[Elecraft] APP NOTE: Using the KFL1-4 over a wide temperature range

[Wayne Burdick]

BACKGROUND

The K1 four-band module (KFL1-4) uses very high-Q band-pass filters.
These "tight" filters provide excellent rejection of out-of-band signals
and ensure good transmit spectral purity.

The down side to very high-Q filters is that wide variations in
operating temperature may cause the filters to become slightly de-tuned.
This is very unlikely to affect receiver performance. However, maximum
transmit power output might be reduced. The exact low or high
temperatures at which some power degradation might be noted will vary
from one KFL1-4 module to the next, because the temperature coeffients
of the trimmer capacitors vary. 

This application note describes two ways to improve the operating
temperature range of the four-band module.

METHOD #1

The simplest way to increase the operating temperature range is to
decrease the Q of the band-pass filters slightly. This can be done by
increasing the size of the small coupling caps in each filter. We have
carefully tested this modification, and found that it had no adverse
effect on transmit or receive performance. These changes alone should
allow operation at significantly higher and lower temperatures, and
will also provide somewhat flatter power output across a given band.

Here are our recommended values:

C9 = 2 or 2.2 pF
C10 = 4.7 or 5 pF
CJ = 2.7, 3.0, or 3.3 pF
CK = 4.7 or 5 pF

You can either replace the existing capacitors or add small capacitors
in parallel on the bottom of the board (this is much easier). If you
wish to make these changes but don't have one or more of the required
capacitors, Elecraft will send them to you free of charge. Send your
request to support@elecraft.com.

METHOD #2

APPLICABILITY: If your KFL1-4 (or K1-4) kit was shipped after December
13th, 2001, you already have the more temperature-stable trimmer
capacitors described below. The new trimmers have a blue body; the old
trimmers were white.

If you anticipate operating in wider temperature extremes, you may need to
replace the original trimmer capacitors on the KFL1-4 board with a more 
temperature-stable type. The original trimmers were 8-50 pF, Elecraft 
p/n E540000. The replacement type is 1-40 pF, Elecraft p/n E540002. 
This difference in maximum capacitance will not affect KFL1-4 module alignment.

We will supply you with a full set (16) of the new trimmers at low cost
($10 plus shipping). Call Elecraft to order, or send e-mail to sales@elecraft.com.

Note: these temperature-stable trimmers are *not* required for other
circuits in
the K1 (or the K2), since these circuits operate at lower Q than the K1's
band-pass filters.

Before you attempt to install new trimmers, you'll need to *completely*
remove all solder from the mounting holes using solder wick or a vacuum
desoldering tool. Otherwise, you won't be able to seat the new trimmers.
Straightening the bends in the new trimmers' leads may also be helpful.
If all else fails, push very gently downwards on the trimmers while
heating the pad from the back side of the board, but be careful not to
use excessive heat, which could melt the plastic bodies. After
installing 
the new trimmers, re-align the module per the KFL1-4 manual.