Anyone make PCBs with Othermill?

[Clifford Heath]

On 13/07/15 07:59, Tom Swift wrote:

Phil Hobbs <pcdhobbs@gmail.com> wrote:
For FET probes, I use Tek P6249s from eBay (4 GHz, about $250). For
slower stuff, i.e. with my 500-MHz scopes, I generally use P6201s (900
MHz).
I need to get higher, but I don't want to spend $28k on a set of 4 used
1134A's. If the 34143 works, it could save a bundle.

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to keep
the inductance and capacitance down. I documented and published here my
project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input capacitance,
making the probe useless above 350MHz.

Obviously I should have used much smaller (physically) coupling
capacitors than 0805, a shorter tip, and a FET with smaller input
capacitance. It's possible I could also have used an RC combination to
form a voltage divider with the input impedance (at a cost in noise).

However I think you'll get my point that the probe performance isn't
mainly about the FET, but about what comes before it. There's a good
reason why folk who can make 7GHz probes can charge $x,000 for them.

 

[Clifford Heath]

On 13/07/15 10:54, Tom Swift wrote:

Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to keep
the inductance and capacitance down. I documented and published here my
project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input capacitance,
making the probe useless above 350MHz.

If 10pf resonates with the probe tip at 350MHz, the inductance is 20.67nH.

Fr = 1 / (2 * pi * sqrt (10e-12 * 20.67e-9)) = 350,065,968.699 Hz

You are not going to get that with a straight piece of wire 7mm long.

Maybe you are using a ground lead that is 10" long?

It was lower than I expected too, but that's what we measured.
I welcome your comments on the design and construction. I'm not
claiming any special expertise; this was my first project into
VHF, let alone higher. The earthy side of the probe is a pogo pin,
the signal side just a header pin, soldered both sides on the PCB.

<http://cjh.polyplex.org/electronics/RFCascodeProbe/>

Clifford Heath.

 

[Clifford Heath]

On 13/07/15 09:59, Tom Swift wrote:

Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to
keep the inductance and capacitance down. I documented and published
here my project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input
capacitance, making the probe useless above 350MHz.

Obviously I should have used much smaller (physically) coupling
capacitors than 0805, a shorter tip, and a FET with smaller input
capacitance. It's possible I could also have used an RC combination to
form a voltage divider with the input impedance (at a cost in noise).

However I think you'll get my point that the probe performance isn't
mainly about the FET, but about what comes before it. There's a good
reason why folk who can make 7GHz probes can charge $x,000 for them.

The only reason these companies charge $X,000 is because we let them.

By "let them", you mean that you decide it's cheaper than developing
your own.

You can put a 450 ohm resistor at the probe tip and drive a coax
terminated in 50 ohms. This can easily get above 7 GHz. The only problem
is the 20 dB attenuation. It can drop a signal into the noise.

Right. That's why $$$ probes put the FET just a few mm from the tip.

According to phil, the ATF3143 has very low input capacity. Even lower
if it is operated as a source follower with a guard shield around the
tip that is driven by the source. There goes your capacitance.

You won't maximise bandwidth that way though (Phil's amps optimise
sensitivity and noise, not bandwidth). Unless you have the source->gate
feedback very finely-tuned to get close to the FETs bandwidth (and risk
oscillation), the buffering effect may be slower than the signal - so
the tip looks reactive at high frequencies anyhow.

What really dismays me is to see HP solder long leads to the input of a
7 GHz probe and show a picture of it connected to a pcb with the caption
that the bandwidth is now only 1.2 GHz. Why spend the money in the first
place? Makes no sense to me. Please see Figure 1-13 on page 25 of

http://literature.cdn.keysight.com/litweb/pdf/01134-97013.pdf

To be fair, they do say that this configuration is "For very widely
spaced targets" - which is not conducive to high-bandwidth probing.

This is supposed to be high tech? These people are supposed to know what
they are doing in handling wide bandwidth signals? And they are teaching
newbies how to probe fast signals?

I am rapidly loosing my confidence in these companies. I think all the
guys who knew what they were doing have retired or have gone into real
estate.

I look forward to seeing your published designs that do better.

