laboratory use.
The principal function of a frequency synthesizer is to furnish
all or most conversion frequencies needed by a radio set. A secondary function is
to stabilize all operating frequencies furnished by the synthesizer.
The simpler
types of frequency synthesizers are usually found in tactical radio sets where
space and weight are paramount factors.
Further, frequency synthesizers in
tactical radio sets normally serve these sets in transceiver configurations; that
is, the frequency synthesizers furnish identical frequencies to the transmitter and
receiver sections of the radio sets.
Transceivers are designed for half-duplex
operation, where each operator takes his turn at receiving or sending. In fixed-
station equipment, the frequency synthesizers are far more sophisticated.
Moreover, fixed-station operation normally calls for frequency synthesizers having
identical stability and frequency increment characteristics for transmitters and
receivers.
This arrangement permits wide separation of the transmitters and
receivers, using different frequencies for transmission and reception.
This
combination allows full-duplex operation where each station is capable of sending
and receiving simultaneously.
For convenience, we will classify synthesizers as
direct and indirect. The direct class furnishes a signal directly from a crystal-
controlled oscillator or from a combination of two or more crystal oscillators.
The output frequency is obtained by combining the outputs of the several
oscillators in a mixing and frequency-selecting circuit.
The indirect class
derives a signal from a variable-frequency oscillator (VFO), which is indirectly
stabilized by a crystal oscillator or a combination of crystal oscillators.
In
practice the classifications of direct and indirect are unimportant, since circuit
design sometimes incorporates significant features of both types.
a. Direct.
In direct frequency synthesis, the frequencies of one or more
crystal oscillators are divided, multiplied, and added to produce the desired
output, while undesired output frequencies are attenuated by fixed or variable
filters. A great number of frequencies can be produced, each related by an exact
predetermined ratio to the highly stable crystal oscillators. Since the stability
of any selected output frequency depends entirely on the stability of the various
crystals, great care usually goes into the preparation of these crystals. The most
favorable frequency for each crystal is chosen, and the crystal is ground to a
precise frequency.
The entire crystal assembly is then enclosed in an oven
controlled by a complex circuit, so as to maintain nearly uniform temperature.
Although direct synthesis is a more straightforward method of generating the
desired frequency, the elimination of spurious frequencies in this method is
difficult.
b. Indirect.
In an indirect system, a VFO is locked by a control loop,
containing a frequency discriminator and a phase detector, to a signal derived from
a crystal-controlled frequency standard. As in the direct method, the crystals are
enclosed in a temperature-stabilizing oven. The control loop performs an automatic
frequency control (AFC) function so as to stabilize the output frequency at any
setting of the VFO frequency-selecting control knob.
Thus any selected frequency
within the range of the VFO will be stabilized by the AFC feedback circuit.
However, the output frequency is not firmly locked by the crystal-controlled
frequency standard, nor is the output frequency determined by the crystal
oscillator. The VFO determines its own output frequency, but it references on the
crystal oscillator to benefit from the inherent stability of the crystal or
crystals.
327 L2
15
Previous Page
