For this purpose, construct the
test circuit of figure 1.
This is a simple Hartley-style crystal oscillator. You will require
access to either a frequency counter or a general coverage receiver
(ask a neighborhood ham to allow you to bring over your crystals
to his shack and test them for few minutes). Mark each crystal
with a number and solder it into the circuit (donít use a crystal
socket). Connect the 9 volt battery and measure the frequency.
If you are using a receiver, find out the frequency on which the
crystal is absolutely zero. Note the frequencies with the 33 pf
capacitor in series and shorted. You will have a pair of frequencies
for each crystal. Select four crystals with pairs of frequencies
that match within 50-40 Hz of each other. A fifth (for the carrier)
oscillator crystal should be within 100 hz of the other four selected.
Calculate the value of the capacitors
of Fig.2 like this:
1. Calculate the average frequency shift of the four chosen crystals
as F (in KHz).
2. C1 = 21 * F, C2 = 40 * F. Choose
the nearest available fixed capacitor. If you canít find a fixed
capacitor within 10% of this value, then parallel two capacitors
to achieve the capacitance.
For instance, in the case of the first prototype, we measure an
average of 5KHz of shift. Thus, the capacitors calculated were
107pf and 200pf. We used 100pf and two parallel 100pfs as a substitute
for 200pf capacitors. These calculations are for 200 Ohms termination.
For a complete discussion of this design method, you are referred
to the excellent paper by Craver in the Communications Quarterly
of 1993, Winter.
design without Toroids
It was decided
to use broad-band techniques where suitable and keep the circuit
free of too many critically tuned circuits. We decided to investigate
the TV baluns as cores for broadband transformers. The TV baluns
as small ferrites as shown in the picture.
Almost all the broadband transformers
are bifilar. Two (the modulator and the transmit mixer cum product
detector) are trifilarly wound. They are simple to produce.
Making a bifilar transformer:
two lengths of 36 swg copper enameled wire.
them together. Tie one end to a nail.
►† Twist the wires
together so that they cleanly have about 8-10 turns per inch.
►† Check that the wires
are evenly twisted (although there will be more twists towards
►† If the balun
core is mounted on a PCB, cut it out with a cutter and remove
all the original windings.