Help! uWave xistor bias oscillates!

I'm fooling around, trying to measure the S params of an HXTR-6101,
Low
noise npn uWave transistor. The transistor breaks into low freq
oscillations as I increae the base current. I've got it in a stripline
fixture, I made. I'm feeding the base from a supply,thru a 50KOhm, to
a
..1uF & 22 uF cap to ground and 47uH coil to the base stripline run.
Same with the collector. How can I stop the low freq oscillations? You
can see a drawing of the stripline here:

http://sdeyoreo.tripod.com/id18.html

Thanks.

 

[Joerg]

Hello,

The emitter side looks like it's just two strips. Do you have a ground
plane? This stuff really needs a full ground plane.

If there is no ground plane it is possible that the circuit breaks into
RF oscillations, upon which the current depletes the 22uF cap on the
collector side because the 1k may not allow enough current to sustain
sufficient voltage. Then the voltage drop causes the oscillation to
stop, the cap charges back up and the whole cycle begins anew. Kind of a
"pumping" oscillation. On the scope you may only see the low frequency
pumping.

Regards, Joerg

http://www.analogconsultants.com

 

On Wed, 27 Apr 2005 21:17:21 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello,

The emitter side looks like it's just two strips. Do you have a ground
plane? This stuff really needs a full ground plane.

There is a full ground plane underneath. Vias connect the emitter

strips to the ground plane. ( I drilled holes and soldered wire.
Spaced the holes close to each other.) Also, the ends of the emitter
strips are connected by foil to the bottom ground plane.So both
emitter strips are

If there is no ground plane it is possible that the circuit breaks into
RF oscillations, upon which the current depletes the 22uF cap on the
collector side because the 1k may not allow enough current to sustain
sufficient voltage. Then the voltage drop causes the oscillation to
stop, the cap charges back up and the whole cycle begins anew. Kind of a
"pumping" oscillation. On the scope you may only see the low frequency
pumping.

That's EXACTLY what I see, a sort of pumping! Maybe I should move the

22 uf to the other side of the 1K?

Regards, Joerg

http://www.analogconsultants.com

 

Oh, and the return to the powersuplies is soldered to the emitter
strip next to the caps. Also, seperate supplies are used for Vb and
Vc.
On Wed, 27 Apr 2005 21:17:21 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello,

The emitter side looks like it's just two strips. Do you have a ground
plane? This stuff really needs a full ground plane.

If there is no ground plane it is possible that the circuit breaks into
RF oscillations, upon which the current depletes the 22uF cap on the
collector side because the 1k may not allow enough current to sustain
sufficient voltage. Then the voltage drop causes the oscillation to
stop, the cap charges back up and the whole cycle begins anew. Kind of a
"pumping" oscillation. On the scope you may only see the low frequency
pumping.

Regards, Joerg

http://www.analogconsultants.com

 

[colin]

<sdeyoreo@hotmail.com> wrote in message
news:u2tv6111bmbnffe50uikicv5iev58r6oa8@4ax.com...

I'm fooling around, trying to measure the S params of an HXTR-6101,
Low
noise npn uWave transistor. The transistor breaks into low freq
oscillations as I increae the base current. I've got it in a stripline
fixture, I made. I'm feeding the base from a supply,thru a 50KOhm, to
a
.1uF & 22 uF cap to ground and 47uH coil to the base stripline run.
Same with the collector. How can I stop the low freq oscillations? You
can see a drawing of the stripline here:

http://sdeyoreo.tripod.com/id18.html

Thanks.

This will undoubtedly oscillate at uhf+ frequency unless you have some sort
of rf load at the collector.
With no load the inductor privodes 90' phase lead at the collector, the
collector base capacitance provides an aditional 90' phase lead back to the
base and you have a nice oscillator.

Colin =^.^=

 

On Thu, 28 Apr 2005 01:04:57 GMT, "colin"
<no.spam.for.me@ntlworld.com> wrote:

This will undoubtedly oscillate at uhf+ frequency unless you have some sort
of rf load at the collector.
With no load the inductor privodes 90' phase lead at the collector, the
collector base capacitance provides an aditional 90' phase lead back to the
base and you have a nice oscillator.

