Michael; Let me make a few comments about this circuit.
> I'm trying to understand a bit more about temperature-related VFO > drift in the K2. My K2 sits in the basement where it is currently seeing > stable ambient temperatures around 67F. When using 10 MHz WWV as a > reference, I see approximately 40 Hz of drift (10,000.03 to 9,999.99) > over a 30 to 45 minute period from a cold start. I am not transmitting > during this period, I assume this is just due to a slight heating (can't > be much of a temperature change) of various resistors, capacitors, > inductors, and varactors. This is clearly in-specification as it is much > less than the 100 Hz that is specified in the manual. ;^) A K2 or K2/100 will warm up about 5-10F above the local ambient temperature due to the internal power dissipation; more options installed will push this toward the higher number. > > My K2 is serial number 4137, so it has had the Temperature-Compensated > PLL Reference Upgrade from the start. The Temperature-Compensated PLL > Reference Upgrade documentation says that you can check the VFO drift by > transmitting for a while to raise the temperature of the radio 15 or 30 > degrees F _AFTER_ you have allowed a warm-up period of 30 or more > minutes. This appears to infer that there is a difference in the nature > between warm-up drift and post warm-up drift. If so, can someone explain > what that difference is? There is no difference in the root cause of the drift. This adjustment procedure was designed so that you could make a consistent measurement of drift as you adjusted the value of RA. It is possible to reduce the 20 meter drift to less than 50 Hz with a 50F temperature swing. My K2 has been tested with this mod over a temperature range of 40F to 130F. The drift is not as well compensated below 50F, so you will see somewhat more drift if you operate outside in the winter. > > I haven't performed the formal post warm-up drift testing, but I plan > to give it a try to see how stable the VFO is after warm-up. This raises > another question... Will making any changes to resistor RA on the PLL > thermistor board have any affect on the 40 Hz of warm-up drift (as > opposed to the post warm-up drift), or is this something that can't > really be addressed by the Temperature-Compensated PLL Reference > circuit? Reducing the drift following the test procedure will also reduce the warmup drift. The warmup drift on my K2 is less than 20 Hertz as determined with the front panel frequency display and WWV. > > Thanks, Michael N9BDF > > PS It was nice to see that resistor RA was placed at the top of the PLL > thermistor board -- makes it much easier to fine tune the temperature > compensation! :^) > This was intentionally done in the design so that any K2 user could improve the performance with a little additional work. That is also why RA is a 1/4 watt resistor and all of the other ones on the circuit board are 1/8 watt resistors. -John KI6WX _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
