Author Topic: Looking for a dynamic signal analyzer with extended bandwidth  (Read 18496 times)

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Online nfmax

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #25 on: October 22, 2017, 07:48:11 am »
Have you considered using a lock-in amplifier as an alternative? If your light source is modulated with a 'simple' waveform (e.g. Rather than a pulse compression coding) nothing else comes close in terms of signal recovery. But of course it is only a single frequency measurement, and requires a reference signal.
 

Online nctnico

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #26 on: October 22, 2017, 10:47:20 am »
One note on the Pico scopes: you have to check very carefully if they use ADCs with the specified resolution or specificy resolution after doing oversampling. When using oversampling you may not get the extra bits if there isn't enough noise in the signal.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #27 on: October 22, 2017, 12:03:45 pm »
Have you considered using a lock-in amplifier as an alternative? If your light source is modulated with a 'simple' waveform (e.g. Rather than a pulse compression coding) nothing else comes close in terms of signal recovery. But of course it is only a single frequency measurement, and requires a reference signal.

Hi, I have a couple of lock-in amplifiers. I agree to extract the signal nothing is better. But I am more trying to understand the impact of changing variables (loading resistors, feedback capacitors, light intensity, modulation frequency...). Currently I can see the degradation in the signal, but I do not always see clearly why. By visualizing the entire spectrum, I might get a more clear idea...

 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #28 on: October 22, 2017, 12:09:36 pm »
One note on the Pico scopes: you have to check very carefully if they use ADCs with the specified resolution or specificy resolution after doing oversampling. When using oversampling you may not get the extra bits if there isn't enough noise in the signal.

Yes, this is correct. That is the biggest advantage over the 4262, it uses a real 16-bit AD, hence also the lower sampling rate only.
The 5000-series uses AD-converters of the other channels in parallel to get increased resolution.
On top of that, both have an enhanced resolution mode were oversampling is used (+4 bits max). This allows both to get up to 20 bits theoretically.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #29 on: October 22, 2017, 12:11:52 pm »
No users here that use the HP 89441A or 89410A? I am still very temped by this solution also, but would like to read some user feedback (google turns up nothing, probably because these are quite old and not commonly used in the DIY community)

Edit: just noticed in the TEA thread user Berni snatched one of these recently (or posted recently about is)...
« Last Edit: October 22, 2017, 12:16:46 pm by _Wim_ »
 
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Offline David Hess

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #30 on: October 22, 2017, 05:28:21 pm »

It is actually much worse than that.  A wide bandwidth (200 MHz) CMOS operational amplifier used for the high impedance input buffer might have a 5nV/sqrHz white noise specification but a 10 MHz corner frequency yielding 1000nV/sqrHz of noise at 10 Hz.  Discrete designs even from 40 years ago are *much* better than this.

Unfortunately, these kind of details are typically not published for the scopes (and other gear).

To answer the question of low frequencies, yes, I need them. Light sources used for testing are typically modulated with frequencies between 300Hz an 20Khz, so this range is very interesting for me. My initial thought was to just by a dynamic signal analyzer, but there just so limiting in higher frequencies. Still, noise wise, they are probably still by far the best for my application...

I was not even being hypothetical; the Cleverscope design uses an OPA2355 as the input buffer.  This does not necessarily make it bad; who knows what the Picoscopes use?  And an integrated wide bandwidth low input current transimpedance amplifier used for wide bandwidth photodiode amplification will have a similarly high noise anyway.  Noise is a major problem with photodiode amplifiers which I assume is why you are interested in this.

Maybe they use a separate low frequency correction loop which became common in oscilloscope designs starting in the 1980s in place of expensive matched dual JFET to lower DC drift.  One way to find out would be to short the input and run an FFT on the resulting 20 MHz bandwidth limited noise; this should be done anyway to find out exactly what the lowest detectable noise level will be at different frequencies.

A sure way to solve this is to implement a simple external preamplifier.  It just has to have enough gain to overcome the noise of the test instrument and it removes the requirement for a 1 megohm input.  Also keep in mind that a 1 megohm input when used with a probe or cable will have considerable input capacitance.  A x10 probe solves this but increases the input noise by 10 times.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #31 on: October 22, 2017, 06:39:12 pm »
A sure way to solve this is to implement a simple external preamplifier.  It just has to have enough gain to overcome the noise of the test instrument and it removes the requirement for a 1 megohm input.  Also keep in mind that a 1 megohm input when used with a probe or cable will have considerable input capacitance.  A x10 probe solves this but increases the input noise by 10 times.

