Help! uWave xistor bias oscillates!

[[email protected]]

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

 

[[email protected]]

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?

 

[[email protected]]

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.

 

[colin]

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 =^.^=

 

[[email protected]]

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.

 

[Mark]

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

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 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.

Mark

 

[[email protected]]

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.

 

[John Miles]

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

 

[[email protected]]

John Miles wrote:

[snip]

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

John's right. Unless these are really special parts the likes
of which I've not seen, the 47uH bias inductors will self-resonate
at frequencies much lower than your 1,700 MHz test frequency.

That's bad.

Ditto for those 22uF and even the 100nF capacitors, which are
resonators here, not capacitors at all. The inductors, meanwhile,
will be *highly* capacitive at 1,700 MHz.

Cures? One might add resistors in series with the inductors,
the larger the better, use a skinny trace instead of or in
conjunction with the inductor in the bias lines, use much smaller
inductor values, and use real RF bypass caps, e.g. 22pF & 100pF.
(100pF = 1 ohm at 1,700 GHz)

Were it me, I'd ditch the inductor feeding the transistor's base
entirely & feed said transistor directly from the 50k-50k divider,
eliminating undesired resonances and reducing the opportunity for
unwanted coupling/feedback in the bargain.

Best luck!
James Arthur

P.S. John's also right about putting the inductors at right
angles to minimize coupling, and shielding and separating help
too, but not before taking the measures above.

 

[sdy]

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?

 

[[email protected]]

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?

Good work on the oscillations.

The 47nH inductors still make me uneasy. A quick scan
through the DigiKey catalog shows self-resonant frequencies
as low as 260 MHz (muRata LQM18NN47) to 1,800 MHz (Panasonic
ELJ-RE47NGF2) to 3,000 MHz (Panasonic ELJ-NK47NAF). Be sure
to check the SRF of the units in your circuit. Non-shielded
inductors invite unwanted coupling, while magnetically-shielded
units have low SRFs.

The collector inductor has the benefit of increasing the
available gain, which might justify its use over a resistor
of equivalent impedance (e.g. 500 ohms). A mischief-reducing
compromise is to use the inductor and a small (22-100 ohm)
resistor in series.

The base-feed inductor, however, offers no advantage, and
several dangers that I can see over simply connecting the
base to the 50k-50k divider, which offers higher impedance
& isolation than the inductor anyhow.

As to matching, the matching needed for best noise figure
is commonly not the same as the matching for best power transfer
(i.e. highest gain), but matching for best gain is a passable
starting point for experimentation. The optimum match varies
with Vcc, f, and Icc. If you're lucky the vendor's datasheets
will offer parameters close to your intended region of operation.

Way back when, I wrecked a small box of MRF571s one after
another learning these same things ... darned if those
amplifiers don't love to scream.

Best,
James Arthur

 

[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

 

[[email protected]]

Hello,


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

Thanks for the link Joerg. The MLC caps are better than I
remembered. Especially heartening was the apparent lack of
parallel resonances in Fig. 5, which were my chief concern.
I've encountered such resonances in days gone by and marvel
at these plots.

Even so, the 0603 was measured to have an effective series
inductance of 0.87nH, making a 100nF unit series-resonant
at 17MHz and, more importantly, contributing 9.3 ohms of
reactance at 1.7 GHz. That's not horrible, but it's not
fantastic -- power supply feedthrough and/or unexpected
feedback are possible.

One can parallel caps to reduce the inductance, as you
say, or, in many instances, deliberately select a lower-valued
cap, using series resonance to advantage. An example of this
can be found here:

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.

Best Regards,
James Arthur

 

[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

 

[[email protected]]

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.

Agreed: that the packaging itself limits ESL to finite values.

It looks to me like the physically smaller caps _are_
somewhat better. Table 1 in the AVX paper shows 0.87nH
for the 0603, vs: 1.2nH for the 1206, while Figure 4
depicts the effects of this.

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.

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?

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...Arse+****+Industries"&rnum=2#4a0130bb27b8f415

Grins,
James Arthur

 

[Jeroen Belleman]

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...Arse+****+Industries"&rnum=2#4a0130bb27b8f415

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

Regards, Joerg

 

[[email protected]]

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 ;-)

Best,
James Arthur

 

[[email protected]]

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?