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CIRCUIT DESCRIPTION
Radio Basics:
We'll take the circuit section by section; the letters show which part of the
block diagram we're explaining in each section.
The AR2 is a simple super-heterodyne receiver. A heterodyne receiver is a
receiver that first converts the desired received frequency into an IF
frequency, or Intermediate Frequency. Many radios are designed this way due
to a variety of reasons, but primarily because it is easy to perform filtering and
amplification on the IF signal rather than the RF signal because the IF is a
single frequency or small band of frequencies, whereas RF is quite a bit wider
and usually much higher in frequency than IF.
To create an IF frequency we use a mixer (D) to down-convert the band of
interest to the single IF frequency. A mixer is a non-linear device, meaning it
will distort the incoming signal with an applied signal. In the case of a mixer
used on a radio this means you will have two different frequencies on the
inputs, which results in four signals on the output. The received frequency is
connected to one input and the local oscillator or LO is supplied to the other.
On the output you will have these two important frequencies as well as the
sum and difference between the two frequencies.
IF
Fr
LO
F
Figure 1
IF
Fr
F
Figure 2
Fi
Sum
LO
Fi
Sum
For example the aircraft band goes from
118MHz to 139MHz, and we plan to use
an IF frequency of 10.7MHz, we need to
have an LO of either 10.7MHz above the
frequency we wish to receive, or
10.7MHz below. It is a matter of
preference in which mixing byproduct you
wish to work with, but in the case of the
AR2 we use an LO of 10.7MHz above the
frequency we wish to receive. This
means that the LO needs to tune from
128.7MHz to 149.7MHz. This is called
high-side mixing since the LO is above
the frequency of interest. The LO is
generated using a phase locked loop
and voltage controlled oscillator on the
AR2 (J, D).
Let's say for example that we wish to
listen to the control tower at ROC
(Rochester International Airport) at
118.300MHz. The LO frequency would
AR2 • 8
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