QRP Zero-beat Indicator for the Elecraft K2
This is a
circuit that builds on KR5L’s zero-beat indicator circuit. He
suggested trying
a CMOS version of the 567 instead of the standard bipolar version, and
here it is:
the QRP version of his circuit. It draws only 0.9 mA at 5V compared to
about 20
mA/8V for the original circuit. 
Compared to the original circuit, C2 and C3 have been divided by about eight, and C1 has been divided by about two, give or take values found in my junk box. Component values:
U1 – LMC567 PLL
R1a – 2.2 k
R1b – 5 k pot
R2 – 22 k
R3 – 220 ohm
R4 – 220 ohm
R5 – 390 k
C1 – 0.15 µF
C2 – 1 µF
C3 – 3.3 µF
C4 – 0.1 µF
C5 – 100 µF
C6 – 470 µF
In addition R3 and C6 have been
added. They add a time constant of 0.1 sec or so, but unlike just
increasing
the output filter capacitor C3, this gives a fade-out when the LED
extinguishes, and results in a smoothness to the display much like the
indicator on the FT-1000. The value for R3 has been found from the
maximum
output current of 20 mA of U1, and is obtained if C6 has been charged
to 5 V
and is then discharged through U1’s output.
R5 has also been added for increased
sensitivity. I found that my circuit required a little over comfortable
speaker
level to trigger before R5 was added. Values as low as 270 k can be
used and
give even higher sensitivity, but also more triggering on noise.
The output is connected to the
right-most LED
of the K2,
an idea that I first heard from
W7DZN. The output goes to pin 20 of the DS2 LED array on the Control
Board. In
the ‘Nite’ setting, the LED is fed from 2.7 V
through a 120 ohm resistor, while
in the ‘Day’ mode it is fed from 4 V. R4 results in
less than 50% of the
current in the LED than in the usual S-meter, and makes it easier to
see that
this segment has a different function.
Vcc is connected to 5V from the output
of U8 on
the RF board (across C90 on the underside of the board), and the audio
input is
connected to the output of the AF power amplifier. The circuit is
mounted on
the original N0SS/KR5L PCB.
The frequency can be adjusted to match
the
sidetone in two ways: By measuring the output of pin 5 with a frequency
counter
and divide the frequency by two, or by increasing the sidetone level in
Test
mode until it turns on the LED, and then tune R1b carefully back and
forth
until one has found the center between the low and high turn-on
frequencies.
Reference: KR5L’s circuit/PCB: http://home.earthlink.net/~n0ss/kr5l_cw_tuning_ind.pdf
Correction 9 June 2002: C4 and C5 have their values interchanged, C4 = 100 µF and C5 = 0.1 µF. Not that I think it matters too much, with the input resistance of 40 kohms, either value is fine for C4, and the same goes for the power supply decoupling C5. So if you built it already, with incorrect values and it works fine, then don’t worry!
This article was printed in Sprat, Summer 2002 and in a German translation in QRP-Report, 3-2002 (Note that R3 was incorrectly labeled 220k).
Change June 2004:
Recently, I moved the zero-beat indicator to a board mounted on the back of the K2's front panel board. I also added an audio buffer for an AF output through the mic plug that is independent of the AF pot setting. The zero-beat indicator uses the same signal, and its indication is therefore independent of audio pot setting. See K2 Audio Output for details and new pdf-file with schematic.
19 May 2011.