NOISE AND DISTORTION MEASUREMENTS FOR A SYSTEM.
MEASURING CNR, CSO OR CTB WITH A SPECTRUM ANALYSER
EQUIPMENT NEEDED
Spectrum Analyser
Tuneable or fixed channel bandpass filter
Low noise post amplifier
Two way radios
PERSONNEL NEEDED
This procedure requires two technicians. One technician takes measurements with the spectrum analyser at the test site. The other technician works at the headend turning power to the channel being tested off and on as directed. The two technicians should communicate via two way radio or telephone.
SPECTRUM ANALYSER ADJUSTMENT
Set your spectrum analyser in accordance with the manufacturers recommendations for Carrier to noise, Composite second order and Composite triple beat.
The technician at the test site should follow these steps.
1.To measure Carrier to noise, Composite second order or Composite triple beat, a spectrum analyser needs an input signal of at least 30dBmV.
At the headend, the broadband signal appears at the level well above 30dBmV
2.Introduce
the broadband signal into the bandpass filter. This filter, whether fixed
or tuneable, must pass the 7MHz channel you have chosen for testing.
3.Introduce
the bandpass filter’s output into the spectrum analyser
input.
4.Tune the spectrum analyser to the visual carrier
of the of the channel being tested. Position the visual
carriers
peak on the centre vertical
graticule line of the display.
5.Tune the bandpass filter to ‘peak’ the
visual carrier, so it attenuates the visual carrier as little as possible.
6.Adjust
the analyser’s built in attenuator and reference level
control to place the visual carriers peak on the top horizontal
graticule line. This will be the reference line for the
measurement.
7.Tell the technician at the headend to turn
off the power for the channel being tested.
8.Read the
relative level of undesired signal component. FOR CARRIER TO NOISE MEASUREMENT
Count down in dB from the top graticule line to the noise floor. Read the noise floor approximately 2MHz away (to the right) from the centre graticule line.
FOR COMPOSITE SECOND ORDER MEASUREMENT
Count down in dB from the top graticule line to read the level of composite second order distortion at these four frequencies: 250kHz to the right and to the left of the centre vertical graticule line and 1.25MHz to the right and to the left of the centre vertical graticule line. Note the worst of these four readings if they differ.
NOTE : This is dependant on channel allocation present when testing.
FOR COMPOSITE TRIPLE BEAT MEASUREMENT
Count down in dB from the top graticule line to read the level of composite triple beat. Take this reading on the centre vertical graticule line.
9.Subtract the spectrum analyser’s correction factor from the reading taken in step 8. The result is the ratio in dB of desired signal to the particular undesired signal component (for example, Carrier to noise, Carrier to composite second order or Carrier to composite triple beat)
10. Tell the technician at the headend to turn the power for the test channel back on. |
