Time-Domain Reflectometry: debugging 2.7 GHz oscillations and Murata ferrite beads...

[Phil Hobbs]

So I\'ve been wringing out a new APD front end. It\'s a 500-MHz TIA with
a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
at about 1 MHz. Changing the drain current changes the duty cycle of
the squegging, and reducing it below about 2 mA makes it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current. All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at higher
gain, but the oscillation frequency doesn\'t move around. The waveform
is more or less sinusoidal-looking, but that\'s not surprising since the
vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
oscillation doesn\'t depend on whether the TIA is connected or not.

Lower-speed front ends based on the same sort of transistor are famously
stable--one of them runs the bootstrap across a two-inch-long FFC cable
going to a MPPC on a cold plate. They also work great as the bottom
device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of my
trusty Tek SD-24 TDRs, like so:
<https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails. Not bad for 20 cents.)

Here\'s the result:
<https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png>
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
by a delayed and not-too step open-circuit reflection and some ringing.
Interestingly the reciprocal of the round-trip delay is right around 2.7
GHz, which would make sense with an open-circuit transmission line
resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors. Our faves are the Murata BLM1xBA
series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads. From top to bottom at
the beginning of the falling-edge transient:
BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
BLM15BA050SN1D (5 ohms @ 100 MHz, green);
BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

<https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png>

The BA-series beads show a lot better high-frequency impedance than the
BB-series ones, despite their datasheet curves being very similar. The
BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked. Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to be
less likely to oscillate at 5 GHz.

Comments welcome.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'ve been wringing out a new APD front end. It\'s a 500-MHz TIA with
a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
at about 1 MHz. Changing the drain current changes the duty cycle of
the squegging, and reducing it below about 2 mA makes it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current. All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at higher
gain, but the oscillation frequency doesn\'t move around. The waveform
is more or less sinusoidal-looking, but that\'s not surprising since the
vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
oscillation doesn\'t depend on whether the TIA is connected or not.

Lower-speed front ends based on the same sort of transistor are famously
stable--one of them runs the bootstrap across a two-inch-long FFC cable
going to a MPPC on a cold plate. They also work great as the bottom
device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of my
trusty Tek SD-24 TDRs, like so:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails. Not bad for 20 cents.)

Here\'s the result:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
by a delayed and not-too step open-circuit reflection and some ringing.
Interestingly the reciprocal of the round-trip delay is right around 2.7
GHz, which would make sense with an open-circuit transmission line
resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors. Our faves are the Murata BLM1xBA
series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads. From top to bottom at
the beginning of the falling-edge transient:
BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
BLM15BA050SN1D (5 ohms @ 100 MHz, green);
BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png

The BA-series beads show a lot better high-frequency impedance than the
BB-series ones, despite their datasheet curves being very similar. The
BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked. Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to be
less likely to oscillate at 5 GHz.

Comments welcome.

Cheers

Phil Hobbs

A bootstrapped pour seems like a great idea. I did that on my
triggered Colpitts oscillator to reduce the effective/terrible FR4
capacitance. But it made things unstable for some reason, so I
reverted to cutting some chunks out of the ground and power planes and
temperature compensating out the positive TC of the FR4.

A couple of those bead TDRs look like capacitors to me.

I like gate resistors to tame phemts, but we mostly work with big
signals so a few more nV of noise is no big deal to us. I\'m using 499
ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
as a resistor.

Gotta study your post in more detail when I have more time.

TDR rocks.
--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Gerhard Hoffmann]

Am 31.03.22 um 18:58 schrieb John Larkin:

On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'ve been wringing out a new APD front end. It\'s a 500-MHz TIA with
a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
at about 1 MHz. Changing the drain current changes the duty cycle of
the squegging, and reducing it below about 2 mA makes it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current. All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at higher
gain, but the oscillation frequency doesn\'t move around. The waveform
is more or less sinusoidal-looking, but that\'s not surprising since the
vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
oscillation doesn\'t depend on whether the TIA is connected or not.

Lower-speed front ends based on the same sort of transistor are famously
stable--one of them runs the bootstrap across a two-inch-long FFC cable
going to a MPPC on a cold plate. They also work great as the bottom
device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of my
trusty Tek SD-24 TDRs, like so:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails. Not bad for 20 cents.)

