7182 buzz appears after K69XV installed

[W4sc]

The PROBLEM

Installed the K60XV and the old 7182 (7181 ->7184 ) Buzz appears. Just like
a bad penny.

Jack W4EFZ related to me that this was an old problem.  The buzz is caused
by the LO and Reference frequency being co-incident in the 12 Mhz range when
the radio was tuned around 7182 and coupling back into the PLL.  My K2 did
not exhibit this problem until K60XV mods were made, Proper PLL voltages
across all bands were also difficult to obtain.

Investigation of the original fix raised some question about component
values chosen for the PLL VCC filter  (100uh and .001uf).  Looking at the
specs on 100uh molded inductors of the physical size used that are available
from several manufactuers revealed that the self resonant frequencies were
in the 12 to 14 Mhz range. As I recall an inductor at SRF and above has a
low impedance (Atleast that's what I recall).  It seems that this is just
the opposite of what one would want to keep the LO frequency out of the PLL
VCC supply pin

How it was cured

1) After much thought, I decided that an inductor with an SRF above the
highest LO frequency (about 25 Mhz) would be best... Also I wanted to
shorten the lead length as much as possible.  An SMT inductor was chosen
with a value of 18uh and SRF of 27Mhz. Its foot print is a 1210 size
(120mils x 100 mils) and fit perfectly between the VCC pin of the PLL chip
and the VCC buss.

2) After a little more thought I decided to increase the bypass capacitance.
I just picked a .033uf as it was the first one greater than .01uf and less
than 0.1uf in the supply box that I came up with.  It is in a 1206 size case
and is just long enough to reach from the PLL chip VCC pin to the ground via
originally used.

3) I also added a 0.1uf capacitor (0805 size) from the VCC buss to the same
ground via.  The solder resist will need to be removed from the VCC buss
trace in order to install. The 0805 fit perfectly.

4) Reading the fix for the 7182 buzz,  there were instructions to trim PLL
chip leads an keep lead lengths short.  The added 68 pf capacitor used
had short leads,, but it looked like a big old lollipop antenna sitting
there.  It was replaced with a SMT 0805 68pf  NPO capacitor.  The 0805 fit
nicely between the PLL chips' leads.

5) Looking at the two varicaps and 10k resistor added for 60 meter coverage,
which were supplied with the K60XV, and again remembering something about
shorts leads,,, They were replaced with SMT components.  Two MMVL109
varicaps in SOD23 package and a 10K 1206 packaged resistor were used.  The
lead lengths were reduced considerably as a result, plus the components are
now right down on the PWB surface.

The payoff.

1) NO MORE 7182 BUZZ (PERIOD)

2) The PLL voltages on all bands came up considerably. After setting up the
PLL (L30) on 160 and 80 meters, all other bands fell into place and WELL
with the specified ranges with plenty of margin. No difficulties at all.

Do not try this at home

Unless you have a GOOD microscope (No, NOT the one your kid got with their
biology set for Christamas last year), or EXTREMELY GOOD maginifier, or the
eye of the 6 million dollar man, AND good soldering SKILLs, AND the proper
soldering tools, I would not reccommend trying to make this change as
oulined here.  The parts are VERY SMALL.  I used a 2 to 10x inspection
microscope and soldering rework station at my work.

Unless you ARE having the PLL problems outlined, I would not reccommend
making this change.  If it ain't broke, don't fix it.

If you need this fix

Get the proper tools, and have the proper skills, or find the proper
experienced help.

Changing the leaded conponent values to meet the criteria outlined above
would probably suffice in the PLL VCC filter.  Keep ALL leads on ALL changes
made with the leaded components as SHORT as possible.  Use a smaller 68pf
leaded capacitor on the PLL.

For your reading, some applicable information.

http://focus.ti.com/lit/an/scaa048/scaa048.pdf

http://www.analogzone.com/hft_1124.pdf

Food for thought

Don't argue with success.

73

de 2577

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