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An amateur radio 40m band temperature compensated variable frequency oscillator (VFO) with drive-level optimized for opposite sideband suppression for use as the local oscillator (LO) of a high frequency (HF) IQ image reject mixer is considered in this work. The first problem of this thesis was the discovery of the HF IQ mixer’s opposite sideband suppression dependence on LO drive-level. As a solution to this problem, the author built a fixed drive-level VFO for the optimal opposite sideband suppression. This then led to the second problem of this thesis; the discovery by this author of the well-known problem of VFO instability in the particular case where the frequency drift primarily arises from ambient temperature variation. This historically rich and technically challenging problem illustrates the difference between an actual high performance 40m JFET Hartley VFO, and an academic textbook schematic. The textbook circuit with standard components was abysmally drifty, and this thesis demonstrates application of theory to reduce the drift to −4.25 ppm/◦C and +8.5 ppm/◦C for the 40m frequency ranges: 7.05 – 7.22 MHz, and 6.95 – 7.13 MHz, respectively. Drift was also reduced to −1.42 ppm/◦C with styrofoam packaging on tank components for the frequency range of 7.05 7.22 MHz. The author constructed a temperature chamber to perform the temperature compensation tests, which is described in this work. The use of boiling water to relieve mechanical stress in the VFO components was investigated in this work, and measured results are presented. The iterations of measurements, compensation calculations, and the final achieved drift are also described in this work. Above 40 dB opposite sideband suppression from the HF IQ mixer was achieved with the frequency stabilized VFO of this work, designed for optimized LO drive-level in the range of +2.25 +5 dBm. Additionally, within this optimized LO drivelevel range, opposite sideband suppression levels of above 50 dB were achieved.

HF IQ Mixer VFO Temperature Compensation and Drive Level Optimization for Opposite Sideband Suppression