On Tue, 30 Jul 2013 14:55:23 -0700, lw1ecp wrote:

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

You mentioned cell phones and other wireless devices. How about police
cars or other radio equipped vehicles driving by? What about other
sources of RFI (TV sets, light dimmers, power tools, etc.)?

Since you used "ugly construction", I assume that your device isn't
shielded. Put it in a cardboard box wrapped in aluminum foil. Ground
the foil to the circuit and filter all the wires going in and out. If
done carefully, this may point to the source of your problem.

Or, maybe not. I was once called on to find the cause of low frequency
noise in a large instrument that was being developed. The noise would
come and go. I spent several days without much luck until I decided to
put the preamp in a box to shield it from the room lights (some
transistors, ICs, and diodes are light sensitive). The noise went away.
But removing the box and turning off the lights didn't make it go away.
Then I noticed that the noise appeared only when the air conditioning fan
came on. Since putting it in a cardboard box made the noise disappear,
it couldn't have been an electrical problem. I visually examined the
board and found that the designer had used Z5U capacitors in a filter
circuit. Replacing these with (much larger, requiring a new board
layout) polyester capacitors solved the problem.

The explanation: Z5U capacitors are sensitive to just about everything.
They change value with temperature, voltage, and frequency. They are
also piezoelectric. In this case, it was the piezoelectric effect. The
capacitors were picking up the air pressure changes caused by the air
conditioning and injecting stray voltages into the circuit.

Not all ceramic capacitors are like this. The C0G types are among the
best capacitors that are readily available. X7R types are intermediate.

Z5U capacitors are excellent for power supply bypassing but not much else.

--
Jim Mueller

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