High speed, high voltage amplifier for cap. load

[[email protected]]

Okay, sorry it took so long. I'm going to take your advice and try the
AD815 circuit. So, some answers to your questions...

How does the trap and the vacuum system go together?

Take a look at the three jpegs at
http://www.stanford.edu/~jwodin/lineartrap/LinearTrapPics/

The linear trap is mounted inside the vacuum system. The 136Ba+ ion
can be trapped in this linear trap anywhere between 1e-10 torr to 760
torr Helium (has to be very clean, so the system is pumped down with
the Pfeiffer turbo pump, as shown, and then baked for about a week at
150C). Residual gas partial-pressures (non-Helium) need to be below
1e-8 torr, or else the ion can capture on one of them, or be ejected
via a collision. As you can see, the ion is injected on the left hand
side, and is radially confined with the RF field. It is free to move
axially, which is why we need the DC potential (also shown) to bring
the ion down to the observation area, where it is viewed by exciting it
with 493nm and 650nm lasers and looking for it's fluorescence.

I'd like to know more detail about those connectors and feedthroughs.

The vacuum feedthroughs that we are using are shown in the picture, but
you can see details at

http://www.mdcvacuum.com/urd/uniface.urd/ecf0070w.display?6.1.c.1

We use this because it is rated to UHV (ultra high vacuum ~1e-10 torr,
which means no finger grease allowed, all metal seal, etc.)

As far as actual electrical connections, my current thought is at

http://www.stanford.edu/~jwodin/lineartrap/LinearTrapPics/cutaway2.JPG

Each pin on a 10-pin feedthrough carries RF+DC for one pair of
electrodes. A kapton insulated wire
(http://www.mdcvacuum.com/urd/uniface.urd/ecf0070w.display?6.1.e.1)
connects the feedthrough pin to an electrode, and another short wire
jumpers that electrode to it's brother. Similarly, each pin on another
10-pin feedthrough carries DC for the other pair of electrodes, and is
connected in a similar way.

Note that I assume there will be heavy RF pickup on the electrodes that
are supposed to have only DC on them, so I was thinking of using your
circuit with the addition of 0.1uF caps (outside the vacuum system of
course, since caps are no good for vacuum). The naïve hope is that
any RF pickup on E2+E3 will go right through the 0.1uF cap (>> 25pF)
instead of re-radiating into the trap (so that the ion truly sees only
DC on those rods). I don't know if this will work.

.. | A1
.. |__________|\____ T1 ___________| E1+E4
.. | | |/ | # | | |
.. | inv #||# 330 | (Set 1)
.. | | A2 #||# 10W |
.. | |___|\____| #___|________________________| E2+E3
.. | |/ | _|_ |
.. | AD815 | 1nF_|_ 0.1uF -,-
.. | dc1 -,- |
.. | |___| E1+E4 gnd
.. | ac sig 100k | |
.. | bus ,--/\/--' (Set 2)
.. | |
.. | |____________________________| E2+E3
.. | | _|_ |
.. | dc2 0.1uF -,-
|
Gnd


The electronics (RF,DC) will be put into an enclosure very close to the
system (~1ft). I was going to use some mil-spec amphenol connectors on
the box (see upper picture at
http://www.stanford.edu/~jwodin/lineartrap/equipment/AmphenolPanelMount.pdf
, though with 10 pins) and then for the mating cable
connector,(http://www.insulatorseal.com/urd/uniface.urd/ecf0070w.display?9.1.2.2).

Yes, but you still want to shield those lines,
transmitter/antenna city, you know, otherwise.

and to carry the signal to the feedthroughs on the vacuum system, I was
going to use some multi-conductor cable with grounded outer
foil-shielding, like Belden 9935
(http://www.stanford.edu/~jwodin/lineartrap/equipment/Belden9935.pdf).
I figure that at 1-10MHz, if my cables are a few feet, they don't
have to be waveguides, so non-coax should be fine. The foil sheath on
the Belden cable should be good enough to not make the cables antennas.

Whew, okay, that's a lot of info...

 

[[email protected]]

whoops, that circuit looks funny.