 

[Tom Swift]

Phil Hobbs <hobbs@electrooptical.net> wrote:

On 7/11/2015 8:48 PM, Tom Swift wrote:
Tom Swift <spam@me.com> wrote:

Thanks very much for your excellent explanation. I will have to see
if LTapice can duplicate the device curves for the ATF38143 and
examine this further.

I looked at the performance as an emitter follower and it didn't seem
all that bad to me. Some results are at

http://s000.tinyupload.com/?file_id=54130764029191346435

That's just SPICE, though. Experimentally the follower performance of
an ATF38143 isn't as good as 0.9 gain. I don't think it follows the
standard MESFET model that well.

Have to try it and see.

One problem with probing wideband signals with a 500 ohm resistive
probe is the 20 dB loss of signal. If the signal is weak already, it
may be difficult to see in the noise. One solution is to se an active
probe, but these can be extremely expensive at microwave frequencies.
I decided to see if the ATF34143 might be useful as a source
follower.

46EB29C7.ASC ATF34143_chip Source Follower @ 117mA

This shows the performance as a wideband source follower with VEE
adjusted for 0V DC offset. The input is 5V p-p or about 18 dBm. The
output is about 4.53V p-p, or about 17.1 dBm. This is a loss of
around 0.9 dB, which could be similar to loss of a short length of
miniature coax at microwave frequencies connecting the probe to the
receiver.

For that use, it's probably fine, but a follower with a gain of 0.9
isn't useful to me very often, even if the ATF38143 were really that
good. I'm usually interested in accurate followers, for instance in
bootstraps. Some of them have wideband gains of more than 0.999,
measured by the bandwidth improvement obtained in the bootstrap.
That's very difficult with a pHEMT, even with its drain cascoded or
bootstrapped.

It would be nice if you could do that at 7GHz.

The ic dissipates 384 mW, which is well under the maximum of 725 mW,
but it will require heat sinking.

Are we talking about the same device? The ATF34143 is a single
transistor in a 4-lead SC70. Its max dissipation is listed as 725 mW,
*with the source lead at 25 C*. Since its thermal resistance theta_JC
is specified at 165 K/W (which is probably optimistic), making the
source lead 25C puts the junction at 0.58*165 + 25 = 145 C. Your 384
mW will cook the device in real life--90 C even with an infinite heat
sink.

The datasheet says SOT-343, but it looks the same as the SC70.

Where did you get the 0.58 and what is it for? Your calculation may have
a typo. I get 0.58*165 + 25 = 120.7 C, not 145 C.

On page 6 of the datasheet they run it at 4V and 120mA. Brave people.

For the follower, I get (0.384 * 165) + 25 = 88.36 C. The maximum channel
temperature Tch is 160 C, so if we keep the pins cool there should be
plenty of margin.

Perhaps mount the ic on a small 2 sided pcb that is 0.015" thick or so.
The bottom is copper, the top has the mounting pads for the ic and a row
of ceramic bypass caps for the drain. The other ends of the caps are
soldered to the edge of the pcb, which is soldered to the brass ferrule.
The pcb bottom is soldered to the ferrule, which is then thermally
connected to the coax shield.

This would also provide mechanical support for the probe, which is going
to get rough treatment as the user tries to punch through soldermask or a
layer of copper corrosion.

If you look at Fig. 1 of the datasheet, the spacing between the curves
varies by a good factor of 3, which is the nonlinearity I'm talking
about. It's pretty horrible.

Yes, it really looks bad. But we are running as a source follower, not an
amplifier. Lots of negative feedback.

Not bad for an ic that costs $185 at Newark:

https://octopart.com/search?q=ATF34143

You're pessimistic by 2 orders of magnitude.

Typo. Missing the decimal. If you followed the link it would show the
correct price and distributors.

Unfortunately, the price has changed between last night and this morning.
Newark now wants $2.54

The 38143 is a
smaller device that I pay about 50 cents for.

How well does it work as a follower driving 50 ohms?

