75ohm同轴电缆测试

Belden 75 Ohm Coaxial Testing

Carl W. Dole, Product Engineer

October 23, 2003

Purpose

This short paper describes the common practices used at the Belden Engineering Center to measure Belden’s HDTV 3 GHz coaxial cables for return loss, attenuation, TDR impedance, capacitance, inductance, and velocity of propagation. Return Loss

Equipment: Hewlett-Packard 8714ET network analyzer with options 100 (SRL & Fault Location) and 1EC (75 Ohms) Hewlett-Packard 85036B calibration kit (type N, 75 ohm)

Equipment Setup and Calibration:

1. Set the network analyzer settings as follows:

mode: SRL

Measurement

Start frequency: 5 MHz

GHz

3

frequency:

Stop

points: 1601

Number

of

seconds

20

time:

Sweep

Hz

System

bandwidth: 1200

N(f)

TYPE

kit:

Calibration

2. Perform an OPEN, SHORT, and LOAD calibration of the analyzer at the type N female REFLECTION port (RF OUT)

of the network analyzer.

Sample preparation:

1. Use a sample length of 100 feet or longer.

2. Install type N male connectors on both ends of the sample.

Note: Cable preparation and connector assembly is CRITICAL to the evaluation and performance of the cable assembly. Poorly installed connectors may result in misleading data and indicate the entire cable assembly is

defective when, in fact, it may be a cable/connector interface issue. Consult connector manufacturer’s

termination instructions for details. Turning on the connector model function is used to help eliminate the

connector effect and measure only the cable in the assembly. The difference between the connector model on

and off measurement values is due to the connector and/or assembly termination quality.

Sample test:

1. Connect one end of the prepared sample to the REFLECTION port of the network analyzer. Terminate the other end of

the sample with the type N female TRANSMISSION port (RF IN) of the network analyzer. Alternatively, the cable can be terminated with a high quality (i.e. high return loss) 75-ohm termination.

2. Initiate the analyzer sweep if it is not already sweeping.

3. Perform the CONNECTOR MODEL function in the analyzer. Then, change the reference impedance to 75 ohms. This

is essential so that the measurement will be return loss (RL) and not structural return loss (SRL), even though the display will still say SRL.

4. Record the data.

Attenuation

Equipment Setup and Calibration:

1. Set the network analyzer settings as follows:

mode: TRANSMISSION

Measurement

Start Frequency: 5 MHz

3

GHz

frequency:

Stop

points: 1601

of

Number

seconds

20

Sweep

time:

1

kHz

bandwidth:

IF

Calibration kit: TYPE N(f)

2. Create a through connection from the REFLECTION port to the TRANSMISSION port using a 75-ohm extension

cable. This cable will remain connected for all calibration and testing of attenuation.

3. Perform a through calibration of the analyzer.

Sample test:

1. Connect one end of the prepared sample to the REFLECTION port of the network analyzer. Connect the other end to

the extension cable on the TRANSMISSION port using a female-female 75-ohm type N adapter.

2. Initiate the analyzer sweep if it is not already sweeping.

3. Record the data.

4. Convert the data to decibels per 100 feet.

TDR Impedance

Equipment: Tektronix 11801C digital sampling oscilloscope with SD-24 TDR/sampling head 75-ohm air line Equipment Setup:

1. Connect the air line to one channel of the TDR/sampling head using a piece of 75-ohm coaxial cable.

Sample test:

1. Connect the sample to the open end of the air line. The sample can be the same as used for VSWR and attenuation.

2. Measure the TDR impedance.