A good pre-amp is indeed something that I need to look for. I know Standford Resaerch and EG&G make some very nice ones, typically also fitted with low and high pass filters to limit total noise. I have bid on a few in the past, but never managed the get one.

Noise is indeed a big problem with photo-amplifiers, but I find it a very interesting topic. Currently I am mostly looking at older commercial sensor systems, trying to understand how they were designed. I am quite a beginner at all of this (more of an automation guy than an electronics guy), but by understanding the design choices better will also allow me to purchase the correct sensor platforms during my day automation job. And maybe one day I can make a design of my own, but at the rate I am going, I will not get my hopes up...
Thanks by the way for the cleverscope doc about noise and SNR, I printed it for a good bedtime read  :)
 

Offline David Hess

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #32 on: October 22, 2017, 07:37:03 pm »
Noise is indeed a big problem with photo-amplifiers, but I find it a very interesting topic.

Currently I am mostly looking at older commercial sensor systems, trying to understand how they were designed. I am quite a beginner at all of this (more of an automation guy than an electronics guy), but by understanding the design choices better will also allow me to purchase the correct sensor platforms during my day automation job. And maybe one day I can make a design of my own, but at the rate I am going, I will not get my hopes up...

Burr-Brown published a number of great application notes on this subject which include detailed circuit analysis.  It is a difficult problem because the high capacitance of the photodiode greatly constrains the performance of the amplifier.

Quote
Thanks by the way for the cleverscope doc about noise and SNR, I printed it for a good bedtime read  :)

I was not aware of that document until I ran across it for this discussion.  That Cleverscope publishes so much detail leads me to trust their designs more.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #33 on: October 28, 2017, 09:58:56 am »
Just a quick update, I just ordered a picoscope 5442B here: https://www.reichelt.de/USB-Oscilloscopes/PS-5442B/3/index.html?ACTION=3&GROUPID=5905&ARTICLE=146212&SHOW=0&START=0&OFFSET=16&SID=95Wb13EKwQAT4AADKebe825c2d261a5a91ed39db57cde2a9f417b&CTYPE=1&MWSTFREE=1

The cleverscope also seemed a very good option, but was much more expensive, and seeing the good results egonotto achieved with his 5000 series scope, and the fact that I used a picoscope before and reallly liked it, made me choice the pico.

Many thanks for all of your replies.

 

Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #34 on: June 10, 2018, 11:10:09 am »
Hello,

Did you get similar good results as the ones posted by egonotto with your new picoscope? How is the noise performance compared to the specs?
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #35 on: June 10, 2018, 01:20:07 pm »
Yes, quite satisfied with my purchase.

I will post some pictures with the result I get when I am in the lab. The scope fits my needs, but on the software side of things there could be some improvements (I am mainly missing a good way to add overlays in FFT window to compare results easily). But the great thing about picoscope is that you can download new software versions for free, so any new features implemented in the future you get also.
 

Online egonotto

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #36 on: June 11, 2018, 05:25:31 pm »
Hello,

thanasisk wrote:
"Did you get similar good results as the ones posted by egonotto with your new picoscope?"

Please don't misinterpret my measures. There is a reduced band width and sometimes a kind of averaging. This is not written in the specs.

In the specs you only find typical noise with full band width.
(There is also no accurate description how to measure the noise).

And there is the PicoScope 5000 not so good. And my PicoScope 5243A is a little worse as in the spec.

From full band width noise consideration there seems better oscilloscopes. For instance the new Siglent or the new Rohde und Schwarz RTB2000

Best regards
egonotto
 

Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #37 on: June 11, 2018, 05:48:27 pm »
A specified 70uVrms at 2mV/div at full bandwidth delivers approx 5 bits ENOB according to theory for the 5000 pico.

Is the siglent or r&s delivering better enob despite being 8bit and 10bit scopes?

 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #38 on: June 11, 2018, 07:56:47 pm »
Here are some quick tests I have done. If you want me to test anything else, please feel free to ask:

- FFT with shorted input for 10mV range an 1V range
- FFT 1Mhz sine from internal generator and Rigol DG1032Z
- noise with shorted input 16 bit and 20 bit (oversampling)
- noise when using EG&G premap with gain set to 100, both for 16 bit and 20 bit (remark: bandwidth pre-amp is only 1Mhz)

Remark: no bandwidth limits or averaging
« Last Edit: June 11, 2018, 08:03:15 pm by _Wim_ »
 
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Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #39 on: June 11, 2018, 08:16:05 pm »
Did some more tests:

- in the above during the first tests for noise 2 power supplies were turned on next to the scope, hence the 50Hz noise
- also repeated the tests above at max sample rate (62.5MS/s for 16 bit)

Again no bandwidth limit or averaging...
 
