Taming the IC-746 frequency drift

The goal is to reduce the IC-746 drift during transmit to a minimum. The problem stems from Icom's placement of the RF unit directly below the fins of the PA heatsink with the cooling fan blowing between the two. The fan is turned on during transmit and the heat from the PA even at 5w output causes the rig to drift in frequency. When I started using WSJT, I ran into this problem and it was suggested placing insulating material inside the reference oscillator would reduce the problem. The 30 second period transmit drift was reduced so I left it at that. When I started using JT65 with the 1 minute transmit, the problem became more pronounced. Since Icom did not use temperature compensation techniques such as capacitors which exhibit both positive and negative tempurature curves together, the frequency would drift due to the heating of these components even though the crystal itself was stabilized somewhat by the insulation.

The following are steps I took to resolve the short term (1 minute) drift.

		I have been working on a temperature stabilized Phase Locked local oscillator for my 23cm transverter. I am using the G8ACE lo which is ovenized with a nice circuit. I used one of the spare boards I have and built up the oven circuit and machined an aluminum block to fit around the crystal. I removed the insulation from the inside of the oscillator compartment and insulated the outside of it. I tapped some raw power from the DC filter board so the oven could be left on during power down, regulated it to 10 volts and set the temperature where I though it would suffice. Not too hot but hot enough to keep the drift to a minimum. Put everything back together and tested it out. The top photo shows the oven and controller. The bottom image shows the placement of the 10v regulator. Click on either to see a full image.
		Since I still had objectionable drift, I took two sheets of plastic foam packing material (not bubble wrap) and covered the bottom of the RF Unit. I fully taped the foam to the RF unit on the edge where the fan is located so no air would be blown in between the two. The foam insullation will probably buy you the most bang for the buck (zero cost - most gain) while the oven ensures tempurature stability. Over a 20 minute period of 1 minute transmit at 30w and one minute receive, I saw a total of 10Hz drift. If I were to do this again, I would start with the insulating the bottom of the RF unit first to see if it solves the problem. 2006-10-16 As an update, I removed the heater block from the crystal enclosure and tried running without it. The frequency did drift though less than without the insulation on the bottom of the board so it is definately a combination of the two.