Re: LF: RE: Analog oscillators

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

> One more question: Currently i use the BF862 since it was on hand. However 
> i could use a normal BF245 instead. I thought it could have an advantage 
> since the housing is bigger and so the temperature differences should be 
> smaller. Also the BF862 has a relative high Id, so the power dissipation 
> and hence the temperatures may be higher. Vdd=5V and the osc didn't start 
> to oscillate when using a source resistor of 100 Ohm. When using 50 Ohm 
> then it runs and the DC source voltage is 0.4V, i.e. 8 mA DC drain 
> current. Could be a bit lower.
When you replace the existing inductor with a high-Q one, you will find that 
the loop gain of the circuit is increased due to the higher Q, and you will 
be able to use a lower gain/reduced transistor bias current.
For CW enthusiasts, I am suprised more use is not made of simple LC VFOs. A 
drift of +/- 100Hz during the time taken for a QSO is usually not 
noticeable, and it is easy to achieve this in the LF/MF range. Also, the 
narrow tuning range means an elaborate tuning mechanism with reduction 
gearing is not needed - just a knob with a simple hand-marked scale is 
adequate. It is much simpler than the schemes needed to obtain an LF/MF 
signal by dividing/mixing down the output of an HF rig, etc. A VFO is also a 
much better practical choice than a crystal oscillator, which leaves you 
stuck on a fixed frequency.
Cheers, Jim Moritz
73 de M0BMU