Understanding the distinction between vector signal and radio frequency signal sources is pivotal for various component and system testing applications. Both serve as test signal sources but with unique characteristics. Let’s delve into their respective features.
Exploring Vector Signal Sources:
The vector signal generator, emerging in the 1980s, employs intermediate frequency vector modulation combined with radio frequency down-conversion. Its core principle involves using a frequency synthesis unit to generate a continuously variable microwave local oscillator signal and a fixed frequency intermediate frequency signal. The resulting intermediate frequency signal, merged with the baseband signal in the vector modulator, creates a fixed carrier frequency intermediate frequency vector modulation signal. This signal is then mixed with the microwave local oscillator signal to produce a continuously variable radio frequency signal, which retains the baseband information of the IF vector modulation signal. Post-modulation and filtering, the RF signal is routed to the output port.
Vector signal generators share several components with ordinary signal generators, such as the frequency synthesis sub-unit and signal conditioning sub-unit. However, the key differences lie in their vector modulation unit and baseband signal generation unit. A vector modulator typically comprises four functional units: a local oscillator with a 90° phase-shifted power division unit, two mixer units, and a power synthesis unit. To enhance performance, these units usually feature 50Ω internal loads and employ differential signal driving.
The baseband signal generation unit, integral for creating the required digital modulation baseband signal, can also load user-defined waveforms. This unit generally includes a burst processor, data generator, symbol generator, FIR filter, digital resampler, DAC, and a reconstruction filter.
Unveiling RF Signal Sources:
Modern frequency synthesis technology frequently utilizes indirect synthesis methods, linking the main oscillator source frequency to the reference frequency source through a phase-locked loop. Characterized by minimal hardware requirements, high reliability, and a broad frequency range, the essence of an RF signal source is its phase-locked loop. RF signal sources, a broad concept, encompass any source capable of generating RF signals. Most current vector signal sources fall within the RF band and are thus often referred to as vector RF signal sources.
Contrasting the Two Signals:
a. A pure RF signal source is primarily designed for generating analog RF single-frequency signals, usually not intended for modulated signals, particularly digital modulated ones. These sources typically offer a wider frequency band and a more extensive power dynamic range.
b. In contrast, vector signal sources are tailored for generating vector signals, which are the modulated signals prevalent in digital communications. They support various modulation types like ASK, FSK, MSK, PSK, QAM, and comply with standards like 3GPP LTE FDD/TDD, HSPA/HSPA+, GSM/EDGE/EDGE Evolution, TD-SCDMA, WiMAX, etc. Due to their inherent modulators, vector signal sources generally have a limited frequency range (around 6GHz). Key indicators for these sources include built-in baseband signal bandwidth and the number of signal channels.
In summary, while both vector signal and RF signal sources are instrumental in testing applications, their distinct operational principles, modulation capabilities, and frequency ranges set them apart, catering to different needs in the realm of signal generation and testing.