Multichannel coherent signal generation

Your task

Testing multi-antenna systems such as phased array or beamforming antennas requires a test system capable of providing multiple signals with constant phase relationships between them. The coherent test signals must have a specific or definable phase difference (delta phase) and definable amplitude. Some of the challenges for such a test system include compactness, phase control capability and simplicity in handling. In particular, phase stability between the channels is of importance.

Many applications in the A & D direction finding and mobile communications beamforming sectors demand that the phase relationships between channels be constant over time with minimal deviations, for example as little as < 1° phase drift. Such high phase stability can only be achieved when using a common synthesizer signal (local oscillator, LO) for all signal generators in a test system.

T & M solution

Rohde & Schwarz signal generators present a compact easy-to-use solution for generating phase-coherent signals. The generators R&S^®Sxx-B90 phase coherence option enables LO coupling of multiple instruments by distributing the synthesizer signal of one instrument to the others. Different generator models can be coupled to optimally fit user requirements in terms of number of phasecoherent channels and RF frequency range.

For example, generating four phase-coherent RF signals up to 20 GHz requires one R&S^®SMW200A two-channel vector signal generator and two sets of R&S^®SGS100A RF source and R&S^®SGU100A upconverter. The whole setup acts as one unit, conveniently controlled via the intuitive R&S^®SMW200A touchscreen. The vector signal generator generates two 20 GHz phase-coherent channels and also provides the I/Q baseband signals and the LO signal for the two external RF outputs (R&S^®SGS100A/ R&S^®SGU100A combinations). The relative phases between all four RF channels are set digitally in the baseband on the R&S^®SMW200A. The four-channel system is therefore easy in handling and capable of controlling the phase relationships in a straightforward way.

High phase stability among all RF signals requires a shared LO signal. The easiest way to distribute the LO signal is by daisy-chaining. Each cascaded instrument amplifies the LO signal to maintain a proper LO signal level. To be less prone to temperature effects, the LO signal frequency is a factor of four lower than the wanted RF frequency, contributing to a better phase stability. The LO frequency is multiplied automatically by the external RF outputs.

Due to the shared LO signal, the phase stability is excellent. For example, at constant ambient temperature the measured phase drift between the channels is < 1° over a minimum of 90 min. at 20 GHz RF frequency(see diagram).

The Rohde & Schwarz test solution is scalable to optimally meet customer needs. The number and type of generators needed depends on the required number of channels and RF frequency.

The small form factor and easy handling of this test solution are unique on the market. The generators are conveniently controlled via one user interface that also allows adjustment of the RF signal characteristics, including phase relationships. Its compact size and excellent phase stability make this multichannel solution ideal for generatingphase-coherent signals.