Cheers

Phil Hobbs

 

[Clifford Heath]

On 13/07/15 11:18, Tom Swift wrote:

Clifford Heath <no.spam@please.net> wrote:
On 13/07/15 10:54, Tom Swift wrote:
If 10pf resonates with the probe tip at 350MHz, the inductance is
20.67nH.
You are not going to get that with a straight piece of wire 7mm long.
Maybe you are using a ground lead that is 10" long?
It was lower than I expected too, but that's what we measured.
I welcome your comments on the design and construction. I'm not
claiming any special expertise; this was my first project into
VHF, let alone higher. The earthy side of the probe is a pogo pin,
the signal side just a header pin, soldered both sides on the PCB.

http://cjh.polyplex.org/electronics/RFCascodeProbe/
I'd be very surprised if that circuit went much above 100 MHz.

It measured dead-flat 100KHz to 250MHz, and after tweaking the trimmer,
almost dead flat to 350MHz. Why are you surprised? I thought it might go
higher, even though I didn't need it to.

I'd still like to hear your thoughts on where you think the bandwidth
limitation is coming from.

You can buld narrowband amplifers by hand for frequencies above 10 GHz.
But you are going to have a hard time hand-wiring a broadband amplifier
for frequencies much above 100 MHz and expect it to be flat.

Buy two Minicircuits 8 GHz ERA-1 for $1.37 (Qty 30) and stick them in
series on a good ground plane.
http://www.minicircuits.com/pdfs/ERA-1+.pdf

My objective was mainly to learn how to design such circuits, not to buy
ready-made. And I wanted an impedance higher than 500ohms at least at
HF, to probe some very short antennae.

Clifford Heath

 

[Tom Swift]

Phil Hobbs <pcdhobbs@gmail.com> wrote:

I was looking at the 38143 datasheet until I noticed you were mainly
talking about the 34143, which has a lower max dissipation. I changed
the result but missed the LHS. The calculation is otherwise correct.

The 34143 is higher. It is 725mW, and the 38143 is 580mW. So that's where
the .58 came from. But 0.725*165 + 25 = 144.625 C, which is where the 145
came from.

Now it makes sense. Still under the max channel temperature of 160 C.

There's a good chance the 34143 will run fine at 384mW. It doesn't take
much to cool a 1/4 W resistor at full bore.

I do build 50-ohm stuff, but it's not my main interest.

For FET probes, I use Tek P6249s from eBay (4 GHz, about $250). For
slower stuff, i.e. with my 500-MHz scopes, I generally use P6201s (900
MHz).

I need to get higher, but I don't want to spend $28k on a set of 4 used
1134A's. If the 34143 works, it could save a bundle.

My experience with FET probes is they are usually very noisy. I'm hoping
the 34143 might be quieter.

Cheers

Phil Hobbs

 

[Tom Swift]

Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to
keep the inductance and capacitance down. I documented and published
here my project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input
capacitance, making the probe useless above 350MHz.

Obviously I should have used much smaller (physically) coupling
capacitors than 0805, a shorter tip, and a FET with smaller input
capacitance. It's possible I could also have used an RC combination to
form a voltage divider with the input impedance (at a cost in noise).

However I think you'll get my point that the probe performance isn't
mainly about the FET, but about what comes before it. There's a good
reason why folk who can make 7GHz probes can charge $x,000 for them.

The only reason these companies charge $X,000 is because we let them.

You can put a 450 ohm resistor at the probe tip and drive a coax
terminated in 50 ohms. This can easily get above 7 GHz. The only problem
is the 20 dB attenuation. It can drop a signal into the noise.

According to phil, the ATF3143 has very low input capacity. Even lower
if it is operated as a source follower with a guard shield around the
tip that is driven by the source. There goes your capacitance.

I don't care if it only has a gain of 0.9 or so, as long as it is stable
and reasonably flat with frequency. I can easily calibrate the waveform
to whatever I want in software.

The other big reason is to eliminate the broadband noise from TEK and HP
active probes. They are unusable with moderately low level signals,
especially if you include the ground bounce from switching noise. I was
surprised to find the TEK P6249 actually attenuates the signal by 5X.
Now we are back in the noise again.

What really dismays me is to see HP solder long leads to the input of a
7 GHz probe and show a picture of it connected to a pcb with the caption
that the bandwidth is now only 1.2 GHz. Why spend the money in the first
place? Makes no sense to me. Please see Figure 1-13 on page 25 of

http://literature.cdn.keysight.com/litweb/pdf/01134-97013.pdf

This is supposed to be high tech? These people are supposed to know what
they are doing in handling wide bandwidth signals? And they are teaching
newbies how to probe fast signals?

