[Elecraft] Tx spurs?

[Mike Harris]

G'day,

Don't think this is anything new.


> I recently finished my K2 (3576) and K2/100. Today I received an
> alarming call from a fellow ham about 50 miles away. My transmit
> frequency was 21035 MHz. My friend copied me on 21077, 21056 21035,
> 21013.5 and 20992. I was very surprised and am quite concerned. We tried
> the experiment at 0.1, 10 and 100 watts with the same results.  These
> are all separated by ~21 KHz. Has anyone seen this before? Any ideas?

Previous messages:

<quote>
I have K2 #3070 with SSB, NB and 160 options plus the KPA100. It is truly a
great radio to use, with one exception. I hope someone on the reflector can
help me resolve this problem.

When I called CQ on a "clear" frequency during the recent ARRL DX Phone
contest, another station came on frequency to tell me I was "plainly
audible" 3-KHz down the band - he thought I was CQing on top of him! Sure, I
was running the legal limit, but I've never received such a report in more
than thirty years as a ham and I've only heard such an artefact once in that
time. I considered it to be a serious problem and verified it with another
station after the contest while I was running 80-W barefoot; with an S7
signal, he reported that he could hear a spurious signal "about 12-dB down"
in his IC756PRO.

Upon investigation, I determined the following. Listening at 21.207 in a
second receiver with a short wire antenna about 2-feet from the K2, I hear a
signal that sounds like a collection of carriers when I enter K2 "tune" mode
at 21.207-MHz, with output of about 1-W into a dummy load. The K2 signal
registers about S9+10-dB on the second RX. When I move the K2's transmit
frequency to 21.206, I hear a cleaner "carrier," but now there are spurs
1-KHz above and below the carrier, about 20-dB below the carrier (3-1/2
S-units) in amplitude. When I move down to 21.205, the spurs are 2-KHz from
the carrier, and so on, as I move down the band. Same thing happens when I
move up the band. The farther I move from 21.207, the weaker the spurs
become. To recap, spurs appear at a separation from the carrier equal to the
difference between the carrier and 21.208-MHz. Note that this also means
that there is always a signal - carrier or spur - at 21.207-MHz.

I believe the problem is leakage from the KPA100's HV osc into the VFO
circuit. Note that TX freq. - TX BFO = 21.207 - 4.913 = 16.294-MHz, which is
where I pick up the stray radiation from the HV osc and find that it is
zero-beat with the TX VFO. The problem disappears when the KPA100 is removed
from the K2, even though the auxiliary 12-V, PA, ribbon cable and speaker
connections are still in place between the KPA100 and the K2. The problem
also disappears when I disable the HV osc by disconnecting 12CTL power via
the ribbon cable. I have checked for good ground connections, measuring less
than 1-ohm from the KPA100 shield to the K2 ground, etc. I even bought a
moto-tool to ensure that all of the anodizing and paint were removed from
their respective surfaces and installed ground jumpers between the KPA100
antenna jack and the metal shield and between the shield and the speaker
frame. With a 100-MHz analog scope on the 50-mV/div range, there is no
perceptible ripple on the aux 12-V line between the K2 and the KPA100. The
HV osc is producing 146-V to the TR switch.

To "probe" further into the condition, I connected a �-inch diameter loop of
insulated hookup wire to one end of a 6-inch piece of RG174 and pulled back
about �-inch of shield from the far end. With the KPA100 sitting "dead bug"
on a cardboard box next to the K2, I can reproduce the condition by putting
the loop between the HV osc and the metal shield, then bringing the exposed
end into the "volume" of the open K2 cabinet over the area of the VFO. The
spurs are strongest when the probe is brought close to C68, K15, terminal 2
of T5, and C73, in that order. Interestingly (to me, at least), TX mixer U10
does not seem to be a sensitive area. I re-heated all of the joints in the
VFO area to ensure good connections; no change observed.

Any ideas where I should check next?
<unquote>

<quote>
Brian,

We are investigating this right now, and will let you know what steps are
needed
to cure it.

73,
Wayne
N6KR

Brian Machesney wrote:
>
> I have K2 #3070 with SSB, NB and 160 options plus the KPA100. It is truly
a
> great radio to use, with one exception.... [spur when operating at
21.207].
<unquote>

Been sort of quiet since then.  Apart from fitting the KPA100 externally to
the K2 there hasn't been a cure offered that I know of.  Also been quiet on
the key click problem

<quote>
I was going to wait for Eric to come back from vacation to collaborate with
me
on this, but I had a brainstorm and decided to give it a try. Seems to work
well. We'll have to get a few builders to try the modifications, but here's
a
brief summary for those who may be interested (results are discussed at the
end):

Hardware changes:

1. U10A on the control board is reconfigured as a second-order low-pass
filter
rather than a simple R-C shaping network. This requires one new capacitor
and
three component value changes, including replacing D3 with a resistor. The
result is an approximately symmetrical, rounded trapezoidal waveform with
about
4 ms rise and fall times. The original circuit had a characteristic R-C
exponential decay waveform on the falling edge, which was a principle cause
of
the observed bandwidth.

2. A large capacitor is added from pin 2 of U8 on the Control board to
ground.
(The MAX534's output buffers are stable with any capacitive load.) This
reduces
the slew rate of the output buffer, further rounding the corners of the
keying
envelope. The final rise/fall times are about 4-5 ms.

Firmware change:

A change in transmit signal sequencing is required to take advantage of the
slower rise/fall times. (If the modified hardware is used without the new
firmware, the waveform will be distorted and there will be no improvement in
keying bandwidth.)

Results:

Initial tests show at least a factor of two reduction in the bandwidth of
keying
sidebands, using a method similar to that posted recently by Earl, K6SE. The
signal also sounds very clean (and looks very clean on the scope). Waveform
symmetry is preserved over the full power control range. I haven't done any
extended testing yet, i.e. using spectrogram.

One other point of interest. The bandwidth-limiting technique suggested by
W8JI--routing the CW signal through a narrow-band filter--would be very
difficult to implement on the K2. The transmit signal path cannot be
conveniently be routed through the CW filter, even with the KSB2 option
installed. It would require 10 or so additional parts, and probably a couple
of
coax jumpers, and could degrade the ultimate rejection of both the CW and
KSB2
filters on receive. The solution I described here (turning the shaping
network
in to a 2nd-order LPF) is the only simple method I have found during two
days of
lost sleep and head-scratching. In fact it's probably similar to what W8JI
ended
up doing on his other radios, although I couldn't find a theoretical
description
of his actual modifications, and don't have the FT1000 schematics.

Assuming this change passes muster with our short list of high-power
contesters,
we'll phase it into the K2 and offer some type of mod kit. It's likely that
this
and a few other minor changes will be included in next K2 firmware release.

At present there are two many unknowns for me to give you a date on this.
Please
DO NOT call Elecraft about it--we'll announce it, as usual.
<unquote>

Regards,

Mike VP8NO (IOTA SA-002)
GQRP 10148
QRP-Canada #57
K2/100 #1400