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Free running gate-based VFO

FREE RUNNING GATE-BASED VFO#

gate based VFO

This diagram shown a free running gate-based VFO or oscillator. The power supply to the gates is not shown.

CIRCUIT DESCRIPTION#

The oscillator itself is formed by only one inverting buffer; the other five buffers are used to provide a pad between the eventual load and to create what I call current-thumpers. A Variable Capacitor (shown as a 170 pF capacitor) controls the frequency. But it could have a varicap diode that provides both manual and voltage control over the frequency.

It has relatively high-value shunt capacitors that go from the two sides of the inductor to GND. Even though these affect the frequency of oscillation, they don't have as much effect as the series tuning capacitor. The capacitor on the left-hand side of the inductor has more effect on controlling the frequency than the capacitor on the right-hand side. I think this is because this side is more reactive.

You will observe that the ratio of these capacitors is 2:1. Experiments have demonstrated that this ratio determines the degree of control that the variable capacitor can offer.

TESTING:#

This circuit has been tested using a Schmitt trigger hex buffer package — The 74HC14. It works perfectly well around 7 MHz. Higher frequencies pose a problem due to the need for a small inductor.

Note: We may not need the precision of the varicap for basic operation. If we do not include this, there may be a lack of control at a fine level. In other words, the control available from the polyvaricon variable capacitor may not allow sufficient granularity. This is especially true for CW (Morse) reception.

JITTER AND STABILITY#

It is interesting to note that when we compare a Colpitts oscillator with a Gate-based oscillator, the Colpitts suffers from worse jitter. I find this fascinating because a Gate-based oscillator consists of multitudes of transistors; these toggle on and off, producing, at least in my mind, a higher probability of spurious and unwanted variations. However, this does not really create the jitter I expected. Indeed, it appears that this type of oscillator is very stable.