Spectrum Analyzer Basics for RF Engineers

The spectrum analyzer is a common tool for any RF engineer. It’s used for viewing RF signals in the frequency domain. Essentially, it converts time-domain signals into the frequency domain for display and analysis. When it comes to power measurement, a spectrum analyzer displays power levels with an accuracy of approximately +/- 0.5 dB.

It’s worth noting that spectrum analyzers aren’t the most accurate for frequency measurements. For precise frequency measurements, tools like a frequency counter are a better choice.

Spectrum Analyzer basics

The block diagram above highlights the major components found in a typical spectrum analyzer. These include:

• RF input attenuator
• Mixer
• IF filter
• Detector
• Video filter
• Local oscillator
• Sweep generator
• LCD display

While not explicitly shown in the figure, an IF (Intermediate Frequency) gain amplifier is essential after the mixer stage.

The figure depicts a simplified spectrum analyzer. As illustrated, the mixer generates both sum and difference components of the input frequency signals. The desired IF frequency is then filtered out using an IF BPF (Band-Pass Filter). The resulting signals are displayed on a screen (historically a CRT) with frequency on the X-axis and power on the Y-axis.

Measurement Using Spectrum Analyzer

To perform a measurement, connect the signal you want to analyze to the RF Input port of the spectrum analyzer. Then, configure the following settings:

• RF Center Frequency: This should match the RF input frequency you want to measure.
• Frequency Range or Span: This determines the range of frequencies displayed and should cover the frequency you’re interested in.
• RBW (Resolution Bandwidth) and VBW (Video Bandwidth): These are crucial for resolution and signal smoothing.
• Sweep Time: The time it takes to sweep across the displayed frequency span.

For instance, if you have two signals that are 100 kHz apart, a 100 kHz RBW will clearly separate them. A wider RBW might make them appear as a single signal, making it hard to differentiate.

VBW, on the other hand, is a low-pass filter that smooths out the signal. You can also use video averaging to improve the resolution of weak signals in the presence of noise. For example, a 3 kHz filter might not resolve closely spaced frequency signals that can be resolved using a 1 kHz filter.

Sweep time needs to be set correctly. Sweeping too fast can cause a drop in displayed amplitude and a shift in the indicated frequency.

Another important setting is Reflevel, which represents the power level at the top of the screen. The relationship between RBW and Sweep time is:

$Sweep \; time = \frac{1}{\sqrt{RBW}}$

The spectrum analyzer finds uses in frequency measurements, spurious signal detection, harmonics analysis, and phase noise measurements.

Tektronix has developed real-time spectrum analyzers (e.g., RSA 3300A, RSA 3408A, RSA5000, RSA6000) that offer more powerful capabilities compared to conventional spectrum analyzers.

Here are some important specifications of a spectrum analyzer:

• Frequency Tuning Range: Should encompass all frequency components of the signal being measured.
• Frequency Accuracy and Stability: Needs to be more stable and accurate than the signal being measured.
• Sweep Width: The frequency band the unit can sweep without needing readjustment.
• Resolution Bandwidth: Narrow enough to resolve different spectral components of the signal.
• Sensitivity and/or Noise Figure: Allows observation of very small signals or small parts of large signals.
• Sweep Rate: The maximum rate is determined by the settling time of the filter that sets the resolution bandwidth.
• Dynamic Range: The difference between the largest and smallest signals the analyzer can measure without readjustment.
• Phase Noise: The analyzer’s conversion oscillators should have spectral purity greater than the signal being measured.
• RF Measurements tutorial: This rf measurements tutorial covers gain, spurious,harmonics,1dB compression point,noise figure,image rejection,return loss, phase noise,group delay,frequency stability,TOI,AM to PM conversion and more RF device measurements.