Spectrum Analyzer basics

Spectrum analyzer is the common tool for any RF engineer. It is used for viewing RF signals in frequency domain. Spectrum analyzer converts time domain signals into frequency domain for display and analysis.

As far as power measurement is concerned, spectrum analyzer displays power levels with accuracy of approx. +/- 0.5 dB. It is not very accurate for frequency measurements,for which other means such as frequency counter can be the best option.

Spectrum Analyzer basics

The block schematic mentions major components in a typical spectrum analyzer. They are RF input attenuator, mixer, IF filter, detector, video filter, local oscillator, sweep generator and LCD display. The IF (Intermediate Frequency) gain amplifier is not mentioned in the figure but it is very much required after the mixer. Figure depicts simple spectrum analyzer, as shown in the figure, mixer produces both sum and difference components of the input frequency signals. From this wanted IF frequency is filtered out using IF BPF. The signals are applied to CRT for displaying X(frequency) and Y(power) data.

Measurement using spectrum analyzer

To perform measurement signal to be measured is connected to RF Input port of the spectrum analyzer and following need to be set appropriately:
•  RF Centre frequency (should be equal to RF input frequency to be measured)
•  Frequency range or Span (should cover wanted RF frequency to be measured)
•  RBW and VBW
•  Sweep time

For example, if two signals are 100 kHz apart, a 100 kHz RBW will easily separate the responses. A wider RBW makes two signals appear as one signal and difficult to distinguish them. VBW is the low pass filter, which helps smooth out the signal. It is also possible to do video averaging on our spectrum analyzer to improve resolution of weak signals in the presence of noise. For example 3KHz filter does not resolve very smaller frequency signals which can be done using 1KHz filter. Sweep time is the time required to sweep LO frequency across the displayed frequency span. Sweep time need to be set appropriately as sweeping the sepctrum analyzer too fast causes a drop in displayed amplitude and a shift in indicated frequency. There is one more setting called Reflevel, which is the power level at the top of the screen.

Sweep time = 1/sqrt(RBW)

The spectrum analyzer is used for frequency measurement,spuriousm harmonics and phase noise measurements. Tektronix has developed realtime spectrum analyzer(model numbers RSA 3300A, RSA 3408A, RSA5000, RSA6000) which is more powerful compare to conventional spectrum analyzer.

Following are important specifications of spectrum analyzer:

• Frequency tuning range - to include all of the frequency components of the signal to be measured.
• Frequency accuracy and stability - to be more stable and accurate than the signal to be measured.
• Sweep width - the band of frequencies over which the unit can sweep without readjustment.
• Resolution bandwidth - narrow enough to resolve different spectral components of the signal.
• Sensitivity and/or noise figure - to observe very small signals or small parts of large signals.
• Sweep rate - maximum sweep rate is established by the settling time of the filter that sets the resolution bandwidth.
• Dynamic range - the difference between the largest and smallest signal the analyzer can measure without readjustment.
• Phase noise - a signal with spectral purity greater than that of the analyzer conversion oscillators cannot be characterized.

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. Read more


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