Hi,
I would like to know the different between the signal outputs at RF probes specifically how the signals look like with GS and GSG probes. Suppose, I have a signal generator generating 2 Vpp signal with 0.5 V dc offset. I assume that with a GS probe the signal is similar to the input signal generated out of the source; But how does it appear for the GSG probe?
Thanks,
-Rakesh.
You can't tell, unless you know what the circuit you are measuring looks like.
Some circuits only have a signal and one ground, and a GS probe will measure this better than a GSG probe will, which will probably have additional capacitance in the ground circuit.
If there are two grounds, then the GS probe will have additional inductance (perhaps) in the ground path and that will affect the measurement. In the microwaves 101 site recommended in the answer to your similar question it points out that some circuits with two grounds only get one ground at the test point because of cost cutting. In that case the damage has been already done by missing out one of the grounds and the signal can't be measured properly at the test point whatever probe you use, but the GS probe will be best.
To get the best measurement the probe has to mimic the same current flow pattern that is on the tracks it is going to probe. Then it will measure the correct impedance and mode pattern.
Your question seems to miss the point of these probes. They are to measure microwave or mm signals, not snail-pace square waves. At frequencies below half a GHz you won't notice any difference and have wasted your money on the probes.
Thanks for the detailed answer. In my application, I have a device with GSG pads and would like to use the GSG probes to supply signal from the function generator. It seems that from your answer in third passage, a GS probe can be used to send RF signal to the device with GSG pads. Am I correct?
Also when you say "mimic the same current flow pattern that is on the tracks it is going to probe", Can you further elaborate what this means?
Thanks,
-Rakesh.
in the GSG configuration half the ground current is in each side of the ground. If you try to join this to a GS configuration half the ground current has to stop dead. This will generate a reflection and imbalanced currents creating modes which may propagate but with a different velocity. Microwave signals require that modes and impedances are matched. If you are not working at frequencies that high it doesn't matter much. Ai 100 MHz the current can take a few cm extra to get to the other ground if it needs to and it will not make any difference.
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