SGC SG-2020 TEST DOCUMENT, Installation And Operation Manual

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© 1998 SGC Inc
SGC Inc. SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
P.O.Box 3526, 98009 Fax: 425-746-6384 or 746-7173 Tel: 425- 746-6310 or 1-800-259 7331
E-mail: sgc@sgcworld.com Web site: http://www.sgcworld.com
14.0 CIRCUIT DESCRIPTION
The SG-2020 uses a single conversion up-converting design. The
IF is at 60 MHz. Bi-directional circuitry is used in the IF and filter
chain.
Selectivity is provided by a 7 pole ladder filter at 60 MHz and vari-
able SCAF digital filters at the baseband.
The synthesizer is a single loop design. The basic synthesizer uses
10 KHz steps with intermediate steps of approximately 10 Hz
obtained through direct microprocessor control of the reference.
14.1 RECEIVE
An incoming signal is routed through the selected low pass filter.
The low pass filters are used on the receive and transmit. Next,
lightning and static protection circuitry, a high pass filter to avoid
interference from strong broadcast stations and then to the custom
high performance double balanced diode ring mixer. The mixer
converts the incoming signal up to an IF of 60 MHz.
The synthesizer tunes from 61.8 MHz to 89.7 MHz. This corre-
sponds to signals from 1.8 MHz to 29.7 MHz. The synthesizer uses
a Motorola IC and a 19.44 MHz VCXO reference. Particular atten-
tion is given to provide the lowest possible phase noise.
The 60 MHz IF signal goes through a strong bipolar amplifier to the
7 pole ladder filter. The noise blanker samples the 60 MHz signal
and gates noise pulsed after the first stage of the filter. The crystal
filter is set to 2.7 KHz bandwidth and its principle function is to
reject the opposite sideband. Overall receiver bandwidth and filter
shape factors are established by the digital SCAF filters.
From the crystal filter, the signal is routed through a high gain, low
noise IF amplifier. Wide range AGC control is provided and IF
clipping avoids overload and distortion.
The product detector is a standard diode ring mixer. The audio
chain consists of a preamplifier, the filters and an output amplifier.
Most existing HF transceivers are designed with 2, 3 or 4 IF stages.