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AM + 450kHz
Detector +
Audio Out
Since the AR2 is a relatively complex (and educational!) product, it's easiest to
show you how things work by using a block diagram and explaining the
different sections. This is a simplified block diagram, but is suitable for our
needs.
Block A. RF band pass pre-filter. This is one of the two filters that helps
reject image frequencies. Note the PCB layout has coils built right in! This
makes for very precise, repeatable coils, and since the inductance for this
½
design needed to be very small, this was a lot easier than installing a 1
turn
coil.
Block B. RF amplifier. This amplifier makes up for the "insertion loss" of the
two filters, and increases the radio's sensitivity to weak signals. Insertion loss
is defined as how much desired signal is lost by a filter or device.
Block C. RF band pass secondary filter. This section further reduces the
possibility of image frequencies being received.
Block D. IF Mixer/VCO. This section consists of an NE602 mixer/oscillator
that can be tuned using what is called a varactor diode in the oscillator
section. A varactor diode is a reverse-biased diode that changes capacitance
according to the reverse voltage. The higher the voltage, the wider the
insulation (depletion layer) and the smaller the capacitance. Thus as the
voltage goes up across the diode, so does the frequency! The tuned inductor
sets the center frequency of our tunable band. We are limited to a little more
than 21MHz, but we are never sure where this range is due to part variances.
The coil allows us to get the 21MHz range centered in our band of interest so
that the frequency will always remain locked throughout the band.
Block E. 10.7MHz IF filter. This is a special ceramic 10.7MHz filter for
narrow-band AM. This helps in rejecting adjacent channels which are only
25kHz away. The filter is 13kHz wide, meaning 6.5kHz to either side.
Block F. This is a simple, single transistor IF amplifier. It has quite a lot of
gain to make up for the insertion loss of the mixer/VCO as well as the
10.7MHz IF filter. This is the advantage of using a low frequency IF since a
simple transistor amplifier works well here.
AR2 • 11
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