Ramsey Electronics FM30, Руководство пользователя

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FM30
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8
on pin 7 needs to be filtered and amplified for good control, and that is
provided by Q2 and Q3, along with R18, R17, C34, R20, C38, R21, and C33.
All of these parts together act like a low pass filter to slow the tuning time
enough for our multiplex signal to still be transmitted, but the tuning signals to
not be transmitted. The parts in this section are carefully chosen for best
operation in audio performance, but have the drawback of being really slow to
tune from one end of the dial to the other. To speed it up would mean to loose
some of the low frequency response of our audio signal. In the case of the
FM30, we have made the PLL very slow to pass low frequency audio very well
for best quality. You will just have to be patient when you turn the unit on
before you can begin transmitting. Since most of these transmitters are left
running all the time on the same frequency, we knew this wouldn’t be a
problem for most of you.
Now there are some other tricks going on in the PLL and the VCO circuit
which you may wonder about. What is the purpose of Q4? Well, because we
are tuning a broad range of frequencies in the FM30, the VCO’s tuning
sensitivity vs. frequency can change quite a lot. Without this part a properly
ad/-75kHz FM stereo signal at 108.1MHz may be +/-250kHz by the
time you get down to 88.1MHz! This is unacceptable, because it would be
very hard to know when you were over-modulating without test equipment.
This would also render the quality of signal detector useless that we have on
the audio inputs. Q4 corrects the tuning sensitivity by acting like a variable pot
based on the tuning voltage. As the tuning voltage goes down, Q4 is turned on
more and more, which in turn allows less and less of the multiplex signal that
is injected into the VCO from pin 5 of U3 to reach the VCO. It is a great little
compensation circuit which keeps our signal /-10kHz of error across
the band!
Now that we have our locked frequency and our multiplexed signal added to it
we want to get the level up to where we can transmit it! The level directly out
of U3 on pin 11 is small, just enough to cover a room, so we need to boost it a
little. U8 is a high-gain amplifier which will get up the level for us so the FM30
has a little more “oomph”. However we don’t want too much “oomph” where
we don’t need it, so before the final amp we have D8, which is another special
variety of diode called a pin diode. This diode has a neat way of working like a
variable RF resistor. As you put more and more current through it, more and
more RF will pass through it too. To vary the current through D8, we have U7,
which is a digital to analog converter. This will generate from 0-5 volts in 256
steps, and in doing so, will drive D8 with 256 steps of current control, which
gives us the ability to control output level quite a bit. L5 and C62 prevent RF
from getting back into the gain control of U7 and messing it up.
The higher level RF out of U8 is then sent through a low pass RF filter to
reduce harmonics, and finally to the output jack where you would connect an
antenna. L6 is simply to provide a DC path to ground in case of any static