On Sat, 03 Sep 2011 11:31:51 -0700, Richard Clark
wrote:

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.

I'm sure you are perfectly aware of the single point of failure in
that generality.

I am? Ummm... well, I guess so.

Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).

True. They all have some internal resistance to overcome. However,
that's negligible resistance when compared to that of a static blast.
Static electricity has lots of potential (volts), but is only able to
deliver small amounts of current. That's why we don't get
electrocutes by the potential (voltage) difference between our head
and our feet. Dividing the large voltage, by the tiny current,
results in a fairly substantial source resistance. I'm too lazy to
look it some real numbers, but I'm sure it's in mega ohms. The 47K
resistance, and whatever ESR the 0.01uF contributes, has little effect
on the energy delivered to the shottky diode.

Incidentally, if the source resistance of the static blast was much
less, then the diode would not simply be fried. It would probably
explode.

When we (silverbacks) got into this game, (the preferable) mica caps
were available, snipped out of the nearest sacrificial TV or radio.
Trying to read those several styles of color coding was the biggest
hurdle, but I had plenty in my junk-box.

Dumpster diving in Henry Radio's trash can in West Smog Angeles was
one of my favorite after skool exercises. Salvaging old TV chassis
and dead tubes were the grand prizes. Silver mica caps came a close
second.

Ceramic is ubiquitous, now, and far from choice in these matters,
unless you do deep research (maybe).

Ceramic is cheap. I was a big fan of porcelain caps from AVX in big
power amps. If you wanna handle current, there's nothing better.
Silver mica would get hot, ceramic would explode, and everything else
was either too big or too expensive.

Incidentally, I don't think they make 0.01uF silver mica caps. The
biggest I played with were in antenna tuners at 4700pF (or should I
say uuF for nostalgia purposes).

You got any favorites that respond to this?

I don't understand the question. Favorite what?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 03 Sep 2011 15:51:10 -0700, Jeff Liebermann
wrote:

Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).

True. They all have some internal resistance to overcome.

It goes beyond that. Extrapolating from power applications hides the
defects of ceramic.

At HF/VHF and above, successful applications comes from throwing uF
solutions at pF problems. Ceramic's performance reveals inductive
reactance above 1-10 MHz. ESR also exhibits the same turn-around in
the same frequency range. Ceramic temperature coefficient is (Y5V)
goes into the toilet in weather that most of the south and eastern
seaboard has seen this summer. XR7 voltage coefficient causes
capacity to plummet at the voltages you offer for static. Over time,
ceramics lose capacity for simply having been in service for a while.

Aside from that, they work fine.

I was a big fan of porcelain caps from AVX in big
power amps. If you wanna handle current, there's nothing better.

However, those ceramics are 1,000 times (min.) larger than what you
have recommended. They serve an entirely different agenda.

AVX discusses these issues in much the same terms (for those larger
caps too) at:
http://www.avx.com/docs/techinfo/mlc-tant.pdf

Incidentally, I don't think they make 0.01uF silver mica caps. The
biggest I played with were in antenna tuners at 4700pF

Where there is every chance that one silver mica head-to-head with the
ceramic actually exhibit better performance (protecting the diodes).
Perhaps with the scarcity of silver mica, however, 10uF ceramics would
make do (it is not like any precision is demanded to force a selection
of 0.01uF which is boilerplate recommendation from the 50s).

73's
Richard Clark, KB7QHC

Jeff Liebermann wrote:



Yep. That's a good way to provide some protection. However, there's
no protection while you're juggling connectors when you run the risk
of a static discharge to the center of the coax connector.

I don't recall reading such a procedure in the user manual. However,
there are plenty of warning:
http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
In section 4.1:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR.
and in 5.1:
WARNING: NEVER APPLY RF OR ANY OTHER EXTERNAL VOLTAGES TO THE
ANTENNA PORT OF THIS UNIT. THIS UNIT USES ZERO BIAS DETECTOR
DIODES THAT ARE EASILY DAMAGED BY EXTERNAL VOLTAGES OVER A
FEW VOLTS.
and in 5.2:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR. PROTECT THIS PORT FROM ESD.

Clear enough. It would appear that MFJ is fully away of the fragile
nature of the input circuitry.

I learned to ground everything working on transmitters the size of
houses. The B+ is bled and grounded when you open the door, but you
still ground anything metal before you touch it. It translated nicely to
CMOS procedures on the bench. I am a grounding fool because I know any
conductor can store a charge lethal to solid state and that any friction
produces a charge. (I humidify, too!)

On 04 Sep 2011 14:05:13 GMT, dave wrote:

Jeff Liebermann wrote:
Yep. That's a good way to provide some protection. However, there's
no protection while you're juggling connectors when you run the risk
of a static discharge to the center of the coax connector.

I don't recall reading such a procedure in the user manual. However,
there are plenty of warning:
http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
In section 4.1:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR.
and in 5.1:
WARNING: NEVER APPLY RF OR ANY OTHER EXTERNAL VOLTAGES TO THE
ANTENNA PORT OF THIS UNIT. THIS UNIT USES ZERO BIAS DETECTOR
DIODES THAT ARE EASILY DAMAGED BY EXTERNAL VOLTAGES OVER A
FEW VOLTS.
and in 5.2:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR. PROTECT THIS PORT FROM ESD.

Clear enough. It would appear that MFJ is fully away of the fragile
nature of the input circuitry.

I learned to ground everything working on transmitters the size of
houses. The B+ is bled and grounded when you open the door, but you
still ground anything metal before you touch it. It translated nicely to
CMOS procedures on the bench. I am a grounding fool because I know any
conductor can store a charge lethal to solid state and that any friction
produces a charge. (I humidify, too!)

