Re: P6021 Noise Problem?

Tom Lee

Do you live relatively near to an FM radio station?

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 2/20/2021 13:08, Chappy wrote:
I recently acquired a P6201 active 1X FET probe which appears to have never been used. The inspection tag was still attached, the accessory pack was still sealed, and there was not a single mark on the probe's ground sleeve. All functions of the probe are working but it has a 2mVpp high frequency noise component riding on any signals that I measure.

The manual states a 'Noise Tangental' of 300uV (150uV RMS), a figure well below the noise signal that I am seeing.

I have tried the probe on both my 7403 through a 7A18, and a new Keysight DSOX1102G with the same results. I have tried powering the device with a Tektronix bench power supply, and a home made unit with the same results. Interesting thing is that the noise level riding on the signal is almost non existent when I feed it a supply of +/- 5V. As I increase the supply voltage above this the noise becomes more apparent. Based on this observation I believe that i can rule out noise picked up by the cable between the amplifier and the probe. I have also scoped the power supply rails with negligible abberations on the supply traces on a 2mv/div scale. With the Keysight scope (BW = 70Mhz) a screen capture shows that the lowest component of the noise starts at approx 100MHz.

I am starting to think that there may be components inside either the amp or probe that have degraded over time. Could a degraded or failed electrolytic capacitor contribute to this noise? Or could it be due to a damaged FET or BJT?

If anyone here has one of these I would welcome your input before I go chasing a problem which perhaps is completely normal for this probe. Once again, its physical appearance indicated that it has never (or hardly) been used.


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