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Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #40 on: June 11, 2018, 11:40:37 pm »
Thank you Wim for the interesting measurements!

May I also ask for :

- +-10mV range, shorted input, BW limit ON, 16bit/62.5MS/s: measure AC RMS + P-P noise  [to see how effective is the BW limit on noise reduction]
- same with +-1V range [to see how the input frontend noise changes]
- FFT with 1kHz sine 2Vpp from internal generator: measure THD,THD+N,SFDR, (+AC coupling to remove DC offset?) [to see the performance of the gen]

 

Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #41 on: June 12, 2018, 12:47:36 am »
As a side note,  sampling at 62.5MS yields a low effective bandwidth of approx 12.5MHz. In that case not using the BW limit will cause aliasing (thus affecting the noise measurements). Perhaps decreasing the number of bits from 16 to e.g 10 to increase the sampling rate sufficiently would show a different picture?
 

Offline KrudyZ

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #42 on: June 12, 2018, 03:32:40 am »
Another oldie that would fit the bill:
HP3585A @0 Hz - 40 MHz with tracking generator, 1M Ohm input in addition to 75 and 50 Ohm.
Very heavy. Repairable with available schematics.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #43 on: June 12, 2018, 06:53:08 pm »
Thank you Wim for the interesting measurements!

May I also ask for :

- +-10mV range, shorted input, BW limit ON, 16bit/62.5MS/s: measure AC RMS + P-P noise  [to see how effective is the BW limit on noise reduction]
- same with +-1V range [to see how the input frontend noise changes]
- FFT with 1kHz sine 2Vpp from internal generator: measure THD,THD+N,SFDR, (+AC coupling to remove DC offset?) [to see the performance of the gen]

Here you are...
 
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Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #44 on: June 12, 2018, 07:03:25 pm »
As a side note,  sampling at 62.5MS yields a low effective bandwidth of approx 12.5MHz. In that case not using the BW limit will cause aliasing (thus affecting the noise measurements). Perhaps decreasing the number of bits from 16 to e.g 10 to increase the sampling rate sufficiently would show a different picture?

Here is the measurement on different sample rates and resolutions. No BW limit configured for all of them. 14 bit seems to be not the ideal choice becasue of low sampling rate and higher nosie...
 
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Offline Marco

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #45 on: June 12, 2018, 09:12:25 pm »
How about the Red Pitaya? Has 14 bit 125 MSPS.
 

Online egonotto

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #46 on: June 13, 2018, 12:12:58 am »
Hello,

Red Pitaya works with digital zoom. You have +- 1V with 14 Bit. The more sensitive V/div settings are only zoom.

Best regards
egonotto


 

Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #47 on: June 13, 2018, 10:24:30 am »
I think the SNR of the 5000, because the design utilizes multiplexed ADCs, does not improve in higher ranges.  Did you get any significant  benefit from utilizing the preamp?

Perhaps the following test would be appropriate:
1. 20mV sine gen 1khz -> channel 1 of ps5000 (+-10mV range)
2. 20mV sine gen 1khz -> preamp 100x  -> channel 1 of ps5000 (+-1V range, custom 1:100 probe configuration in settings)

For both cases measure SNR. BW limit on at 16bits mode, no hires or averaging.
 

Offline thanasisk

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #48 on: June 13, 2018, 11:50:58 pm »
I calculated the ENOBs for your scope.

16bit@2mV/div@20MHzLIMIT yields 6.19bits ENOB
16bit@200mV/div@20MHzLIMIT yields 8.86bits ENOB

So if you were to use the preamp, a maximum of approx. 2.5bits ENOB could be gained depending on the noise of the preamp.
 

Offline _Wim_Topic starter

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Re: Looking for a dynamic signal analyzer with extended bandwidth
« Reply #49 on: June 14, 2018, 08:07:55 pm »
I think the SNR of the 5000, because the design utilizes multiplexed ADCs, does not improve in higher ranges.  Did you get any significant  benefit from utilizing the preamp?

Perhaps the following test would be appropriate:
1. 20mV sine gen 1khz -> channel 1 of ps5000 (+-10mV range)
2. 20mV sine gen 1khz -> preamp 100x  -> channel 1 of ps5000 (+-1V range, custom 1:100 probe configuration in settings)

For both cases measure SNR. BW limit on at 16bits mode, no hires or averaging.

Yes, quite a significant benefit. Attached the comparison.

Setup:

DG1032Z 2Vpp sine => Attenuator /100 => Preamp x100 => Picoscope  (with custom 1:100 probe)
DG1032Z 2Vpp sine => Attenuator /100 => Picoscope


 
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