We are starting a new series of posts on the blog under the title – RF Performance Parameters. The purpose of this series is to:
- Introduce the deci-Bel, the unit relevant to RF measurements.
- Describe the major RF performance parameters that are used to benchmark the performance of both the Transmitter and Receiver sections of a radio.
- Compare the various performance levels of a Portable radio, a Mobile radio and a Base Station in terms of these parameters.
- Examine how these performance parameters manifest themselves within a radio system, ie: a network containing Base Stations, Mobiles and Portables.
After reading all the posts in this series, you should be able to:
- Read the RF Specification sheet for a radio and understand what all the numbers mean.
- Understand what levels of performance represent good, moderate or poor performance for Portables, Mobiles and Base Stations.
- Compare the RF Specification sheets for a Tait radio and similar products from our competitors and understand the relative advantages / disadvantages of each.
- Understand, from the RF performance specifications, the relative advantages / disadvantages of a Tait radio system (ie: a network containing Base Stations, Mobiles and Portables) compared to a radio system offered by one of our competitors.
We will start by looking at the deci-Bel, the unit most commonly used to specify the RF performance parameters of a radio. Then, we will describe at a very basic level, the operation of a radio using a simple block diagram. A quick comparison of the three major types of radios designed and manufactured by Tait – Base Stations, Mobiles and Portables – will then be given, outlining the uses and limitations of each. Then we will be at a point to start covering the major RF performance parameters relevant to Transmitters and Receivers we will describe what these mean and give typical performance levels for Base Stations, Mobiles and Portables. For the sake of simplicity, we will consider only analog FM in this series.
Once this is all understood, we will finally examine how these performance parameters manifest themselves within a radio system, i.e. a network containing Base Stations, Mobiles and Portables.
First, lets look at exponents:
If we write 102, we mean 10 * 10 = 100
If we write 103, we mean 10 * 10 * 10 = 1000
So, the exponent (2 or 3 in the example above) tells us how many times to multiply the base (10 in the example) by itself.
In these examples 2 is the logarithm of 100, i.e. log10 (100) = 2 and 3 is the logarithm of 1000, i.e. log10 (1000) = 3
We can also have negative logarithms. If we write 10-2, we mean 0.01, or 1/100
In other words 10-n = 1 / 10n
The graph below shows the relationship between a number (X) and its logarithm (log X).
The basic fact to remember is when the exponent increases by 1, this represents a factor of 10 increase and when the exponent decreases by 1 it represents a factor of 10 decrease.
An example of a logarithmic scale is the Richter scale used for measuring earthquakes. Basically, an earthquake with a magnitude of 7 is ten times stronger than an earthquake with a magnitude of 6.
Now we have understood logarithms, we can move on to look at the main unit used to quantify RF Performance Parameters, the deci-Bel (dB).
The deci-Bel is a relative unit used to represent the relationship between two quantities, basically:
dB = 10 log10 (P1/P2)
In RF measurements, it is used to represent the relationship between two power levels, i.e.
dB = 10 log10 (P1/P2)
In the simplest case, P1 could be the output power of an RF Amplifier, and P2 could be the input power.
Some useful relationships to remember are:
- If 10 log10 (P1/P2) = 10dB it means P1 is ten times greater than P2
- If 10 log10 (P1/P2) = -10dB it means P1 is ten times less than P2
- If 10 log10 (P1/P2)= 3dB it means P1 is double that of P2
- If 10 log10 (P1/P2) = -3dB it means P1 is half that of P2
The Unit of RF Level (dBm)
A common form of specifying an RF power level is dBm. All this means is the ratio of a given RF power level relative to 1 milliWatt (mW), i.e.
Power (dBm) = 10 log10 (P1/1mW)
1µW = -30dBm
10µW = -20dBm
100µW = -10dBm
1mW = 0dBm
10mW = +10dBm
100mW = +20dBm
1W = +30dBm
We will continue this series next week.