Experimental Base-Bias Tuned VFO - Supplemental Page

Introduction

This page contains some data, schematics and photos which supplement the main web page.

The first 3 items below are a SPICE simulation of the prototype VFO. You can see that the voltages at the base, collector and emitter current vary as the 10K pot is turned.

The next 3 bitmaps concern the VFO shown below:

The circuit, start-up, and saturation (collector dropping below the base voltage) are all shown for a tune voltage of 1.85 volts. I assumed a Q of 200, which I modeled with a parallel R of 16.7K across the coil. [Rp=QwL where w=Omega=2pF]. If the Q would have been high, I would have been into deeper saturation with the 1.85 volt conditions. I found, as you did, that there was no saturation with 1.06, 1.26 volts. At 1.76, it was just on the edge. That is, the bottom tip of the collector sine wave was hitting the base voltage. Now all of this is just simulation. Measurements are always better. Also, your measured results may well change with a different 2N3904.

Analysis by W7ZOI