Analog modulation versus vector modulation
An analog signal generator varies one property of the carrier at a time: amplitude for AM, frequency for FM, or phase for PM. That is enough for many tasks, and it keeps the architecture simple. It is not enough for modern digital communications or for realistic radar and electronic warfare signals, where amplitude and phase change together in arbitrary patterns that no single analog parameter can describe.
Vector modulation solves this by controlling amplitude and phase simultaneously and independently. Any point in the complex plane becomes reachable, which means any modulation scheme that can be described mathematically can be generated. This is the difference between approximating a waveform and producing it exactly.
The I/Q representation
Vector modulation works in terms of two components. The in-phase component, I, is the part of the signal aligned with a reference carrier. The quadrature component, Q, is the part shifted ninety degrees from it. Together I and Q define a vector whose length is the instantaneous amplitude and whose angle is the instantaneous phase. Steer that vector over time and you have produced an arbitrary modulated signal.
This representation is powerful because complex schemes reduce to streams of I and Q samples. A 256-QAM constellation, an OFDM symbol, a chirped radar pulse, or a frequency-hopped waveform all become sequences of I/Q values fed to the modulator. The Model 875 carries an internal I/Q modulator with deep waveform memory and very high data rates, so it can play both custom waveforms and standards-based signals such as GSM/EDGE, 3GPP, HSDPA, CDMA2000, and GPS, alongside ASK, FSK, MSK, and PSK schemes.
Wideband modulation and deep memory
Two properties decide how demanding a vector source can be: modulation bandwidth and memory depth. Wide modulation bandwidth lets the source reproduce signals that occupy large instantaneous spans, which matters for wideband radar, modern wireless, and realistic interference. Deep waveform memory lets it play long, non-repeating sequences without looping, which matters when a short repeating pattern would betray the simulation or fail to stress the device under test.
The 875 pairs wideband support with deep memory and adds wideband linear chirp generation, intra-pulse modulation, and pulse shaping. The result is a source that can produce the inside of a pulse, not just its envelope, which is exactly what receiver and EW testing demands.
Pulse descriptor word playback for EW
Electronic warfare lives on dense, dynamic signal environments. A modern scenario may contain many emitters, each with its own frequency, power, pulse width, and timing, all changing as the scenario evolves. Describing that environment pulse by pulse, in waveform samples, would be impractical. The pulse descriptor word, or PDW, is the answer.
A PDW is a compact record of the parameters of a single pulse: pulse width, amplitude, frequency, angle, and time of arrival. A list of PDWs describes an entire emitter, or an entire battlefield, as a stream of instructions rather than raw samples. The Model 875 accepts PDWs uploaded over Ethernet or USB and plays them back in real time, and a fast control port allows PDWs to be streamed live so the scenario can react. This is how a single source reproduces an agile, multi-emitter threat picture for receiver and EW system test.
Model 875 vector capabilities
The table summarizes the vector and modulation capabilities most relevant to communications and EW work. Confirm option availability and exact figures against the current datasheet.
| Capability | Model 875 |
|---|---|
| Frequency range | 10 MHz to 40 GHz (options 875-4 through 875-40; down to 100 kHz with option 100k) |
| Channels | 1 desktop; 2, 3, or 4 phase-coherent in 2U rackmount |
| Modulation types | I/Q vector; pulse; analog AM/FM/PM (option MOD); internal digital schemes; avionics DME/VOR/ILS (option AVIO) |
| Digital standards | GSM/EDGE, 3GPP FDD, HSDPA, CDMA2000, GPS; ASK, FSK, MSK, PSK |
| PDW playback | Upload via Ethernet or USB (option PDW); real-time input via Fast Control Port (option FCP) |
| Switching speed | 200 ns modulation switching; under 2 µs frequency loop with option UFS |
| Output power | -50 to +20 dBm (down to -90 dBm with option PE; -120 dBm with PE2) |
| Phase noise at 1 GHz, 1 kHz | -130 dBc/Hz (close-in improved with option LN) |
Plan a vector source
Vector and PDW configuration is best scoped against the signals and scenarios you intend to generate, since the options interact. Berkeley Nucleonics application engineers can map your communications standards or EW threat list onto a Model 875 configuration. For specifications and quotations, contact info@berkeleynucleonics.com or call 800-234-7858. The full family overview is on the RF & Microwave Signal Generators documentation page.