Such an extreme RF environment is not necessary to blow up the diodes.
None of the 3ea MFJ-259B boxes that I replaced required a transmitter
the size of a house to blow up. Much as the protective procedures
that you are recommending are genuinely useful, the instrument first
has to protect itself.

Assuming they were all fried by ESD, I tried to conjur a method that
would protect the existing design. As simple bleeder resistor to
ground will only help under trivial situations. The worst case
senario, of holding the instrument in one hand, and plugging in a
PL259 that is connected to an ungrounded antenna with a large static
charge, is all too common. Changing to an type-N connector will help
because the grounded shield is connected first, instead of the center
conductor, as in the SO-239.

Back to back diodes might work if the RF levels are low enough. The
non-linearity of the diodes will cause measurement accuracy problems
and rectify any off frequency RF going into antenna connector.

I'm tempted to try 1:1 RF broadband transformers which should work
over octave frequency ranges. Better yet, a tuned 1:1 RF xformer, to
improve the front end selectivity so that it can be used in an RF
polluted environment[1]. That would work, but will also be very
clumsy and expensive.

I know of several devices where failure is sufficiently common, that
spare parts are included with the instrument, and the components are
in easily accessible sockets. While not a great solution, it does
make some sense.

Other than attaching a grounded anchor chain to the MFJ-259b, spraying
holy water around the area to increase humidity, or carrying various
anti-static protection devices, do you have any suggestions as to how
the instrument could better protect itself from ESD?






[1] Attaching a wattmeter to the typical VHF antenna on a mountain
top that is colocated with FM/TV xmitters will show a watt or three of
RF. The front end of the MFJ-259b is not going to like that.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 03 Sep 2011 22:29:49 -0700, Richard Clark
wrote:

At HF/VHF and above, successful applications comes from throwing uF
solutions at pF problems.

That's not a problem. In order to get obtain decent bypassing across
5 octaves of bandwidth (2-30MHz), one needs to have multiple capacitor
values and types in parallel. The self-resonant characteristics of
the capacitors is the limiting factor. At some frequency, every
capacitor, and its associated lead inductance, will exhibit an
impedance dip commonly known as series resonance. Below this
frequency, the capacitor will look ummm... like a capacitor. Above
this frequency, it will be more like an inductor.
http://www.ecircuitcenter.com/Circuits/cmodel1/cmodel1.htm

None this has anything useful to do with the 0.01uf caps in the
instrument. The diodes are in series with 47K resistors, which are
much larger than any inductive reactance that the 0.1uf bypass
capacitor might present. Since the MFJ-259b only works well up to
maybe a 10:1 VSWR or 5Kohms, the 47K is sufficiently larger than
whatever reactance is presented by the 0.01uf to make the capacitor
characteristics to not be an issue.

While component selection and circuit design are interesting topics,
the current problem is MFJ design quality, MFJ-259b, ESD protection,
and chronic detector diode failures.





--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

Apparently you missed my previous rant on the topic. See the
schematic extract of the RF section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
Notice the directly connected diodes. The diodes in question are
Avago HSMS-2820 zero bias shottky diodes.
http://www.avagotech.com/docs/AV02-1320EN
15V Max PIV is rather low. It won't take much voltage at the antenna
go exceed 15V. The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.
An important clue is that BOTH D3 and D4 appear to be blown each time,
which implies an external failure, not a component failure.

Argh. I just started working on the analyzer, and found a few errors,
all of which are my mistakes.

1. The antenna analyzer that arrived (3 days late) today is not the
expected MFJ-259B but an MFJ-269. The difference is that the MFJ-269
goes up to UHF frequencies. The front end is similar, but not
identical.

2. The MFJ-269 has a type-N connector, while the MFJ-259 has a UHF
SO-239 connector. So much for the idea of substituting a type-N
connector.

3. Despite my idea of installing a bleeder resistor on the antenna
connector to drain off the static charge, the antenna connector
already shows 50 ohms resistance to ground. I missed this path
because of the rather difficult to read schematic of the MFJ-259 RF
section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
The path is from the antenna connector, through R24, L11, and then to
ground.

The MFJ-269 schematic is easier to read at:
http://802.11junk.com/jeffl/crud/MFJ-269-RF-section.jpg
which goes through R88, L12, and then to ground.

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I haven't finished working on the MFJ-269 quite yet. A report and
photos when I'm done.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I haven't had any problem with the analyzer since installing my dummy
load on it.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?


- 73 de Mike N3LI -

Mike Coslo wrote:

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I haven't had any problem with the analyzer since installing my dummy
load on it.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?


- 73 de Mike N3LI -

Is it possible that 50 Ohms at DC is much higher Z at MF/HF/VHF?

Mike Coslo wrote in
:

But that brings up something interesting. You're reading 50 Ohms on
the input connector? Isn't that going to make *all* readings somewhere
near 1:1?

If you look at the circuit, there is a DC path from the centre pin, via the
50 ohm resistor that forms part of the measurement bridge, and an RFC to
ground. That would look like 50 ohms at DC, but it will not prevent the
measurement bridge working in the way you suggest.

Owen

Jeff Liebermann wrote in
:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

But the reality is that people do damage these things... and so the
method you suggested earlier is not likely to be sufficient to protect
them.

I am careful to avoid connecting an instrument of this type to an
antenna system unless I have drained any static charge first, and avoid
other transmitters on air nearby.

An easy trap is to connect the analyser or the like to an antenna, then
start working on the antenna without considerig the risk of introducing
a spike during the work.

Painful, but worth disconnecting immediately after each measurement. Not
as painful if BNC connectors are used.(Why did they build those things
with SO239... don't answer!)

Owen