FM30
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6
surrounded by a bunch of resistors and capacitors. This arrangement of parts
is a low pass filter. This filter is designed to help reduce high-frequency audio
signals from mixing with the stereo pilot signal and producing mixing products
of various frequencies. Ok, perhaps that is too much to bite off. In layman's
terms, this low pass filter prevents high frequency audio signals like symbols
and chimes from getting garbled up during the creation of a stereo signal
before transmission, so it increases the audio quality of the final transmitted
signal.
This signal is then piped off to U3, the BH1415F stereo modulator IC. We
wont get into that quite yet, because we have another circuit of importance
before this that you will find to be very helpful to you during day to day use!
Take a look at D9, D10, and R39. These are peak detectors that sample both
the left and right channels and combine them together, which is presented on
pin 3 of U5:A. The levels that these diodes detect are then compared against
the constant voltage level seen on pin2 of U5:A. Right now R33 and R38 are
not used, but are there in case the threshold needs to be changed in future
revisions. Your product uses the 2.5V bias reference because it happened to
be the correct value to detect audio peaks. Notice I used the term compare;
U5:A is set up to be a comparator since it has no negative feedback.
Once the voltage on pin 3 surpasses that on pin 2 (our reference), the state of
the output pin 1 of U5:A goes high (5V). If the voltage on pin 3 goes below that
on pin 2, the output goes low (0V). The comparator is set up with these
specific diodes and voltage reference so that audio peaks surpassing a very
specific level flip the comparator state back and forth. This change of state is
then sent to the microcontroller U1, and the program in there counts how
many times this happens in an interval of time and then computes a quality of
signal value for you. In our case, the comparator is designed to detect peaks
over +/-75kHz of deviation, which is the standard bandwidth used by radio
stations. If you run the volume up too high on the FM30, the comparator
reference level is surpassed often, the micro counts this, and an indicator on
the display will show a poor quality of signal indication.
Speaking of the reference voltage, all of the analog circuitry needs to be
biased up at 1/2 of the supply voltage so that your audio signal has the
greatest possible dynamic range. We would like the analog signals to be able
to go all the way from 0V to 5V, and in order to do this with minimal distortion
we need 2.5V. This bias voltage is generated with a simple resistor divider
consisting of R22 and R24, and then noise filtered with C42 and C43. This
voltage is then “amplified” with U5:D which is set at a gain of 1 and then
distributed to the rest of the analog circuitry.
This leads us to the next important part of the circuit, the microcontroller. This
section is essentially the “brains” of the show. The microcontroller does
several things all of the time; it checks for button presses, it counts the quality
