Capacitance of RF probes?

[palmeiras]

Hello Guys,

I’m planning to measure the output of a VCO directly on wafer, using a probe station (RF probes). What is common impedance that I should expect from a RF probe?
1pf in parallel with 1Mohm?


I know that DC probes have very high capacitances and because this I’m going to avoid it.

Thank you,

 

[goldsmith]

Dear palmeiras
Hi
Wen you're going to buy a probe , it will has a guidance manual , you can see some curves in that , and some notes about frequency response of your probe . but if you loose your user manual , you can measure it , but it is a bit hard . ( and i think if you loose it's datasheet you can refer to the site of manufacturer .)
Best Wishes
Goldsmith

 

[volker_muehlhaus]

What is common impedance that I should expect from a RF probe?

Usually 50 Ohm

 

[palmeiras]

Hello guys!

Thanks for your replies. This is the origin of my doubt. I though inicially that it would be only 50 ohms. To match with the spectrum analyzer impedance. However, I was reading the manual of this probe **broken link removed** , and its says that is around 1 pf and 1MHz. But it is not WAFER probe.

So in summary, doesn't it have any capacitance?

Do you have a link of any good RF-Wafer probe?

Thank you,

 

[E Kafeman]

The Agilent probe presents a low RF load when measuring in existing circuits. It is handheld.
Probe input impedance: 1 MOhm//0.7 pF
Probe output impedance: 50 Ohm
RF wafer probing tools is in my opinion, not anything that is handheld and mostly a question about what kind of tip to use: **broken link removed**
If you are RF measuring a wafer as a kind of wafer functional verification are these tips connected to external equipment that provides correct impedance/voltage for what actual wafer pin is expecting and it can be both RF out as well as RF in that is injected this way.
Also for handheld high impedance probes can often tip be replaced for better grounding close to actual measurement point. This reduces problem measuring ground currents related to other active circuits on the pcb.

 

[palmeiras]

Hello Kafeman,
Thanks for your reply.
Is this value a standard? Or does RF wafer probes of different brands have different values?
So… if we plan to inject and measure RF signal, do we have to buy both types of probes? Input and output? I’m right?
Best regards,

 

[E Kafeman]

No there is no standard for the high impedance input reactive values. Capacitive load is generally less for high frequency probes. 50 Ohm output is dominating as active output impedance.
Wafer probing tips are mostly of passive type and can be used for either output or input. Type of footprint of tip can be important depending on how the contact-points on wafer are shaped.

 

[palmeiras]

Hello Kafeman,

Thanks again for your information.
Do you mean that the same probe can be used for input and output? And when acting as input, the resistance is nearly 1Mohm with 0.7 pF, but when acting as output, the impedance is predominately 50 ohms?

 

[FvM]

I guess, you understood, that the said RF proble incorporates a buffer amplifier. Thus it's surely not suited as a signal source to the chip.

 

[palmeiras]

Hello FvM

Thanks very much!
So… please, consider that I’m going to measure the output of my VCO using a RF probe. Then, my VCO will see an output impedance of 50 (RF probe) || 50 ohms (Spectrum analyzer), ~ 25 ohms (resulting of the parallel combination)?

Actually I will also need to connect a DC blocking capacitor before the Spectrum Analyzer.

In addition, how much of parasitic capacitance would you expect for RF pads, cables, etc?

 

[E Kafeman]

A active high impedance probe have a buffer amplifier, probe input impedance is therefore not related to probe output impedance.
VCO will only see the high impedance side of the probe, while spectrum analyzer will be loaded by 50 Ohm from probe.
A active probe can handle a maximal voltage at input, AC or DC. If VCO voltage is higher then what the probe is specified to handle is not a serial capacitor any protection. From data sheet for the probe can also be read or calculated its maximum output level. If that is higher then what analyzer can handle, can both attenuators and dc-blocking be used between probe and analyzer. It will not affect impedance load at probe input.
Capacitive load in RF pads for wafer probing can be read in data sheet for the pads, but is usually very low, much lower then for the probe buffer amplifier input capacitance.

 

[FvM]

There seem to be a misunderstanding. You have been talking about "the probe acting as output" which in my understanding describes driving the chip with external signal. Connecting the probe to the VCO output involves "the probe acting as input", or in other words as a high impedance load.

 

[palmeiras]

Hello E. Kafeman and FVM,

Thank you very much for this discussion. Actually I think that I made a big confusion.
As I’m going to measure the VCO directly on the wafer, I going to use a passive RF WAFER probe (for instance, **broken link removed** ), as exemplified by Kafeman. And then, how could I model it in my electric simulation?

I’ve read the manual in the above site but there isn’t too much information about it. It is more focus on reflections.

Thanks again,