|Product Performed to Expectations:||2|
|Specifications were sufficient to design with:||5|
|Demo Software was of good quality:||2|
|Demo was easy to use:||2|
|Support materials were available:||1|
|The price to performance ratio was good:||7|
|TotalScore:||19 / 60|
I didn't mean to publish this, so it's a work in progress.
Let me start by saying, I mainly applied for this RoadTest because I wanted to test the Frequency Response Analyzer (FRA) of the new 1000X series scopes. My review is solely of that feature. In fact, my RoadTest application was only about 1 page long, and the focus was this single paragraph:
“My idea for testing this unit is rather simple: I would like to have fun with the DSOX1102G by using its Frequency Response Analyzer to test both passive and active filters. I'm thinking some basic LR, CR, and LCR passive filters and a few active filters such as Butterworth and Chebyshev. I'd like to compare my hand measurements to the automated measurements. I have a few projects where I would utilize the oscilloscope as well, one of which is troubleshooting a continually rebooting Samsung cell phone. I'm trying to keep this application short enough such that you will see practical, achievable goals for this RoadTest. If you are concerned about the quality of the review, simply check the previous reviews I have completed from the list above.”
THIS REVIEW FOCUSES ON THE FRA FEATURE OF THE DSOX1000 SERIES
Once I started using the FRA I soon realized a few things. Firstly, this feature of the 1000X is not only poorly documented, it’s practically undocumented. I had to email Keysight to get info and the response I got was:
“First, you asked about available documentation. There isn’t yet a lot of documentation specific to the DSOX1000’s FRA feature. Additional application notes are coming. For now, page 71 in the User’s Guide describes how to set the different parameters for the measurement, such as the input / output channels, amplitude, load setting, etc.”
So, the documentation is coming. I read the manual before even applying for the RoadTest and I just assumed I missed some documentation, but no, that single page in the manual is all the documentation available for the FRA. I’ve reproduced that single page below:
Fortunately, the scope itself shows you how to make the connections; that is, how to hook up the source, Channel 1, and Channel 2. A whole Scope Month video was dedicated to the FRA and it was even shown on the kick-off video. I know, because I watched all the videos. I wasn’t getting satisfactory results with my FRA, so the agent from Keysight suggested I hook up a resistive divider and measure its frequency response.
Measuring a Resistive Divider
I used the topology of Figure 1 for the resistive divider. I did not use a breadboard, instead I used screw terminals to connect the two resistors together as shown in Figure 2. Scope probes were set to 1x attenuation.
Figure 1: Resistive Divider Setup. R1=R2=100k.
Figure 2: Implementation of the Resistive Divider.
Figure 3 shows the response from the FRA for the circuit of Figure 1 with R1=R2=100k.
Figure 3: Frequency Response of the Resistive Divider. Gain is the blue curve and Phase is the Orangish curve.
As always we ask "What do we expect for a result?". I expect the gain to be a flat -3 dB, until at a higher frequency where some parasitic effects may cause some deviation. I have no idea how the gain curve ever got higher than 0dB in this test ... it's a passive circuit. The probes may be influencing the result; I've contacted Keysight support. Let's assume the gain plot is my fault, and look into the phase response. I expect a flat line at 0 degrees for the phase until at higher frequencies where parasitics cause a deviation. The phase has two very wrong points.
I took a video of the analyzer running and grabbed the waveforms from the two points of interest. Figure 4 and 5 show the waveforms for frequencies of 158 kHz and 400 kHz. Yellow is the input and green is the output. The measurements taken at 158 kHz and 400 kHz are clearly wrong. For 158 kHz, the reported phase shift is 80°, but it is clearly closer to 30°. At 400 kHz, the reported phase shift is -5.34°, but it is clearly closer to 45°.
Figure 4: FRA waveforms for f=158 kHz. Reported phase is 80°. Visual inspection shows the phase is closer to 30°.
Figure 5: FRA waveforms for f=398 kHz. Reported phase is -5.34°. Visual inspection shows the phase is closer to 45°.
So, there's a bug or glitch occurring, which isn't a big deal IF they fix it. After seeing the above plots I thought I could go back and reduce the effect of the outliers by adjusting the frequency steps of the test. This was the worst part of the RoadTest because YOU CAN'T ADJUST ANYTHING EXCEPT THE AMPLITUDE FOR THIS TEST!!! Are you kidding me!?!?! After talking with Keysight, I found out that YOU HAVE TO PURCHASE THE NEXT MODEL UP to get the ability to change the frequency steps. Once again, are you kidding me? Don't made a big deal about the frequency analyzer if it doesn't have basic functionality like changing the frequency steps. Call it a script or something else, because it sure isn't an Analyzer. I am so disappointed, because I was really looking forward to this feature. To find out it's a crippled script being called an analyzer that you can only really use in a more expensive scope just outraged me, again, because Keysight made such a big deal about the feature.
Some time passes...
I've worked with the resistive divider some more. For some reason, I get a different response when I plug the scope into a UPS. I've also separated the cables of the scope to prevent crosstalk. Figure 6 shows the latest result with a 9V amplitude setting. Figure 7 shows a noisier version of the same response by using the recommended 200 mV drive signal. Figure . The gain starts flat at -6.5 dB,
Figure 6: FRA of Resistive Divider using 9V drive signal.
Figure 7: FRA of Resistive Divider using 200 mV drive signal.
I didn't mean to publish this, so it's a work in progress.