Oscillator

I'm blogging to help me keep track of a project to build a HF radio for my Summits on the Air outings. I've been drawing on the experiences of others that I can find on Google, so I feel that I should post some things I've learned to share in kind.

First, I considered building around some passive double-balanced mixers, but this requires either purchasing monolithic units or purchasing two transformers per mixer, along with diodes. In either case, I could hardly justify, to myself, paying $10 in shipping for $5 worth of parts. So, I started on a Gilbert cell. Inspired by this work, I'm pretty dead set on a single-ended Gilbert cell mixer.

There's one problem. All of the radio bands worth considering for SOTA are above 7 MHz, probably realistically above 14 MHz and my cheap, ancient signal generator only works up to 1 MHz, so testing my mixer will require a new oscillator.

It's worth mentioning, before putting my first circuit diagram up, that I was inspired by the story behind the NorCal 2N2/XX transceiver and a sale on Newark to use mostly PN2222A transistors in my designs.

The oscillator is as follows:

Oscillator Circuit Diagram

Q1 and Q2 are both PN2222A transistors. L1 is a hand-wound coil, C1 is derived from observed circuit behavior. This circuit was an adaptation of this work.

The simulated output is:

Simulated Oscillator Behavior

The blue trace is the voltage at the Q2 collector. The red trace is the voltage on the Q1 base.

I measured the voltage on the Q2 collector in the constructed circuit:

Measured Voltage on Q2 Collector

I'm not sure how to reconcile this terrible photograph of the scope screen with the simulated behavior. To make matters worse, connecting the oscilloscope to the tank causes the resonant frequency to jump to ~25 MHz, so I've got to buffer the tank voltage to measure it. This will undoubtedly load the tank with more capacitance and reduce the maximum frequency of the oscillator. I hope that the output will be much more sinusoidal, and maybe it will be worthwhile. If the maximum frequency is too low, I can wind a new inductor.