I am rapidly loosing my confidence in these companies. I think all the
guys who knew what they were doing have retired or have gone into real
estate.

 

[Tom Swift]

Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to keep
the inductance and capacitance down. I documented and published here my
project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input capacitance,
making the probe useless above 350MHz.

If 10pf resonates with the probe tip at 350MHz, the inductance is 20.67nH.

Fr = 1 / (2 * pi * sqrt (10e-12 * 20.67e-9)) = 350,065,968.699 Hz

You are not going to get that with a straight piece of wire 7mm long.

Maybe you are using a ground lead that is 10" long?

 

[Tom Swift]

Clifford Heath <no.spam@please.net> wrote:

On 13/07/15 10:54, Tom Swift wrote:
Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to
keep the inductance and capacitance down. I documented and published
here my project to build an amplified probe using a BF862 (10pF
input capacitance), with a 7mm probe tip and nominal 10K impedance.
The inductance of that length of probe resonates with the input
capacitance, making the probe useless above 350MHz.

If 10pf resonates with the probe tip at 350MHz, the inductance is
20.67nH.

Fr = 1 / (2 * pi * sqrt (10e-12 * 20.67e-9)) = 350,065,968.699 Hz

You are not going to get that with a straight piece of wire 7mm long.

Maybe you are using a ground lead that is 10" long?

It was lower than I expected too, but that's what we measured.
I welcome your comments on the design and construction. I'm not
claiming any special expertise; this was my first project into
VHF, let alone higher. The earthy side of the probe is a pogo pin,
the signal side just a header pin, soldered both sides on the PCB.

http://cjh.polyplex.org/electronics/RFCascodeProbe/

I'd be very surprised if that circuit went much above 100 MHz.

You can buld narrowband amplifers by hand for frequencies above 10 GHz.
But you are going to have a hard time hand-wiring a broadband amplifier
for frequencies much above 100 MHz and expect it to be flat.

Buy two Minicircuits 8 GHz ERA-1 for $1.37 (Qty 30) and stick them in
series on a good ground plane.

http://www.minicircuits.com/pdfs/ERA-1+.pdf

Put a 450 ohm microwave resistor on the end of a 50 ohm coax and connect
to the first ERA. Verify it is terminated in 50 ohms. Use capacitive
coupling between stages, LF cutoff frequency of your choice. Trim the
gain at the outout to 0 dB and you are done.

DC coupling is more difficult.

> Clifford Heath.

 

[Tom Swift]

Phil Hobbs <hobbs@electrooptical.net> wrote:

On 7/12/2015 7:59 PM, Tom Swift wrote:
According to phil, the ATF3143 has very low input capacity. Even
lower if it is operated as a source follower with a guard shield
around the tip that is driven by the source. There goes your
capacitance.

Bootstrapping is hard to do at gigahertz frequencies, because the
strays bite you. There isn't really such a thing as "high impedance"
above a few hundred megahertz, except in very special cases. Your
average chip resistor has a parasitic capacitance of 0.08 pF, give or
take, so a 1k resistor sitting on its own has a corner frequency of
about 2 GHz.

The bootstrap is not necessarily to get a high impedance from the point
of the circuit. It will be in a 50 ohm environment anyway. The problem
is the current through the ground wire. This creates a voltage that
lifts the probe off ground and puts stray currents on the outside of the
shield.

With the 34143, I don't anticipate having any components between the
gate and the test point. The source is right there, so the connection to
the guard is very short. The probe diameter is as small as possible so
it can get into tight spaces. This means the actual probe tip can be
very short and still hit the connection when it is next to a thick ic or
some large component such as a heat sink.

The ATF38143's input capacitance is below 1 pF, so with a nanohenry of
parasitic inductance from the leads and (very small) probe pins, that
comes in around 7 GHz, with a Q of about 1 at 50 ohms.

I assume the 34143 is about the same. 7 GHz is all I'm trying to reach
with the first version. I looked at Avago's site to see if I could find
if they sell bare dice but had no luck.

