The Phase Locked Loop Synthesizer - Swann CUBIC ASTRO 102 BX Service Manual

OCUBIC
CCMMUNICATIONS
SIIWAIfI
DIVISION·
shop, yet commercial instruments are prohibitively expensive! For that re-
ason, the circuit of such an instrument is included here, so that it may be
built up locally and added to the shop equipment inventory for such service.
Referring to the accompanying diagram, we see that the 2N1671 is a uni-
junction transistor, connected in an oscillator circuit. The 0.22 mfd cap-
acitor is charged through the lOOK and lM resistors, with the latter made
adjustable to control the charging rate. When the charge reaches a certain
level, the UJT fires, discharging the capacitor, and generating a sharp
pulse of current through the 56 ohm resistor. This in turn fires the 2N5060
delivering thereby a relatively large current pulse through the inductor.
This pulse is used to simulate the impulse noise. Level is adjusted by the
50 ohm potentiometer, and the small wattage 27 ohm series resistor acts as
an output fuse, for protection from the circuit to which it is connected.
Power is shown as two. 9-volt batteries in series, although a suitable AC
supply delivering approximately 18 volts IC at a few milliamperes would also
be suitable.
THE PHASE LOCKED LOOP SYNTHESIZER
>-......
.-0"
'.f.
BASIC PHASE LOCKED
LOOP
P""••
Comparator
"rot
Electronic phase locked loops (PLL) have been in use since the 1930's
where they were used for radar synchronization. In later years, they came
into more widespread use as the synchronization system in television re-
ceivers and other applications. It is only in recent years that they have
found widespread use as frequency synthesizers, where they have found many
applications in satellite communications systems, airborne navigation sys-
tems,
FM
communications, and, most recently, in
HF
SSB communications equip-
ment.
In the basic PLL system, the output of a local voltage controlled osc-
illator (VCO) is compared with an incoming, or reference, signal, and if the
two are not of the same frequency and phase, an error'signal is generated to
tune the VCO in such a direction, and to the required degree, that they 00-
come so. This is the system used in television sync systems, and is shown
in the accompanying diagram.
The usefulness of sueh a simple
system is limited, however, to such
synchronization tasks; however, it
was found that, if a means of divid-
ing the output frequency of the VCO
could be inserted in the path be-
tween the VCO and the comparator,
the output of the VCO could 00 made
to automatically assume a frequency
that was a multiple of the reference
frequency, that multiple being equal
to the division factor of the divid-
er circuit.
By making the divider circuit programmabIe, that is, making it so that
the division ratio could 00 varied at will, and making the VCO of an approp-
riate frequency range, true frequency synthesis could be achieved; that is,
the frequency of the VCO could
be
made to vary, in integer multiple steps
of the reference frequency. This scheme is shown on the following page.
7-7