4
SPECIAL CIRCUIT FEATURES
The Atlas 180 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. Operating under license from Southcom, Atlas
Radio has access to the very latest state-of-the-art circuit
designs which have been tested, proven, and type
accepted fro military and commercial use.
Receiver input circuit:
Referring to the block diagram, 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 kc I.F. Thus, the overload and cross modulation
problems commonly encountered with an RF 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 180 will continue receiving signals in the presence
of extremely strong adjacent channel stations that would
overload, cross modulate, or desensitize other receivers.
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 180 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 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.
Seelctivity: Following the low noise first I.F. amplifier,
the signal passes through the crystal lattice filter, a highly
sophisticated package designed specially for the Atlas 180
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 cycles was carefully selected to
provide audio response from 300 to 3000 cycles in both
receive and transmit modes. While occupying slightly
more bandwidth than a 2100 or 2400 cycle filter, it has
been convincingly proven that the transmission and
reception of the audio frequencies between 2400 and 3000
cycles provides a substantial improvement in weak signal
readability.
At the same time, the improved fidelity of voice
communication is readily noticeable.
The 6 db bandwidth of 2700 cycles is backed up
by a 6 to 60 db bandwidth ratio of only 1.7 (shape factor),
and ultimate rejection greater than 110 db. It is this
extremely steep skirt selectivity which will reject strong
adjacent channel signals.
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.
This new development is responsible for great
simplification of the transceiver circuit, leading to fewer
components, lower cost and greater reliability.
In receive mode the first mixer heterodynes the
antenna signal with VFO injection. In transmit mode the
first mixer functions as a balanced modulator with carrier
oscillator injection and mic. amp. input. In both modes the
first mixer output is at the intermediate frequency (I.F.) of
5520 kc.
In receive mode the second mixer functions as a
product detector with carrier oscillator injection. Its
output couples audio frequencies to the receiver audio
system. In transmit mode the second mixer heterodynes
the I.F. signal with VFO injection. Its output is now at the
transmit frequency, and is coupled through tuned circuits,
preamplifiers, driver stage, and power output amplifier.
Oscillator switching is accomplished with four
F.E.T.s resulting in very low intercoupling between
oscillators.
Transmitter Broadband Circuitry: The amplifier stages of
the transmitter provide full power output over the entire
1.8 to 15 mc range, and require no tuning. Tuned circuits
between the second mixer and the transmitter amplifier
module select the desired mixer product and reject the
unwanted products. These tuned circuits are band
switched and provide full coverage of each band. They
are double tuned and over coupled, requiring no further
adjustment after being factory set.
Harmonic output from the power amplifier is
suppressed by a band switched two section low pass filter.
This filter is connected between the power amplifier
output and the antenna terminal. The low pass filters and
power amplifier are both designed for a 50 ohm load. It is
important that the load be quite close to 50 ohms, non
reactive, in order to operate at full rated power.
Receiver Broadband Circuitry: The receiver input filters
are band switched, and provide full band coverage
without need for a panel peaking control. In addition, the
signal passes through the low pass transmitter filter,
suppressing possible interference from strong local VHF
signals.