Here\'s the result:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
by a delayed and not-too step open-circuit reflection and some ringing.
Interestingly the reciprocal of the round-trip delay is right around 2.7
GHz, which would make sense with an open-circuit transmission line
resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors. Our faves are the Murata BLM1xBA
series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads. From top to bottom at
the beginning of the falling-edge transient:
BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
BLM15BA050SN1D (5 ohms @ 100 MHz, green);
BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png

The BA-series beads show a lot better high-frequency impedance than the
BB-series ones, despite their datasheet curves being very similar. The
BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked. Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to be
less likely to oscillate at 5 GHz.

Comments welcome.

Cheers

Phil Hobbs

A bootstrapped pour seems like a great idea. I did that on my
triggered Colpitts oscillator to reduce the effective/terrible FR4
capacitance. But it made things unstable for some reason, so I
reverted to cutting some chunks out of the ground and power planes and
temperature compensating out the positive TC of the FR4.

A couple of those bead TDRs look like capacitors to me.

I like gate resistors to tame phemts, but we mostly work with big
signals so a few more nV of noise is no big deal to us. I\'m using 499
ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
as a resistor.

Yes, at frequencies where it works like a resistor, it creates
noise like a resistor.

V1 is only there as a compiler pleaser, needed for syntax.

<
https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
>

chears, Gerhard

 

[Phil Hobbs]

Gerhard Hoffmann wrote:

Am 31.03.22 um 18:58 schrieb John Larkin:
On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'ve been wringing out a new APD front end.  It\'s a 500-MHz TIA with
a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
at about 1 MHz.  Changing the drain current changes the duty cycle of
the squegging, and reducing it below about 2 mA makes it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current.  All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at higher
gain, but the oscillation frequency doesn\'t move around.  The waveform
is more or less sinusoidal-looking, but that\'s not surprising since the
vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz.  The
oscillation doesn\'t depend on whether the TIA is connected or not.

Lower-speed front ends based on the same sort of transistor are famously
stable--one of them runs the bootstrap across a two-inch-long FFC cable
going to a MPPC on a cold plate.  They also work great as the bottom
device in a cascode with a 45-GHz SiGe NPN.  (The NPN needs a
base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of my
trusty Tek SD-24 TDRs, like so:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails.  Not bad for 20 cents.)

Here\'s the result:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
by a delayed and not-too step open-circuit reflection and some ringing.
Interestingly the reciprocal of the round-trip delay is right around 2.7
GHz, which would make sense with an open-circuit transmission line
resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors.  Our faves are the Murata BLM1xBA
series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads.  From top to bottom at
the beginning of the falling-edge transient:
BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
BLM15BA050SN1D (5 ohms @ 100 MHz, green);
BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png


The BA-series beads show a lot better high-frequency impedance than the
BB-series ones, despite their datasheet curves being very similar.  The
BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked.  Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to be
less likely to oscillate at 5 GHz.

Comments welcome.

Cheers

Phil Hobbs

A bootstrapped pour seems like a great idea. I did that on my
triggered Colpitts oscillator to reduce the effective/terrible FR4
capacitance. But it made things unstable for some reason, so I
reverted to cutting some chunks out of the ground and power planes and
temperature compensating out the positive TC of the FR4.

A couple of those bead TDRs look like capacitors to me.

I like gate resistors to tame phemts, but we mostly work with big
signals so a few more nV of noise is no big deal to us. I\'m using 499
ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
as a resistor.



Yes, at frequencies where it works like a resistor, it creates
noise like a resistor.

V1 is only there as a compiler pleaser, needed for syntax.


https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
    

Comes up blank for me. But it\'s true--there\'s a very general theorem of
classical thermodynamics that states that *in thermal equilibrium* any
process that can dissipate power will also produce fluctuations with
average power kT/2 per classical degree of freedom.

You can derive the sqrt(4kTR) formula by considering an isolated RC
circuit, setting the mean energy on the capacitor as kT/2, and computing
what the resistor\'s noise power has to be in order to keep the mean
energy constant.

As soon as you apply power, of course, all the nice theorems go
away--for instance, a diode-connected transistor looks like a resistor
whose value is 25 mV/I_C, and whose temperature is T_J/2 (150 K at room
temperature).

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Gerhard Hoffmann]

Am 31.03.22 um 20:50 schrieb Phil Hobbs:

Gerhard Hoffmann wrote:

Yes, at frequencies where it works like a resistor, it creates
noise like a resistor.

V1 is only there as a compiler pleaser, needed for syntax.


https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
     

Comes up blank for me.

Works for me. 2k5 * 1k5 screen dump.
called up 7 times upto now.
But Flickr is sloooooow!

