Direct Frequency Modulation of Crystal Controlled Transistor Oscillators

Three types of HF and VHF crystal controlled transistor oscillators are experimentally studied to assess their frequency shift bandwidth and stability properties. The capacitance bridge oscillator exhibits a bandwidth under certain circuit conditions of 0.1 per cent of center frequency, with amplitude modulation of 1 db, but degraded frequency stability. Combining this oscillator with another exhibiting superior stability in the low VHF band results in a hybrid oscillator circuit in which frequency can be shifted 0.02 percent with good power regulation. Frequency stability is 0.46 ppm/
\degC
and 3 ppm/10 per cent change in supply voltage. Most of the experiments were conducted at 35 Mc, but higher order modes to 150 Mc were also investigated.