Colin =^.^=

Sorry, each port is connected to the network analyzer (50 ohms) thru a
DC block. Also, there is a ground plane underneath. The emitter strips
are connected to the ground plane thru vias and the power supplies
return is soldered to the emitter strip next to the caps. Seperate
supplies are used for base and collector.

 

On 27 Apr 2005 18:47:22 -0700, "Mark" <makolber@yahoo.com> wrote:

are the holes for the emitter grounds right near or under the emitter
lead...

They're awfully close to the emitter leads, without getting in the way

of the leads. The holes are spaces like 1/4 in apart, 4 - 5 holes up
each strip. then foil bend over the edge, connecting each strip to the
ground plane.

you need to have basically NO inductance in series with the emitter


also try adding a ferrite bead in on the leads feeding the Dc to the
base and the collector, place the beads directly at the microstrip.

I have beads I'll try in the AM.
I agree, you have a RF oscillation that is "pumping". You need to stop
the RF oscilation., It may help to identify the RF oscillation
frequency with a spectrum analyzer.

If nothing else, I can do another board with the shortest emitter

strips possible.
Oh, I'm trying to get S params at 1700 MHz.
>Mark

 

[John Miles]

In article <u2tv6111bmbnffe50uikicv5iev58r6oa8@4ax.com>,
sdeyoreo@hotmail.com says...

I'm fooling around, trying to measure the S params of an HXTR-6101,
Low
noise npn uWave transistor. The transistor breaks into low freq
oscillations as I increae the base current. I've got it in a stripline
fixture, I made. I'm feeding the base from a supply,thru a 50KOhm, to
a
.1uF & 22 uF cap to ground and 47uH coil to the base stripline run.
Same with the collector. How can I stop the low freq oscillations? You
can see a drawing of the stripline here:

http://sdeyoreo.tripod.com/id18.html

Thanks.

The emitters are directly bonded to the ground plane, right? Emitter
followers can oscillate if there's any inductance in the base lead.

I first heard of that phenomenon in Pease's book on analog
troubleshooting, but I'm sure it's well-known and/or obvious to many.
There's a nice explanation at
http://home.tiscali.be/kpmoerman/electronics/notes/efollow/efollow.htm .
Could that be what's going on?

Apart from that: 47 uH sounds like a heck of a lot of inductance for
microwave work. Those bias chokes may have significant parasitics. Who
knows... they might act like coupled resonators at certain frequencies.
At a minimum they should be oriented perpendicular to each other. Can
you try different choke values to see if the oscillation moves in
frequency?

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

 

[Joerg]

Hello,

No oscillations!!! Thanks to all! I removed the bypass caps and went
with 22pF and 100 pF, as per James Arthur, and all is well now! My
mistake, the coils are 47nH, not 47 uH.. No I can fool around some more
and try stub tuning for a good 50 ohm match. I'ld like to try a noise
figure measurement. I have an HP noise source and NF meter. I assume I
need to tune to a good match for the noise test?

Great!

Just one word about ceramic caps. Even a 0.1uF shouldn't have let you
down. Check AVX corporation under Tech Papers for the article by
Dr.Jeffrey Cain about parasitic inductance. Figure 5 is pretty
interesting here:

http://www.avx.com/docs/techinfo/parasitc.pdf

Any cap of reasonable size, such as 0603 has an inductance just by its
sheer size, forming a "loop". So it helps to have several in parallel.
Also, it just has to be SMT. Through hole is going to be next to
impossible at 1700MHz.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello James,

http://www.atceramics.com/technicalnotes/circuit_designer.asp

in the document titled
"Circuit Designer's Notebook - Capacitors in Bypass Applications."

Figure 2 shows a net impedance of just about 2 ohms for a
single 100pF bypass capacitor at 1.7 Ghz, a 4-fold improvement.

Thanks. That is an interesting paper. What struck me in the AVX paper
was that smaller SMT caps weren't really any better at high frequencies.
Much is probably attributable to the packaging itself. So smaller should
be better.

Just curious: Where does your email address come from? Dagmar is, at
least in Europe, a female name but James certainly isn't. Or is it a
company name?

