Auto Input Range

Hi Matt,

I have run sweeps with auto-ranging on my 402 and it worked fine, but now when auto ranging is on I hear the relays click at a range change and then I get and error message indicating an overload like it did not switch. This is happening when the signal into the analyzer goes from under 0DBv to over 0DBv. I manually change ranges it seems to range fine indicating the relays are not stuck. Any idea if I’m doing something wrong or if its a timing issues / hardware problem?

Jerry G

Every time time that happens to me- relays clicking in and out, I increase the attenuation until it stops, or lower the level of the input signal…

The whole point of letting it auto range is so that it can change ranges through a sweep without having to manually do it and they set it up that way to increase accuracy. If I have to manually set the range it is far less valuable.

I guess I have not used the auto range feature- I start out with max attenuation and then decease it as needed to get what I think is the best response for the condition I am testing. I typically have plenty of time to spend with a DUT so that feature is not one I have noticed, other than the attenuator relays clicking when I have overloaded the QA402… :cold_sweat:

Hi @ggorman, do you see the “RANGE” message appear on the upper left of the graph while it’s going through the measurements?

Which plugin specifically are you using that is causing this, and what is your approximate DUT gain?

Hi Matt,

Sorry for the long delay. I’m testing a filter it has a very low end roll off so as I testing higher in Freq the level moves up to about 0dbV I here the relays click but then I get the error. Basically when anytime it needs to change ranges on its own this happens.

Hi @ggorman, which plugin is this?

If I use the QA402 in Auto Range, and I measure very low voltages of just mV range, and then I measure 100Vrms because of an accident, what happens? Does the Input have an OverVoltage protection?

The QA402 will weather a +32 dBV (max input) event just fine if you are in the most sensitive mode (0 dBV full scale input). Beyond that, it could fail depending on where the cycle the fault was introduced. If you touched a probe to a 1 kHz 100Vrms sine (+40 dBV) while in 0 dBV input mode right at the peak of the sine (140V), the input resistor would probably open up. And the input capacitor might be damaged.

Your protection is greatly extended if the input attenuator is on (full scale input set to +24 dBV or higher). If you see the green “ATTEN” light on the QA402 front panel, a single accidental probe slip to 100Vrms shouldn’t be a problem.

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Thanks Matt! This makes it clear what to expect :slight_smile: