You are correct that switching supplies usually have R/C oscillators.
However, after reaching a stable temperature, and with constant load,
the drift could be quite small, perhaps tens of ppm, because the control
supply is often well regulated, either from the main output, or by a
linear regulator on the control chip.
In a few cases, e.g. computer monitors, the power supply runs at a
submultiple of the sweep frequency to reduce displayed noise, and
so is effectively controlled by a crystal in the computer driving it.
It's possible that the supply is locking to a multiple of the mains
frequency. Are you sure that the sound card sampling rate is accurate,
and that the software is reporting the frequency correctly? There is
probably a big signal at 15625 Hz that you can use for a quick check.
But in any case, your setup should make it very easy to see if the
source is local. Just use a laptop, running on batteries. The rest
of the system is passive! Rotate the loop for a null and note
the bearing. Move the rig a hundred meters, perpendicular to the
apparent station direction. See how much the null angle changes.
If it's coming from far away, this is a quite interesting signal.
If it is local, try flipping the main switch to your house to see
if your own gear is the culprit.
----- Original Message -----
From: "Alberto di Bene" <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 26, 2003 2:01 PM
Subject: LF: Re: VLF signals
Matti and Stewart,
thanks for your answers. Just one question Stewart. You say that :
In effect, the input bridge is a balanced mixer,
multiplying the 20870 Hz carrier by the distorted mains
So the noise conducted to the mains is primarily
20870 Hz odd multiples of the mains frequency.
But, if this were the case, and given that the switching frequency is not
crystal-controlled, shouldn't I see waving lines ? What I see are perfectly
straght lines, no signs of ondulations....
I have noticed another signal, may be Matti has an explanation
for this too :
73 Alberto I2PHD