FM-100 Page 7
If you are a close observer, you will notice that U4 derives its clock signal from
the PLL. The PLL has a nifty feature of a buffered divided crystal output to drive
external devices such as microcontrollers. This is another part of the 32 bits we
send to the microcontroller other than the divide by N info. When first powered
on, the XCLK line is at the lowest possible divide by C frequency, which is
6MHz / 16 or 375KHz. This runs the microcontroller at a pretty slow rate until
we can send out new information using a C of 4, giving us a clock of 1.5MHz.
This clock frequency is not only used by the microcontroller U4, but also by U9
and U10, the switched capacitive filter circuitry that we will talk more about in a
minute.
U1 is a serially accessed EEPROM, which stores our frequency settings for the
next time we power up the FM-100. This prevents us from having to go through
setting the proper frequency every time we turn off the FM-100. The frequency
data is only set to the EEPROM when the user exits the setup mode, and a
confirming beep from SP1 lets the user know it is written.
U6, the BA1404 FM stereo transmitter IC is what does all the work of creating
your stereo subcarrier, and doctoring your audio signals for transmission. The
BA1404 was originally designed to run from a couple of 1.5 volt batteries, so it
was made to be run at a low voltage. We decided to use a 2.5 volt regulator
(VR1) to give us the proper voltage at the greatest noise immunity from the
power supply. The low level RF output of U6 is then fed to Q2 and surrounding
parts to boost it to a more reasonable level for filtering. Since the RF output of
the BA1404 is not the cleanest, much filtering is required to remove unwanted
transmissions that are outside of the FM band.
The primary filter for the job is the bandpass filter consisting of C84, L5, L4,
C82, L6, and C85. This has a lower cutoff at about 70MHz, and an upper cutoff
at 120MHz. The output of this filter is sent to a class C amplifier consisting of
Q1 and R32. This gives us a higher level output which is adequate for local
broadcast. But before going to the antenna, we low pass filter the output at
110MHz. This gets rid of the unwanted upper harmonics that are very present
in a class C amplifier. Harmonics are multiples of the primary frequency and the
primary one we are trying to get rid of is the second harmonic (F x 2), which in
our case winds up in the aircraft band. It is extremely important for us not to
interfere with ANY other transmissions in ANY band. These filters will keep us
from doing any harm outside our intended transmission band.
Now that we’re done talking RF, we can get on with the audio. We will begin
with the microphone amplifier. U7, a voice detector chip is used in this circuit. It
has a couple features that we use in the FM-100 that really help us out. One
feature is the AGC or Automatic Gain Control. This prevents us from
overloading the audio circuitry by getting excited and yelling in the microphone.
When the amplitude of the signal coming from the microphone increases, the
gain of the microphone amp decreases, keeping its output relatively the same
over varying input levels.
Содержание FM-100
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