Regards, Joerg

http://www.analogconsultants.com

 

[Jeroen Belleman]

dagmargoodboat@yahoo.com wrote:

Agreed: that the packaging itself limits ESL to finite values.
[...]
the device to "wide and short," which is effectively
paralleling devices as you've already suggested. I
poked briefly about for these--and they do exist--but I
didn't find any right off.

Syfer makes 0612 sized capacitors.

http://www.syfer.com/pdf/lowinductance38.pdf

Jeroen Belleman

 

[Joerg]

Hello James,

Even better than "small" is to change the aspect ratio of
the device to "wide and short," which is effectively
paralleling devices as you've already suggested. I
poked briefly about for these--and they do exist--but I
didn't find any right off.

At RF that would almost be like placing caps parallel.

The real trick with RF is to use the circuit board itself. Areas such as
the supplies on the OP's circuit board should be large and form a
substantial capacitor with the ground plane underneath. Above 1GHz this
can then become the major decoupling contributor while the 0.1uF just
takes care of other noise coming in from the outside. Cost would be next
to nothing.

Then there is shielding. A nice metal strip between base and collector
can be very useful, at a few pennies a pop. Once when I absolutely could
not find any metal at a client's site we all shared a can of anchovies
during our lunch and cut up the can later. Problem was, we had to drink
lots of water that afternoon.

Umm...there is/was a method to my registration, and I had to be
_someone_, so I applied the principles which Genome has already
spoken to rather elegantly in the thread "Nationals Webench:

http://groups-beta.google.com/group/sci.electronics.design/browse_thread/thread/fbd2de29732123e2/4a0130bb27b8f415?q=genome+%22Arse+Fuck+Industries%22&rnum=2#4a0130bb27b8f415

ROFL. I believe someone did the same on the Freescale site lately.

Regards, Joerg

http://www.analogconsultants.com

 

On 29 Apr 2005 16:22:45 -0700, dagmargoodboat@yahoo.com wrote:

Joerg wrote:
Then there is shielding. A nice metal strip between base and
collector
can be very useful, at a few pennies a pop. Once when I absolutely
could
not find any metal at a client's site we all shared a can of
anchovies
during our lunch and cut up the can later. Problem was, we had to
drink
lots of water that afternoon.

Love it! Once upon a project I fashioned cavity filters from
Clabber Girl Baking Powder cans--tin-plated light gauge steel--and
copper refrigeration tubing. Q of 300 at 900MHz with insertion
loss <2dB; they saved the day. These days, however, projects are
smaller... I use Altoids cans ;-)

6 meter beer can cavity resonators?
Best,
James Arthur

 

On 29 Apr 2005 21:13:50 -0700, dagmargoodboat@yahoo.com wrote:

6 meter beer can cavity resonators?

That's an awful lot of beer -- we'd need all of s.e.d.
to empty it!

IIRC, it did take a few cans, didn't it? ( maybe that's why I don't

IIRC!)

Grins,
James Arthur

 

[Joerg]

Hello James,

Thanks for the tip Jeroen. AXV makes 'em too, as I
discovered on the next page of their .PDF. Table 2
lists an ESL of 610pH for their 0612 package, and
the text projects 530pH for a 0306.

For GHz circuits I always try to create enough of an island that the
capacitance between this island and the ground plane kicks in. These
ultra high frequency caps can be pricey.

Regards, Joerg

http://www.analogconsultants.com

 

[John Woodgate]

I read in sci.electronics.design that dagmargoodboat@yahoo.com wrote (in
<1114897329.443327.95110@g14g2000cwa.googlegroups.com&gt about 'Help!
uWave xistor bias oscillates!', on Sat, 30 Apr 2005:

Thanks for the tip Jeroen. AXV makes 'em too, as I discovered on the
next page of their .PDF. Table 2 lists an ESL of 610pH for their 0612
package, and the text projects 530pH for a 0306.

Your parts are better: Syfer specs 500pH for their 0612.

The inductance is just a function of physical size. A difference of 500
pH to 610 pH for same-size components is difficult to explain.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Larry Brasfield]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote
in message news:CUvUdfDELIdCFw6y@jmwa.demon.co.uk...

