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|Simple Frequency Stabiliser by Olivier F5LVG|
|Written by Hans Summers|
|Friday, 04 September 2009 21:46|
Olivier Ernst, F5LVG sent me the following article about his simple frequency stabiliser, built according to the original Huff Puff theory (as opposed to the more complex "Fast" method). He used ordinary diodes as varicap diodes, a zener for the main VFO tuning and a red LED for the stabiliser correction. See my page Common Diodes as Varicaps for information about my own experiments.
A Simple Frequency Stabiliser
Olivier ERNST, F5LVG, 2 rue de la philanthropie, F-59700 Marcq-en-Baroeul, France
If you have made an LC oscillator that drifts you need a frequency stabilizer. Perhaps you need a simple frequency counter to. Here is your solution.
The front end adapts the VFO signal to the 74HCT7474 (figure 1). A frequency reference is obtained by a 74HCT4060 that divided the crystal frequency by 16384. This frequency reference is applied to the 74HCT7474. This IC compares both signals. It is a dual D type positive edge triggered flip-flop : during the positive edges of the clock, the Q output takes the value of the data input and keeps it until the next positive edge of the clock. If the reference and the VFO are in phase, the Q output remains constant. If the VFO and the reference are not in phase the Q output fluctuates between the high and the low level. In that case, after the 1 uF capacitor, there is a series of positive and negative pulses. Only the positive pulses go through the BD137. These pulses are integrated by the 680 uF capacitor and applied to the VFO varicap. The voltage across the varicap tends to increase, consequently, the stabilizer tends to increase the VFO frequency. When the VFO frequency and the reference are again in phase, no pulses are applied to the varicap. This demonstrates that the stabilizer is able to correct a negative drift. To correct a positive drift, the 680 uF capacitor is connected so that the voltage applied to the varicap is 0.33 V when the system starts, and after tends to decrease. Therefore, even if the VFO has a positive drift without the stabilizer, it has a negative drift when connected to the stabilizer.