Friday, January 5, 2018

Cheap Siglent scope in a QRO environment

I've had my $380 Siglent oscilloscope for about six weeks now and have mostly used it to look at signals on my uBITX and various 630m projects. Yesterday I took it to work because we had a problem with one of our radars and I wanted to see how the scope performed in an RF-noisy environment.

The problem was that one channel of the radar was operating at too low of an RF output and with too wide a spectrum. Before even getting on site I knew that only two things are likely to cause this and an oscope is needed to verify, correct and align the new component. For official use, we have a calibrated Tektronix oscope but I wanted to compare readings and presentations of signals with the cheap Siglent.

The first image shows the problem and the traces agree with the more expensive Tektronix. The pink trace is an inverted (it's actually a negative pulse) representation of the voltage pulse being sent to the cathode of the radar's klystron amplifier. This is the pulse that forward biases the klystron.

The yellow trace is the low power RF input that is to be amplified by the klystron and fed to the antenna. It is deformed, thus the problem with low power and a wide bandwidth. It is also supposed to be centered within the biasing pulse.

Shape and parameters of RF drive pulse after replacing PFN

After replacing the pulse-forming network, the input RF looked good and all that had to be done was adjust the bracketing of the two pulses and adjust the PFN for the proper pulse width and the klystron cavities (the six black knobs) for minimal rise and fall times. This was done with the Tektronix scope and then looked at with the Siglent. The biggest difference between the two was the fall time of the pulse which was 137nS on the Siglent, 132nS on the Tek - a 3% difference.

Both scopes allow measurements of various pulse parameters to be made with cursors or, at the push of a button, a text screen can be displayed. The values displayed in text agreed with what I measured manually.

While these alignments were being performed, the radar channel adjacent to us was radiating with 1.3 million watts and much QRN was being generated by the focusing coil as well yet no degradation in the Siglent's performance was seen as a result. This makes me all the more confident in the Siglent, especially for ham use.
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2 comments:

  1. Well, not that I understand anything of it. It seems though you have a very interesting job. Nice to see there is not much difference between a cheap scope and a calibrated Tek these days. 73, Bas

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    1. It's been a fun career but I'm looking forward to retirement soon. 73 and HNY!

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