Build A 100Khz Crystal Calibrator


Build A 100Khz
Crystal Calibrator

By

There is a great deal of old
amateur gear which many amateurs have decided to restore and
bring back to life. While much of the early amateur transceivers
work just fine they usually lack a digital readout and must rely
on analog dials for tuning. The problem of dial calibration is
complicated by the non-linear effects of tuning capacitors. This
month’s circuit is a 100Khz crystal calibrator using an
inexpensive microprocessor crystal and CMOS IC’s which are
readily available at Radio Shack.

The main problem with building a 100Khz oscillator is the
unavailability of 100Khz crystals. Even if you find a vendor
willing to cut such a crystal for you, plan on paying $20 or more
not including shipping charges. The circuit in Figure 1 uses an
inexpensive 8Mhz microprocessor crystal which can be easily
obtained from most parts suppliers for about $1. Using a 74HCT393
binary counter IC, we can easily divide down the 8 MHz signal
from our crystal into 100Khz or almost any frequency we need.

The circuit in Figure 1 uses a couple of NAND gates (74HCT00 IC)
for our 8Mhz crystal oscillator. Capacitor C2 actually helps us
tune the crystal to the exact frequency, Use any value of C2 from
about 22pf to about 82pf to get the oscillator on frequency. In
the prototype, 68pf worked fine for most of the crystals tested.
For an exact frequency, replace C2 with a 22pf and add a 50pf
trimmer capacitor in parallel. By adjusting the 50pf trimmer
capacitor, we can easily get the crystal exactly on frequency.
The first NAND gate is our oscillator while the second NAND gate
acts as a buffer and conditions the signal. This signal is then
fed into the clock pulse input of one of the 4 bit binary
counters in IC2. By taking the output from the Q3
signal, we have now divided the signal by 16 giving us 500Khz.
This signal is now fed into the clock pulse input of the second 4
bit binary counter. In the first counter we tied the MR (clock
reset) line to ground. In the second counter, we need the count
to reset when we reach a binary five, which will allow us to
divide the 500Khz by 5. For this counter, we used the last 2
remaining NAND gates in IC1 to detect the desired value. When we
reach the correct reset interval, the MR line goes high resetting
the counter to zero and allowing us to effectively divide by 5.
The 100Khz output is taken from the Q2 line and is
coupled through capacitor C4.

If you prefer, this circuit can be easily changed to a 50Khz
calibrator by wiring pins 13 and 14 of IC1 into pins 10 and 8 of
IC2. See Figure 2 for details. This arrangement makes the second
counter reset at binary 10 which divides the 500Khz by 10 giving
us the desired 50Khz.

The HCT series CMOS logic IC’s require a 5 volt power supply just
like the old TTL logic series. In this circuit we used a 5 volt
zener diode, D1, along with resistor R2 to get 12 volts down to 5
volts. Capacitor C3 is used primarily for bypassing the
oscillator and must be used in this circuit. If you prefer to
power this circuit from a 9 volt battery, then reduce resistor R2
to 220 ohms for best performance.

If you plan to install this circuit inside a transceiver, feed
the output directly to the receiver front end. Make certain you
have connected it after the TR relay so that the circuit doesn’t
get zapped by RF from the transmitter side. Also, you will want
to install an ON/OFF switch which will interrupt the 12V line
going to the circuit. As with all static sensitive CMOS IC’s, use
special handling precautions and do not leave out those IC
sockets.

This circuit has been found to generate accurate birdies at every
100Khz and will be an excellent aid in getting that old rig right
on frequency. The accuracy of the 100Khz birdies will depend on
how close the 8Mhz oscillator is on frequency. With the
difficulty in getting crystals for specific frequencies, this
circuit gives new meaning to the words "Divide and
Conquer".

DE

Build A 100Khz Crystal Calibrator

Build A 100Khz Crystal Calibrator #2

Parts List

C1
 
22 picofarad
ceramic disk capacitor

C2
 
68 picofarad
ceramic disk capacitor (see text)

C3
 
.1
microfarad ceramic disk capacitor

C4
 
220
picofarad ceramic disk capacitor

R1
 
10 Meg ohm
1/4 watt 5% resistor

R2
 
470 ohm 1/4
watt 5% resistor

D1
 
5.1 volt
zener diode 1N4733A (RS 276-565)

IC1
 
74HCT00 quad
2 input NAND gate IC (RS 276-2801)

IC2
 
74HCT393
dual 4 bit binary ripple counter IC (RS 276-2884)

Y1
 
8 MHz HC49U
microprocessor crystal (MOUSER 520-HCA800-20 or DIGI-KEY
X021-ND) The auther has a large number of these crystals
in stock and will make them available at cost upon
request.


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