Oscilloscope input noise comparison

[ci11]

My first guess is that it's correlated with the sampling clock (2.5GS/s -> 4 interleaved ADCs at 625MS/s)
that wouldn't be so strange but geez.. i would have expected better.
Shall i power up my ancient boat anchor and try to get some data, just for kicks ?

That thought did not come to mind since this HDO is supposed to use a single TI ADC12D1600 per channel, for dual 1.6 GS/s or interleaved 3.2 GS/s. 625MHz is not an easy guess. But perhaps you're right - only LeCroy knows.

[JPortici]

mine is just a guess a peak at a hint right of 600MHz suggest 625 which is a divider of 1250 and 2500, the latter being a very common sampling frequency for oscilloscopes.
I don't know about the HDO internals unfortunately

EDIT: I looked at the HDO4032 datasheet: Sample Rate (Single-shot) 2.5 GS/s

[_Wim_]

Today I was measuring a high frequency spectrum with my Picoscope 5442B, and started wondering the following:

According to the datasheet the 5443B and 5444B have a better time base accuracy than my 5442B (50ppm for the 5442B, and only 2ppm for the higher end models). When I run a high bandwidth FFT, I always see some spurs around 33Mhz (see attached screenshots with shorted input). These are the worst in 12bit mode, and get better in 15 and 16 bit mode (due to paralleling of the ADC's?). I was wondering if these are simular on a 5443/5444 model or if these are related to the lower quality clock?

[Performa01]

FFT spurs are not related to time base accuracy.
The close-in phase noise is related to time base stability, but certainly not visible in a wideband spectrum plot without a signal.

The reason for spurs with a signal applied are non-linearities of the ADC. But without any signal, the non-linearities are pretty much irrelevant and all spurs can be attributed to internal interference signals.

Since the Pico 5000 series use some tricky configurations of the ADC cores in order to get the various resolutions, the 12-bit mode obviously generates more interference than other modes.

By the way, here you can see what a true 16 bit ADC in a Pico 4262 (which is optimized for low noise and low distortion) can do with regard to noise floor and spur level (with the same 50ppm time base tolerance!):


Pico_4262_FFT_Noise_50Ohm_term

[_Wim_]

FFT spurs are not related to time base accuracy.
The close-in phase noise is related to time base stability, but certainly not visible in a wideband spectrum plot without a signal.

The reason for spurs with a signal applied are non-linearities of the ADC. But without any signal, the non-linearities are pretty much irrelevant and all spurs can be attributed to internal interference signals.

Since the Pico 5000 series use some tricky configurations of the ADC cores in order to get the various resolutions, the 12-bit mode obviously generates more interference than other modes.

By the way, here you can see what a true 16 bit ADC in a Pico 4262 (which is oiptimized for low noise and low distortion) can do with regard to noise floor and spur level (with the same 50ppm time base tolerance!):

Thanks for this plot and the explanation. If only the 4262 would have had more bandwidth! Sometimes I do which I had bought that one, but the additional bandwidth made me choose the 5442B.
I agree this is related to the trickery, but do not understand why the 12bit mode is worse, because that sample rate is supported directly by the ADC without trickery (unless the ADC does some internal trickery off course).

[Performa01]

I agree this is related to the trickery, but do not understand why the 12bit mode is worse, because that sample rate is supported directly by the ADC without trickery (unless the ADC does some internal trickery off course).

It could be different signal paths for clock and input signals (even on the chip) with slightly more crosstalk in 12-bit mode?

-110 dBu isn't a very high level after all (<2.5µVrms or <7µVpp). In the +/-10mV range, this is equivalent to some -80dBFS. PicoTech specifies only 60dB SFDR (but that is irrelevant without a signal) and 110µVrms noise in 12-bit mode. So there is no specification for spurs without a signal, but I guess the -80dBFS is simply the performance limit of a 5000 series PicoScope.

[thanasisk]

Would be very very interesting to plot the low frequency input noise density  of the scopes, i.e. dc to 500 kHz. Is this possible  already with the same measurement files?

[Performa01]

Would be very very interesting to plot the low frequency input noise density  of the scopes, i.e. dc to 500 kHz. Is this possible  already with the same measurement files?

No it isn't. We need limit the record length - hence the sample rate - or the files would get huge.

Because of the the split-path design of the input buffer (and the FET amplifiers required for this), the noise in the DC/LF path is very high. So the noise plot below some 10kHz will most likely be ugly for just about any reasonably modern standard scope.

[nctnico]

Would be very very interesting to plot the low frequency input noise density  of the scopes, i.e. dc to 500 kHz. Is this possible  already with the same measurement files?

No it isn't. We need limit the record length - hence the sample rate - or the files would get huge.

Because of the the split-path design of the input buffer (and the FET amplifiers required for this), the noise in the DC/LF path is very high. So the noise plot below some 10kHz will most likely be ugly for just about any reasonably modern standard scope.

