Measuring groundless differential PCB transmission lines with the CITS880s
Application Note AP8509 

Operating principle of groundless differential probes for CITS880s (2015 editions onwards)

Polar CITS880s controlled impedance test system offers calibrated measurements on groundless differential pairs when used in “groundless differential” mode in conjunction with Polar G-Series ergonomic groundless differential probes.

Polar series of GDPS (groundless diferential) probes provides a measurement solution for situations where there is no physical access to the ground plane, or where a ground plane is not present, i.e. a balanced line with equal width signal and return path. (Testing and modelling impedance of differential pairs without ground AP153)

The GDPS probe converts the single ended signal from the CITS880s to a differential signal that is applied to the coupon under test. The reflected signals from the coupon are converted back to single ended as they return through the probe to the CITS880s.

Single ended to differential conversion is thus made at the probe – which inherently removes the skew variation that arises with cable wear and movement.

The enhanced waveform of the CITS880s is necessary to optimise the performance of this probe, and the CITS880s software post processes the acquired data in order to determine accurate measurements.

The time proven RF technique employed in the probe allows handling this type of conversion in RF systems. Polar have adapted it to the two way passage of signals through the CITS probe to provide a better solution to situations where access to the ground plane is not possible. It is especially beneficial in that it permits these measurements to be made with a single coaxial cable to the probe – thus giving the operator a more ergonomic and cost effective solution to this type of measurement.

Correlation between CITS880s and Atlas

The impedance of several fixed resistors and PCB differential coupons was measured using a Polar CITS880s and compared with a Polar Instruments Atlas Insertion Loss Test System with and without a ground connection.

The results are shown below (impedance in Ohms).

Correlation between CITS880s and Atlas

 

Groundless measurements

Measurements with ground

 

CITS880s

Atlas

CITS880s

Atlas

Resistor (68 Ohm)

67.8

68.5

N/A

N/A

Resistor (100.7 Ohm)

99.4

99.7

N/A

N/A

Resistor (150.1 Ohm)

147.1

145

N/A

N/A

Coupon A

103.3

104.5

104.1

104.9

Coupon B

103.4

104.6

104.6

104.7

Coupon C

90.6

91.1

90.6

91.1

Coupon D

84.5

85.9

85.5

85.9

Coupon E

96.4

97.7

97.6

97.8