How does a signal source analyzer work?

     Signal source analyzer is a commonly used instrument for measuring crystal oscillators, PLLs, clock circuits, and phase noise. As a comprehensive measuring instrument, signal source analyzer provides all necessary measurement capabilities, including:
    1) Phase noise
    2) Frequency, phase and power, transient parameters of signal source
    3) Frequency, RF power and DC current
    4) Spectrum monitoring
    5) AM noise measurement

    6) Baseband noise measurement

    Figure 1 is a functional diagram of a signal source analyzer.

Figure 1 Functional diagram of signal source analyzer

     The signal source analyzer uses a coherent receiver method to reduce the local noise of the instrument. The structural schematic is shown in Figure 2. This technology can basically eliminate system noise. The signal source analyzer has two independent signal channels and a built-in reference source. If a higher frequency range is required, the E5053A can be used. The E5053A has local oscillators (LOs) to down-convert the signal to obtain uncorrelated signals. If the two signals are uncorrelated, after they pass through the vector addition circuit, the total noise power from the reference source can be reduced by vector averaging, and at the same time, the noise signal from the device under test (DUT)
will be highlighted. The degree of noise elimination depends on the number of correlations. Performing 100 correlation operations can reduce the noise platform by 10dB, and performing 10,000 correlation operations can reduce the noise platform by 20dB.

Figure 2 Principle structure diagram of signal source analyzer

    The signal source analyzer uses dual channel measurements to fully characterize the characteristics of frequency conversion signal sources. In wideband mode, all behaviors of frequency hopping can be observed. In narrowband mode, detailed information of frequency, phase and power over time can be analyzed. These measurements can be performed simultaneously and displayed with multiple traces. This allows designers to quickly evaluate the dynamic response of synthesizers, LO circuits and transmitters.

Figure 3. By measuring simultaneously in wideband and narrowband, detailed information on frequency, phase, and power over time can be analyzed.

    How to test fast frequency conversion sources is a measurement problem at present. These signal sources can lock the frequency in sub-microseconds and are mainly used in fields such as high-speed wireless data communications and aviation/defense radar. To meet this need, signal source analyzers can provide 8 nanosecond sampling rates, which will provide enhanced sampling resolution and better frequency resolution, thus meeting the requirements for characterizing the characteristics of high-speed frequency conversion sources in the future (the frequency resolution is 7 kHz in the 8 nanosecond sampling rate and 0.2 Hz in the 8 microsecond sampling rate).

    The hardware trigger input port on the analyzer allows the signal source under test to change synchronously with the measurement trigger. The pre-trigger capability it provides can be used to observe various phenomena before and after the event. The video trigger capability helps to quickly check the frequency hopping behavior of the signal source under test.

    In addition to accurate phase noise measurement capability, the signal source analyzer also features an AM noise measurement mode and a baseband measurement mode to effectively investigate potential noise sources on RF signal sources. AM noise can be measured at the RF input port using the same connections as phase noise measurements. Low frequency noise (1 Hz to 100 MHz) is measured at the BNC baseband input port through a DC-coupled 50 ohm impedance.

    The frequency, power, and DC power measurement capabilities of the signal source analyzer are tuned to characterize oscillators (from fixed oscillators to voltage-controlled oscillators). Frequency, RF power, and DC current (at the DC supply voltage port) measurements can be synchronized with voltage sweeps on the DC control voltage or DC supply voltage. Only one test connection is required to get a trace curve of each parameter on the screen. The following parameters can be measured:

    1) Relationship between frequency and DC control (tuning) voltage
    2) Tuning sensitivity (relationship between frequency and change in DC tuning voltage)
    3) Relationship between frequency and DC power supply voltage
    4) Frequency shift (relationship between frequency and change in DC power supply voltage)
    5) Relationship between RF power and DC control (tuning) voltage
    6) Relationship between RF power and DC power supply voltage
    7) DC current (at the DC power supply voltage end)