Harmonics and Spurs from Transmitters

An ideal transmitter would deliver the exact signal that you intended, with no additional unwanted components. Unfortunately, there are no ideal transmitters. In reality, a transmitter can have undesirable signal components like excessive harmonics and spurs. Luckily, there are a few ways that we can identify and minimize them.

Transmitters commonly use amplifiers to boost the signal strength. Unfortunately, most amplifier designs will add and amplify the harmonics of the output signal. Earlier we discussed superposition of sinusoidal waveforms and how harmonics of a sine wave can be built up to create different waveform shapes. A harmonic is simply a waveform with a frequency that is an integer value of the intended signals frequency.

Tech Example: If we look at a sine wave with a fundamental frequency of 10MHz, the second harmonic is a 20MHz sine wave. The second harmonic is two times the fundamental frequency, the third is three times, etc..

Figure 9: Sine wave with a 10MHz Frequency (yellow) and a second sine wave with a 20MHz Frequency (blue)

Here is a screen capture of a 10MHz sinewave from a high quality RF source:

Figure 10: 10MHz sine input into a spectrum analyzer. Note 1^st and 2^nd harmonics.

The 2nd Harmonic has a frequency that is twice the fundamental (20MHz) and the 3rd Harmonic is three times the fundamental (30MHz). Although we programmed the source to output at 10MHz, there are still some additional components to the output sinewave.

Searching for Harmonics

When searching for harmonics, it is important to widen the frequency span on the analyzer so that you can capture them. If the fundamental frequency of your transmitter is 100MHz, it may be wise to look at a span from 100MHz to 500MHz or greater, so that you can capture a larger span of potential harmonics.

Harmonics also tend to be significantly lower in power than the fundamental frequency. Note how the power level drops significantly between the fundamental (-10dB) and the harmonics (-64dBm, -73dBm) above. This can make them difficult to capture using an oscilloscope. Lowering the RBW value and using preamplification (if available) will lower the noise level of the analyzer and help to isolate these low powered signals.

Tech Note: If you experience issues with excessive harmonics, many can be minimized by using filters or alternative transmitter designs.

Spurious Emissions

Spurious emissions (spurs), can also be problematic. A spur is typically the unwanted result of nonlinear components in a circuit or transmission path. Nonlinear components can include:

• Amplifiers
• Mixers
• Diodes

Also, spurs can be be created by oxide layers on the mating surfaces of cables and adapters.

Hunting for spurs is very similar to hunting for harmonics. Configure the spectrum analyzer span to cover a frequency range wide enough to cover the expected location of the spurs and lower the noise floor by using the RBW and preamplifier. Unlike harmonics, which are at frequencies that are even multiples of the fundamental, spurs can be caused by different events so their locations are at less predictable frequencies.

Mixers

Many spurs are products of intentional or unintentional mixing of signals. Investigating areas where there are mixing products from known signals is a good starting spot. Earlier we introduced mixers and mixing products. In the most simple case, a mixer takes two signals as inputs and the resultant output contains the original signals as well as the addition and subtraction of the inputs.

Figure 11: A Simple Mixer

While it may be difficult to eliminate the spurs all together, they can often be minimized by filtering. Other small details like connector torque and aperture cleanliness can also be culprits.