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


Introduction:

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.

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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.

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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.

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5. Press Normalize-> Normalize on

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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.

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Step 2: Measure

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

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

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

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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;

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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).

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