Hi!. I want to tie a ~40MHz VCO to a frequency stabiliser. A PIC measures initial f and then makes corrections. Resolution is 10Hz. The PIC count is shown on a LCD module.
The VCO is a classical JFET Hartley. Gate: 100k and 1N4148 to GND, 100pF to LC end. Source: to 25% tap.
After warmup, the tens of Hz show an acceptable jitter of +/-10Hz most of the times. But suddenly I witness a BAD worsening, the tens become mad and even the hundreds are affected by jitter. This can last several minutes. I go for a walk, come back, and peace reigns again.
I read about flicker (1/f, Random Telegraph) noise, but this behavior is strange. I made sure Vdd is clean, there are no cellphones or other wireless devices around, there are no creeping surface currents because I used "ugly construction" in the air. Replacing the varicap/varactor with a fixed C made no difference.
This improved somewhat by connecting the gate directly to the hot end with no R nor diode, and inserting a parallel 1kohm/1nF in series with the source, in order to provide low-frequency degeneration.
I made sure the level at the PIC input is ample. A 43MHz xtal oscillator gives always rock-steady reading.
- Is there a name for this "intermittent-1/f" noise?.
- Has anyboy had succes in reducing it?.
- Off topic (not radio): how can a PC read the values from the PIC to the LCD?. E. g. via parallel port, with some code written in ASM, and booting in DOS to keep Windows out of the way.
Many thanks!. Daniel LW1ECP