Ramsey Electronics SR-1, Инструкция по эксплуатации, Страница: 6 / 22

SR-1 – 6
at least $100 for a receiver offering a significant improvement over your
trusty SR-1.
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 SR-1
Receiver!) To learn more about the hobby of ham radio, write ARRL
(American Radio Relay League), 225 Main Street, Newington, CT 06111.
CIRCUIT DESCRIPTION
The NE602 IC is a combination oscillator-mixer. Signals from the antenna
are peaked by L1 and fed to pins 1 and 2. The oscillator frequency is
determined by C3,C4,L2 and the varactor tuning network (D1,R2,etc.). The
use of a hyper-abrupt style varactor diode permits a tuning range of over 2.5
Mhz by R2. The mixer output is applied to the 260 Khz bandpass IF
transformer and amplified by Q2 and Q3. The AM audio is detected by D2
and preamplified by U2(A) before being boosted to speaker level by the
LM386 IC. U2(B) and Q1 provide AGC (automatic gain control).
The very low 260 Khz IF affords exceptional gain and selectivity
characteristics and also offers a sort of "bonus" that would not be normally
welcome in a receiver intended for communications and listening for very
weak signals. In tuning your SR-1 receiver, it is useful to understand that you
will hear any signal at the oscillator frequency PLUS 260 Khz and also at that
frequency MINUS 260 Khz. This is not a problem for general listening to
shortwave broadcasts, as long as you realize that you'll find every broadcast
twice as you pass through the 2.5+ Mhz range of the Tuning control. The
circuitry required to minimize this "image" reception would defeat the goal of
economy and simplicity intended for the SR-1. More elaborate superhet
receivers deal with this phenomenon by using a higher intermediate
frequency, several IF stages and very exact tuning of the antenna input
circuit. However, you will find the SR-1 to give perkier reception with simple
antennas than do many much more costly portable receivers.