.. | A1
.. |__________|\____ T1 ___________| E1+E4
.. | | |/ | # | | |
.. | inv #||# 330 | (Set 1)
.. | | A2 #||# 10W |
.. | |___|\____| #___|_____________________| E2+E3
.. | |/ | _|_ |
.. | AD815 | 1nF_|_ 0.1uF -,-
.. | dc1 -,- |
.. | |___| E1+E4 gnd
.. | ac sig 100k | |
.. | bus ,--/\/--' (Set 2)
.. | |
.. | |_________________________| E2+E3
.. | | _|_ |
.. | dc2 0.1uF -,-
|
Gnd

 

[Winfield Hill]

Jesse Wodin wrote...

whoops, that circuit looks funny.

. | A1
. |__________|\____ T1 ___________| E1+E4
. | | |/ | # | | |
. | inv #||# 330 | (Set 1)
. | | A2 #||# 10W |
. | |___|\____| #___|_____________________| E2+E3
. | |/ | _|_ |
. | AD815 | 1nF_|_ 0.1uF -,-
. | dc1 -,- |
. | |___| E1+E4 gnd
. | ac sig 100k | |
. | bus ,--/\/--' (Set 2)
. | |
. | |_________________________| E2+E3
. | | _|_ |
. | dc2 0.1uF -,-
. |
. Gnd

Funny? No, it looks OK to me. Thanks for the detailed description
with links and drawings, it was very entertaining! And everything
you've described looks OK. The capacitive coupling from one line
carrying RF to another carrying RF gave me pause, until I realized
they're both carrying the same RF voltage, even if derived from a
different amplifier. So that's OK. The capacitance to ground is a
necessary evil, and what we've worked to address with enough muscle.

As for that muscle, I'm cautiously-hopeful that the AD815 amplifiers
will be up to the task, although John Jardine's comment about their
figure 15 says we should ask someone at Analog Devices, or take some
measurements, or both.

Amplifiers that can deliver sizable current at high slew rates and
high frequencies and with >30V supplies are hard to find, and often
become discontinued (witness the nice types formerly from Elantec)
without replacements, so I viewed the AD815 as a keeper and a part I
should add to our shelves. The SIP package with heatsink-tab Analog
offered at first (I have a copy of the older rev-b datasheet) is by
far preferable to the 24-lead thermally-enhanced SOIC package they're
continuing to sell. So I therefore went looking for any old stock,
found some, and purchased 25 pieces ($10) of the staggered-lead pkg,
on a lifetime-buy basis. I can send you the source info later from
work.

When they come we can check out that figure 15 business. Perhaps in
my copious free time :>) I can also design a transformer so we can
test the whole circuit to be sure it's up to snuff. If we do this
quickly you can decide if you want to get your own lifetime-buy of the
discontinued AD815 SIP-tab staggered-lead parts before they disappear.

 

[[email protected]]

As for that muscle, I'm cautiously-hopeful that the AD815 amplifiers

will be up to the task, although John Jardine's comment about their
figure 15 says we should ask someone at Analog Devices, or take
some measurements, or both.

Well, I ordered some AD815AY amps from Rochester, so I figure I can
hook them up and test to see if the large signal freq. response is as
it says. I figure that if the response is as it says, that I can build
a transformer that gives enough voltage gain at 10 MHz, and then when I
want to run lower, I just lower the source voltage (easy to do, since
everything is run by cpu).

Amplifiers that can deliver sizable current at high slew rates and
high frequencies and with >30V supplies are hard to find, and
often become discontinued

why is that? They seem like useful objects... though maybe only to me.

I can also design a transformer so we can
test the whole circuit to be sure it's up to snuff. If we do this
quickly you can decide if you want to get your own lifetime-buy
of the discontinued AD815 SIP-tab staggered-lead parts before
they disappear.

That would be great help, since I've never designed one before. I was
thinking that since I'm going to be making of order 10 or more of these
things, that the transformers should be as uniform as possible over the
set -- so, I was thinking of just buying them from some place like
coilcraft. Of course, I'm also open to winding my own.

