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About 250 MHz probes...

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About 250 MHz probes...

Postby jom » Fri Jul 13, 2007 2:21 am

Is there a way to get the schematic of the the 250 MHz probes that come with the 5203? I would like to use them (because they are very good probes) for use with a 50 Ohm input spectrum analyzer (not the Picoscope but a "real" one). I don't have anything else handy to do some board probing and I need a higher input impedance probe to use in a 50 Ohm system.

The documentation on the probe says it is a 10 MOhms with a 1 MOhm input (X10 setting) so how does the loading change if it's a 50 Ohm spectrum analyzer instead of a scope? Basically, I'm trying to calibrate the probe for this use so I can compare the value on the analyzer with what I'm really getting.

As an example, the analyzer has a test output at 20MHz at -10 dBm and with the probe I get -35 dBm on the analyzer. So the the "loss" is about 25 dB. Does this make sense with what is inside the probe??


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About 250 MHz probes

Postby PeterF » Mon Jul 16, 2007 10:22 am

Any 250Mhz 10:1 proble for a scope will be totallly unsuitable for a 50 ohm input Spectrum Analyser. The probe has a 9Meg resistor in parallel with a 10pF variable capacitor, the pair in series with the input signal. The input impedance of a scope is 1Meg in parallel with 40pF. The 9M/1M combination gives a 10:1 attenuator & the 10pF capacitor at half scale(4.5pF) gives also a 10:1 attenuator with the 40 pF input capacitance. Adjusting the value of the variable capacitor balances the HF/LF response of the probe. With a 50 Ohm input impedance, this balance is shot to pieces. The resistive attenuation is over 100dB and the Capacitive about 25dB, depending on the setting of the variable capacitor and the input impedance. The matching will be horrible! You need a fully 50 ohm probe system or a properly designed impedance matching attenuator.
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