Topband: Receivers with a Twist

[Tom Rauch]

> As a K2 owner, I'd have to give a definite "yes" to that.
However, there
> is one major flaw with the K2 on transmit - when using a
K2 to drive a
> kilowatt amplifier, the strong electromagnetic field from
the amp's HV
> transformer gets into the PLL circuit of the K2 causing
what sounds like
> noise on the K2's CW sidebands, extending out at least
+/-2 kHz.

Interesting.

The ICOM 751 series has the same problem when the internal
switching supply is used.   ICOM positioned the transformer
in the switching supply right over the VCO coils. I had to
redrill the holes in mine and rotate the SMPS 90 degrees.

By the way, defects like this are not visible on a scope.
The carrier looks pure. They are FM, not AM.

MFJ has a similar problem in the small transceivers they
sell. Not only will a power supply nearby FM modulate the
VFO, the radio's internal speaker magnet changes the VFO
frequency. This causes the radio to change frequency
depending on the location of steel around the radio and
drift in frequency as the cabinet changes size with heat.
If you turn the volume up the sidetone shakes the cabinet,
moves the speaker, changes flux in the VFO coil, and FM
modulates the signal.

Small inductors often change inductance when subjected to
strong external magnetic fields. Something to keep in mind.
Mu metal or even regular soft iron or steel around the VCO
or VFO coil will probably help a great deal.

73 Tom