CCWN 76:65
                                                   
SYNTHESIZER FOR 5 TO 5. 5 Mhz                                     
           BY RAY PETIT
                                                 
        The diagram on the next page is of a 5 to 5.5 MHz
synthesizer which delivers output in steps of 100 Hz,
approximately 1 volt r.m.s. into 50 ohms, suitable for the local
oscillator of a double balanced mixer driving a 9 MHz i.f. for
the 20 and 80 Meter Ham bands. I have not actually built this
circuit, but it is representative of many designs I have put
together in the course of my work, and it should not present any
problems.          The synthesizer requires a single 12 volt
power supply, a 1 kHz TTL reference signal derived from the CCW
station frequency standard, and five digits of frequency-set
information, TTL compatible for the programmable counter. This
can be supplied from either straight toggle switches or from
thumbwheel switches (ECD complement output, common grounded).     
    The VCO is a MC1648P using a Motorola MV104 dual varactor
diode and a coil to resonate at 50-55 MHz with a control voltage
input from 2 to 10 volts. The VCO is controlled in steps of 1
kHz. The upper SP8640 (95H90) divides the VCO output by 10,
yielding 5 to 5.5 MHz in 100 Hz steps.          The lower SP8640
and all the remaining counters function as the programmable
divider for frequency-setting purposes. Although this particular
circuit is set for 5 to 5.5 MHz, it can operate up to 16 MHz by
changing the VCO coil and the "1 MHz" data inputs, using a
74LS193 for presets from 10 to 16.           The phase detector
is the RCA CD4046AE. The transistor driving pin 3 of the detector
is a level translator, from TTL levels to a 12 volt swing. The RC
network connected to pin 13 and going to the VCO is remarkably
simple for the performance it produces using the unique charge
pump detector of the 4046.           Layout, shielding, and
bypassing in this type of circuit is very critical. There are
three separate power supply regulators, each heavily filtered.
All are required to keep counter noise from appearing in the VCO
output as spurious sidebands.           At the 5 MHz output of
this circuit it should be possible to get 80 dB suppression of
the 1 kHz reference frequency spurious output, and phase noise
from the circuit should be about 100 dB/Hz a few kHz away from
the carrier. Although this latter figure is nowhere near
state-of-the-art, it is a very respectable result for such a
simple circuit.           Just think: there are one thousand CCW
channels this synthesizer can be set to in a 100 kHz segment of a
ham band' The output will be just as accurate and stable as the
station frequency standard. A comment about harmonics. The output
of this unit has a lot of harmonics, but if the signal is used as
the local oscillator driving a double-balanced mixer, the
harmonics Will make virtually no difference assuming that there
is some form of preselection on the input signals. If an
application requires a harmonic-free output, suitable low-pass
filters producing at least 60 dB suppression can be assembled
easily.