How to Use a Spectrum Analyzer
Name ______
Date ______
Stuff Needed To Do This Lab
The instructor supplies this:
One or more spectrum analyzers that will read signals from 80 MHz to 3 GHz.
An 802.11b/g wireless access point.
Introduction to Spectrum Analysis
Sometimes we need to see the electromagnetic waves used by wireless systems to send data. Similar to an oscilloscope that is used to examine an electrical signal as it varies over time, a spectrum analyzer is used to capture and display the waveforms of radio frequency signals. This is a graphical representation of the signal's amplitude or signal energy as a function of frequency, not time. The spectrum analyzer is to the frequency domain as the oscilloscope is to the time domain. With an oscilloscope when measuring in the time domain, all frequency components of the signal are summed together and displayed. In the frequency domain, complex signals that are made up of more than one frequency are separated into their frequency components, and the signal level at each frequency is displayed. The spectrum analyzer is a passive device. It does not do anything to the signal. It just displays it. A spectrum analyzer looks like this.
The three things most commonly measured with a spectrum analyzer are:
Modulation
The signal modulation shows the transmitted waveform.
Distortion
Distortion is seen as something altering the expected waveform.
Noise
Noise is everything the spectrum analyzer or radio cannot decipher. What we see as noise may be someone else’s useful signal.
No spectrum analyzer is easy to use until you understand what they measure and the usage of the four basic controls. Once you do, they are really quite easy. These controls are:
Frequency Selection
The frequency selector control is used to find the center of the range of frequencies of interest.
Span Control
The span control then adjusts this range to set the area to examine. In other words, it adjusts the size of the window.
Attenuation or Reference Level
This control sets the sensitivity level for the on-screen display. So that the entire signal will be displayed on the screen, the signal may have to be reduced or attenuated. This is often called setting the reference level
Peak Hold
For signals that move around, such as FHSS and DSSS, a control is required to capture each small part of the total signal and hold that part while the rest of the signal appears over time.
Let’s look at some common signal patterns as seen on a spectrum analyzer.
First we will look at some FM radio and TV station signals. After those we will look at the types of signals more commonly seen in wireless data networks. These are frequency hopping and direct sequence spread spectrum signals.
The most basic type of signal is one that operates with high peak power at a single frequency. FM radio is an example of this. The following are some FM radio signals that range from 88 MHz to 108 MHz.
For these measurements the center frequency was set to 98 MHz. The span was set to 2 MHz. The reference level was set to -30 dBm.
Here is a trace of the FM radio band taken in Fort Worth, Texas.
This view is similar to a group photograph. By zooming out you can see everyone in the group. Like this.
By reducing the span to 200 KHz the individual FM stations begin to show up more clearly. In this example the center is 96.3 MHz. Off to the right a station shows up at 96.7 at lower power and local high power station at 97.1. To the left is a low power station at 95.9 MHz.
By reducing the span it is similar to zooming in to just show part of the group. Like this:
Reducing the span to 50 KHz only one station at 96.3 MHz shows up.
Next let’s look at some TV station signals.
This is analog TV channel 4 with channel limits of 66 to 72 MHz and a visual carrier at 67.25 MHz.
This is analog TV channel 5 with channel limits of 76 to 82 MHz and a visual carrier at 77.25 MHz.
This is analog TV channel 8 with channel limits of 180 to 186 MHz and a visual carrier at 181.25 MHz.
This is analog TV channel 11 with channel limits of 198 to 204 MHz and a visual carrier at 199.25 MHz.
This is analog TV channel 13 with channel limits of 210 to 216 MHz and a visual carrier at 211.25 MHz.
This is analog TV channel 21 with channel limits of 512 to 518 MHz and a visual carrier at 513.25 MHz.
This is analog TV channel 4 with channel limits of 584 to 554 MHz and a visual carrier at 549.25 MHz.
This is analog TV channel 33 with channel limits of 584 to 590 MHz and a visual carrier at 585.25 MHz.
This is analog TV channel 39 with channel limits of 620 to 626 MHz and a visual carrier at 621.25 MHz.
In data networks a range rather than a single frequency is used. When FHSS – Frequency Hopping Spread Spectrum is used the signal moves from single frequency to single frequency in a pattern. The use of each frequency is at high peak power, but only for a short time. Then another frequency is used.
Here is an example of such a signal with the peak hold on for 20 seconds with a tight span setting.
With the peak hold on for 30 seconds.
