On 10.04.22 21:03, g...@hoffmann-hochfrequenz.de wrote:
Am 2022-04-10 18:09, schrieb use...@teply.info:
On 10.04.22 04:47, g...@hoffmann-hochfrequenz.de wrote:
Am 2022-04-09 20:35, schrieb Lux, Jim:
On 4/9/22 10:03 AM, use...@teply.info wrote:
Recently I was discussing some measurement results with my
colleagues as we're trying to come up with a low noise JFET which
can successfully be integrated into a SiGe BiCMOS process, and
quite often we're also struggling to identify why exactly variant A
has significantly lower noise than variant B, or why a new approach
does not improve noise the way it was expected.
So from a manufacturing process design point of view, achieving low
1/f noise indeed is closer to sheer dumb luck than the proverbial
"more art than science" suggest.
This is very, very true. Some manufacturers get very low noise or very
low leakage (or both), essentially by being "lucky". From what I've
been told, there's no good models, nor predictions - so people share
"lore" of "if you get these 2Nxxxx FETs from the mfr in England,
they're really good" until they aren't. There isn't enough market
for these, so I suspect research money to "solve the problem" isn't
available.
Buy a life time supply while they are available. One reel will
probably do.
Unfortunately, that's not always an option. Sometimes you only learn
that Part A is exceptionally good only when it isn't anymore. One of
our customers got bitten by that, relying on parts which exceeded
specs until they got transferred to another fab. Afterwards, the part
still met their specs, but didn't meet the customers requirements. Of
course the manufacturer put in some effort trying to make the device
as good as it was because it was a good customer, but there's only so
much effort you can justify for 100k parts a year. For small-value
varicaps, where you can dice easily 30k-50k pieces out of a single
wafer, the customer would happily have bought a full manufacturing lot
had they known before the fact.
Like all those microwave MMICs with low noise, they worry about 100
MHz and up (if not 1GHz), they certainly don't worry (or control) for
noise at 5 MHz, or where the 1/f knee is. So just because you got good
results with a batch of them, the next batch might not. It's not even
clear you could come up with a standardized test method, because the
noise depends on a lot of other factors (drain current, for instance).
> When it changes from lot to lot, then you have lost. You cannot catch
that on the wafer tester. No one can pay for the tester time.
A simple BJT or FET circuit is allotted a ms or so in total, maybe.
You cannot measure 1/f in the 100 Hz range in that time. The picture
of the FET amplifier I had 3 days ago took 35 minutes, per trace.
By far, most of the wall time is aquisition time for the lowest octaves.
It depends a bit on the needs. Whether we're talking about DC, analog,
RF or digital testers for example. But even for the most complex
tests, tester time is cheap compared to engineering time for test
setup.
I've worked for a tester manufacturer near Stuttgart for a project, and
their customers had a completely different view about this. :-)
I had a mid-scale mixed signal tester just for myself to exercise my
software; only half a day per week for someone else's regression tests.
In our hall there were 300 engineers, almost entirely for software, only
15 or so for hardware design. A different customer of mine was quite
proud on their E5052B; in the neighbor cubicle someone sent half a dozen
of them one morning to calibration. That did not create any bottleneck.
That all must be paid for. Then you arrive at options: Normal vector
speed is <crawl>. For $$$ you get 10eXYZZY vectors at warp speed. Once.
My VNA cannot handle mixers because of a missing software option. As much
as I hate it, I can understand it.
But of course you're right, as 1/f takes long, one will have to trade
test time versus lower frequency corner and sample count. Doing full
manufacturing screening (as in test every single manufactured device)
is prohibitively costly more or less independent of lower frequency
corner for all but the most demanding applications. Testing a dozen
devices out of a full wafer (with, say, 10 k devices per wafer) is
manageable. Last year I measured a few samples for 1/f down to 0.01
Hz. That's a matter of starting the measurement on friday afternoon
and then going home for the weekend. Next sample on the next weekend
;-) There was no point in starting the measurement before friday
afternoon in any case as during the week the environment was more
noisy than the DUT...
Reminds me of our bit error rate measurements in fiber optics. We had
to make sure, that NOBODY, not even the CEO took his cell phone into the
lab.
If it was only cellphones it would have been too easy. Shielding 900 and
1800 MHz is not really a problem. But removing the crud the MBE
downstairs leaves on the mains lines is something that I didn't manage
successfully yet. But on weekends there's nobody operating it 8-P
Can you tell us what you used as bias and power supplies for these 0.01 Hz
measurements? A tiny hint? Please, pretty please?
Sure I can. It's not much black magic involved after all in the parts I
know: For Collector Bias, I used a stack of rechargeable batteries and a
few relays to change voltage, plus a DMM (HP 34401A) to measure the
actual voltage and current. For the base, two of these had to do
together with another relay-switched resistive divider. That has been
provided by our customer along with the DUTs. I believe it was 2V
lead-acid batteries, as they were a tad heavy. On the last sample I also
tried one of our Agilent E5270B SMU Mainframes, and worked quite okay as
well as the DUTs were not as low noise as has been hoped. But with that
I only went down to 0.1 Hz due to time constraints. You have to keep in
mind however that the test system in use (Proplus 9812DX) also does some
filtering (unfortunately I can't give you details here as I don't know
exactly) on the supplies, which probably helped as well.
At least I'm not aware of situations where the E5270B actually posed a
limit here.
Bests,
Florian DH7FET
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