But this would need to be measured with the bandwidth limit on and the samplerate needs to be 100Ms/s or more otherwise the HF noise will alias into the lower frequencies.

[egonotto]

Hello,

I have on my 5243A and 5444B strong spur at 31.25 MHz.


Best regards
egonotto

[_Wim_]

I have on my 5243A and 5444B strong spur at 31.25 MHz.

Just zoomed in, exactly 31.25Mhz here also

[egonotto]

Hello,

31.25MHz * 4 = 125MHz
Perhaps this has to do with the clock frequency of the ADC



Best regards
egonotto

[_Wim_]

Hello,

31.25MHz * 4 = 125MHz
Perhaps this has to do with the clock frequency of the ADC



Best regards
egonotto

If that is the case, these spurs should move to a different frequency when you run an FFT with 200MHz bandwidth, because I expect the sample rate will then be 400MS/s, which is not dividable by 31.25. Or does is sample at 500MS/s and throws away a bit of the FFT?

Could you post a plot to 200MHz to check this with your 5444B?

[Performa01]

If that is the case, these spurs should move to a different frequency when you run an FFT with 200MHz bandwidth, because I expect the sample rate will then be 400MS/s, which is not dividable by 31.25. Or does is sample at 500MS/s and throws away a bit of the FFT?

Could you post a plot to 200MHz to check this with your 5444B?

PicoTech have pioneered the fully digital trigger system, which requires the ADC to run at full sample rate all the time. This sample data stream is fed into the digital trigger system. Only after that, the sample data get decimated if required to fit shorter record buffer lengths, so we get the effective sample rate at this point.

In other DSOs, where the FFT is not a completely independent operating mode like it is for PicoScopes, one additional decimation step is required whenever the max. FFT length is less than the record length.

Apart from that, the old 3206B samples at 500MSa/s and the FFT shows quite a few strong spurs, starting at 31.25MHz again. That’s probably just the frequency of the clock generator signal, which is then multiplied by a PLL in order to get the required ADC sample clock.

See the noise plot from 1kHz to 200MHz:


Pico 3206B FFT 50Ohm 1kHz-200MHz

EDIT: Please keep in mind that this plot is not directly comparable to the 4000 and 5000 series, because the most sensitive input range is +/-50mV (10mV/div) for the 3206B.

[_Wim_]

I sniffed a bit around internally with a DIY near field probe, but could not locate any strong 31.25MHz interference signal. The closest I could find was the 1.5Vdc switchmode regulated which had a spur at 31.20MHz, but also at around 29, 27... that were as strong, and we see only the one at 31.25MHz.

I also enabled all four channels (so only 1 ADC per channel running at 125MS/s, the spur at 31.2MHz remained. So it seems the ADC must do some interleaving internally for the 12 bit mode, but the datasheet is a bit confusing about this topic:

The high speed modes all utilize interleaving to achieve high sampling speed. Quad channel mode interleaves 2 ADC branches, dual channel mode interleaves
4 ADC branches, while single channel mode interleave all 8 ADC branches. In precision mode interleaving is not
required and each ADC channel uses one ADC branch only.

So it seems ADC1 consists of 2 ADCs, and only one of them is used is precision mode (14 bit). This also corresponds with the reduction in spurs in 14 bit.

[jpb]

I am interested in these Picoscope results as I'm thinking of getting the 4262 for ADEV and phase noise type measurements but would like more than 2 channels if possible.

I have a couple of questions.

Looking at the data sheet for the 5000 series, it looks as if 16bit mode only works with 1 channel - is this correct? (i.e. if you're in 16 bit mode you can only have one out of four channels operational) If it is correct, then for 16 bit (below 5MHz) you can't get more than 2 channels (4262) anyway.

Secondly, the relatively noisy plots you posted _Wim_ are labelled "shorted" while the plots you posted Performa01 are labelled as being terminated in 50 ohms. The input impedance of the scopes is 10M not 50 ohms but I wondered if this made a difference or perhaps "shorted" just means a 50 ohm terminator anyway.

[2N3055]

3406D MSO 500MSa/s from 1kHz to 200MHz 50 Ohm terminator on input (I didn't have a short handy)

[Performa01]

Looking at the data sheet for the 5000 series, it looks as if 16bit mode only works with 1 channel - is this correct? (i.e. if you're in 16 bit mode you can only have one out of four channels operational) If it is correct, then for 16 bit (below 5MHz) you can't get more than 2 channels (4262) anyway.

Yes. You can get more than 2 channels only in 14 bit mode. But then, the 15 and 16 bit modes appear to be extreme trickery and I don’t see a real advantage using them.

I think that the 4262 is rather unique and cannot be compared to the 5000 series scopes in any way.

Just look at the dynamic performance specifications in the data sheet to get the idea.

