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Dependence of a scope's frequency response on vertical attenuator setting

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In the thread Which oscilloscope is better?, I argued that to state that the analog front-end of the Rigol DS1052E has 160MHz bandwidth, you need to test every vertical attenuator setting:

--- Quote from: saturation on January 27, 2011, 06:28:08 pm ---However, I'm not sure doing a frequency sweep across different attenuator setting is necessary since the attenuator is calibrated in v/div and its accurate to within its spec, suggesting one setting is approximate good for all settings.

--- End quote ---
From past experiences, I got the impression that there was a significant difference in frequency response between various vertical attenuator settings. To quantify this, I tested the frequency response of a Tektronix TDS-220. This is a 100MHz/1GS/s DSO, the closest I could find to the current cheap low-end scopes by the likes of Rigol, although I would expect Tek's analog front-end to be superior to Rigol's. The relatively low bandwidth (100MHz) makes it easier to test, since you need to generate signals with a constant amplitude at frequencies beyond the scope's bandwidth.

I used a Tektronix SG-503 leveled sine wave oscillator (verified to be reasonably close to spec with a 500MHz DSO, but I don't have a spectrum analyzer or scope with bandwidth well beyond 250MHz to accurately verify the 3% amplitude flatness from 50kHz to 250MHz). It was connected to the scope with about a meter of RG-58 coax, and it was terminated with a Tektronix 011-0049-01 50-ohm feed-through terminator at the scope end (I only tested channel 1). The scope was set to average over 16 waveforms, and was set to measure frequency and Vp-p. I set the scope to each vertical setting, adjusted the SG-503 output amplitude to be slightly less than 8 divs high, and then increased the frequency (and adjusted the horizontal setting to show a few periods).

The first graph shows the frequency response at each vertical setting, the second shows the -3dB point versus the vertical setting. The vertical settings above 500mV/div were measured with slightly less accuracy, since the generator only goes up to ~5.5V, not enough for 8 divs. This made the (8-bit) measurements less accurate, especially at 5V/div.

Excluding the settings below 10mV/div (which are limited to 20MHz), the -3dB point varied between 119MHz and 158MHz (2V/div and 5V/div may have been even higher if the generator could generate a signal with a larger amplitude), so depending on the vertical setting, I could claim this scope has 158MHz bandwidth or 119MHz. For a proper measurement, you should actually sweep the environmental conditions over the full operational temperature/humidity range, but I don't have an environmental chamber. It's quite likely that my lowest -3dB point would have been even lower in that case, perhaps barely above the specified 100MHz.

I was impressed by the triggering, it had no problem triggering at 270MHz as long as the amplitude was at least 1.5 div (which required significantly increasing the vertical setting). At 270MHz, there is almost no increase in on-screen amplitude when increasing the vertical sensitivity from 20mV/div to 10mV/div, since the amplitude decreases from ~113mV to ~59mV (this difference is also visible in the first graph). From 50mV/div to 20mV/div, the amplitude decreases from ~189mV to ~113mV, not that much change on screen either. This is what prompted me to do these measurements.

Nice data.
It is fairly common for the bandwidth figures of scopes to not apply to the lowest few ranges as you have found. They will often have different bandwidth (and noise) figures for these lower ranges.
Presumably because those ranges are amplified instead of simply attenuated down to a base level range.


Great writeup!

Awesome data and test setup!  Thanks a million for clarifying the issue you raised.  There are very few DSO: gain vs frequency response analyses in any google search, so I presume these are very rare and worth a detailed expose.

If others have sources, please link them here, particularly say for LeCroy, R&S and other scope makers.

Agilent has many 'Agilent vs Tek ' themed product application notes [ i.e., ersatz propaganda?] here's a snapshot of a very relevant one.

See photos #1, the Agilent curve vs photo #2, Tek curve.

Its coincidental it reflects a distribution of frequency vs gain similar to the different experiences I've had with the Rigol 1052E [assuming Rigol follows Agilent'd requirements in design, having once been a source for Agilent's low end scopes] and alm's experience with his Tek scope, brought up in another thread.

Agilent maintains a smaller, i.e., tighter, spread of gain vs frequency until its roll off.  Note these high end Agilent and Tek scopes are brick wall filtered scopes, not Gaussian -3dB roll offs.  But at issue is not the filter but the variability in frequency response of the vertical amp gain. Agilent prefers amps have a flat response; one can see there are differences in frequency for each vertical setting, but falls within ~ +/- 0.5dB across its vertical settings [~ 6%].

The Tek has a similar spread of 0.5 dB but varies ~ +/- 1.5dB, past 13 GHz.  It has interesting implications for measurement.

Although we are considering 100 MHz DSOs, if this design philosophy is carried through their scopes line by Agilent and Tek, then even in the case of the low end 1052E vs Tek 220, it explains my experience of a flat response until -3dB on my Rigol and the more varied response alm's had on his Tek.  Looking at the 0.5dB variation is  ~ +/- 6% of reading on the scope divisions, that variation is close to the posted 1052E spec of 4% + other corrections for range, see Photo #3.

I'll try to reproduce some of alm's test as best possible given my meager gear and graph them, and reconfirm those impressions.  More later as I've time to analyze alm's good work in detail.

The roll of -3dB is from the front end filter, but this variation in gain vs frequency is something from the amplifier design.  It shouldn't make a difference between any scope in the Rigol 1000E lineup, but as I know, know one has really tested it like alm's procedure here, other than the scope manufacturers themselves and rarely publicize it [ except for the Agilent application note].


--- Quote from: Mechatrommer on January 31, 2011, 02:00:11 pm ---nice. what about rigol 1052e hacked to 1102e?

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