I read in sci.electronics.design that dagmargoodboat@yahoo.com wrote (in <1114897329.443327.95110@g14g2000cwa.googlegroups.com&gt
about 'Help! uWave xistor bias oscillates!', on Sat, 30 Apr 2005:

Thanks for the tip Jeroen. AXV makes 'em too, as I discovered on the next page of their .PDF. Table 2 lists an ESL of 610pH
for their 0612 package, and the text projects 530pH for a 0306.

Your parts are better: Syfer specs 500pH for their 0612.

The inductance is just a function of physical size. A difference of 500 pH to 610 pH for same-size components is difficult to
explain.

Difficult, yes. For several reasons, it is easy to believe.

The inductance contributed by the interleaved portion
electrodes would be affected by the number of leafs
and their dimensions. Capacitors with the same outside
dimension can vary in both respects.

The results might be obtained by trying to measure the
inductance from different portions of the endcaps.

There is also an interesting issue in the definition of a
two terminal inductance -- one which I would like to
see resolved if that is possible. Presumably, when an
inductor is measured, (especially a small valued one),
inductance of the rest of the circuit (the test equipment
and connecting cables and fixture) is supposed to be
eliminated from the stated result. This is commonly
done by placing some object other than the DUT into
the unknown position and "nulling", where the object is
assumed to have known (or 0) inductance. However,
unless that object and the DUT have the same magnetic
field interaction with the unchanging portions of the
measurement circuit, the result will be skewed as it is
dependent on the test circuit geometry.

This is obviously a bit academic for normal inductors,
wound to increase internal inductance. But for bits of
metal that resemble a straight conductor, I do not see
a good way of glossing over the issue if one cares to
distinguish 500 pH from 610 pH and claim either to
be right or wrong.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[John Woodgate]

I read in sci.electronics.design that Larry Brasfield
<donotspam_larry_brasfield@hotmail.com> wrote (in
<f78de.11$wu5.1279@news.uswest.net&gt about 'Help! uWave xistor bias
oscillates!', on Sun, 1 May 2005:

Difficult, yes. For several reasons, it is easy to believe.

For a particular meaning of 'believe'. I think you imply that the
difference is due to different methods of measurement, and thus the
'lower' inductance part may not in fact be better. If so, I entirely
agree.

The inductance contributed by the interleaved portion electrodes would
be affected by the number of leafs and their dimensions. Capacitors
with the same outside dimension can vary in both respects.

With the same dielectric, the options are few. We don't know if the
dielectric are the same.

The results might be obtained by trying to measure the inductance from
different portions of the endcaps.

Indeed.

There is also an interesting issue in the definition of a two terminal
inductance -- one which I would like to see resolved if that is
possible. Presumably, when an inductor is measured, (especially a
small valued one), inductance of the rest of the circuit (the test
equipment and connecting cables and fixture) is supposed to be
eliminated from the stated result. This is commonly done by placing
some object other than the DUT into the unknown position and "nulling",
where the object is assumed to have known (or 0) inductance. However,
unless that object and the DUT have the same magnetic field interaction
with the unchanging portions of the measurement circuit, the result
will be skewed as it is dependent on the test circuit geometry.

This is why test fixtures with precise dimensions and mechanical
stability are used for measurements of small inductors and capacitors.

This is obviously a bit academic for normal inductors, wound to
increase internal inductance. But for bits of metal that resemble a
straight conductor, I do not see a good way of glossing over the issue
if one cares to distinguish 500 pH from 610 pH and claim either to be
right or wrong.

I wouldn't gloss over it at all. I'd ask the manufacturers if their
measurements are traceable to national standards, and if so, how.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Fred Bloggs]

Larry Brasfield wrote:

The inductance contributed by the interleaved portion
electrodes would be affected by the number of leafs
and their dimensions. Capacitors with the same outside
dimension can vary in both respects.

There is such a thing as simulation, and of course, the capacitor
manufacturer employs competent engineers for whom these things are not
as mysterious as they are for you. When they embark on a new product
line specifically for low ESL high frequency use, these various factors
are considered beforehand and not after the fact.

The results might be obtained by trying to measure the
inductance from different portions of the endcaps.

Not quite- the probing and frequencies make this the least significant
error of the measurement.

