Author Topic: Tek P6243 probe distortion question  (Read 249 times)

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Offline TimFox

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Tek P6243 probe distortion question
« on: July 19, 2021, 09:43:59 pm »
I recently purchased a Tektronix P6243 x10 high-Z probe and the required 1103 TekProbe power supply from eBay.  So far, I have been very careful with it, never connecting it to a signal with a DC component, and obeying the input limits. 
The power supply signal output must be connected to a DC-coupled 50 ohm load:  I have used a TDS 350 DSO with a 50 ohm BNC terminator and a Marconi 2380/2382 400 MHz spectrum analyzer that has a 50 ohm input. 
So far as I can tell, the unit is performing well, up to the frequency limits of my equipment.  I have always connected the probe input through the "KlipChip" and Y-lead adapter provided.
Before doing some spectrum-analyzer measurements where harmonic content is important, I thought to measure the THD with an -hp- 339A distortion analyzer (adding a 50 ohm termination at the analyzer input terminal).  The generator output is terminated in 600 ohms, the probe goes to the resistor leads via the KlipChip, and about 3 feet of coax goes to the terminator at the analyzer input.
The maximum output of the generator is 3 V rms (8.5 V pk-pk) into 600 ohms, well within the P6243 input ratings:  +/- 15 V "no damage", +/- 8 V for <4% linearity error.  The graph of typical linearity error vs input voltage shows inflections at about -6 V and +8 V.
My results:  339A at 90 kHz (100 kHz maximum) with only 400 Hz high-pass filter engaged.  All voltages rms.
Vin     Vout       THD
3 V     300 mV  0.065%
1 V     100 mV  0.064%
0.3 V  30 mV    0.092%
0.1 V  10 mV    0.013%

Not bad, but why so high at 0.3 V?  I continued at lower voltages
Vin          Vout     THD
10 mV     1 mV    0.017%
30 mV     3 mV    0.014%
100 mV  10 mV   0.013%
200 mV  20 mV   0.014%
250 mV  25 mV   0.0145%
270 mV  27 mV   0.0135%
280 mV  28 mV   0.0135%
290 mV  29 mV   0.0135%
300 mV  30 mV   0.092%  (again!)

There is no amplifier diagram in the manual, since it is a proprietary hybrid module.
My question is why there is such a sudden discontinuity at 300 mV rms = 848 mV pk-pk input?
I can't find any fault in the 339A analyzer when repeating these measurements.

Offline David Hess

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Re: Tek P6243 probe distortion question
« Reply #1 on: July 20, 2021, 02:48:15 am »
Active probes are not particularly low distortion and not optimized for it.  My guess is that two different distortion mechanisms are constructively and destructively interfering to produce that hump in distortion with amplitude.

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