A spectrum analyzer is an instrument that shows signal strength (amplitude) as it varies by signal frequency. The frequency comes on the horizontal axis, and the amplitude is displayed on the vertical axis.
Applications of Spectrum Analyzer
- Measurement of Harmonic Distortion
- Analysis of Pulse Modulated Signal
- Abnormal Oscillation in an Oscillator etc.
- Occupied Bandwidth by a signal
Important functions used in making measurement
- RBW – Resolution Bandwidth
- VBW – Video Bandwidth
- SPAN – Span of Frequency band to be measured
- MARKER – Used to make Measurements
- CENTRE FREQ. – Used to select Frequency of Interest
- PRESET – Initializes the Spectrum to Default Settings
RBW & VBW
These are nothing but Collection of Band Pass Filters which are adjusted for better resolution and measurement of the signal.
Press “Centre Frequency “Button of Spectrum Analyzer and enter the desired Frequency which can be seen at the middle bottom part of Screen in Spectrum Analyzer.
Press the “SPAN” Button and enter the band (e.g. 200 KHz) in which you want to see the Signal, Span is calculated from leftmost corner of Screen to Rightmost Corner of the Screen of Spectrum
SETTINGS OF RBW AND VBW
- In order to measure Received Signals properly we have to adjust these two parameters for making exact calculations.
- By Convention the Ratio of RBW/VBW is 100:1, and in HECL we prefer keeping RBW as 1 kHz and VBW as 10Hz, if we choose very large SPAN then the time of Sweep becomes very large.
This is the time taken in completing one Trace i.e. from the Leftmost corner of Screen to Rightmost Corner. While making a measurement if you are Getting UNCAL (Not calibrated) then you need to select Auto Option in Sweep Time Menu (it makes Calibrated).
To Measure C/N of any carrier press Marker and select NORMAL marker, put this Marker on the Noise Floor and select DELTA MARKER Option from the same Menu (a Diamond shaped marker or a line) you will see Diamond Marker will get still at that point and now move the “Knob” on the front panel and Normal Marker starts moving giving you the difference between DELTA & NORMAL marker in both Vertical as well as horizontal axis.
EXAMPLE – How to measure C/N of OCC with Spectrum
- In Window 5, OCC is at 80 MHz
- Press “CENTRE FREQ” button and put 80 MHz
- Press “SPAN” button and put 100 kHz
- Press “RBW “Button and put 1 KHz
- Press “VBW” button and put 10 Hz
- Select “SWEEP” and press AUTO option
- Let the Trace gets complete at least once
- Now Select NORMAL Marker from Marker Menu and set it on the Noise Floor and press DELTA Marker Option in the same Menu, Move the Control Knob until you reach the Peak of the OCC i.e. at 80 MHz and read the Difference on the Top Right most Corner of the Screen, this value gives you C/N of OCC
How to Save a Trace in the Spectrum Analyzer
- Press “SAVE” button
- Select “Internal Trace” Option from the Menu
- Select “A” from the Menu and you will be asked for Register number in which you want to save the Trace (Range of Registers is 01 to 52)
- Enter Register Number say #07 and press “ENTER” key
- The trace will be saved in Register No 7