The other big reason is to eliminate the broadband noise from TEK and
HP active probes. They are unusable with moderately low level
signals, especially if you include the ground bounce from switching
noise. I was surprised to find the TEK P6249 actually attenuates the
signal by 5X. Now we are back in the noise again.

It's meant to work with DC-coupled, 50-ohm scopes such as the TDS
694C.
You can roast the input resistors if you put too much voltage on
them,
so for a general-use probe, an attenuator makes sense.

Aside from microwave radar and avionics transponders, I don't see much
need for high voltage on circuits running at GHz frequencies. Mostly low
level oscillators and some fast logic.

> (Scopes are pretty noisy in any case.)

That's the noise I am talking about when the signal is attenuated.
That's why the 34143 is so attractive.

I was also hoping to use the probe with my HP 8566 and also start work
on a microwave VNA. It would be nice to be able to look at a filter and
see if it was really doing what it should.

Cheers

Phil Hobbs

 

[Tom Swift]

John Larkin <jlarkin@highlandtechnology.com> wrote:

> Why not use an opamp? ADA4817 maybe.

Is that for the high frequency side?

Only CAD $5.79 at Newark. I didn't realize they were so inexpensive.

https://octopart.com/search?q=ADA4817

2 pA input current and 4 nV/sqrt(Hz) is not bad. Only problem is the 1 GHz
bandwidth. I need to go higher.

But I'm seriously thinking of a split channel. Some low frequency op amp
for the DC, and perhaps a nice Hittite (ADI) to get to 30 GHz or so.

Please be assured I pay careful attention whenever you and Phil talk about
splitting the channel into high and low frequency and the difficulties in
recombining them. An old HP journal article also talks about this
technique. I think the rolloff was at around 1 MHz, but I forgot the
instrument it was used in.

I already looked at the noise figure for the ERA-1+: 5 dB. That's not bad
for a cheap 8 GHz version to get started. Maybe it's time to stick it in
LTspice and see how it would work.

Overload recovery is going to be interesting.

 

[Phil Hobbs]

On 7/12/2015 7:59 PM, Tom Swift wrote:

Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to
keep the inductance and capacitance down. I documented and published
here my project to build an amplified probe using a BF862 (10pF input
capacitance), with a 7mm probe tip and nominal 10K impedance. The
inductance of that length of probe resonates with the input
capacitance, making the probe useless above 350MHz.

Obviously I should have used much smaller (physically) coupling
capacitors than 0805, a shorter tip, and a FET with smaller input
capacitance. It's possible I could also have used an RC combination to
form a voltage divider with the input impedance (at a cost in noise).

However I think you'll get my point that the probe performance isn't
mainly about the FET, but about what comes before it. There's a good
reason why folk who can make 7GHz probes can charge $x,000 for them.

The only reason these companies charge $X,000 is because we let them.

You can put a 450 ohm resistor at the probe tip and drive a coax
terminated in 50 ohms. This can easily get above 7 GHz. The only problem
is the 20 dB attenuation. It can drop a signal into the noise.

According to phil, the ATF3143 has very low input capacity. Even lower
if it is operated as a source follower with a guard shield around the
tip that is driven by the source. There goes your capacitance.

Bootstrapping is hard to do at gigahertz frequencies, because the strays
bite you. There isn't really such a thing as "high impedance" above a
few hundred megahertz, except in very special cases. Your average chip
resistor has a parasitic capacitance of 0.08 pF, give or take, so a 1k
resistor sitting on its own has a corner frequency of about 2 GHz.

The ATF38143's input capacitance is below 1 pF, so with a nanohenry of
parasitic inductance from the leads and (very small) probe pins, that
comes in around 7 GHz, with a Q of about 1 at 50 ohms.

I don't care if it only has a gain of 0.9 or so, as long as it is stable
and reasonably flat with frequency. I can easily calibrate the waveform
to whatever I want in software.

The other big reason is to eliminate the broadband noise from TEK and HP
active probes. They are unusable with moderately low level signals,
especially if you include the ground bounce from switching noise. I was
surprised to find the TEK P6249 actually attenuates the signal by 5X.
Now we are back in the noise again.

