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I am now doing a project to measure the phase difference in my circuit. Figure 1 is my circuit. My target is to know the phase difference between the output of the DAC and the input of the ADC. I have known the phase of the output of the DAC, phase shift of the analogue circuit and that the modulator has no phase shift. And the output signals from the DAC is lower than 20kHz.

My questions are:

what phase shift would the SMA connector cause normally?(SMA connector is 5-1814400-1 from TE connectivity) And

How about the RF port? What phase shift would it cause?

Does anyone have idea about my questions or idea about how can I solve the problems? (I will try my best to update all information in need)

Thank you very much!

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1.

More information:

The cable between the SMA and RF port is about 30cm long. But I can`t find any more detail about it. The modulator and analogue circuit are connected through fibre and PD. The analogue circuit, SMA connector and MCU are inside one PCB board.

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  • \$\begingroup\$ Is the SMA connector part of the modulator or part of the MCU? What cables are connected between the MCU, the connector, and the modulator? \$\endgroup\$ Commented Feb 11, 2015 at 5:15
  • \$\begingroup\$ @ThePhoton：the SMA connector is part of the PCB board,and the board includes the MCU and the analogue circuit. About the cable, I don`t know what kind of it and I know less about the cable. It is about 30cm long. \$\endgroup\$ Commented Feb 11, 2015 at 5:56
  • \$\begingroup\$ The 30 cm cable has a much bigger phase effect than the sma connector. \$\endgroup\$ Commented Feb 11, 2015 at 5:59
  • \$\begingroup\$ Also the modulator also probably has more phase effect than a single connector (doesn't it have connectors itself?) so if you're neglecting its delay, you really shouldn't worry about this connector. \$\endgroup\$ Commented Feb 11, 2015 at 6:01
  • \$\begingroup\$ @ThePhoton: how serious would the phase effect caused by the cable be? \$\endgroup\$ Commented Feb 11, 2015 at 6:03

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At 20 kHz, the phase shift of the SMA connector is negligible. The length of the signal path is maybe 5 mm. The dielectric constant of the PTFE (Teflon) material in the SMA connector is about 2.0. So the delay through the connector is about 23 ps. That's about 20 microradians of phase delay at 20 kHz.

Even at higher frequencies, the delay through the SMA connector is probably quite small compared to the delay through the cable that's connected to the connector, which you've said nothing about.

If you were working at a higher frequency where 20 ps made a difference, then you'd have a problem. Because the delay induced by a connector like this depends on the footprint on the PCB where you mount it as well as the construction of the connector itself. And because the two sides of the connector aren't the same type of waveguide.

The best way to determine the characteristics of the connector would probably be to build two additional "test coupons" onto your pcb. Each coupon would be a length of trace with a connector on each end. The two traces would be different lengths. By measuring the S-parameters of the two coupons with a network analyzer, you'd have enough information to cancel out the affect of the traces and determine the characteristics of the connector.

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  • \$\begingroup\$ Thank you very much! But would the connector or the RF port introduce low-pass effect due to the parasitic capacitance and resistance? I just know less about the material and the construction of connector or RF port. \$\endgroup\$ Commented Feb 11, 2015 at 6:23
  • \$\begingroup\$ It's probably about the same as the other things --- way too small to worry about at 20 kHz. To say more, you'd have to tell us something about it besides that it's a "rf port". \$\endgroup\$ Commented Feb 11, 2015 at 6:26
  • \$\begingroup\$ Sorry for that. I am just learning the construction of the RF port. So by now I can`t tell any more about it. Anyway, Thank you very much! \$\endgroup\$ Commented Feb 11, 2015 at 6:35
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I don't think either the SMA connector or RF port will introduce phase shifts because they are not filtering, amplifying or otherwise processing the signal in any way. They simply pass it along, like a wire. Therefore the phase shift caused by increasing the path length will be practically negligible, but let's calculate it anyway.

Your minimum period is 1/20kHz = 50 us. If you assume your signal propagates at the speed of light, and your SMA connector adds ~2 cm to your path, then your signal will take approx 67 ps to travel through the connector. This is 1.333e-6 of your period or a phase shift of approximately 0.00048 degrees. And for signals with frequencies less than 10 kHz, the period will be longer and the phase shift will matter even less. I don't think you have much to worry about.

Unless you use really long cables, the phase shift from the DAC output to the ADC input (not due to your analog circuit) will be negligible.

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  • \$\begingroup\$ The cable is about 30cm long. So does it matter? And do you mean that RF port also would not introduce serious phase shift? \$\endgroup\$ Commented Feb 11, 2015 at 6:11

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