Author Topic: Rigol RP7080S Active Probe Advice?  (Read 1956 times)

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Offline NF6X

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Rigol RP7080S Active Probe Advice?
« on: November 02, 2016, 12:55:24 am »
In the day job, I'm debugging a low-powered 32.768 kHz crystal oscillator circuit which is having start-up issues. I have a Rigol DS4024 oscilloscope on the bench. Its RP3500A passive probes load the oscillator circuit too much, preventing start-up (which is already marginal), and damping the oscillations severely once the oscillator is running. I think I will need an active probe to be able to observe the oscillator as I work on both load capacitor tuning and RTC block configuration in the firmware, though I'm open to advice regarding other probing techniques.

The Rigol RP7080S looks like a natural fit since it is designed to work with the scope I have. I'm having trouble finding any reviews of it, though, so I'm hoping that somebody here is familiar with it and can share their experiences and opinions.

One of the specification lines in the user's manual concerns me: "input noise" is specified as "70mVpp", which seems much too high. Is that a typo?! For comparison, I looked up the specifications of the least expensive Keysight active probe I could find. The Keysight N2795A datasheet specifies its "probe noise" as "< 2.5 mVrms (referred to input)". I'm not sure if the Rigol vs. Keysight noise specifications refer to the same parameter under the same conditions.

Offline Someone

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Re: Rigol RP7080S Active Probe Advice?
« Reply #1 on: November 02, 2016, 02:41:56 am »
Its hard to find reviews on any professional measurement hardware, but good to see Rigol finally offering some active probes. They're closer to the Keysight 1130B product (with many identical specifications) but even with the 1130B input noise of 3mV over a smaller operating voltage its still has a much higher signal to noise ratio. Using off the shelf parts I'd expect them to be getting better noise specifications since their input impedance is 25kOhm to ground, so it might be a typo.

Offline ADT123

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Re: Rigol RP7080S Active Probe Advice?
« Reply #2 on: November 02, 2016, 05:41:49 am »
As your frequency is only 32kHz you can probably cheat and get away with using a passive probe.  Check the spec of the passive probe for its tip capacitance (eg 12pF) and then check what the cap to ground value is in the oscillator circuit (eg 22pf).  Replace the cap to ground with one that in this example would be 10pf so that when the scope probe is attached the total load is back to 22pf.
Disclaimer:  I have worked for Pico Technology for over 25 years and designed some of their early oscilloscopes. 

Happy to answer Pico related questions when I have time but I am on eevblog in a personal capacity as electronics is a hobby.

Offline MarkL

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Re: Rigol RP7080S Active Probe Advice?
« Reply #3 on: November 02, 2016, 09:41:22 pm »
You might also want to consider getting the Rigol T2R1000 Tektronix probe to Rigol scope adapter for US$299 and then pick up a used Tektronix active probe.

A Tek P6245 can be had for around $100 to $150: 1.5GHz, 1Mohm, <1pF, and is supported by the T2R1000 for calibration and offset control.

The adapter opens the door for a lot of other good probe bargains too.

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