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Use RF switch module to switch 125M Hz differential signals

A customer has 1 pair output signal. The signal is differential, 900mVp-p, 125 MHz.
He sends to output signal to his DUT with 4 differential channels and uses switch to read the signal back in turn.
He needs to measure the voltage waveform to get the return Vp-p to determine pass or fail.

According to his needs and tutorial "Advanced Signal Routing with the NI PXI-2593 and NI SCXI-1193 RF Switches
", we have pxi-2593 or scxi-1193, which can be configured as 2xN topology to switch differential signals. In the spec, the insertion loss is 0.9dB when the signal is less then 500 MHz.

I have some questions:
1. Does that mean the voltage error is around 10%? (Accoreing to dB=20log(V/Vref)
2. What else parameter should I pay attention when I want to know the voltage error?
3. If the customer needs the voltage error less than 5%, do we have any solution?

Thank you for help.

Henry
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@chlin12 wrote:
A customer has 1 pair output signal. The signal is differential, 900mVp-p, 125 MHz.
He sends to output signal to his DUT with 4 differential channels and uses switch to read the signal back in turn.
He needs to measure the voltage waveform to get the return Vp-p to determine pass or fail.

According to his needs and tutorial "Advanced Signal Routing with the NI PXI-2593 and NI SCXI-1193 RF Switches
", we have pxi-2593 or scxi-1193, which can be configured as 2xN topology to switch differential signals. In the spec, the insertion loss is 0.9dB when the signal is less then 500 MHz.

I have some questions:
1. Does that mean the voltage error is around 10%? (Accoreing to dB=20log(V/Vref)
2. What else parameter should I pay attention when I want to know the voltage error?
3. If the customer needs the voltage error less than 5%, do we have any solution?

Thank you for help.

Henry




Hello Henry,

You have a couple of options. All of our RF switches are single-ended coax (one signal line surrounded by a ground). To route a differential signal, you'll need to split the pair out to two coax cables and use two banks of multiplexers. Since you only have a 4x1 requirement, you could use one PXI-2593 (dual 8x1) or two PXI-2590 (single 4x1 each) modules. The SCXI counterparts have more banks.

The characteristic impedance of these switches is 50 Ohms. What is the differential impedance of your input signal? Ideally, this split configuration would work best if the differential impedance was 100 Ohms (each half run downs a 50 Ohm line with no coupling between them). The more impedance mismatch you have, the more reflection and signal loss will occur.

As for the insertion loss, you are right that the voltage at 125 MHz will be attenuated by as much as 10% (typical is more like 5%), but it is repeatable. If you calibrate the system (including the switch and cables), you should be OK. The insertion loss through the switch, like contact resistance, is stable over the relay life.

What is the frequency content of this signal? Is it sinusoidal at 125 MHz, or a square wave with faster edges?

Hope this is useful.

Charles
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