If we do this quickly you can decide if you want to get your own
lifetime-buy of the discontinued AD815 SIP-tab staggered-lead parts
before they disappear.

What exactly is a lifetime-buy? Just a bulk order? I'm pretty sure my
prof. would be interested in that if this works.


Also, one last out-of-left-field approach. So, we have the following
earth-shattering (esp. the A150) amps sitting around lab

ENI 240L (http://www.bellnw.com/products/5939/)
ENI 350L (http://www.bellnw.com/products/7562/)
ENI A150 (http://www.bellnw.com/products/5427/)

which all like 50Ohm output. These are the amps that I was considering
using in the circuits at the beginning of our converasations. Soo,
here's the hairbrained idea. What if I do the following (only showing
the RF part)

..
.. |\ _______
.. +-----|/------| Tuner |------|
.. | Amp +-------+ |
.. | +--|
.. Vac
.. |
.. Gnd

where Tuner is a HAM radio antenna tuner (e.g. a'la
http://www.ldgelectronics.com/autotuners.html). For example, looking
at the Z-100 autotuner, it claims that it can convert an antenna load
from 6 Ohm to 800 Ohm from about 1MMz to 54MHz. Then, I could just use
a honker amp, go through this tuner, and the amp should see 50 Ohm. I
would have mentioned this earlier, but it was just suggested to me
today.

 

[[email protected]]

By the way, any of you guys are welcome to come visit our lab over here
at Stanford in the physics department. Not too much to see right now,
since we're moving from the basement to the 1st floor (yeah lab
upgrade!), though most of the time, we have some other ion traps up and
running, so you can come see a single atom, and a pretty kick ass blue
laser (here's a slide from a talk I gave awhile ago with a CCD image of
a single ion:

http://www.stanford.edu/~jwodin/Pictures/ion.jpg

 

[[email protected]]

Do you happen to know any part numbers for some crt cathode drivers?

 

[Winfield Hill]

[email protected] wrote...

That would be great help, since I've never designed one before.
I was thinking that since I'm going to be making of order 10 or
more of these things, that the transformers should be as uniform
as possible over the set -- so, I was thinking of just buying
them from some place like coilcraft. Of course, I'm also open
to winding my own.

These are easy to wind because they don't have very many turns.

What exactly is a lifetime-buy? Just a bulk order? I'm pretty sure
my prof. would be interested in that if this works.

That's the term for your decision about how many you'll need to
last you the rest of your life, the project's life, the product's
life, or whatever. You put those on the shelf and stop worrying
about whether they'll be discontinued, replaced, or whatever.

Also, one last out-of-left-field approach. We have the following
earth-shattering (esp. the A150) amps sitting around lab

ENI 240L (http://www.bellnw.com/products/5939/)
ENI 350L (http://www.bellnw.com/products/7562/)
ENI A150 (http://www.bellnw.com/products/5427/)

which all like 50 Ohm output. These are the amps that I was
considering using in the circuits at the beginning of our
converasations. Soo, here's the hairbrained idea. What if
I do the following (only showing the RF part)

.
. |\ _______
. +-----|/------| Tuner |---+--|
. | Amp +-------+ |
. | +--|
. Vac
. |
. Gnd

where Tuner is a HAM radio antenna tuner (e.g. a'la
http://www.ldgelectronics.com/autotuners.html). For example,
looking at the Z-100 autotuner, it claims that it can convert an
antenna load from 6 Ohm to 800 Ohm from about 1MMz to 54MHz. Then,
I could just use a honker amp, go through this tuner, and the amp
should see 50 Ohm. I would have mentioned this earlier, but it
was just suggested to me today.

I can't advise you on the use of one of those tuners, except to
say this means you can only use one frequency at a time, I assume.
Or is the question, how often and fast do you change frequency and
how fast does this autotuner tune?

I can advise you on the use of high-power wideband 50-ohm RF amps
in your application: it might be reasonable. First, consider,
25 watts on a 50-ohm terminated line is 35.3Vrms = 100Vpp, which
is exactly your required voltage. You'd put the power terminator
as close as possible to the electrodes. The main problem is your
huge 400pF capacitance, which with a 50-ohm source (we assume the
RF amp isn't back terminated) places you down -3dB at 8.33MHz.

