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Design >> Analog Design >> popcorn noise (RTS noise?) in parasitic VNPN BJTs
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Message started by aaron_do on Nov 23^rd, 2009, 3:54am

Title: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 23^rd, 2009, 3:54am

Hi all,

do we normally find popcorn noise in parasitic VNPN transistors? I was running some measurements on a circuit which used such transistors, and found what I could only describe as popcorn noise.

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by loose-electron on Nov 23^rd, 2009, 8:36am

Absolutely!

Read up on shot noise in bipolar transistors.

Don't expect it to be modeled properly in Spice however.

-jerry

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 24^th, 2009, 1:13am

Hi,

thanks for the reply. I read up a little about shot noise, but discarded it as it is supposedly white, right? I have included a plot of the output noise PSD. As you can see there are what appears to be tones at a few frequencies. The only noise I know which could exhibit this is burst noise.

Either that or the noise is coming from my measurement setup, but my setup appears to be quite clean of noise (I checked). I'm powering the DUT using AA batteries connected to linear regulators with lots of shunt caps. The only equipment used are a signal generator and a spectrum analyzer.

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by pancho_hideboo on Nov 24^th, 2009, 7:07am

(1) What do you observe in DUT ?

(2) What purpose do you use signal generator in your spectrum measurement ?
It seems you don't use signal generator in this measurement.

(3) Observe spectrum of ground without DUT or witout biasing to DUT.
When you try this, do spectrum spikes disappear ?

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 24^th, 2009, 8:00am

Hi pancho_hideboo,

the circuit is actually an LNA and mixer, so the signal generator provides the LO. I haven't checked the spectrum of ground, but i've checked some other nodes (LNA output node and a biasing node from a constant-gm biasing circuit) and no spikes are observed. I suppose that rules out noise on the ground node right? I've also tried turning off the LO and changing the biasing of the LNA. In neither case was the 3.5 MHz noise signal affected.

Unfortunately I can't run any further measurements on this circuit (in the near future) unless I can come up with a good explanation for this noise. Its on-wafer testing at my University, and there's a pretty big queue to use the probe station.

I'm not so sure that's its burst noise any more since the same noise component appeared on a separate chip. I'm not sure if it was the same wafer.

I didn't quite understand question (1).

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by pancho_hideboo on Nov 24^th, 2009, 8:11am

aaron_do wrote on Nov 24^th, 2009, 8:00am:

I didn't quite understand question (1).

You observe IF output of "LNA+Mixer" in the conditions of the followings ?
- RF is no signal feed with termination of 50ohm
- LO is drived at some frequency by signal generator

aaron_do wrote on Nov 24^th, 2009, 8:00am:

I suppose that rules out noise on the ground node right?

What do you mean ?

aaron_do wrote on Nov 24^th, 2009, 8:00am:

In neither case was the 3.5 MHz noise signal affected.

What do you mean ?
You mean spike at 3.5MHz never disappear in any case ?

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 24^th, 2009, 4:22pm

Quote:

You observe IF output of "LNA+Mixer" in the conditions of the followings ?
- RF is no signal feed with terminated of 50ohm
- LO is drived at some frequency by signal generator

Correct

Quote:

What do you mean ?

If the 3.5 MHz signal were on the ground line then I expect it would affect all nodes, not just the output node. Is that correct?

Quote:

You mean spike at 3.5MHz never disappear in any case ?

Correct.

cheers,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by pancho_hideboo on Nov 25^th, 2009, 2:31am

aaron_do wrote on Nov 24^th, 2009, 4:22pm:

If the 3.5 MHz signal were on the ground line then I expect it would affect all nodes, not just the output node. Is that correct?

I think so.

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 25^th, 2009, 4:16am

I just realized that it might be common-mode oscillation which has been incompletely canceled by the differential operation. This would agree with the fact that my op-amp appears to have pretty poor IIP3. Its strange though since I normally overcompensate the CMFB loop since I only use it to set the output common-mode DC value...

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by Ken Kundert on Nov 25^th, 2009, 11:54am

Arron,
It also might be due to a resonance in the supply lines, which would cause peaking in the noise.

-Ken

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 25^th, 2009, 5:47pm

Hi Ken,

thanks for the reply. pancho_hideboo also pointed out that the ground line may not be clean, but as I mentioned, the 3.5 MHz noise only appeared at the output. I was only able to check two other nodes, but neither showed any evidence of a 3.5 MHz peak in the noise.

cheers,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by pancho_hideboo on Nov 25^th, 2009, 11:18pm

Try to vary power supply voltage value to DUT.
Frequency of spike is still same ?

