Products > Test Equipment
Rigol DS1000Z series buglist continued (latest: 00.04.04.04.03, 2019-05-30)
Njk:
Greetings!
DS1054Z is a new instrument to me. Owned Tek TDS3054B previously. In comparison, Rigol seems noisy but it also has more memory, more SW candies so it has its pros and conrtras, as usual. The scope is of its latest HW generation (I presume), board version 0.1.4.
I've enabled all the options, including 500 uV/div (which works fine, BTW, taking its limitations into account) and installed the firmware of the day, v.00.04.05.SP2.
Now I'm evaluating the FFT capabilities. It's not essential feature for a scopes but anyway.
In DS1000Z, the FFT function can operate in two modes, the Trace mode and the "Memory*" mode. The Trace mode looks like a finished product, At least, it works in a consistent manner. But it's actually a legacy mode, with poor resolution in the frequency domain. The user's manual for MSO5000 series provides better coverage for the matter.
FFT resolution = (FFT sampling rate) / (points number)
In the Trace mode, the FFT function fetches a points from the display memory, so the FFT sampling rate is actually equal to the beam sweep data rate, 100/(T scale) in points/s, and the points number is equal to 1200 (12 horizontal grid divisions, 100 two-pixel point images per division). From that, max. theoretical FFT resolution bandwidth for each T scale can be calculated. For instance, on the smallest T scale:
5 ns/div -> 20 GPts/s -> 16.666 MHz, while the finest FFT F scale setting is 20 MHz/div.
On the largest T scale (not considering the slow sweep scales):
100 ms/div -> 1 kPts/s -> 0.83 Hz, while the finest possible F scale is 1 Hz/div.
So in the Trace mode, on each T scale, the FFT resolution is almost equal to the smallest F scale. Not good, but it would be pointless to try to increase the resolution adding some AI rather than more data, because that would likely result in even more poor spectrum plot. Instead, Rigol finally decided to implement fetching a points from the acquisition memory in their entry level scopes, adding the "Memory*" mode in DS1000Z. In that mode, FFT sampling rate is equal to the instrument's current sampling rate, and the max. points number is equal to 16K, which is about 14 times greater, allowing for a much more accurate FFT.
Specifically, there are two features in DS1000Z, that sources their input data in the acquisition memory, the Frequency Counter, and the FFT function. The Counter works fine, but my experience with FFT in the Memory mode is mixed. I've a feeling that there is a large room for improvement in the SW.
For instance, the book reads "You can adjust the center frequency and horizontal scale at the same time using the horizontal scale knob". It's natural for a user to expect that the displayed readings for the frequency and sampling rate FFT scales will change accordingly. In the Trace mode, that's in fact so. In the Memory mode, when the instrument's sampling rate has changed by the T scale knob rotation, the readings does not change. More precisely, sometimes they changes, sometimes not and sometimes they changes in wrong way. It's not consistent, and the only reliable way to update the readings is to turn the FFT function off and on again, losing all the cursors, configured for the Math window.
But that's the most innocent defect, though it's annoying. On rare occasions, the spectrum trace does not appear at all when the function has cycled on/off/on. Typically, it does, and in that case I've noticed similar inconsistency trying to control the FFT settings. With the same combination of sample rate, mem depth and T scale, sometimes it's possible to configure FFT for desired center frequency and scale, sometimes not, because of too high granularity or because the banner "Parameter Limited!" pop-ups.
I'm failed to figure out a pattern so far. Perhaps it depends on how lucky the scope was at the boot time or how long the user played with the buttons and knobs. Once I witnessed the "Memory leak in FFT" pop-up message. Something is definitely wrong there. Poor software, but it's good enough to make a problems difficult to report because of the sometimes keyword.
I'm hoping someone here who has more experience with that instrument can provide more insights so we can work out a meaningful communiqu to Rigol's attention.
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WBR
Njk:
Ok, found a way to navigate around the defects and use the feature.
The trick is in proper sequence. When you'd initially switched to the Memory mode, it's necessary to set the Center frequency at zero, and then set the F scale as desired. Then you can set desired center frequency. Otherwise, it could not be possible because of high granularity of allowed center frequency values. Pretty weird. Looks like missing initialization.
A post without images looks boring now. This is the FFT plot for 1 V 1.1 kHz sine wave signal (img0.png):
Notice the discrepancy in measured amplitude values between the FFT and the RMS item. It's a pure sine wave from a pure analog high quality signal generator, there are no other harmonics, why the 3 dBv difference?, Actually, it's also weather-dependent, could be 0 dB on another day. Perhaps a calibration issue, that's Rigol, never mind.
It's interesting the amplitude value calculated by the FFT can be more accurate, probably because it uses data from the acquisition memory (img1c.jpg):
The Frequency counter also seems working fine for that frequency (img2c.jpg):
Finally, let's increase the amplitude above the channel's clipping level to see the harmonics (img3.png):
Edit: actually, the interdependency of Center frequency and F scale settings is documented, but in the user's manual for DS2000A series: "Note: Setting the horizontal position can changing the center frequency of the FFT operation results indirectly."
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WBR
Njk:
Another puzzle. What prevented Rigol from (optionally) dismissing the input signal trace for full screen FFT plot? Displaying both of them can result in total mess on the screen.
Found a workaround. Switch the display in the Dots mode and reduce the beam intensity down to zero. Fortunately, the intensity control does not affect a math traces. For better effect, play with the acquisition mode setting (seemingly it does not affect FFT when it's at the Memory mode). The result: a nice full-screen view.
My apologizes if that trick is long time obvious.
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WBR
tautech:
:-//
Don’t these have a Exclusive FFT mode that doesn’t show the source waveform ?
RoGeorge:
That's a nice trick :-+, I don't think DS1054 has the option to display FFT only.
In fact, the FFT functionality in Rigol DS1054Z is unusable in practice. It works, but it's too cumbersome to operate the settings, too low resolution, and too slow to make any use of it, other than maybe a didactic demonstration.
The most useful trick with FFT on DS1054Z is to avoid the FFT entirely. ;D
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