But 8.3MHz is close to 10MHz and this gives us hope that a simple
series peaking inductance can deliver the signal unabated to the
electrodes up to 10MHz. For example, a 0.63uH series inductance
would resonate with 400pF at 10MHz. Your 50-ohm termination can
provide the needed Q-damping for the L-C resonance.

Playing with spice to evaluate the best flat response curve in the
4 to 10MHz region, 0.7uH looks good, assuming a pure 50-ohm source.
It peaks to +4% at 5MHz and then drops to -18% at 10MHz (compare
to -36% at 10MHz with no series inductance). You can experiment
with this stuff on your computer.

It'd probably be best to break up your capacitances into groups
and feed each one with its own inductor. For example, with your
16 electrode sets, a simple calculation gives us 25pF each, which
means 11uH in series at each electrode set. Or eight sets, 50pF
+ 22uH, etc. You can measure each one, at the 50-ohm terminator,
including its coax cable (which just looks like more capacitance),
and calculate the best inductor for that set. Perhaps you'll use
small tunable inductors, adjusted to flatten the frequency sweep.

If -20% at 10MHz is unacceptable, or you need a flatter frequency
response, you'll have to lower the RF node resistance at the coax
junction point. You can use a transformer (e.g., a Jerry Sevick
transmission-line type to avoid leakage inductance) or a resistive
network. Either way, you'll then need more RF power to get 100Vpp.

 

[Robert Latest]

electrodes. A kapton insulated wire
(http://www.mdcvacuum.com/urd/uniface.urd/ecf0070w.display?6.1.e.1)

Kapton, by the way, is the same thing as "polyimide", which is a
standard insulation material for transformer winding wire. In my
UHV days (1e-11 torr pressure range) I've exclusively used that
wire instead of the ultra-expensive stuff the specialized vacuum
companies try to sell you. Foil-wrapped has some mechanical
advantages over laquered though.

robert

 

[colin]

Do you happen to know any part numbers for some crt cathode drivers?

This was the sort of one I was thinking of,
http://www.national.com/pf/LM/LM2412.html
however it only seems to have an output swing of 68v with an 80v supply, I
think older drivers used to handle wider voltages but I cant find them,
certainly the old discrete driver transistors were rated about 250v and used
supplies of >100v.

maybe its still of some interest to you anyway, you might be able to peak it
enough, or just use it in place of the ad815s with op transformers.

Incidently out of interest I ran my idea of chained bootstraped op amps
through ltspice and it looked surprisingly good although I since found out
the model for the LT1210 has grnd referenced nodes wich makes the results
less reliable.

Colin =^.^=

 

[Winfield Hill]

colin wrote...

This was the sort of one I was thinking of,
http://www.national.com/pf/LM/LM2412.html

One word. Capacitance, my good man, capacitance. Did you
notice the test capacitance for those drivers? It was 8pF.

Incidently out of interest I ran my idea of chained bootstraped
op amps through ltspice and it looked surprisingly good ...

How much capacitance to ground did you add to your modeling,
for the floating supply chokes or common-mode transformers
being driven at 10MHz?

 

[colin]

colin wrote...

One word. Capacitance, my good man, capacitance. Did you
notice the test capacitance for those drivers? It was 8pF.

Hi, True 8pf isnt much although its still requires .1 amp, also their test
ciruit does have quite an involved output ciruit, but considering they are
rated for up to 200mhz pixel clock I would think they would be able to cope
with considerably higher capacitance at only 10mhz, they have fixed gain so
there is no feedback loop to become unstable with higher capacitances. They
dont however give much information about what the SOA is although they say
that resistive loads can be used for other aplications limited only by
thermal considerations.

They are quite cheap and used enmass, and as theres 3 in a package you could
distrubute the electrodes as necessary, I would be inclined to think that
devices in the same package would have close enough charecteristics to be
paralleled fairly easily.

However as they dont have the voltage swing I was thinking they did they
seem far less atractive, as they still need either a transformer or
peaking/matching circuit, otherwise they would of been a neat solution.

