EMRFD Message Archive 12904

Message Date From Subject
12904 2016-06-03 09:22:29 Ashhar Farhan a simple overtone oscillator
for the on-going HF1 transceiver effort, we were looking for an overtone oscillator that is not very critical to set and doesn't need an air-core inductor to avoid shielding it.

it turns out that a very simple arrangement can result in 3rd overtone oscillations. a simple Colpitts crystal oscillator with very small feedback capacitances can reliably get to the 3rd overtone of almost any crystal.

the base-emitter capacitance is entirely skipped (the internal base-emitter capacitane of the transistor is good enough) and a 10pf capacitance between the emitter and ground is all that is needed. 

here is the circuit :

a few interesting observations :

1. the emitter resistance has to be substantially higher than the emitter-ground capacitor's reactance at the fundamental frequency. 
2. the output is low, but clean. 0 dbm is typical.
3. the BC547s with the an Ft 150 Mhz needed 10 pf of capacitance, the 2N3904s needed jsut 5 pf (at 10pf, the 2N3904 fell repeatedly into fundamental mode).
4. almost all the junkbox crystals between 16 MHz and 24 MHz oscillated. most of these were really low quality, microprocessor stuff. 
5. the choice of Cx depends upon the transistor type as well as the required frequency. however, given the correct value of the Cx (eg 10 pf with BC547) every crystal did the 3rd overtone.
6. crystals from 14.318 MHz (oscillating at 43 MHz) to 24 MHz (oscillating at 72 MHz) worked without tweaking!

- farhan
12907 2016-06-03 22:47:59 mvs_sarma@ymail.c... Re: a simple overtone oscillator
Great experiment Ashhar Farhan. thanks for the observations and they are very much useful.
12916 2016-06-06 08:42:48 Nick Kennedy Re: a simple overtone oscillator