Dr. James C. Garland wrote:
Gang,
I?m pulling my hair out over an RF feedback problem with my K3/100,
and wonder if anybody has any ideas? I use an external station
controller that switches key, microphone, CAT, line in/out, etc. between
different rigs. The microphone audio connects to the K3?s rear panel
microphone jack and when the K3?s power exceeds about 15W (in
SSB), the audio becomes greatly istorted. It does this when all the other
cables to the K3 (except the microphone and power cord) are
unplugged. It also does it when the audio is fed to the K3 via the rear
Line In jack. Everything in my station is well-grounded. Here are some
things I?ve noticed:
1. The feedback still occurs when a military-type completely shielded
100W dummy load is screwed directly to the K3?s antenna port. This
suggests the RF from the K3 is getting into the audio line from some
other source than radiation from the coax cables in the station, most
likely the 12V power cord. To support this conjecture, I also notice that
the feedback threshold (15W) is not changed when the K3 drives a
linear amplifier.
2. I normally use a 30A Kepco 12V power supply to run the K3 and my
other station accessories. The RF feedback problem is improved but
not entirely eliminated if I use a separate 12V supply for the K3. A
common mode choke on the 12V line doesn?t appear to make any
difference.
I just measured the common-mode RF current on the K3's 12V power cord at
about 2mA at 7MHz, 100W into a dummy load. The background level was 1mA,
most of which was coming *up* the line from the SMPS.
The meter consists of an RF current transformer made from a clamp-on
ferrite core, feeding an AD8307 wide-range log detector. It responds to
the total common-mode current from all sources combined, with a
bandwidth from LF to several hundred MHz.
For perspective, about 1mA of assorted RF crud is a typical background
level on computer and monitor cables, including the filtered mains input
or DC output of a good-quality SMPS. Common-mode currents are also
installation-specific, because they depend on the impedance into which
the current is flowing, so in real-life installations it's only
justifiable to compare orders of magnitude (the location of the decimal
point) rather than obsess over exact values.
From that perspective, 2mA on the DC cord of the K3 is not a seriously
high level of RF leakage from a box that is generating 100W of RF. It is
also notable that the output cable leading to the dummy load has only a
few mA of common-mode current (contrast that with 1.4A of
differential-mode RF current flowing *inside* the shield). My IC-746
gave very similar results to the K3.
Therefore it seems unlikely that K3s in general would have enough RF
leakage on the power cord to cause the problems you're seeing, Jim. That
doesn't rule out the possibility of a fault on your particular rig...
but maybe you should be looking for a broken shield connection or
something else that has been disturbed by puling out the previous rig
and installing the K3.
Most likely of all is a broken shield connection on a PL-259, which can
let out high levels of RF current, which then crawls over the whole
station.
3. I can see some modulation-induced fluctuationss on the 12V line with
an oscilloscope that increase with the K3?s power setting. I haven?t
looked at these fluctuations closely enough yet to know whether it?s RF
noise or just audio?frequency fluctuations caused by the K3?s
modulating current draw from the power supply.
To summarize, my conclusions so far are that (1) the 12V power cord is
a source of RF leakage from the K3, even though the K3 has a filter at
its 12V power connector to minimize this leakage; and (2) The
audio/DSP circuitry in the K3 is very sensitive to RF ? much more so
than other transceivers in my station. (I?ve not had this problem with
other rigs.)
Because of this RF sensitivity, one evidently has to be very careful
when hooking accessories to the K3. The front panel microphone jack
is (to me) wired in a curious way, with the Mic ground and PTT ground
(shield) connections floating above the K3?s chassis with a 100uH rf
choke. Similarly, both the ?hot? and ?shield? side of the rear panel mic
input jack has series 100uH rf chokes, which isolate the input from
chassis ground.
The problem with this arrangement is that most accessories that would
connect to the rear panel audio input jacks have single-ended outputs,
with a shielded cable that is directly tied to chassis ground at the
accessory end of the cable. Thus, unless one is very careful, it is easy
to have a situtation where the signal grounds in the audio circuitry of
the K3 can fluctuate with respect to the K3?s chassis ground, and this
may be the source of the RF feedback sensitivity.
Unfortunately, I can?t figure out the detailed mechanism for the
feedback closely enough to figure out a solution. Foir example, I don?t
know whether it?s better to leave the minus side of my 12V power supply
floating, or to tie it to the chassis gound. And even if I left it
floating, it would be tied to the chassis anyway by the other
accessories hooked to it, and this might cause more problems than
grounding it at the power supply terminal.
Think very carefully about that treacherous word "ground"! In reality,
"ground" connections at physically different locations are *never* at
the same potential. Often it doesn't matter... but if the common
negative return for all the accessories is at the PSU, the voltage drop
along the negative return for the K3 will be injecting DC, RF and SMPS
leakage into the entire audio system.
If the PS is allowed to float and the common negative return for all the
accessories is moved very close to the K3, that might solve the
problems.
Somehow, it seems like a wiring change in
the ground configuration of the K3?s audio circutis to improve RF
isolation may be needed to lick the problem completely.
The unwanted impedances in the internal "ground" paths of the K3 are a
weakness that might make a problem appear worse, but I don't think they
are the actual source of the problems.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
_______________________________________________
Elecraft mailing list
Post to: [email protected]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