How much capacitance to ground did you add to your modeling,
for the floating supply chokes or common-mode transformers
being driven at 10MHz?

Well I wasnt too sure what capacitance to add, or to what nodes (10pf+ to
each op might not be unreasonable), seing as though one might need to put
the thing near some metal to heatsink it too, also device inductances might
need to be taken into consideration too, the file is in the ltspice grp, I
tweaked it a bit since then, however as the op-amp model uses internal gnd
nodes I didnt bother to investigate any further. I was mainly interested in
seeing what the current distribution was like. might be useful if you find
you need a freq range too wide for a transformer.

I would sugest going with Win's ideas anyway as there seems less unknowns,
and hes been good enough to work out most things already although you
might consider the lt1210 op amp as a close alternative (single op amp per
package/higher curent)

Colin =^.^=

 

[Winfield Hill]

colin wrote...

I would sugest going with Win's ideas anyway as there seems less unknowns,
and hes been good enough to work out most things already although you
might consider the lt1210 op amp as a close alternative (single op amp
per package/higher curent)

I am finding the 50-ohm wideband power amplifier attractive. But then
they cost the big bucks, and even if there's one or two lying around in
the lab, one hesitates to tie it up permanently in an application that
can be solved with less horsepower. On the other hand, time is money,
and as the ready-made basis for a solution it would take less time.

 

[[email protected]]

hm, sound very attractive, and my advisor loves throwing $$ around...
but it's also fun to make circuits work.

well, I just went down to the Sunnyvale HAM radio outlet here in the
bay area and bought an antenna tuner > Heh heh, going to hook it up
to my monster ENI RF amp with a forward-backward power meter to see if
it actually works... otherwise, gotta get to work with some of these
other ideas (waiting on the AD815 chips till Mon or Tues, though I've
got an APEX PA98 right here, so I'll also start playing with that).

Though... this is California, and I am going surfing in a bit, so we'll
see how far I get

 

[[email protected]]

antenna tuner was "okay," but not great. It reduced the reflected
power at some frequencies, but not at others (doesn't seem to like
<2MHz), and sometimes, it wasn't very low... so I think I'm gonna ditch
that idea and go with others...

 

[Winfield Hill]

[email protected] wrote...

antenna tuner was "okay," but not great. It reduced the reflected
power at some frequencies, but not at others (doesn't seem to like
<2MHz), and sometimes, it wasn't very low... so I think I'm gonna
ditch that idea and go with others...

But don't ditch the wideband power amp just yet, check the comments
on a series L in my post, Message-ID: <[email protected]>
To be clear, I'm talking about adding fixed series inductor(s), with
no tuning vs frequency required.

 

[Fred Bartoli]

[email protected] wrote... ....
I can advise you on the use of high-power wideband 50-ohm RF amps
in your application: it might be reasonable. First, consider,
25 watts on a 50-ohm terminated line is 35.3Vrms = 100Vpp, which
is exactly your required voltage. You'd put the power terminator
as close as possible to the electrodes. The main problem is your
huge 400pF capacitance, which with a 50-ohm source (we assume the
RF amp isn't back terminated) places you down -3dB at 8.33MHz.

But 8.3MHz is close to 10MHz and this gives us hope that a simple
series peaking inductance can deliver the signal unabated to the
electrodes up to 10MHz. For example, a 0.63uH series inductance
would resonate with 400pF at 10MHz. Your 50-ohm termination can
provide the needed Q-damping for the L-C resonance.

Playing with spice to evaluate the best flat response curve in the
4 to 10MHz region, 0.7uH looks good, assuming a pure 50-ohm source.
It peaks to +4% at 5MHz and then drops to -18% at 10MHz (compare
to -36% at 10MHz with no series inductance). You can experiment
with this stuff on your computer.

Going further you can start with a 4th order cheb matching filter and then
taylor it for just 400pF output cap and best output.
The best case would be to have an additional 50R load.