Gerhard

 

[John Doe]

Phil Hobbs wrote:

Gerhard Hoffmann wrote:

https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public

Comes up blank for me.

Here... Fails in Firefox. Works in Microsoft Edge.

 

[John Larkin]

On Thu, 31 Mar 2022 21:11:30 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

Am 31.03.22 um 20:50 schrieb Phil Hobbs:
Gerhard Hoffmann wrote:


Yes, at frequencies where it works like a resistor, it creates
noise like a resistor.

V1 is only there as a compiler pleaser, needed for syntax.


https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
     

Comes up blank for me.

Works for me. 2k5 * 1k5 screen dump.
called up 7 times upto now.
But Flickr is sloooooow!

Gerhard

Fine, fast here with Firefox.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[ehsjr]

On 3/31/2022 3:11 PM, Gerhard Hoffmann wrote:

Am 31.03.22 um 20:50 schrieb Phil Hobbs:
Gerhard Hoffmann wrote:


Yes, at frequencies where it works like a resistor, it creates
noise like a resistor.

V1 is only there as a compiler pleaser, needed for syntax.


https://www.flickr.com/photos/137684711@N07/51974219045/in/dateposted-public/
     

Comes up blank for me.

Works for me. 2k5 * 1k5 screen dump.
called up 7 times upto now.
But Flickr is sloooooow!

Gerhard

Works for me too.
Ed

 

[Clifford Heath]

On 1/4/22 3:09 am, Phil Hobbs wrote:
> Comments welcome.

No technical comment, just this:

Fascinating post, many thanks for sharing.

CH

 

[Phil Hobbs]

Phil Hobbs wrote:

So I\'ve been wringing out a new APD front end. It\'s a 500-MHz TIA
with a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a
strong squeg at about 1 MHz. Changing the drain current changes the
duty cycle of the squegging, and reducing it below about 2 mA makes
it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current. All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at
higher gain, but the oscillation frequency doesn\'t move around. The
waveform is more or less sinusoidal-looking, but that\'s not
surprising since the vertical bandwidth of the scope (a TDS 694C) is
about 3.3 GHz. The oscillation doesn\'t depend on whether the TIA is
connected or not.

Lower-speed front ends based on the same sort of transistor are
famously stable--one of them runs the bootstrap across a
two-inch-long FFC cable going to a MPPC on a cold plate. They also
work great as the bottom device in a cascode with a 45-GHz SiGe NPN.
(The NPN needs a base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of
my trusty Tek SD-24 TDRs, like so:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails. Not bad for 20 cents.)

Here\'s the result:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD)
followed by a delayed and not-too-
steep
open-circuit reflection and some ringing. Interestingly the
reciprocal of the round-trip delay is right around 2.7 GHz, which
would make sense with an open-circuit transmission line resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors. Our faves are the Murata
BLM1xBA series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads. From top to bottom
at the beginning of the falling-edge transient: BLM18BA100SN1D (10
ohms @ 100 MHz, light orange); BLM15BA050SN1D (5 ohms @ 100 MHz,
green); BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (
5
ohms @ 100 MHz, yellowish).

https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png



The BA-series beads show a lot better high-frequency impedance than
the BB-series ones, despite their datasheet curves being very
similar. The BLM15BA050 shows a bit of undershoot near 1.5 ns, but
the others are all basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical
at this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked. Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to
be less likely to oscillate at 5 GHz.

Comments welcome.

(Fixed a couple of typos that might impede understanding)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

John Larkin wrote:

On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

So I\'ve been wringing out a new APD front end. It\'s a 500-MHz TIA with
a pHEMT bootstrap, and wants to oscillate at 2.7 GHz with a strong squeg
at about 1 MHz. Changing the drain current changes the duty cycle of
the squegging, and reducing it below about 2 mA makes it stable.

The oscillation frequency doesn\'t change much (5% or so) with drain
current. All of which suggests that the oscillation is due to some
reasonably-sharp resonance someplace--the squegging gets worse at higher
gain, but the oscillation frequency doesn\'t move around. The waveform
is more or less sinusoidal-looking, but that\'s not surprising since the
vertical bandwidth of the scope (a TDS 694C) is about 3.3 GHz. The
oscillation doesn\'t depend on whether the TIA is connected or not.

Lower-speed front ends based on the same sort of transistor are famously
stable--one of them runs the bootstrap across a two-inch-long FFC cable
going to a MPPC on a cold plate. They also work great as the bottom
device in a cascode with a 45-GHz SiGe NPN. (The NPN needs a
base-stopper bead, but the pHEMT doesn\'t.)

