Ramsey Electronics FM100B, Инструкция по эксплуатации

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FM100B
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list the part numbers here since they are easy to spot in the boxed off area.
The graph shows the low-pass response of the audio filter. You can clearly see
the two notches that combine together to give a nice sharp low pass response.
Part of the output of U4:D (the output of the low pass filter) is monitored by the
level indicators. IC U7:B, D5, R71 and C69 comprise a peak hold detector. Part
U7:B and D10 make up a real diode, meaning there is no .7 volt drop that is
normally associated with a diode. Because the diode is accounted for in the
feedback of the opamp its forward voltage drop is nullified. The “real diode” will
charge C69 quickly on positive going signals without discharging it on the
negative swings. The discharge cycle (or rate) is left solely up to R71. The
larger the value, the longer the time the peak hold function is. The voltage on
the peak hold is then observed by using an LM3915 bargraph display driver
(U9) and a ten segment LED bargraph. This part is pretty self explanatory, it’s
really just a voltage meter with a log scale instead of a linear one.
The rest of the audio from U4:D enters U2 (the stereo modulator IC) and is
converted to RF for transmission. This audio can also be monitored on J6’s
RCA audio monitor output which is at line level by this point. You can use a
tape recorder on these or a headphone amplifier to hear what you are
transmitting before it goes out over the air!
Now we can go back to the microphone amplifier. U12 is a microphone
conditioner IC. It has a feature that we use in the FM100B that really help us
out by eliminating a lot of manual work. This is the AGC or Automatic Gain
Control. This prevents us from overloading the audio circuitry when we get
excited and yell into the microphone. When the amplitude of the signal coming
from the microphone increases, the gain of the microphone amp decreases to
keep its output relatively the same over varying input levels.
U13 is the voice detection IC. Voice detection is used to make the Auto AGC
feature of the FM100B. Essentially pin 7 of U13 goes high when there is a
varying signal level seen on the microphone (as compared to the constant level
of background noise). When pin 7 goes high, it turns JFETS Q7 and Q8 on
making them act like voltage controlled pots. The more they are turned on, the
less resistance is seen from source to drain. This has the effect of muting the
audio from the line level inputs and allowing only the microphone to be heard
when there is an active voice signal detected.
U1 (the microcontroller) has the ability to override this feature by turning on
transistor Q5 and pulling the gate inputs of Q7 and Q8 low. This prevents them
from turning on no matter what the output of U3 tries to do. The microcontroller
can also mute the microphone audio by setting the MIC_MUTE line high. This
turns on Q2 and grounds out all the microphone audio. R30 and C25 smooth
out the switching transitions so that there is very little popping heard like is
apparent in most switches. This MUTE line is also used to turn on the
microphone line when the speaker sounds a tone to prevent it from being