This gives you something like this:


.--------------. 680nH 1uH
| ___ | ___ ___
| .-|___|--------------UUU--+--UUU-----+--------.
| | 50R | | | |
| /+\ | | .-. | electrode cap
| ( ) | --- 50R | | ---
| \-/ | 520p --- Load| | --- 400p
| | | | '-' |
| | | | |
'--------------' | | |
HF amplifier === === ===
GND GND GND


There you have 0/-4% over a 10.5MHz BW.

It'd probably be best to break up your capacitances into groups
and feed each one with its own inductor. For example, with your
16 electrode sets, a simple calculation gives us 25pF each, which
means 11uH in series at each electrode set. Or eight sets, 50pF
+ 22uH, etc. You can measure each one, at the 50-ohm terminator,
including its coax cable (which just looks like more capacitance),
and calculate the best inductor for that set. Perhaps you'll use
small tunable inductors, adjusted to flatten the frequency sweep.

You can do that too and the components values become 11uH, 16uH, 33p, and
800R for each of the 16 cells.
The load resistor then have only 1.5W power dissipation.

 

[Winfield Hill]

Fred Bartoli wrote...

Winfield Hill wrote...

Going further you can start with a 4th order cheb matching filter
and then taylor it for just 400pF output cap and best output.
The best case would be to have an additional 50R load.

This gives you something like this:


.--------------. 680nH 1uH
| ___ | ___ ___
| .-|___|--------------UUU--+--UUU-----+--------.
| | 50R | | | |
| /+\ | | .-. | electrode cap
| ( ) | --- 50R | | ---
| \-/ | 520p --- Load| | --- 400p
| | | | '-' |
| | | | |
'--------------' | | |
HF amplifier === === ===
GND GND GND


There you have 0/-4% over a 10.5MHz BW.

That's a nice idea, but your addition of a 50-ohm source impedance
(which I doubt can be assured from a typical RF power amp) gives
you a great advantage.

In fact, with an assured 50-ohm source, the 50-ohm termination now
presents a 25-ohm impedance to the 400pF. This means that a straight
connection to the 400pF with no inductance is down only 14% at 10MHz,
and with a 0.135uH inductor in series it's a virtually perfect +0.3,
-0.6% response, and thus better than your network.

Apparently Jesse would need to characterize the Zout of his amplifier
before calculating his best solution, or do an empirical evaluation.
That is, try it.

 

[Fred Bartoli]

Fred Bartoli wrote...

That's a nice idea, but your addition of a 50-ohm source impedance
(which I doubt can be assured from a typical RF power amp) gives
you a great advantage.

I've never used wideband RF amps and have simply assumed they could handle
the power.
I'm apparently mistaken.

In fact, with an assured 50-ohm source, the 50-ohm termination now
presents a 25-ohm impedance to the 400pF. This means that a straight
connection to the 400pF with no inductance is down only 14% at 10MHz,
and with a 0.135uH inductor in series it's a virtually perfect +0.3,
-0.6% response, and thus better than your network.

Oops. Overlooked this.
But then if the RF amp doesn't cope with the 50R load you're stuck too.
Unless designing the filter for a higher impedance load, 100R for ex.

.--------------. 680nH 1.3uH
| ___ | ___ ___
| .-|___|--------------UUU--+--UUU-----+--------.
| | 50R | | | |
| /+\ | | .-. | electrode cap
| ( ) | --- 100R | | ---
| \-/ | 470p --- Load| | --- 400p
| | | | '-' |
| | | | |
'--------------' | | |
HF amplifier === === ===
GND GND GND

This will give you +0.7%/-3.3% over the bandwidth and -2.5% up to 9.9MHz.
Unfortunately the input impedance drops down to 26R at 5MHz and we're almost
back to square one. Almost because its phase is better behaved than the
series L network (+/- 20 degrees) and is almost resistive.

Apparently Jesse would need to characterize the Zout of his amplifier
before calculating his best solution, or do an empirical evaluation.
That is, try it.

Sure.

 

[[email protected]]

okay, thanks so much for all the help! I got the AD815s in, so I'm
taking a look at transformers now as well. Also, I'm taking a closer
look at your power amp comments as well... any suggestions for winding
a transformer for the AD815's? Should I use ferrite cores?

Jesse