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 25^th, 2009, 11:52pm

Hi pancho_hideboo,

unfortunately, I cannot re-assemble my test setup at the moment. I did try changing the DUT supply and I did'nt notice any changes in the frequency of the 3.5 MHz noise, but I didn't record any values so i can't say for sure. I used a supply-independent biasing which means that the gain of my amplifiers wouldn't have been affected much by changes in the supply voltage.

thanks for the help,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Nov 28^th, 2009, 5:55pm

Are the frequency spikes constant, or do they vary?

Do you have a metal trash can you can put over your whole setup?
A metal trash can (with liner so you can remove the trash:) is a firm requirement for an analog lab.

Steve

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 29^th, 2009, 12:29am

Hi sos,

do you think the circuit might be picking up interference? The 3.5 MHz spike doesn't seem to move.

Anyway the probe station chamber should shield the wafer quite well from interference...i think.

cheers,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Nov 29^th, 2009, 3:30pm

Yes, interference is my thought. First thing I go for is the metal trash can. Mine walked off one day, I found facilities using it at the paper shredder. I told them it was lab equipment (fortunately I had put a "Calibration not required" sticker on it)...

Maybe your setup is already shielded as you say. I think the spikes are a little suspect. The other thing I do a lot is poke at my setup with tweezers (use them to connect my body capacitance to the circuit). Most nodes this just causes big problems, but sometimes it is good for finding bad connections. Gives you a feel for what's going on...

Steve

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Nov 29^th, 2009, 3:41pm

Re-reading the thread, I'd have to agree with Ken's comment--making an analog setup stable on a wafer prober can take some doing. One thing you can do to test for oscillation is wind a few turns of magnet wire (maybe 1cm diameter) and solder it to a connector so you can plumb it into the input of a spectrum analyzer. Use this as a sniffer to check your circuit for oscillation without having to probe it directly (you often stop the oscillation due to probe loading).

Oscillation caused by probe parasitics is sounding lots more likely than burst or popcorn noise.

You could also try packaging a bunch of chips and sorting for yield in packaged form. If it is the probe parasitics causing problems, it could take a lot of time on the machine to fix.

Steve

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 29^th, 2009, 5:04pm

Hi Steve,

thanks for the help. Actually I was also thinking that maybe Ken was right. I was going to hook the power supply board up to a network analyzer and see if there are any high impedance frequencies.

Another thing is that I used electrolytic capacitors for the power supply bypassing. Is this going to affect the oscillation frequency? I was told I should switch to tantalum...

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Nov 29^th, 2009, 5:58pm

sos wrote on Nov 29^th, 2009, 3:41pm:

That's the coil that's 1cm in diameter, not the wire...

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 29^th, 2009, 6:06pm

Is there a way to check for supply line resonance on the PCB without using a network analyzer? Bear in mind there isn't necessarily any oscillation, just resonance...

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Nov 30^th, 2009, 10:22am

aaron_do wrote on Nov 29^th, 2009, 5:04pm:

I can tell you up front that if you hook a network analyzer up to all that wiring on the prober, you will get weird results. What do they mean--who knows? I favor a different approach. I wasn't kidding about the tweezers. Hold the tweezers and lean the same hand on some grounded surface. If you touch the supply pins and get changes in your noise output, this is pretty good evidence that your supply bypassing is suspect (maybe not 100% of the problem but significant).

The frequency where bypass capacitors become ineffective is pretty much where they go self resonant (or resonant with the external lead length on the prober). Electrolytic caps have higher internal inductance than tantalums. We typically use 4-10uF tantalums plus 0.1uF ceramic or film caps in parallel, on each supply to ground. The simple minded idea is that when the big cap quits working due to resonance, the little cap takes over (which has a much higher self-resonance), but this is a simplification. Actually, the resonance on the big cap is still there and can cause problems in some cases. I'd go with tantalum plus 0.1uF in parallel, as close as you can get to the chip (I realize there are probe pins in the way). Keep lead lengths (supply and ground side) as short as possible. You can't get rid of the probe card, but no meandering traces!

If you still have problems, sometimes you can add resistors to damp out resonance. One place is to add a series R-C between the supply pin and ground--like maybe 25 Ohms in series with 0.1uF. You could also try supply to supply if you have split supplies (ground on your board may not have any relation to what the chip is seeing as ground). The other place you could put resistors is in series with probe pins. There are obvious problems with this idea--you're very limited how much resistance you could put in a current carrying line, like the supply. You can put them in series with high impedance pins like inputs, but be careful you don't build a filter and ruin what you're trying to measure.

