SECTION 4 CIRCUIT THEORY
4-1. INTRODUCTION
The Atlas transceiver employs several unique features in its circuit design which lead to exceptional performance.
Most of the circuitry is directly descended from similar equipment manufactured for military and commercial
markets by Southcom International, Inc., of Escondido, California. Les Earnshaw, ex ZL1AAX is President and
Director of R&D of this company. Operating under license from Southcom, Atlas Radio has access to the very
latest state-of-the-art circuit designs which have been tested, proved, and type accepted for military and
commerical use. Figure 4-1 illustrates the modular design and plug-in P.C. boards of the Atlas transceivers.
4.2. RECEIVER INPUT CIRCUIT.
Referring to the block diagram illustrated in Figure 4-2, notice that there is no preamplification of the signal. After
passing through input tuning circuits, the signal is coupled directly into a double balanced diode ring mixer where
it is heterodyned to the 5520 kHz I.F. . Thus, the overload and cross modulation problems commonly encountered
with an R.F. Amplifier stage are largely eliminated. This has always been somewhat of a problem with vacuum
tube R.F. Amplifiers, and a much more serious problem with transistor or F.E.T. Amplifiers. With its advanced
front end design the Atlas transceiver will continue receiving signals in the presence of extremely strong adjacent
channel stations that would overload, cross modulate, or desensitize other
receivers.
4-3. SENSITIVITY
As with most new developments in technology, it may be difficult to accept the fact that a proper receiver can
exhibit good sensitivity without a stage, or more, of R.F. amplification prior to frequency conversion. The fact is
that the Atlas is at least as sensitive as the best of the tube or solid state receivers having R.F. Amplifiers. This is
due largely to the very low noise figure of the double balanced diode ring mixer, followed by a low noise I.F.
Amplifier. Sensitivity is rated at 0.3 microvolts for a signal-plus-noise to noise ratio of 10 dB. Typical
measurements will read 0.15 to 0.2 microvolts.
4-4. SELECTIVITY
Following the low noise first I.F. Amplifier, the signal passes through the crystal ladder filter, a highly
sophisticated package designed especially for the Atlas transceiver by Network Sciences, Inc., of Phoenix,
Arizona. Here is where superior selectivity has been tailored to take full advantage of the extremely wide range of
signal levels that the front end design is capable of handling. A 6 dB bandwidth of 2700 Hertz was carefully
selected to provide audio response from 300 to 3000 Hertz in both receive and transmit modes. While occupying
slightly more bandwidth than a 1200 or 2400 Hertz filter, it has been convincingly proven that transmission and
reception of the audio frequencies between 2400 and 3000 Hertz provides a substantial improvement in weak
signal readabilty. At the same tune, the improved fidelity of voice communications is readily noticeable, and helps
account for the report of "broadcast quality" from the Atlas. The 6 db bandwidth of 2700 Hertz is backed up by a 6
to 60 db bandwidth ratio of only 1.6 (shape factor), and ultimate rejection greater than 130 dh. It is this extremely
steep skirt selectivity, illustrated in Figure 4-3, which will reject strong adjacent channel signals.
45. OSCILLATOR SWITCHING
The unique method of changing from receive to transmit mode by switching the carrier oscillator and VFO is
illustrated in the block diagram, Figure 4-2. This new development is responsible for great simplification of the
transceiver circuit, leading to fewer components, lower cost, and great realiability.
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