Kenwood TM451 TCXO Upgrade

Being used in a mountain-top repeater, our TM451 transceivers suffered severe frequency shifts as the temperature varied. They are exposed to temperatures as low as -15C in winter and as high as 45C in summer, and the original crystal oscillator is too sensitive to this variations. We measured the repeater to be as high as 2.5Khz up or down in frequency depending on ambient temperature, and we looked for a solution.

We thought about using crystal heaters, the very nice ones available from Khune electronics, but finally decided to use TCXOs to help preserver battery power. I was lucky to find a pair of cheap 12.8Mhz NKG VC-TCXOs from eBay, and this is how I installed them and the results obtained.

The Modification

The TCXO output is injected to the circuit in the crystal oscillator connection.

After checking that levels where compatible, one of the legs of the original crystal is cut and the small-diameter coax is soldered to the PCB pad. The 5V neccesary for the TCXO are taken from a nearby solder joint.

A small PCB with just a capacitor and two resistors for the 1/2 divisor that biases the VC adjust terminal is placed on the other side of the rig.

The Results

To measure the improvement in frequency stability, I run two tests, before and afte the modification. The rig went to the freezer, down to -10C and was keyed until it reached 45C, while monitoring temperature and frequency. Temperature was measured using a low thermal mass temperature to voltage converter TC1047 connected to an Agilent 34401A bench meter. Frequency was measured with an Agilent 5835A counter. Both meters were connected via GPIB to a PC running EZGPIB where a small program was used to record the data every 5 seconds. I ploted the data in the following chart, which shows quite a dramatic improvement in frequency stability.