3. Q-Multiplier
I sought a low distortion, high Q, negative resistance oscillator as the heart of this receiver and came up with this fun Colpitt’s variant. You can spend years learning, building and testing a ton of oscillator topologies and I plan to work towards this over time.
Above — (FFT) The worst case distortion of the Colpitt’s Q-multiplier at 6 MHz : 2nd harmonic = -46 dBc. With lower amplitude, it may drop as low as –55 dBc across the 4.9- 8.33 MHz tuning span.
The regen control changes the oscillator amplitude. 1 annoyance with a Colpitts — as you adjust the amplitude or “regen” control potentiometer, the oscillator frequency changes since bias changes affects the transistor input capacitance (mainly through collector to base inter-element capacitance at the pn-junction).
Above — A diagram showing the various internal and parasitic capacitances that affect a transistor circuit.
To reduce the tank tuning effects from bias change, I employed 3 strategies that worked:
- High fT transistors (low input C)
- Cascode oscillator topology (reduces Miller effect)
- High tank C to L ratio [(weak effect) — also minimizes stray C plus may boost resonator Q and decrease phase noise]
Initially, some friends reacted negatively to my transistor choice — the 2SC3355, a low–noise 6.5 GHz BJT. Yes, that’s a bit crazy fT-wise, but I bought 20 for $1.00 from a dying electronic store’s bargain bin and quite frankly — they’re amazing. I ran ferrite beads and VHF-UHF bypass to prevent parasitic oscillations and sniffed out none with my spectrum analyzer and DSO.
All my other BJTs above the fT of the PN5179 and BF199 are SMT parts. I wanted the RF board to only house leaded parts soldered in 100% Classic Ugly Construction plus — no low-Q, unknown temperature coefficient cut or glued pads anywhere on the RF board. Actually, leadless SMT parts probably offer the better BJT choice with respect to wiring parasitics.
It also might be better to run MPSH10 (PNP), BF199, PN5179 or other transistors with an fT between ~1- 2.5 GHz compared to my mega fT choice to reduce the chance of spurious oscillations while still fronting a low input C — I’ll leave transistor choice up to you.
