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SR2 – 7
varactor diode tuning. The varactor (D3) forms an L/C (Inductor/Capacitor)
tank circuit with T3. Increasing the voltage on the varactor diode with R21
(Tuning Pot) increases the capacitance of D3 thus increasing the frequency
output of the LO section.
Now that we know the two signals coming into the Mixer stage, both the Fc
(receive carrier frequency) and the LO (generated local oscillator), we can
better cover its operation. The mixer takes these input signals and performs
a few very basic operations. The technical explanation of how the mixer
combines these signals through Fourier Series is interesting but very drawn
out. The point of using the block diagram, however, is to simplify matters.
Therefore, the function of the mixer is to obtain the ‘product’ and the ‘sum’ of
the input signals. This means you take the input signals and merely ‘add’
and ‘subtract’ their values to determine what you get on the output. The
general formulas to use are quite simply, ‘Fc + LO = Mixer Output’ and ‘Fc –
LO = Mixer Output’.
The realistic output of the mixer stage unfortunately has other signals
besides the ones we want. This brings us to the next stage, Intermediate
Frequency Filtering. An Intermediate Frequency (IF) is a signal somewhere
between the RF signal Fc we started with and the final audio message we
are trying to get. The desired IF we are dealing with is a fixed number, such
as 455 kHz. As long as the proper relationship between Fc and the LO
exists, the IF value will stay constant. Due to the fact that the IF frequency
stays the same all the time, the Intermediate Frequency Filter can be very
narrow. The filter will remove any other signals coming from the mixer that
are not in the proper pass band and yield a clean signal for further
processing.
The next stage is an Amplifier with an adjustable gain feedback loop. The
gain control is dependant on the amount of signal being received (this
Automatic Gain Control ‘AGC’ will be covered in a moment). The amplifier
boosts the signal level of the incoming IF and gives us a stronger signal to
work with.
After the amplifier stage is another Intermediate Frequency Filter. This helps
remove any unwanted residual signals still present and cleans up the
amplified IF for a high quality signal.
At this point the audio signal we are trying to obtain is riding on the IF signal.
The Demodulator circuit finally extracts the message from the IF through a
process called envelope detection. Now that we have our message back in
the audio realm, it is directed through the audio circuitry to the speaker
output.
Wait, there are still a few sections we have not discussed!
The demodulated audio branches off before the audio circuitry and is used
to perform some useful functions. The RSSI LED, Received Signal Strength
Indicator Light Emitting Diode, gives us a general signal level feature. The
stronger the signal we receive, the brighter the LED will glow. This is great
for help pulling in those weak transglobal transmitter signals. The gain of the