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dB is the most common measurement parameter used for RF amplitude

It is

a power ratio defined as:

dB= 10Log(Pout/Pin)

Notice that dB is a ratio not a specific quantity but let's see how it is used

Since P=(V2)/R

dB=10 Log((Vout2)/Rout)/((Vin2)/Rin)

If we assume that Rout = Rin

dB=10 Log(Vout/Vin)2

dB=20 Log(Vout/Vin)

Many times we shorten these formulae by using a fixed reference.

The two most common are:

dBm, which means dB relative to 1milliwatt = 10 Log Pout/1*10-3

dBuv, which means dB relative to 1 microvolt = 20 Log Vout/1*10-6

Some Examples

If you change the signal voltage by:

1/2 = -6dB.

1/10 = -20dB.

1/100 = -40dB.

2 = +6dB.

10 = +20dB.

100 = +40dB.

Frequency and Wavelength

Frequency is 1/time that it takes for an oscillating signal through a particular

state or voltage and return.

Wavelength is the distance a wave travels from a particular state or voltage until

it reaches the same state or voltage. Wavelength is important for calculating

possible sources of emission problems.

In air, the relationship between wavelength and frequency is:

(Speed of light)/F = (3*108)/F


1MHz has a wavelength of 3*108/1*106 = 300 meters

10 MHz has a wavelength 3*108/10*106 = 30 meters

30MHz has a wavelength of 3*108/30*106 = 10 meters

This last wavelength is an important one. The emissions test for commercial

and industrial products starts at 30MHz and the

most common measurement range is 10 meters.

The FCC and European emissions test is to be a far field test.

If you recall from any school training about antennae, we would like an antenna to be 1/4 or

1/2 wavelength long. I would like for you to have a feel for antenna effectiveness.

One of the most powerful stations in my area is KNX-1070. Calculating

the wavelength of 1070KHz we find.

Frequency = 1070KHz Wavelength = 280.4 meters

Other Interesting Frequencies

Frequency = 100MHz Wavelength = 3 meters

Frequency = 1GHz Wavelength = .3 meters = 11.7 inches


The Mathematics of digital signals is interesting. Most of us don't think about the frequency content of such signals, but it is very important to do so when considering emissions. The voltage of a perfect square wave can be represented mathematicaly as the limit of a sum:

SIN(wt) + 1/3 SIN(3wt) + 1/5 SIN(5wt) + 1/7 SIN(7wt) + …

where "w" is the radian frequency or 2*PI*F

Notice that these are the odd harmonic or odd multiples of the basic frequency out to infinity. The higher the multiple the lower the voltage amplitude.

The reality, however, is that digital signals are not perfect square waves and therefore they start with a DC component which is approximately 1/2 the amplitude of the power supply voltage, but beyond that all of the even harmonic are present also. The best defined or maybe we would call it the prettiest digital signal would be defined by the limit of the sum of the following equation:

= 1/2VDD + SIN(wt) + 1/2SIN(2wt) + 1/3 SIN(3wt) + 1/4 SIN(4wt) + 1/5 SIN(5wt) + …

Due to distortions, rarely do digital signals appear with exactly the proper amplitude, but the frequencies are right on the money. Usually all of the frequencies beyond a certain harmonic can be measured in the emission spectrum until the voltage amplitude is very small and normally the odd harmonics are higher than the even ones. Each harmonic seeks and I can almost say actively seeks an antenna appropriate for that frequency. We often measure frequencies out to the 100th to 200th harmonic.

There are other sites of interest:

EMI Labs sites: (This is not intended to be a complete list of labs but these are one that I contact often)

Southern California

CKC Laboratories:

DNB Engineering:

Garwood Labs:



TUV America/Product Services:

State of Washington

Acme Testing