The difference with this one may be that there\'s a bootstrapped pour
under the summing junction components, driven by the pHEMT source.

Sooo, I took a bare board, bodged in a U.FL micro coax connector from
the bootstrapped pour to ground, and hung it on the front of one of my
trusty Tek SD-24 TDRs, like so:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRSetupSm.png>.

(U.FLs are super useful for this sort of thing--far better than coax
pigtails. Not bad for 20 cents.)

Here\'s the result:
https://electrooptical.net/www/sed/beads/BootstrapPourTDRsm.png
(green curve: U.FL unplugged; white curve: board attached).

There\'s a capacitive dip (3.0 pF as measured on a Boonton 72BD) followed
by a delayed and not-too step open-circuit reflection and some ringing.
Interestingly the reciprocal of the round-trip delay is right around 2.7
GHz, which would make sense with an open-circuit transmission line
resonator.

I also did some measurements of the Murata beads we use as base/gate
stoppers for microwave transistors. Our faves are the Murata BLM1xBA
series.

The following scope photo shows a TDR of a short piece of 0.080\"
hardline with various low-Z Murata ferrite beads. From top to bottom at
the beginning of the falling-edge transient:
BLM18BA100SN1D (10 ohms @ 100 MHz, light orange);
BLM15BA050SN1D (5 ohms @ 100 MHz, green);
BLM18BB100SN1D (10 ohms @ 100 MHz, purple); and
BLM15BB050SN1D (10 ohms @ 100 MHz, yellowish).

https://electrooptical.net/www/sed/beads/MurataBLM15-18-050and100beadsTDRsm.png

The BA-series beads show a lot better high-frequency impedance than the
BB-series ones, despite their datasheet curves being very similar. The
BLM15BA050 shows a bit of undershoot near 1.5 ns, but the others are all
basically monotonic at late times.

Interestingly the BLM15 (0402) and BLM18 (0603) beads look identical at
this resolution--the traces lie right on top of each other.

Fun stuff, and it\'ll be more fun once I get the resonance problem
knocked. Putting the pHEMT source connection near the middle of the
pour instead of at one end will help, I expect--this part ought to be
less likely to oscillate at 5 GHz.

Comments welcome.


A bootstrapped pour seems like a great idea. I did that on my
triggered Colpitts oscillator to reduce the effective/terrible FR4
capacitance. But it made things unstable for some reason, so I
reverted to cutting some chunks out of the ground and power planes and
temperature compensating out the positive TC of the FR4.

A couple of those bead TDRs look like capacitors to me.

The higher-Z BB-series parts are more resistive than the BA-series
ones--poking around on these pages is pretty illuminating.

<https://ds.murata.co.jp/simsurfing/gateway.html?partnumbers=%5B%22BLM18AB050SN1D%22%5D&rgear=suaykx&rgearinfo=com>

(You can replace the part number with the one you actually want to
see--getting the same plot by dorking with the web page is much slower.)

A five-ohm bead that peaks at nearly 100 ohms resisitive out at 4 GHz or
so is a super useful part.

I like gate resistors to tame phemts, but we mostly work with big
signals so a few more nV of noise is no big deal to us. I\'m using 499
ohms in one case! I guess a hi-Z bead makes as much hf Johnson noise
as a resistor.

Gotta study your post in more detail when I have more time.

TDR rocks.

Yup, especially for time-domain stuff. Stuff like stability analysis is
a bit subtler using TDR vs. a network analyzer. I should break out my
HP 70820A (a 40 GHz scope/network analyzer/spectrum analyzer based on
samplers).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Cursitor Doom]

On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]

Obviously you do a *lot* of testing and development, Phil. Which
particular area of electronics do you find most stimulating to tackle?
(if any nowadays given your extreme level of familiarity with the
subject, that is).

 

[Phil Hobbs]

Cursitor Doom wrote:

On Thu, 31 Mar 2022 12:09:19 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]

Obviously you do a *lot* of testing and development, Phil. Which
particular area of electronics do you find most stimulating to tackle?
(if any nowadays given your extreme level of familiarity with the
subject, that is).

I mostly like building complete instruments. I\'ve been doing a lot of
front ends lately, which is getting boring, so we\'re trying to pour a
bunch of concrete so that we can turn round a customized front end for
folks made from nice tested building blocks.

I thought this oscillation thing was fairly interesting, and was looking
for some input from other folks. (Maybe even Joerg, if I can entice him
away from doing fast laps around his bierkeller on the mountain bike.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com