So you have read the specs of your brand new Spectrum Analyser on the datasheet but how can you test it for yourself?


A filter is a universal, two-port, frequency-selective device used in a wide variety of electronic circuits and applications. A low-pass filter is one which passes
signals with a frequency lower than a certain cut-off frequency while attenuating the higher frequencies. Low-pass filters are often used in radio transmitters,
audio devices, and communication systems. They can be viewed as a two-port network when using a spectrum analyser to characterize. The following steps are
used to measure the low-pass filter characteristics using the GPS-SA3000XX spectrum analyser. The GPS-SA3000XX will require the Tracking Generator (Option).
In this example, the GPS-SA3000XX spectrum analyser will be used to characterize a 450 MHz low-pass filter.

Step 1: Normalize

In order to ensure accurate measurement results of the amplitude-frequency characteristics of the filter and avoid being affected by the frequency-related
responses associated with the connecting cables, the instrument and cables first need to be normalized.

1. Connect the instrument and cables as shown below.



2. Set the start frequency, stop frequency, resolution bandwidth:
a) Press Frequency-> Start Freq-> 0 Hz;
b) Press Frequency-> Stop Freq-> 1 GHz;
c) Press BW-> 1 MHz
These values can be entered from the keyboard or the universal control knob.




4. Enter an appropriate tracking generator output power. For example, -10 dBm.
Press TG level-> -10 -> dBm
Excessive signal power may damage the device under test so the output power should not exceed the maximum power rating of the DUT.


5. Press Normalize-> Normalize on


6. Press Norm Ref Pos -> 90%. This will allow for a convenient location on the spectrum analyser screen to observe the filter’s frequency response curve.


Step 2: Measure

1. Connect the filter (device under test) as shown below:


The frequency response curve of the low-pass filter is now displayed on the screen without pressing any additional keys.


2. Enable the standard preamplifier to lower the DANL (displayed average noise floor). Notice the difference.


3. Measuring the 3 dB Bandwidth of the filter:
a) Press Marker -> 0 dB;
b) Press Marker Fn-> N dB BW -> -3dB;
The 3-dB bandwidth of the filter is now automatically calculated and displayed: 444.000032 MHz;


4. Measure the out-of-band degree of attenuation at different bandwidths.
a) Press Marker-> 444.000032 MHz;
b) Delta-> 400 MHz;
Notice that the filter’s attenuation level is displayed at a frequency 400 MHz higher than the cut-off frequency of 444.000032 MHz (-48.42 dB).