What is a thermistor?

From Zerobeat

The device listed here as an upgrade "Elecraft K2 Temperature-Compensated PLL Reference Upgrade" was added to production K2's somewhere around serial number 3300.

It transformed stock VFO stability that was marginal to good based upon any specific K2 construction, to very good for all K2's in which they are installed.

Posted by: Eric / WA6HHQ Posted On: 04/25/03 Subject: Elecraft K2 Temperature-Compensated PLL Reference Upgrade

Message Posted:

We've just completed field testing of an easy-to-install upgrade to the K2's PLL synthesizer stage. The upgrade uses a thermistor and a few other components to temperature-compensate the varactor-tuned crystal reference oscillator. The compensation circuit reduces the reference oscillator's temperature coefficient by as much as a factor of 10. This will be of particular interest to those who use the K2 with the KPA100 option, or who frequently operate outdoors or mobile. All of the components are mounted on a small circuit board which replaces one of the existing resistor networks. This results in a clean, no-wires installation. The changes do not increase current drain.

The PLL upgrade will be included free of charge with all new K2 kits starting today (s/n 3446 and up). Existing K2 owners can purchase the upgrade kit for only $19 (order number E850138). The upgrade kits are now in stock.

You can get more information at: http://www.elecraft.com/order_form_parts.htm#K2%20Parts

We'd like to thank John Grebenkemper, KI6WX, who designed the temperature-compensation circuit, as well as all of those who field-tested the upgrade kit.

73, Wayne, N6KR Eric, WA6HHQ

Drift comparisons

Here are some drift comparisons with other transceivers for reference. Values are Hz from startup reference frequency after 15 minutes, 30 minutes then 40 minutes. Receive only.

 Drake R4C (stock)         150/300/500 
 Yaesu FT101EE (stock      180/280/300 
 Elecraft K2 (stock)       110/150/170 
 Ten Tec Corsair 2 (stock) 050/100/150 

 --- Dividing line here for acceptable ssb/cw ---

 K2 with thermistor        30/40/No further drift after 30 minutes 
 C2 with K4DPK TCXO        30/50/No further drift after 30 minutes
 OMNI 564 with TCXO        Less than 20 cycles   
 IC756Pro                  Less than 20 cycles  

The Corsair II with stabilizer also drifted an additional amount (20-40 cycles approximate) when transmitting for 5 minutes or more, then returned to the original receive frequency after 5 or 10 minutes of cool down time.