DSO Quad bandwidth

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bielec
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Re: DSO Quad bandwidth

Post by bielec » Fri May 20, 2011 11:35 pm

I have modified my scope to increase the bandwidth as follows....

For ch A.
Remove C73
C11=1.5pF
Across R17 3.3pF
Across R21 22pF
C9=1.5pF
Across R15 3.3pF
Across R19 12pF

For ch B.
Remove C74
C12=1.5pF
Across R18 2.2pF
Across R22 15pF
C10=1.5pF
Across R16 2.2pF
Across R20 12pF

I now get about 10MHz sinewave bandwidth on all ranges and good pulse response with minimal overshoot and rounding. Trigger and good display of sub 100nS pulses is now possible. As delivered, the scope had less than 1MHz B.W. and poor risetime which varied depending on range.

I would suggest the above values are used as a guide, for experimentation, and I hope it may help others get better performance.

up2spex
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Re: DSO Quad bandwidth

Post by up2spex » Sat May 21, 2011 3:32 am

Greetings all,
I've been monitoring all your comments and investigating your data concerning the Quad bandwidth topic.

I ordered a Quad, but not received it yet.

I have the schematic, component datasheets, and have been using Slimfish's "Quad.zip" file for analysys. Thanks Slimfish, you saved me the trouble.

This is my $.02

In my mind, the first logical step is the 72Ms/s sample rate issue.
I like lygra's take on this and the sampling chart.
I personally would want an accurate graphical representation of a signal, otherwise, want's the point?
My Instek GDS-1062A has a sample rate of 1Gs/s and 60MHz bandwidth.
With a 60MHz signal, that works out to 16 samples per cycle, and that many samples per cycle on the Quad would be for a 4.5MHz signal.
I don't see the Quad being useful beyond 10MHz.

Now for the OPA2354 opamp issue.
With all range switches open, the amp has a gain of 10, the datasheet shows a small signal bandwidth f-3db = 10MHz at a gain of 10.
Also I noticed the power supplies are 3.6 and -3.0 which exceed the max specified voltage of 5.5V, but not the absolute max rating of 7.5V.
I'm wondering if this will effect the slew rates/bandwidth because operating a transistor close to breakdown voltage increases leakage current possibly causing slower response.

As far as testing an actual Quad, I have a scope and a Parallax Propeller proto-board, there is a demo for synthesizing square waves up to 120MHz

Tina-TI testing confirms a lot of what everyone has been saying.
I plan to modify Slimfish's circuit to include a scope probe and function generator impedence.
I'm aware that spice programs don't include parasitic signals and stray capacitance, Spice is for before/after picture, looking for improvement. The designer should know the limits of spice and compensate for it in the real world.

I'll post more when I have more, Thanks

lygra
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Re: DSO Quad bandwidth

Post by lygra » Sat May 21, 2011 8:32 am

up2spex wrote:Also I noticed the power supplies are 3.6 and -3.0 which exceed the max specified voltage of 5.5V, but not the absolute max rating of 7.5V. I'm wondering if this will effect the slew rates/bandwidth because operating a transistor close to breakdown voltage increases leakage current possibly causing slower response.
Nice find on the op-amp supply voltage specifications. The Quad -3V comes from a non-regulated TC1221 which provides -5V light load, and slumps with more current draw. I want to confirm that it is -3V when I get my replacement Quad (supposedly next Monday according to DHL). I will also confirm the 3V6 voltage with a fully charged battery.

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slimfish675
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Re: DSO Quad bandwidth

Post by slimfish675 » Mon May 23, 2011 4:04 pm

Hi,

Bielec, twisted wires as small capacitors... that's a very neat trick (also an old school one :-)). Good work!!!

My question (as you are very handy with hw) is whether using small valued resistors could avoid the use of the three compensation capacitors... (as in U17). Other thing is if you have 0603 resistors of proper values...

Unrelated, ADC decoupling is not bad (22uF+1uF), but for high frequency (>3 MHz) one expect also a smaller capacitor (10nF) which would provide the fast transient current for conversion peaks.

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Re: DSO Quad bandwidth

Post by lygra » Fri May 27, 2011 9:11 am

Today I attempted to do some Quad measurements (fresh out of the box sys 1.31, app 2.30. fpga?? (I wanted a base-line before I changed anything)) but I ran into a couple of snags:

1. All my waveform displays appear to be 1/10 of the sine wave input signal. Where do you set the 1x or 10x probe type? I can't find a menu choice for that in the Quad Manual 0.91b. Is there a 10x probe selection option?

2. The Auto trigger mode is very flaky. It has trouble finding sync and many times displays garbage when it appears to be synched, but after a few seconds it may lock in and display properly. If it doesn't lock in properly, then you must move the trigger level out of the signal and then bring it back and try again. Success rate appears to be about 30%

3. The Normal mode trigger on the other hand appears to lock properly and always displays an appropriate waveform.

4. The Quad-measured Vpp does seem to track the Quad displayed waveform amplitude properly, but the Quad-measured Vpp appears to be off by a factor of 20, 1/20th of the actual input signal; or to put another way, 1/2 of the 1/10 amplitude observed waveform.

