ANTENTOP- 01- 2004, # 005

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 (don't 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 can't 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.

Broad-band 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:

►Take two lengths of 36 swg copper enameled wire.

 ►Hold 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 the ends).

 ►If the balun core is mounted on a PCB, cut it out with a cutter and remove all the original windings.

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