Basic knowledge and usage precautions of Keysight spectrum analyzer

Spectrum analyzer is an essential instrument for measuring high-frequency signals and is widely used in the RF field. The most basic function of a spectrum analyzer is to discover and measure the frequency and amplitude of a signal and display it in a graphical manner. A spectrum analyzer can measure a variety of characteristic parameters of RF signals, including frequency, frequency selection power, bandwidth, adjacent channel power, modulation waveform, field strength, etc.

basic knowledge

(1) Frequency range

The signal frequency range that can be analyzed when the spectrum is working is the selection index of the spectrum. It must be ensured that the test signal is within the working frequency range of the spectrum;

(2) Input power

The input power of the spectrum is divided into average continuous and pulse input power. Average continuous power refers to the maximum power value of the signal that the instrument can continuously input, and pulse input power refers to the value of the pulse input power that the spectrum can measure (strictly comply with the pulse width and duty cycle parameters required by the manufacturer). The input power is generally expressed in dBm, which is a value that considers the absolute value of power. The calculation formula is: 10lg (power value/1mw). For example: 0dBm=1mW, 20dBm=100mW, 30dBm=1000mW=1W;

(3) Input impedance

The terminal impedance presented by the analyzer to the signal source. The rated impedance of RF and microwave analyzers is usually 50Ω. For some systems (such as cable TV), the standard impedance is 75Ω. Impedance mismatch will cause large measurement errors and even interfere with circuit operation.

(4) Average noise level (DANL)

The average noise level is equivalent to the size of the spectrum's own noise. The choice is related to the minimum signal amplitude measured by the engineer. Ideally, the smaller the DANL, the better. However, as the DANL becomes smaller, the more complex the technology required, the more expensive it becomes. Measuring is like sailing, where we can only see the reef when the sea level is lower than the reef.

(5) Preamplifier

Adding a small signal amplification module in the spectrum can improve the sensitivity of the system (preamplifier and Agilent spectrum analyzer), which is mainly used to measure small signals;

(7) Tracking source

Add a frequency sweep signal source synchronized with the spectrum in the spectrum. After adding the tracking source, scalar network parameter measurements can be performed. For example, the frequency characteristic curve of the unit under test (such as amplifier circuit, filter circuit) can be tested, and the reflection coefficient, return loss, and standing wave can also be measured with the standing wave ratio test kit.

Notes on using Agilent spectrum analyzer:

First, on the premise of fully understanding the functional indicators/operating specifications of the instrument:

1. Before use, estimate the input signal power/level and confirm that the instrument can be used to measure the signal to be measured; it is strictly forbidden to input signals that exceed the allowable range of the instrument;

2. When the measurement result is significantly different from the expected result, first check the instrument status and the measured signal for any abnormalities to avoid more serious faults;

3. Start with a small input signal and then gradually increase it;

4. To add a large signal, you need to add an attenuator, attenuation probe/cable, etc. to the input port.