Gas station without pumps

2016 December 15

Function generator bandwidth of Analog Discovery 2

Filed under: Data acquisition — gasstationwithoutpumps @ 15:56
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The network analyzer function of the Analog Discovery 2 USB oscilloscope makes it easy to characterize the function generator’s bandwidth—just connect the function generator to the input channel (making sure that the input channel is not specified as a reference) and do a sweep.  The only choice is whether to use the wires that come with the basic unit or the optional BNC adapter board and scope probes.  I tried it both ways (and with both 1X and 10X settings of the scope probes), using 1V amplitude on the waveform generator one in all cases:

There is not much difference in the bandwidth between 10X probes and wires (both high impedance) (8.5–8.8MHz bandwidth), but the 1X scope probes provide higher bandwidth—higher than the 10MHz measurable with the network analyzer.

There is not much difference in the bandwidth between 10X probes and wires (both high impedance) (8.5–8.8MHz bandwidth), but the 1X scope probes provide higher bandwidth—higher than the 10MHz measurable with the network analyzer.

I tried loading the function generator with resistors, but this made essentially no difference in the frequency characteristics. It isn’t the 1MΩ resistance of the scope that matters, but the capacitance of the oscilloscope plus probe.

So I tried adding capacitive loads and found that I got a very clear LC resonance. With a 330pF load, I got the peak near 10MHz to approximately cancel the drop:

The resonance around 9.1MHz with a 330pF load is actually a little too strong and over-corrects for the drop in bandwidth. Adding 6.8Ω in series with the 330pF capacitor makes a load that nicely compensates for the inductance of the wires.

A resonance around 9.1MHz with a 330pF capacitor implies an inductance of about 0.93µH, which is in a reasonable ballpark of the sort of inductance one would expect for 80cm of wire (4 wires each about 20cm).

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