Basics of a Spectrum Analyzer

 

Spectrum analysis basics



What is a frequency range?


Imagine a giant motorway, several kilometers wide, with thousands of lanes. On this motorway, every imaginable kind of vehicle can be found: motorcycles, cars, trucks, etc. To not let them get into each other’s way, every lane is reserved for only a single group of road users: e. g., lane 1 only for cyclist, land 3 only for pedestrians, lane 40 only for trucks etc. Depending on the traffic caused by the individual groups, these lanes also have different widths: For example the land reserved for cyclists is far narrower than that reserved for trucks, etc. High frequency ranges, and the road users are applications (for example, a cell phone, a microwave oven, a radio-controlled car lock, in effect all appliances that somehow work with radio waves).

So, every application has its own frequency range for exclusive use. By assigning a separate frequency range for each application, conflicts between different “Road users” can be avoided, so that e. g. a cell phone cannot be disturbed by a microwave oven.

 

 

Big difference between exposure limits



Back to our motorway: Of course, all road users also have their own specific speed limits. For our example, a pedestrian may only walk at up to 5 km7h. Cars, in change, may speed at up to 250 km/h. Exposure limits for radio applications work similarly: Here, however, the word “speed” is replaced by the transmitting power. E. G., a broadcast station may have an enormous transmitting power of 1 000 000W or more, in contrast, a radio-controlled car lock only a few mW (1mW=0.001W) etc. 3 examples of exposure limits in practice.

 

 

Frequency range (MHz)

Application

Power limit (W EIRP)

1880 – 1900

DECT phones

0.25

2320 – 2450

Amateur radio (11 cm)

750

5725 – 5825

WLan 802.11a

0.025

 

 

It is easily visible that each radio application may only use one exactly defined frequency range. Also, the high differences in admissible transmitting power are noticeable.

 


Application of spectrum analysis

 


There are 2 main reasons for application of spectrum analyses

a.    You would like to know which radio applications are active
b.    You would like to measure the exposure caused by each of the radio applications separately, e. g. for evaluating exceeding of exposure limits.


Regarding a.:
Let’s reconsider our “giant motorway” example:
Remember that every lane was only intended for use by a single kind of vehicle. Now imagine that a huge bridge crossed this motorway, with you standing on the bridge and looking down on the motorway.

Now, for example, you would like to know exactly what is happening on the motorway, and this for every single lane. However, the motorway is incredibly wide, so you would need rather good binoculars to be able to even look a few km far. Let’s just imagine that your binoculars have a range of 6km (6000m). Now you would like to know how much traffic there is on a specific lane and how fast it is travelling. So, you’ll take a piece of paper and write down the number of the lane and the data you evaluated. Starting at lane 1, you see: nothing – OK, let’s go on with lane 2 – nothing either. Now lane 3 – OK, there is some traffic going at 18 km/h. Continuing with lane 4 – Nothing, etc. Until you have reached the last lane. What have you accomplished now? You’ve performed an analysis of the entire range of lanes from 0 – 6 km. Or, in other words: You performed a range analysis. As you know, to analyze something means breaking it down into smaller parts which can be evaluated. In this case, the 6 km wide motorway was that big “something” and the smaller parts were the individual lanes. The word “range” can now be replaced by the word “Spectrum” and there we are. You have performed a spectrum analysis.

If you now in addition have a lanes plan telling you which lane is assigned to which kind of vehicle, you can exactly determine what kinds of vehicles have just been travelling.

Spectrum analysis in high frequency technology works exactly like that: There are „lanes“, here as well. Though, these lanes are called frequency ranges. The width of these frequency ranges is measured in the unit Hz (Hertz). However, as the frequency ranges are mostly found in high Hz ranges. Writing them in plain Hz would require huge numbers. Thus, the unit Hz is often extended to MHz (1.000.000Hz) and GHz (1 000 000 000Hz). Like this, the whole thing becomes much clearer. So, 1 000 000 000Hz can also be written as 1 000MHz or 1GHz.
?

The different kinds of vehicles are called radio applications and have their own abbreviations: e. g. the radio application “UMTS” (the new, digital mobile communications standard) has its own frequency range which spans 1900 to 2200MHz (1.9 – 2.2GHz).

The speed at which the vehicles are travelling can now be replaced by a new expression: the signal strength or level.

So far, we have now explained the used expressions and units. Now, high-frequency analysis works just as our motorway example:

For example, our measurement device should evaluate all frequency ranges from 1MHz to 6 000MHz (in pictures, our 6 000 m wide motorway). Step by step, every frequency range is evaluated precisely. First: 0 to 1MHz, then 1MHz to 2MHz etc. until 6 000MHz. Also, the signal strength of every frequency range is exactly stored. Like this, we also learn what signal strength was present on which frequency range.

 

 

 


Real-world examples



Let’s assume that we want to exactly evaluate the frequency range from 1GHz to 6GHz, and that the following 3 radio applications were active simultaneously with various signal strengths (in practice it will mostly be a lot more different applications.

Frequency range (MHz)        Application            Reading
1.880 – 1.900            DECT portable phone    40
2.320-2.450                Amateur radio (11cm)    20
5.725 – 5.825            WLan 802.11a        80

How can this be visually displayed on a measurement device? Well, first, we will map the frequency range from 1GHz to 6GHz on a line from left to right (X-axis):

 

 

Ok, this was still pretty simple. Now, we tag each of the 3 applications depending on their frequency on the right spot of the X-axis and can thus see where they can be found:

 

 

Well, this was again pretty simple. And finally, we display the strength of each of the 3 readings as vertical bars on the Y-axis.

 

 

Additionally, we have also adapted the width of each vertical bar to the width of the respective frequency range of each radio application (the so-called bandwidth.

DECT only has 20MHz (1.880 – 1.900MHz = 20MHz) of bandwidth, a very small range. Amateur radio, in contrast, already uses a far higher bandwidth (2.320-2.450 = 130MHz) usw.

We can now see all information concerning these three signal sources.

In practice, this will look similar on the SPECTRAN Spectrum Analyzer display.

 

 

In this example, we also have 3 main signal sources (from left to right):

Signal 1:      942 MHz with -63dBm
Signal 2:   2.024 MHz with -23dBm
Signal 3:   5.823 MHz with -42dBm

These are displayed as vertical bars. The same rule as in our example applies here, too: The higher the measured signal strength, the higher the bar. Further information regarding each of the bars is displayed from left to right as markers in the upper display area.

Marker 1 (the first bar from the left) is displayed 942MHz; strength -63dBm.
Marker 2 (the second bar) is displayed 2024MHz; strength -23dBm.
Marker 3 (rightmost bar) is displayed 5823MHz; strength -42dBm.



The configured frequency range is constantly being scanned. Thus, the display will also constantly change. This is recognizable because of a small dot above the graphics display which moves from left to right. This procedure of continuous scanning is called “sweeping”.



So, what kinds of information have we acquired now?

a)    In the entire frequency range from 0 – 6GHz, there are 3 main signal sources.

b)    The frequency and signal strength of all 3 sources is exactly known.

So, we have acquired a quick overview of what is active in this frequency range.

As the exact frequency of the signal sources is now know, it is easy to determine the exact applications emitting these signals (see also our frequency tables on chapter 12.2. or the more extensive frequency tables on the Aaronia website www.spectran.com).

On the basis of these frequency tables, we can determine for e. g. 942MHz:

937.6 to 944,8MHz = GMS900 (DL) T-Mobile

Meaning that this is a GSM900 cell tower (DL-Download) of the provider
“T-Mobile”.