This topic describes the use of T-parameters (time-domain parameters) and S-parameters (scattering parameters) to identify Time Domain Reflectometry (TDR) and Time Domain Transmission (TDT) waveforms. During measurements, an N1055A or 54754A TDR module sends stimulus steps to the DUT and captures TDR and TDT response waveforms. These traditional TDR/TDT waveforms are time domain parameters waveforms that display the relationship between stimulus and response waveforms (voltage-ratio). This provides an intuitive view of a DUT's impedance profile (reflection and transmission characteristics) versus distance.

If option 202 is installed, time domain parameter waveforms can be transformed to the frequency domain as S-parameters, which are power-ratio measurements that include magnitude and phase. S-parameters describe how a test device modifies a signal. Typical S-parameter measurements include return loss, insertion loss, near-end cross talk (NEXT), and far-end cross talk (FEXT).

Measurement Names

The identification scheme used for S-parameters is also used to identify time domain parameter waveforms. The signal input to a device is referred to as the stimulus and the signal received from the device is referred to as the response. Measurements names begin with the capital letter "T" for time domain parameters and "S" for S-parameters. The letter is followed by numbers that represent the ratio of the signals on the device's input and output ports. For example, the T21 time domain parameter describes an insertion loss measurement made with the stimulus applied on Port 1 and the response on Port 2, as shown in the following figure. The output port's number is listed first followed by the input port's number. In normal usage, the ratio notation 2:1 is shortened to 21. The following picture shows how a T21 / S21 test setup would be represented in FlexDCA's DUT layout panel in the TDR Setup dialog box.

In a single-ended device, one transmission line carries data to a port which is comprised of one terminal. On a single-ended device with one port, only the return-loss (reflection) measurement T11 (and corresponding S11 in the frequency domain) is available. On single-ended devices with two ports, measurements T11, T21, T12, and T22 are available with associated S11, S21, S12, and S22 measurements.

 

Balanced Devices

In a balanced device, two transmission lines carry complementary data to a single port which is comprised of two terminals. The two lines can be driven with either a differential (anti-phase) or common (in-phase) stimulus. The advantage of using a balanced device is that a receiver can reject a signal that is common to both conductors. Also, radiation is reduced due to cancellation of electromagnetic fields.

Balanced devices can also be measured during mixed stimulus use, where the input and output stimulus types are different. For example, the input port could be driven differentially while the common response is measured. The S-parameters measure the impedance between the two lines. Time-domain parameter and S-parameter names for balanced ports include two additional letters: C (common) and D (differential):

• SCCxx (common measurement, where xx represents port numbers)
• SDDxx (differential measurement)
• SDCxx (mixed-stimulus measurement: common input, differential output)
• SCDxx (mixed-stimulus measurement: differential input, common output)

As with the port numbers, the convention is to list the letter (C or D) representing the output port stimulus type first followed by the letter representing the stimulus type of the input port.

2-Port Single-Ended Measurements

The following table lists all four possible combinations of time domain parameter and S-parameter measurements on a two-port device. To find the measurement, locate the intersection of the stimulus (input) port column and the response (output) port row. The return loss measurements are S11 (forward) and S22 (reverse). The insertion loss measurements are S21 (forward) and S12 (reverse).

		Time Domain Parameters			S-Parameters
		Stimulus Ports			Stimulus Ports
		1	2		1	2
Response Ports	1	T11	T12		S11	S12
	2	T21	T22		S21	S22

4-Port Single-Ended Measurements

Sixteen different measurements can be made on four-port devices. In addition to the measurements previously described, available measurements include near-end cross talk (S31) and far-end cross talk (S41).

		Time Domain Parameters					S-Parameters
		Stimulus Ports					Stimulus Ports
		1	2	3	4		1	2	3	4
Response Ports	1	T11	T12	T13	T14		S11	S12	S13	S14
	2	T21	T22	T23	T24		S21	S22	S23	S24
	3	T31	T32	T33	T34		S31	S32	S33	S34
	4	T41	T42	T43	T44		S41	S42	S43	S44

1-Port Balanced Measurements

In addition to balanced reflection measurements (for example SDD11), you can perform single-ended reflection measurements on the individual port terminals (S11 and S22) as well as the S21 near-end cross talk measurement between the positive and negative terminals.

2-Port Balanced Measurements

The following tables describe the standard and mixed stimulus measurements.

Differential

		Time Domain Parameters			S-Parameters
		Stimulus Ports			Stimulus Ports
		1	2		1	2
Response Ports	1	TDD11	TDD12		SDD11	SDD12
	2	TDD21	TDD22		SDD21	SDD22

Common

		Time Domain Parameters			S-Parameters
		Stimulus Ports			Stimulus Ports
		1	2		1	2
Response Ports	1	TCC11	TCC12		SCC11	SCC12
	2	TCC21	TCC22		SCC21	SCC22

Mixed Stimulus. Common Inputs, Differential Outputs

		Time Domain Parameters			S-Parameters
		Stimulus Ports			Stimulus Ports
		1	2		1	2
Response Ports	1	TDC11	TDC12		SDC11	SDC12
	2	TDC21	TDC22		SDC21	SDC22

Mixed Stimulus. Differential Inputs, Common Outputs

		Time Domain Parameters			S-Parameters
		Stimulus Ports			Stimulus Ports
		1	2		1	2
Response Ports	1	TCD11	TCD12		SCD11	SCD12
	2	TCD21	TCD22		SCD21	SCD22