It's meant to work with DC-coupled, 50-ohm scopes such as the TDS 694C.
You can roast the input resistors if you put too much voltage on them,
so for a general-use probe, an attenuator makes sense. (Scopes are
pretty noisy in any case.)

What really dismays me is to see HP solder long leads to the input of a
7 GHz probe and show a picture of it connected to a pcb with the caption
that the bandwidth is now only 1.2 GHz. Why spend the money in the first
place? Makes no sense to me. Please see Figure 1-13 on page 25 of

http://literature.cdn.keysight.com/litweb/pdf/01134-97013.pdf

This is supposed to be high tech? These people are supposed to know what
they are doing in handling wide bandwidth signals? And they are teaching
newbies how to probe fast signals?

I am rapidly loosing my confidence in these companies. I think all the
guys who knew what they were doing have retired or have gone into real
estate.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[John Larkin]

On Mon, 13 Jul 2015 01:18:08 GMT, Tom Swift <spam@me.com> wrote:

Clifford Heath <no.spam@please.net> wrote:

On 13/07/15 10:54, Tom Swift wrote:
Clifford Heath <no.spam@please.net> wrote:

The main difficulty in building your own probes is not the buffer
amplifier, but the mechanical construction and impedance management
around the probe tip. The tip needs to be strong, but very small to
keep the inductance and capacitance down. I documented and published
here my project to build an amplified probe using a BF862 (10pF
input capacitance), with a 7mm probe tip and nominal 10K impedance.
The inductance of that length of probe resonates with the input
capacitance, making the probe useless above 350MHz.

If 10pf resonates with the probe tip at 350MHz, the inductance is
20.67nH.

Fr = 1 / (2 * pi * sqrt (10e-12 * 20.67e-9)) = 350,065,968.699 Hz

You are not going to get that with a straight piece of wire 7mm long.

Maybe you are using a ground lead that is 10" long?

It was lower than I expected too, but that's what we measured.
I welcome your comments on the design and construction. I'm not
claiming any special expertise; this was my first project into
VHF, let alone higher. The earthy side of the probe is a pogo pin,
the signal side just a header pin, soldered both sides on the PCB.

http://cjh.polyplex.org/electronics/RFCascodeProbe/

I'd be very surprised if that circuit went much above 100 MHz.

You can buld narrowband amplifers by hand for frequencies above 10 GHz.
But you are going to have a hard time hand-wiring a broadband amplifier
for frequencies much above 100 MHz and expect it to be flat.

Buy two Minicircuits 8 GHz ERA-1 for $1.37 (Qty 30) and stick them in
series on a good ground plane.

http://www.minicircuits.com/pdfs/ERA-1+.pdf

Put a 450 ohm microwave resistor on the end of a 50 ohm coax and connect
to the first ERA. Verify it is terminated in 50 ohms. Use capacitive
coupling between stages, LF cutoff frequency of your choice. Trim the
gain at the outout to 0 dB and you are done.

DC coupling is more difficult.

Why not use an opamp? ADA4817 maybe.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Mon, 13 Jul 2015 04:53:59 GMT, Tom Swift <spam@me.com> wrote:

John Larkin <jlarkin@highlandtechnology.com> wrote:

Why not use an opamp? ADA4817 maybe.

Is that for the high frequency side?

Only CAD $5.79 at Newark. I didn't realize they were so inexpensive.

https://octopart.com/search?q=ADA4817

2 pA input current and 4 nV/sqrt(Hz) is not bad. Only problem is the 1 GHz
bandwidth. I need to go higher.

But I'm seriously thinking of a split channel. Some low frequency op amp
for the DC, and perhaps a nice Hittite (ADI) to get to 30 GHz or so.

The fast Hittites are 50 ohm distributed amps and insane power hogs,
and a real nuisance to bias and use. Probably not what you'd want in a
scope probe.

Please be assured I pay careful attention whenever you and Phil talk about
splitting the channel into high and low frequency and the difficulties in
recombining them. An old HP journal article also talks about this
technique. I think the rolloff was at around 1 MHz, but I forgot the
instrument it was used in.

Splitting shouldn't be a big deal. A scope probe isn't expected to
have 0.1% step response flatness. Somebody, LeCroy I think, makes a
scope with multiple front-end mixers, multiple IF amps and ADCs, and a
recombiner.

