SR2
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6
you, you'll be in a better position to choose a more elaborate receiver. While
there are various multi-band portable radios available, you can expect to pay
at least $100 for a receiver offering a significant improvement over your
trusty SR2.
To learn more about this SWL hobby, look for a copy of "Popular
Communications" at newsstands. An inexpensive and interesting general
introduction to all kinds of radio listening is the book, "Shortwave Listening
Guide" by William Barded, Jr. (1987; Radio Shack Catalog Number 62-
1084). This book also includes helpful introductory information about VHF
monitoring, which you can enjoy with the Ramsey FR-146 and AR-1 receiver
kits, as well as HAM radio, CB, antennas, and other topics.
To learn more about Ramsey Electronics HAM radio kits, write for our
complete catalog. (And, be sure to tell us how you're doing with your SR2
Receiver!) To learn more about the hobby of HAM radio, write ARRL
(American Radio Relay League), 225 Main Street, Newington, CT 06111 or
visit their website: http://www.arrl.org/.
CIRCUIT DESCRIPTION
The following paragraphs describe the circuit operation of the SR-2
Shortwave Receiver kit. Through the use of a simplified block diagram the
basic circuit theory is easy to understand. The full schematic (pg. 14) shows
the details concerning specific operation and component variable
configurations.
Take a moment and examine the circuit block diagram found on page 9. The
simplified signal flow of the block diagram shows the basic sections of the
receiver. The corresponding components are noted under each main block
and can be cross referenced to the schematic
The start of our circuit begins with the Antenna. RF Signals (Fc = carrier
frequency) from the Antenna are applied to the RF Input and Filtering
allowing only the signals of interest to pass through. The high pass filter
helps eliminate unwanted signals picked up by the antenna improving the
overall reception quality of the radio.
After the input signal is filtered, it moves to the Mixer stage. Notice on the
diagram that there are two inputs to the mixer. We have discussed one of
these input signals coming into the mixer but not the other as of yet. Drop
down to the Local Oscillator block. The local oscillator (LO for short) acts as
your tuning control for what frequencies you can receive by generating a
signal on the board close in value to that which will be used by the mixer.
There is a direct relationship between the generated frequency of the local
oscillator (LO) and the exact receive frequency (Fc) you want to listen to.
This will become clear when we finish discussing the block diagram.
The LO section is a Colpitts oscillator that takes advantage of smooth