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Nov 30^th, 2009, 4:44pm

Hi sos,

thanks for the detailed reply. I guess the supply bypassing will have to be redone, but i'll experiment with the tweezers. Seems like you're trying to create an additional load to the power supply with the tweezers to see how it responds right? What if the tweezers are too good conductors? Seems like i'll get a short circuit...

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by sos on Dec 1^st, 2009, 4:34am

aaron_do wrote on Nov 30^th, 2009, 4:44pm:

The tweezers do *not* short anything, sounds like you have the wrong idea. All I do with the tweezers is add parasitics to the supply. The parasitics consist of me--or my hand, depending what I'm leaning on. The tweezers only contact the circuit at a single point--you could use a piece of stiff wire instead, but tweezers have a nice grip. The only purpose is a quick check to see what nodes are sensitive to parasitic loading. Obviously, you don't want to do this when there is enough voltage around to be exciting.

Just to be clear--no short circuit or bridging is allowed. The tweezers are just a means to connect your hand to the circuit as a quick and dirty, mostly medium impedance R, ~C load. Signal nodes are clearly going to be pretty sensitive above 1MHz, but the other nodes should not be.

Steve

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by aaron_do on Dec 1^st, 2009, 6:48am

I see. Thanks for the clarification. I originally thought you meant to connect one end of the tweezers to ground...

thanks,
Aaron

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by loose-electron on Dec 1^st, 2009, 9:17am

Looking at the spectral content I do agree with the group that it is NOT a transistor noise problem, thats for sure.

Welcome to the "noise onion" problem. But at the lower frequencies, it may not be an EMI type coupling issue.

This could be either a low level oscillation within the system, or something coupling in from the outside.

Key thing is you need to selectively divide things up, separate and isolate things until you determine the cause.

First - If it is on a probe station, get the part diced and packaged. Getting things quiet and under control on a probe station is not a realistic place to test things. Get it soldered down onto a test PCB (no socket!) that is well decoupled and well filtered. Right there you may find that solves the problem.

Probe stations are a nightmare of cross coupling, random inductance and other issues not easily controlled. Use them for DC measurements only, and even there, be cautious about resistive contacts and other things.

Once you are on a PCB with a packaged part, get everything going into the part quiet and selectively filtered. If that doesn't isolate things, then its time to put it inside a metal box.

Let us know what happens!
good luck
- jerry

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by rf-design on Dec 14^th, 2009, 7:41pm

Hi Aaron,

the RTS-noise does not have spikes in the frequency domain. It is so called because of the discrete nature of traps. Depending on the effect of the observed quantity like channel conduction current you can see the discrete nature. Trapping aquire and release can have a very wide time spread. In very small devices <25nm a small number electron charges define the amount of current flow in the channel of a MOS. In some experimental techs <10nm RTS current fluctuation could be 30% leaving it also useless for digital design.

In bipolar operation trapping have an effect on the base recombination. So yoo could see RTS-noise in the base current but the effect will be much less than in MOS-operation. So trap rich zones near the surface or isolation oxides touching base regions are sensitive.

In the case of the old lateral PPNP (Lateral Poly PNP) where the lateral base zone is defined by gate ring enclosing the P+ emitter diffusion there are traps similar to MOS operation. The gate electrode is the fifth's connector of this device. If it is sligthly negative biased you surpress the effect of traps below the gate plate. So 1/f-noise is reduced in this parasitic PNP. The brother of this device is the FPNP (Lateral Field Oxyde PNP) wheere the base is made by field oxyde, or STI in more advanced processes. Her you could not improce 1/f-noise by biasing. These improvement are used to lower 1/f-noise in bandgaps in CMOS using the PPNP.

I think you device show similar effects. The peaks in frequency domain are instrumentation noise.

Title: Re: popcorn noise (RTS noise?) in parasitic VNPN BJTs
Post by RobG on Dec 14^th, 2009, 8:03pm

rf-design wrote on Dec 14^th, 2009, 7:41pm:

I think you device show similar effects. The peaks in frequency domain are instrumentation noise.

I can't say one way or another, but I do know if you have a lot of base resistance 1/f noise will ruin your day. High sheet poly also gets pretty 1/ffy. But I think Aaron was more concerned with the peaks than the low freq noise.
rg