I already looked at the noise figure for the ERA-1+: 5 dB. That's not bad
for a cheap 8 GHz version to get started. Maybe it's time to stick it in
LTspice and see how it would work.

The ERAs make good pulse amps. They tend to not be very close to 50
ohm input impedance, usually less.

Good luck getting a Spice model! MiniCircuits occasionally changes the
recipe (or buys them from someone else) now and then anyhow, and
everything changes.

Why not use a passive (resistor) probe and put an amp downstream in a
box if your scope needs it? A little sigal averaging can get the s/n
ratio back.

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

https://dl.dropboxusercontent.com/u/53724080/Gear/HP/HP54006_probe.zip

We sometimes build passive pickoffs and SMB connectors into our PC
boards to let us probe things that would be difficult otherwise.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tom Swift]

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 04:53:59 GMT, Tom Swift <spam@me.com> wrote:

But I'm seriously thinking of a split channel. Some low frequency op
amp for the DC, and perhaps a nice Hittite (ADI) to get to 30 GHz or
so.

The fast Hittites are 50 ohm distributed amps and insane power hogs,
and a real nuisance to bias and use. Probably not what you'd want in a
scope probe.

Only the resistor goes in the probe. The amplifier is in a separate box.

Maybe a pain, but it's the only way I know to get to 30 GHz or beyond.

Splitting shouldn't be a big deal. A scope probe isn't expected to
have 0.1% step response flatness. Somebody, LeCroy I think, makes a
scope with multiple front-end mixers, multiple IF amps and ADCs, and a
recombiner.

Yes, a nightmare to align I'll bet.

Why not use a passive (resistor) probe and put an amp downstream in a
box if your scope needs it? A little sigal averaging can get the s/n
ratio back.

That's exactly what I am talking about. But the probe is also for other
things like a spectrum analyzer, VNA, and anything that needs wide
bandwidth, low noise and flat response.

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

What about a microwave trimmed surface mount? Fig. 6 for a MCT 0603 shows
a 220 ohm is flat to 20 GHz in

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

Maybe there are other versions with good performance at higher resistance
values. Or perhaps a small peaking capacitor could boost the high end on
a 470 ohm resistor is a 450 is not available. The small amplitude error
could be calibrated out with a gain pot.

https://dl.dropboxusercontent.com/u/53724080/Gear/HP/HP54006_probe.zip

We sometimes build passive pickoffs and SMB connectors into our PC
boards to let us probe things that would be difficult otherwise.

Yes, that is the obvious solution for a released pcb. But when you are in
development you need more flexibility. Or even if you get a wierd fault
on a bad board that is not covered by your standard pickoffs.

 

[John Larkin]

On Mon, 13 Jul 2015 16:52:17 GMT, Tom Swift <spam@me.com> wrote:

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 04:53:59 GMT, Tom Swift <spam@me.com> wrote:

But I'm seriously thinking of a split channel. Some low frequency op
amp for the DC, and perhaps a nice Hittite (ADI) to get to 30 GHz or
so.

The fast Hittites are 50 ohm distributed amps and insane power hogs,
and a real nuisance to bias and use. Probably not what you'd want in a
scope probe.

Only the resistor goes in the probe. The amplifier is in a separate box.

Maybe a pain, but it's the only way I know to get to 30 GHz or beyond.

Splitting shouldn't be a big deal. A scope probe isn't expected to
have 0.1% step response flatness. Somebody, LeCroy I think, makes a
scope with multiple front-end mixers, multiple IF amps and ADCs, and a
recombiner.

Yes, a nightmare to align I'll bet.

Why not use a passive (resistor) probe and put an amp downstream in a
box if your scope needs it? A little sigal averaging can get the s/n
ratio back.

That's exactly what I am talking about. But the probe is also for other
things like a spectrum analyzer, VNA, and anything that needs wide
bandwidth, low noise and flat response.

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

Good to 6 or 8 GHz.

What about a microwave trimmed surface mount? Fig. 6 for a MCT 0603 shows
a 220 ohm is flat to 20 GHz in

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

Maybe use a regular surfmount resistor or two in the probe and
equalize out any parasitics in the amp.