                     5000 series         4262
Crosstalk            >400:1              >50000:1
Harmonic Distortion  -70 dB (≥12 bit)    -95 dB
SFDR                  70 dB (≥14 bit)     96 dB
Noise                 70 µV RMS (16 bit)  8.5 µV RMS

Secondly, the relatively noisy plots you posted _Wim_ are labelled "shorted" while the plots you posted Performa01 are labelled as being terminated in 50 ohms. The input impedance of the scopes is 10M not 50 ohms but I wondered if this made a difference or perhaps "shorted" just means a 50 ohm terminator anyway.

Input short or 50ohm terminator don’t make a difference at that level.

3406D MSO 500MSa/s from 1kHz to 200MHz 50 Ohm terminator on input (I didn't have a short handy)

Yes, this is much improved over the older 3000 series. Higher input sensitivity, switchable 20MHz bandwidth limit, twice the ADC sample rate…

Interesting to see the much reduced spurious signals and the spur at 31.25MHz is missing altogether. Probably because of the doubled ADC speed, whereas the various physical sampling speeds in the 5000 series might have different clocking requirements.

[_Wim_]

Looking at the data sheet for the 5000 series, it looks as if 16bit mode only works with 1 channel - is this correct?

Yes, but 16 bit mode makes no real sense for this scope. Above 14 bit there is no performance increase noise floor wise, and give you already 84 db theoretical, with an SFDR or only 70dB, this is more than sufficient. In 14 bit you can use all 4 channels up to 60Mhz. But as stated above, the 4262 has much higher performance than the 5000, but lacks the bandwidth. Depending on your needs, this can be important or not.

Secondly, the relatively noisy plots you posted _Wim_ are labelled "shorted" while the plots you posted Performa01 are labelled as being terminated in 50 ohms. The input impedance of the scopes is 10M not 50 ohms but I wondered if this made a difference or perhaps "shorted" just means a 50 ohm terminator anyway.

I was using a short, not 50 ohm terminator. I have use a 50 ohm terminator in the past also, but result were indentical.

[_Wim_]

[Yes, this is much improved over the older 3000 series. Higher input sensitivity, switchable 20MHz bandwidth limit, twice the ADC sample rate…

Interesting to see the much reduced spurious signals and the spur at 31.25MHz is missing altogether. Probably because of the doubled ADC speed, whereas the various physical sampling speeds in the 5000 series might have different clocking requirements.

Indeed, the 3406D perform quite wel, I am impressed. Still dreaming of an 4262 with the bandwidth of the 5000 series (that is also affordable    )

[jpb]

Still dreaming of an 4262 with the bandwidth of the 5000 series (that is also affordable    )

I'd like a 4 channel 4262 with an external reference input - this is unlikely to happen.

I'm dithering between a 4262 and something like an RME UX usb interface. If I want to measure 3 oscillators with a reference/calibration channel then this needs 4 channels.
The 4262 is the easiest solution but I can only ever measure 1 oscillator against a reference oscillator.
Using an 8 channel audio USB interface the frequency range is limited (1Hz to 80kHz basically), only 2 channels have pre-amps and setting levels is more of a pain. The noise figures are good though and it is a cheaper option.

If I want to measure 3 oscillators in a 3 cornered hat approach then I'd need 3 two channel 4262s as I'd have to use one channel on each for reference but it could be done on a single 4 channel 4262 if such a thing existed.

[egonotto]

Hello,

@_Wim_: "Could you post a plot to 200MHz to check this with your 5444B?"

Perhaps we should ask this problem in the Picotech forum.

Best regards
egonotto

[_Wim_]

Perhaps we should ask this problem in the Picotech forum.

That is a possibility, but as the spurs are "within spec", I do not expect much of an answer. I have been into contact with them a few times (using their feedback functionality build into the software), and although they are quite responsive, the answer is typically not very enlightening.

[Andreas]

Hello,

at these sensitivities also the exact measurement setup is important.
The below 10 MHz peaks on Egonottos plot are shurely from a switchmode supply or from some other radiating cables nearby.
I usually separate carefully the power supply cable and the USB cables on my laptop.
(sometimes I also operate the laptop on battery only).

Attached 2 plots from my PS5444A with 50 Ohms terminator on the input.
One is with PS5444A outside my cookies box.
the other with PS5444A inside my cookies box (USB-cable hanging out)
So already there we have some (small) differences.
I guess that also a ferrrite on the USB-cable gives some improvement.

with best regards

Andreas

[_Wim_]

I guess that also a ferrrite on the USB-cable gives some improvement.

I have a ferrite on both USB and power cable, but when I installed it, I did not see a whole lot of difference. I also looked at the common mode noise "delivered" via the power cable, but did not find any correlation between the spurs and the common mode noise. Should do the same for the USB also. I will try to post some results of this later on.