There is also an interesting issue in the definition of a
two terminal inductance -- one which I would like to
see resolved if that is possible. Presumably, when an
inductor is measured, (especially a small valued one),
inductance of the rest of the circuit (the test equipment
and connecting cables and fixture) is supposed to be
eliminated from the stated result. This is commonly
done by placing some object other than the DUT into
the unknown position and "nulling", where the object is
assumed to have known (or 0) inductance. However,
unless that object and the DUT have the same magnetic
field interaction with the unchanging portions of the
measurement circuit, the result will be skewed as it is
dependent on the test circuit geometry.

As usual, you are fretting like the ignorant wisp of an intelligent
lifeform you are- every single one of the test fixture, fixture probing,
and reference plane extension effects has been characterized accurately
and the impedance analyzer provides for their correction. It is not
necessary to present simple-minded perfect opens and shorts for
calibrations as you suggest.

This is obviously a bit academic for normal inductors,
wound to increase internal inductance. But for bits of
metal that resemble a straight conductor, I do not see
a good way of glossing over the issue if one cares to
distinguish 500 pH from 610 pH and claim either to
be right or wrong.

You should keep the list short and just tell us what you do understand.
As usual, your post is more space-taking, mindless, garbage. It is clear
you have never used an impedance analyzer in your life.

 

[Larry Brasfield]

[Derf transform applied.]

"Fred Bloggs" <nospam@nospam.com> wrote in
message news:42752FAD.10803@nospam.com...

Larry Brasfield wrote:

The inductance contributed by the interleaved portion
electrodes would be affected by the number of leafs
and their dimensions. Capacitors with the same outside
dimension can vary in both respects.

There is such a thing as simulation,

Simulation is not going to have any bearing on how
inductance varies with plate count and dimensions.

.
When they embark on a new product line specifically for low ESL high frequency use, these various factors are considered
beforehand and not after the fact.

Of course, they also take into account their own
production processes and constraints. For low
voltage ceramic caps, uniformity of the dielectric
imposes limits on thickness versus yield. So it
would be naive to think that all ceramic caps of
the same type, size and value are the same just
because cap makers hire competent engineers.

The results might be obtained by trying to measure the
inductance from different portions of the endcaps.

Not quite- the probing and frequencies make this the least significant error of the measurement.

Assertion without evidence or argument. I see
no reason to believe that some measurements
are not made from endcap ends, and others made
from endcap sides. Nor do I see any reason to
believe that the effects of such differences cannot
explain the 110 pH difference reported earlier.

As for "frequencies make this ...", that looks like
irrelevant gibberish to me.

There is also an interesting issue in the definition of a
two terminal inductance -- one which I would like to
see resolved if that is possible. Presumably, when an
inductor is measured, (especially a small valued one),
inductance of the rest of the circuit (the test equipment
and connecting cables and fixture) is supposed to be
eliminated from the stated result. This is commonly
done by placing some object other than the DUT into
the unknown position and "nulling", where the object is
assumed to have known (or 0) inductance. However,
unless that object and the DUT have the same magnetic
field interaction with the unchanging portions of the
measurement circuit, the result will be skewed as it is
dependent on the test circuit geometry.

[derf] every single one of the test fixture, fixture probing, and reference plane extension effects has been characterized
accurately and the impedance analyzer provides for their correction.

It is clear to me that you have missed the point. The
fact that any instrument purports to provide some
kind of correction for fixturing does not eliminate
the definitional issue. The simple fact is that there
is no defined way to partition mutual inductance
between a component and connections to that
component such that a lumped inductance can be
assigned and claimed to belong to the component,
except by fiat.

It is not necessary to present simple-minded perfect opens and shorts for calibrations as you suggest.

Amazing that you take my "known (or 0) inductance"
to be perfect anything other than inductance. You are
so desparate to detect ignorance that you invent it.

This is obviously a bit academic for normal inductors,
wound to increase internal inductance. But for bits of
metal that resemble a straight conductor, I do not see
a good way of glossing over the issue if one cares to
distinguish 500 pH from 610 pH and claim either to
be right or wrong.

[derf] It is clear you have never used an impedance analyzer in your life.

That makes clear the depth of your delusion. I have
used a variety of impedance analyzers, as well as a
few network analyzers for measuring impedance.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.