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K2 Temperature-Compensated PLL Reference Upgrade

Rev. E, March 24, 2005

Description

This upgrade kit significantly improves the stability of the K2’s VFO, reducing drift per unit of

temperature by as much as 90%. It is especially useful with a K2/100, since operation at higher power will

cause faster heating inside the cabinet. The circuit, designed by John Grebenkemper, KI6WX, uses a

thermistor to temperature-compensate the varactor diodes in the PLL reference oscillator.

All parts used in the upgrade are installed on a small PC board that substitutes for resistor network RP3 on

the RF board. (See photo of installed thermistor board on page 4.) You can assemble this small board now

(see page 2), or when it is needed during assembly of the K2. All related manual changes are shown below.

If your K2 is not yet assembled, make the following changes to the Owner's manual:

 On page 47, right column, last step: Make a note to install the thermistor PCB assembly at the

location of RP3, rather than the original resistor network.

 On page 50, left column, first step: Make a note to use only the 12.096 MHz crystal supplied with

this upgrade kit, which is marked ECS 12.09-CD (there may also be a lot number, which can be

ignored). Your K2 may have been supplied with 1 or 2 crystals of a different type. Only one crystal is

now needed (X1). X2 will not be used.

 On page 54, right column, second step: Make a note to substitute a 12-H inductor (supplied with

this upgrade kit) for the original 10-H unit supplied with your K2 for use at L31.

If your K2 serial number is lower than 3000, you should first install the BFO mod kit (Elecraft part

number BFOMDKT). The BFO modifications are already present in K2s with serial number 3000 and

higher. NOTE: The BFO mod kit originally included a 12.096 MHz crystal for use in the PLL reference

oscillator. This crystal is now supplied with the PLL upgrade kit. However, if you installed the crystal from

the BFO mod kit, you will probably not have to remove it. This is covered in the PLL upgrade kit

instructions.

Upgrade Kit (E850138) Parts List

Ref Description Qty Part #

RF-L31 12H Shielded Inductor 1 E690019

RA 33K 1/4W Resistor (org-org-org) 1 E500057

RB 12K 1/8W Resistor (brn-red-org) 1 E500089

RC 2.2K 1/8W Resistor (red-red-red) 1 E500088

RD 1.8K 1/8W Resistor (brn-gray-red) 1 E500087

RE, RF 10K 1/8W Resistor (brn-blk-org) 2 E500092

THM Thermistor, 10K, 3% (small orange body

with black and brown stripes near leads)

1 E500091

RF-X1 Crystal, ECS 12.09-CD 1 E850007

MISC Thermistor PC Board 1 E100152

MISC #24 Green Wire, Insulated 1 ft (30

cm)

E760008

Summary of content (4 pages)

PAGE 1
K2 Temperature-Compensated PLL Reference Upgrade Rev. E, March 24, 2005 Description This upgrade kit significantly improves the stability of the K2’s VFO, reducing drift per unit of temperature by as much as 90%. It is especially useful with a K2/100, since operation at higher power will cause faster heating inside the cabinet. The circuit, designed by John Grebenkemper, KI6WX, uses a thermistor to temperature-compensate the varactor diodes in the PLL reference oscillator.
PAGE 2
Thermistor PC Board Assembly Locate the thermistor printed circuit board. One side will be labeled with THM and the letters A through F. All components will be installed on the labeled side, and soldered on the other side. Install 1/8th-watt resistors RB, RC, RD, RE, and RF in their respective positions as shown on the board. (Be sure to check the color codes carefully.) Do not pull on the resistor leads, as they may be fragile.
PAGE 3
If you determined that you need to remove X1 (and X2, if present): (1) Heat and lift the ground lead soldered to the side or top of X1. (2) Alternately heat each of the crystal leads on the bottom side of the board while gently rocking it back and forth on the top until it falls out. (3) Remove X2 in the same manner (if X2 is present). Locate resistor network RP3, which has 8 pins and is near X1 and L31. RP3 must be removed.
PAGE 4
Theory of Operation The thermistor circuit works by applying a variable offset voltage to varactor diodes D16 and D17 to compensate for drift in the PLL reference oscillator. As temperature increases, the uncompensated oscillator will drift down in frequency. The thermistor causes a slight increase in the bias voltage to these diodes as the temperature increases. The resistor values have been chosen to minimize the drift of the PLL Reference Oscillator over its entire tuning range (typically 10 to 13 kHz).