With peak hold on for 60 seconds.
With peak hold on for 180 seconds during low activity.
With peak hold off.
With peak hold off showing a single frequency.
Unlike the other signals just examined the DSSS – Direct Sequence Spread Spectrum is characterized by a low level signal that is spread over a wide range. As such it looks very much like transient noise. To actually see this type of signal on the spectrum analyzer the peak hold function of the spectrum analyzer must be activated. This function keeps each signal on the screen as long as the peak hold function is activated. By doing this a pattern of the signal begins to appear.
To see the entire ISM 900 MHz span from 902 to 928 MHz set the center frequency to 915 MHz. Set the span control to produce a span of 3 MHz.
To use a spectrum analyzer in the 2.4GHz band for 802.11b signals set the center frequency to 2437 MHz and a 5 MHz per division span. The spectrum analyzer display will be centered on 802.11b channel 6. Each of the five vertical grid lines on either side of the center one will represent that respective channels from channel 1 on the far left to channel 11 on the far right.
To see the entire UNII 5.3 GHz span from 5.250 to 5.350 GHz set the center frequency to 5300 GHz. Set the span control to produce a span of 10 MHz. To see the entire UNII 5.8 GHz span from 5.725 to 5.825 GHz set the center frequency to 5775 GHz. Set the span control to produce a span of 10 MHz. To see the entire UNII 5.3 GHz span from 5.250 to 5.350 GHz set the center frequency to 5300 GHz. Set the span control to produce a span of 10 MHz.
Let’s look at some of these signals. First, with peak hold off.
With peal hold still off, but a transient signal captured.
With peak hold on capturing a single channel
With peak hold on capturing two channels next to each other.
Interference exhibits a strong peak. In this example the oft mentioned microwave oven interference is captured. First one brief view without the peak hold turned on. The next one is a capture over time with the peak hold on.
Let’s look at a practical application of the DSSS type of signal we have just been examining. In the screenshots that follow the middle vertical line is on 2446 MHz. Going to the left from the middle marker each vertical line is -20 MHz from the one to its right. So the first vertical line left of the middle vertical line is on 2426 MHz. Like so:
Right 5 - 2546
Right 4 - 2526
Right 3 - 2506
Right 2 - 2486
Right 1 - 2466
Middle - 2446
Left 1 – 2426
Left 2 - 2406
Left 3 - 2386
Left 4 - 2366
Left 5 - 2346
Sample Trace
Here is a sample trace.
Each of the traces below show the various 802.11b channels by zooming in on just that channels portion of the signal, not the entire display as seen just above.
For example for Channel 1 with a center frequency of 2412 MHz we would expect a peak there with a fall off of 11 MHz on each side.
802.11b Channel 1
802.11b Channel 2
802.11b Channel 3
802.11b Channel 4
802.11b Channel 5
802.11b Channel 6
802.11b Channel 7
802.11b Channel 8
802.11b Channel 9
802.11b Channel 10
802.11b Channel 11
It is very common for two devices to be on the same channel, usually channel 6, as this is the default channel for most manufacturers. As you can see, this is not readily discernible from the spectrum analyzer trace.
802.11b Two Devices on Channel 6
It is possible to use multiple channels in the same physical space if they are far enough separated. Shown next are common channel sets used in the same space to avoid interfering with each other.
Channel 1 and 6
Channel 1 and 11
Channel 1 – 6 – 11
The spectrum analyzer just shown, the Avcom 1727B, cost about $4,000.
A lower cost way is to use the Wi-Spy from MetaGeek. This is a USB device and a program to read it. This device does not have any of the common spectrum analyzer controls. It operates on peak hold by default.
What is the difference between the standard spectrum analyzer and the $400 USB device? In general it is a lack of control over the displayed signal, such as no peak hold or span control. Also the accuracy of the displayed dBm levels is less. But these lower cost devices may be sufficient for basic work such as a site survey for a wireless LAN.
To select a spectrum analyzer you must consider what frequency range needs to be displayed. This is both the basic frequency of the system being examined and any harmonics that need to be seen. The need to examine harmonics can call for a spectrum analyzer that has a range up to ten times the base frequency being examined. In addition consider what is the amplitude range required. This is the maximum input and the sensitivity. Sensitivity is the ability to measure small signals. To what level can the difference between two signals, both in amplitude or dynamic range and frequency or resolution be resolved. This is important so that two signals that are near each other can be distinguished. How accurate are the measurements.