5. All amplitude and frequency measurements of the input signal are conducted off a TEE'd BNC cable to a Tektronix TDS-210 60Mhz scope with 1GS/s sample rate. The input to the Quad uses an MCX to BNC adapter and BNC cable from the TEE that also feeds the Tektronix scope.

Once I get this 1/10 issue resolved, then I will conduct accurate bandwidth measurements at 1Mhz increments. My initial findings using only the display results shows the roll-off (-3db)to start between 3 and 4 Mhz and at 10Mhz the amplitude falls to 40% of 1/10th of the input signal. Once again, all these measurements were conducted with sine wave signals.

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Re: DSO Quad bandwidth

Post by lygra » Sat May 28, 2011 3:22 pm

Found my 1/10 problem to be the 10x setting (by mistake) on the Tektronix. :oops:

The test conditions listed below took about 5 hours:

1. Quad FW = App 2.35, Sys 1.34, FPGA 2.5, used "Normal" trigger mode for sinewaves, "Auto" trigger mode for calibration DC voltage measurements.
2. Performed front-end compensation for best waveform as per factory procedures.
3. Used Tektronix TDS-210 60Mhz 1GS/S scope to monitor sinewave Quad inputs.
4. Used a digital multimeter to measure the DC voltages supplied to the Quad.
5. Using the same DC calibration voltage as an input results in errors. This indicates that the calibration process is not working properly. When you use the same DC voltage to calibrate and then measure, you would expect consistent results, but not the case with these FW versions.
6. Used the Quad Measure Vp-p function and display waveform for results measurements.
7. The 50mv and 0.1v scales have bandwidth issues.
8. The 0.2v scale provides consistent and flat bandwidth for the tested frequencies. :D
9. My Heathkit IG-102 RF Generator can't produce output voltage to properly test the remaining scales.

Conclusions:

1. Throughout these tests I never felt comfortable with the Normal Trigger, however the Auto trigger was much worse. It got so bad that I would turn off the Nano for each new test because I didn't trust that the Vpp was always updating properly. Trigger detection and associated measure updates have been measured and found lacking.
2. Calibration FW and/or procedures still have issues.
3. Flat bandwidth can be obtained on the 0.2v/Div scale, although the DC accuracy if different from RF accuracy.
4. Need to repeat these tests using AC coupling.

Test Results:

Code: Select all

Calibrated Chnl-A 50mv/Div with 286mvDC

applied 286mvDC to input and quad = 268mvDC, same with waveform, accuracy = 94%

Tektronix 100Khz sinewave @ 156p-p mv, quad = 110p-p mv, display = 2.2 div = 110p-p mv, accuracy = 70%
Tektronix 1Mhz sinewave @ 156p-p mv, quad = 62p-p mv, display = 1.1 div = 62p-p mv, accuracy = 39%
Tektronix 2Mhz sinewave @ 156 p-p mv, quad = 42 p-p mv, display = .85 div = 42p-p mv, accuracy = 28%
Tektronix 3Mhz sinewave @ 156 p-p mv, quad = 34 p-p mv, display = .7 div = 34 p-p mv, accuracy = 22%
Tektronix 5mhz sinewave @ 152 p-p mv, quad = 28 p-p mv, display = .5 div = 28 p-p mv, accuracy = 18%
Tektronix 10Mhz sinewave @ 152 p-p mv, quad = 18 p-p mv, display = 18 p-p mv, accuracy = 12%


Calibrated Chnl-A 0.1v/Div with 559mvDC

applied 559mvDC to input and quad = 492mvDC  accuracy = 88%

Tektronix 100Khz sinewave @ 156p-p mv, quad = 112p-p mv, display = 1.1 div = 112p-pmv, accuracy = 70%
Tektronix 1Mhz sinewave @ 156p-p mv, quad = 100p-p mv, display = 1 div = 100p-p mv, accuracy = 64%
Tektronix 2Mhz sinewave @ 156 p-p mv, quad = 76p-p mv, display = .8 div = 76p-p mv, accuracy = 48%
Tektronix 3Mhz sinewave @ 156 p-p mv, quad = 68p-p mv, display = .7 div = 68p-p mv, accuracy = 44%
Tektronix 5mhz sinewave @ 152 p-p mv, quad = 56p-p mv, display = .6 div = 56p-p mv, accuracy = 37%
Tektronix 10Mhz sinewave @ 156 p-p mv, quad = 36p-p mv, display = .4 div = 36p-p mv, accuracy = 23%