30 GHz probing is awfully ambitious.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tom Swift]

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 16:52:17 GMT, Tom Swift <spam@me.com> wrote:

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

Good to 6 or 8 GHz.

How do you know? What do you have to measure it?

I'd go for a Vishay that is specifically designed and trimmed for this
application. Then measure it to make sure.

What about a microwave trimmed surface mount? Fig. 6 for a MCT 0603
shows a 220 ohm is flat to 20 GHz in

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

Maybe use a regular surfmount resistor or two in the probe and
equalize out any parasitics in the amp.

Length problem again. Equalization would be tough to work with at high
frequency. You have a hard enough time trying to keep things flat.

> 30 GHz probing is awfully ambitious.

HP N2803A, 6:1, +/- 0.8V peak. Page 7 of

http://cp.literature.agilent.com/litweb/pdf/5989-6162EN.pdf

I don't know how much they cost. I don't want to know

 

[John Larkin]

On Mon, 13 Jul 2015 17:19:00 GMT, Tom Swift <spam@me.com> wrote:

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 16:52:17 GMT, Tom Swift <spam@me.com> wrote:

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

Good to 6 or 8 GHz.

How do you know? What do you have to measure it?

A Tek 11801 TDR/Sampling scope, around 30 ps net pulser+scope rise
time. And Agilent specs the probe for 6 GHz.

I'd go for a Vishay that is specifically designed and trimmed for this
application. Then measure it to make sure.

What about a microwave trimmed surface mount? Fig. 6 for a MCT 0603
shows a 220 ohm is flat to 20 GHz in

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

Maybe use a regular surfmount resistor or two in the probe and
equalize out any parasitics in the amp.

Length problem again. Equalization would be tough to work with at high
frequency. You have a hard enough time trying to keep things flat.

If you push it into a scope and export the waveform to a PC, you can
do the eq in software. It's the "deconvolution problem." That's a
separate discussion.

https://dl.dropboxusercontent.com/u/53724080/Sampling/TDR_Decon_demo.jpg

30 GHz probing is awfully ambitious.

HP N2803A, 6:1, +/- 0.8V peak. Page 7 of

http://cp.literature.agilent.com/litweb/pdf/5989-6162EN.pdf

I don't know how much they cost. I don't want to know

Buy a sports car instead.



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tom Swift]

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 17:19:00 GMT, Tom Swift <spam@me.com> wrote:

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

Good to 6 or 8 GHz.

How do you know? What do you have to measure it?

A Tek 11801 TDR/Sampling scope, around 30 ps net pulser+scope rise
time. And Agilent specs the probe for 6 GHz.

Sounds kind of a loose way to define the frequency response. First, you
need a VNA to show the frequency response to look for dips in amplitude
or phase bumps caused by missmatch. Next you need the attenuation vs
frequency to verify the attenuator is flat. This is the same measurement
as Fig. 6 in the Vishay document

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

Finally, you need a proper TDR to define the risetime and check for
aberrations.

I don't see the value in paying for an undocumented component that is not
intended for this application. The Vishay has around three times the
bandwidth and is documented and intended for this work. If you run into
problems, you can call them for support. If you called Caddock they would
not have the faintest clue what you were talking about.

I'd go for a Vishay that is specifically designed and trimmed for this
application. Then measure it to make sure.

What about a microwave trimmed surface mount? Fig. 6 for a MCT 0603
shows a 220 ohm is flat to 20 GHz in

http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf

 

[John Larkin]

On Mon, 13 Jul 2015 23:08:05 GMT, Tom Swift <spam@me.com> wrote:

John Larkin <jlarkin@highlandtechnology.com> wrote:

On Mon, 13 Jul 2015 17:19:00 GMT, Tom Swift <spam@me.com> wrote:

The Caddock resistors, at 450 or 950 ohms, make good clean passive
probes.

I worry about the length becoming part of a wavelength at the high
frequency end. I don't think the response would be very flat.

Good to 6 or 8 GHz.

How do you know? What do you have to measure it?

A Tek 11801 TDR/Sampling scope, around 30 ps net pulser+scope rise
time. And Agilent specs the probe for 6 GHz.

Sounds kind of a loose way to define the frequency response.

Then do it your way.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com