. On the other
hand, because the chip is smaller than regular DIP-based
ICs it is possible to fit the entire transmitter
on a small PCB.
BH1417 chip may also be used a stand alone stereo
encoder. The advantage of that is that you have
full freedom of using a transmitter & amplifier
of your choice. You will still have a pre-emphasis,
limiter, stereo encoder and low pass filter in one
small package because very few external components
are required for these blocks. PIN 5 is MPX output
that can be directly connected to an external FM
transmitter through a 10uF cap.
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BH1417
Block Diagram
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Main blocks of BH1417
chip are; pre-emphasis, (audio level) limiter, stereo
encoder (MPX), PLL circuit, oscillator and RF buffer.
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Frequency
Selection / Calibration
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Frequency
selection is very straight forward. Simply select
transmission frequency at which you would like to
transmit, set the combination for 4-DIP switch and
BH1417 will immediately tune to that frequency.
If you can't hear the transmitted audio signal on
your FM receiver then re-adjust 2.5 turn variable
coil until you can hear the signal.
As a test to see how PLL is working you can use
a laboratory power supply to vary the
voltage supply from 4 to 6V. While doing that BH1417
will automatically vary the voltage for MV2109 varicap
diode making sure that there's no frequency drift.
| S1 |
S2 |
S3 |
S4 |
Frequency |
| ON |
ON |
ON |
ON |
87.7 MHz |
| OFF |
ON |
ON |
ON |
87.9 MHz |
| ON |
OFF |
ON |
ON |
88.1 MHz |
| OFF |
OFF |
ON |
ON |
88.3 MHz |
| ON |
ON |
OFF |
ON |
88.5 MHz |
| OFF |
ON |
OFF |
ON |
88.7 MHz |
| ON |
OFF |
OFF |
ON |
88.9 MHz |
ON |
ON |
ON |
OFF |
106.7MHz |
OFF |
ON |
ON |
OFF |
106.9MHz |
ON |
OFF |
ON |
OFF |
107.1MHz |
OFF |
OFF |
ON |
OFF |
107.3MHz |
ON |
ON |
OFF |
OFF |
107.5MHz |
OFF |
ON |
OFF |
OFF |
107.7MHz |
| ON |
OFF |
OFF |
OFF |
107.9MHz |
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Frequently Asked Questions
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This
is list of answers to questions that I have
received through an e-mail so far. If you should
have any other questions please don't hesitate
to e-mail them to me.
What is the range of BH1417 Stereo FM
Transmitter?
Range of the transmitter depends on many factors.
a) Length and type of the antenna (the longer
the better, copper will radiate radio waves
better than aluminum)
b) Supplied voltage (6V Max power output)
c) Weather conditions (clear skies & no
humid, greater distance)
d) Elevation above the ground (the higher the
better)
e) Line of sight (no buildings or trees, greater
distance)
f) Whether external amplifier is used
I would like to be able to transmit
the music to greater distances. Can you provide
any suggestions on how to modify the circuit?
You can connect an external
amplifier based on the common 2N3866 / 2N4427
RF transistors. Please keep in mind that it
is illegal to operate higher power transmitters
in most of the countries without the proper
license.
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Where to get the Parts
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If you are building the
above Stereo PLL FM Transmitter and have trouble
finding some of the components like BH1417,
7.6 MHz crystals, 2.5 turn high precision variable
coil and MPSA13 darlington transistor we are
distributing these components at Electronics-DIY
store.
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