Calibrated Chnl-A 0.2v/Div with 1161mvDC

applied 1161mvDC to input and quad = 984mvDC  accuracy = 85%

Tektronix 100Khz sinewave @ 156p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%
Tektronix 1Mhz sinewave @ 156p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%
Tektronix 2Mhz sinewave @ 156 p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%
Tektronix 3Mhz sinewave @ 156p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%
Tektronix 5mhz sinewave @ 156p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%
Tektronix 10Mhz sinewave @ 156p-p mv, quad = 120p-p mv, display = .6 div = 120p-p mv, accuracy = 77%

lygra
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Re: DSO Quad bandwidth

Post by lygra » Sat May 28, 2011 10:20 pm

Attached are pictures of a 3.55Mhz square wave signal from a function generator. All connections are same as previous post, except that a function generator is generating the square wave signal.

1. The first picture shows the Tek view by itself with both the tee'd BNC cables connected and the Quad MCX adapter disconnected. The rounded edges are due to the stray capacitance of the cabling which does not match the stray capacitance of a standard 1x probe.

2. Second picture shows the Tek view with Quad connected.

3. Third picture shows Quad view with both connected. Please note that calibration has not been conducted on the 0.5V scale. the waveform is the subject of concern here, not the amplitude. My original Engineering model Quad presented with a much better waveform for the same condition as this Beta model. Note that this is a multiple sweep display because the camera shutter allows more than one sweep to be viewed in this picture. This accounts for the strange look due to multiple display sweeps.

4. Fourth picture can be found on the next post, because only 3 attachments are allowed. This picture shows the Quad after being disconnected from Quad MCX adapter and shows the measured remnants from picture #3. This is very disturbing because it is a false measurement in real-time, this is why I suspect all measurements when changing the input signal.

Conclusions:

a. Quad stray capacitance is limiting the input signal and causing Tek measurement errors.
b. Measurement values are not properly updated when signal trigger is lost.
b. Those boxes in picture 3 are due to multiple sweeps. One sweep shows steps, not boxes. What is interesting is that these steps are in the same time domain as the switching noise of the interlace mode. This may indicate that display issues are causing these steps in the interleave mode, and not the interleave mode itself.
Attachments
tek view 3.55Mhz tek only.jpg
tek view 3.55Mhz tek only.jpg (18.28 KiB) Viewed 7814 times
tek view 3.55Mhz both.jpg
tek view 3.55Mhz both.jpg (15.01 KiB) Viewed 7814 times
quad view 3.55mhz on both.jpg
quad view 3.55mhz on both.jpg (31.88 KiB) Viewed 7814 times
Last edited by lygra on Sat May 28, 2011 10:33 pm, edited 4 times in total.

lygra
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Re: DSO Quad bandwidth

Post by lygra » Sat May 28, 2011 10:21 pm

Picture #4 goes with previous post.
Attachments
quad view 3.55Mhz no Quad.jpg
quad view 3.55Mhz no Quad.jpg (24.05 KiB) Viewed 7814 times

bielec
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Re: DSO Quad bandwidth

Post by bielec » Mon May 30, 2011 5:34 pm

Lygra wrote...

"What is interesting is that these steps are in the same time domain as the switching noise of the interlace mode. This may indicate that display issues are causing these steps in the interleave mode, and not the interleave mode itself"

I think these steps are simply the sample steps. I notice you have both chA and chB on so the unit is not interleaving two sets of samples. To see the interleaving effect, you have to "HIDE" chB. Then you will see the sawtooth effect discussed elsewhere and you will notice an apparent doubling of timebase rate. (which I consider advantageous)

Also apparent on jpg 4 is poor risetime and rounding due to the effect of excessive compensation capacitors discussed earlier. After adjusting my values, I acheive a risetime within 2 sample periods for a 3.5MHz waveform of this amplitude. Go on Lygra, get you soldering iron in the back there!

lygra
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Re: DSO Quad bandwidth

Post by lygra » Tue May 31, 2011 9:27 am

bielec wrote:I think these steps are simply the sample steps. I notice you have both chA and chB on so the unit is not interleaving two sets of samples. To see the interleaving effect, you have to "HIDE" chB.
It is quite possible that those are sampling steps as you stated. If you look at the Nano 50Khz square wave display (attached) you will see 25 steps in the display during one sample time = 1us. This square wave on the Nano is a similar percentage of sample rate as the 3.55Mhz on the Quad. Maybe BenF could comment on the reason for these 25 steps in the Nano. I suspect that they are display update steps. Could the Quad processor be running fast enough that you can't see it's display update steps?

I know about the interleave display issues and I am trying to keep this thread on bandwidth topic, that is why that info and associated pictures were not included.

I am going to hold off on the soldering iron while I conduct further testing on the factory issued Quad. ;)
Attachments
Nano dsply steps2.jpg
Nano dsply steps2.jpg (31.58 KiB) Viewed 7746 times

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