Day: April 22, 2018

Amateur Radio, Electronics Workbench

Motorola K1003A Channel Element: Oscillation!

A handful of Motorola K1003A Receive Channel Elements arrived from eBay:

Motorola Channel Elements - overview
Motorola Channel Elements – overview

Having three 13466.666 kHz candidates, two with gold labels (2 ppm tempco) I disemboweled a silver-label (5 ppm) victim:

Motorola Channel Element - silver label
Motorola Channel Element – silver label

They’re well-studied, with readily available schematics:

k1003-schematic
k1003-schematic

For lack of anything smarter, I put a 1 kΩ resistor from RF Out to Ground to get some DC current going, then used a 470 nF cap and 47 Ω resistor as an AC load:

K1003 Channel Element - bias lashup
K1003 Channel Element – bias lashup

Which oscillated around a mid-scale DC bias, but looked ugly:

K1003 Channel Element - 13.4 MHz output - 1k bias
K1003 Channel Element – 13.4 MHz output – 1k bias

Perusing some receiver schematics suggested a heavier DC load, so I swapped in a 470 Ω resistor:

K1003 Channel Element - 470 ohm bias - 13.4 MHz output
K1003 Channel Element – 470 ohm bias – 13.4 MHz output

It’s now running around 3 V bias with fewer harmonics; the scope’s frequency display in the upper right corner seems happier, too.

The receiver will run that through a filter to wipe off the harmonics, then multiply the frequency by three to get the mixer LO.

There are many, many different Channel Elements out there, in receive and transmit flavors, but at least I have some idea what’s going on inside.