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Test equipment signal measuring issue

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moro

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Hello all,
in a production test equipment, on the tested pcbs ( DUT) there is a measurment of a RTC signal ( 32768hz nominal)...

The signal is measured trough a keysight 53220A counter.

The signal is routed from the tested pcb via a test probe trough a 1 meter long wiring harness -> contact switching matrix -> counter

The problem which occurs is on the counter the deviation of the RTC signal is quite large ( +/- 20hz), how can i compensate this wiring capacitance/loss? i tryied to use a 50ohm coaxial cable, but i am limited by the hardware in the sense that i need to go trough the switch matrix, where i have other 300 probe signals ( voltages/pwms).
Any ideas how to improve this issue?

Thanks
 

Hi,

You need an aplifier close to the DUT. High input impedance.

Klaus
 

You don't want a 50 ohm coax cable unless the source can drive 50 ohms, since such a shielded cable has a lot of capacitance.

Is the frequency stable if you measure directly at the PCB source?

What does the signal look like on a scope connected to the end of the cable?
 

Hello
@Klaus- i was thinking of using a Jfet type OpAmp because of high input impedance ( 10^12 ohms) , also i guess its required to have smal capacitance as posible ( 10-12pf)?

Can you recomend some models? I was thinking of OPA series from Texas Instruments

@crustschow - the frequency is stable at the pcb probe side, the variation occures on the instrument side
 

Hi,

@Klaus- i was thinking of using a Jfet type OpAmp because of high input impedance ( 10^12 ohms) , also i guess its required to have smal capacitance as posible ( 10-12pf)?
For impedance calculations you have to take capacitance into account. Without capacitance it is just "resistance".

So, yes, you need low capacitance also.

A inverting circuit has the advantage, that there is a known series resistance....(The OPAMP input capacitance doesn´t care)
A non inverting circuit has the advantage of high input resistance, but the OPAMP input capacitance is completely at your xtal signal.

Do some calculations which one is better for your circuit.. it dpends on how critical your XTAL circuit is on those loads.

Klaus
 

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