Simulating and testing multi-antenna systems such as phased arrays or beamforming antennas requires a test system capable of providing multiple signals with deterministic frequency and amplitude with stable, user-adjustable phase relationship among those signals. AnaPico’s APMS multi-channel signal generators provide a well-designed solution for these applications, packing unique phase coherent signal features in a compact design. The APMS40G-ULNPHS is a compact, four channel, 40 GHz signal generator that fulfills the demanding requirements for many new test applications cost-effectively.
When talking about signals and phase coherence, various terms are sometimes used interchangeably, although each term has a very specific meaning. Here are the important definitions used in this article.
Phase continuity and discontinuity
A signal is phase continuous if, after switching frequency, the phase of the signal is the same as before the switch occurred. If the phase changes after switching, the signal is phase discontinuous.
Phase coherence between two channels
If the phase relationship between two signals remains constant, the signals are considered to be phase coherent.
Phase coherent switching
Phase coherent switching defines the state of the signals’ phase once frequency switching is complete. Two signals at frequency f and with relative phase ϕ are said to be phase coherently switched if the relative phase is again ϕ whenever they go back to frequency f.
A signal has phase memory if, when switched from frequency f1 to frequency f2 and back to frequency f1, the signal’s phase is the same as if it had run continuously at f1.
Phase matched outputs
A multichannel signal generator has phase matched signals if the outputs have 0 degree relative phase at all output frequencies.
AnaPico’s APMS-ULN multi-channel synthesised signal generators are now available with frequency coverage to 40 GHz and with one to four channels in a compact 1U 19 in. rackmount enclosure. They provide tight stability, phase coherence and extremely fast tuning speeds, and each channel’s frequency, phase, amplitude and modulation can be independently programmed. Other features include a compact design, excellent phase noise, high output power, accurately leveled output and simplicity of control.
The exceptionally low phase noise and high correlation of the independent channels yield outstanding phase coherence, both short- and long-term. The high stability synchronization circuit shared among all the channels of a single unit, with proprietary techniques for exact frequency synthesis, ensures little systematic phase drift between channels, even after hours or days of uninterrupted use. Some applications require more than four independent outputs maintaining phase stability over long time periods. The APMS-ULN offers a dedicated clock synchronisation mode, using two ports on the rear panel to maintain phase coherence among a cascaded group of APMS-ULN sources. In this way, the unique features of the APMS can be scaled to virtually any number of channels.
To demonstrate the phase stability over time, Figure 1 shows the measured phase difference between two 5 GHz output signals over 10 hours. The excellent phase stability between two individual channels of the APMS is shown by the blue trace. Similarly, the excellent stability when synchronising two separate units is shown by the green trace. For comparison, phase locking two independent signal generators using an external, 100 MHz reference results in significant phase drift— several hundred milliradians—shown by the red trace. Synchronising with a common 10 MHz reference yields even worse performance.
In addition to the excellent channel-to-channel phase stability, the APMS supports both phase coherent switching and phase memory (see Figure 2).
Its channels can be synchronised to maintain a defined phase relationship at all times at any set frequency. As an example of phase coherent switching, consider two channels set to the same frequency f1, with a phase offset of ϕ degrees. After switching both channels to any other frequency and then back to the initial frequency f1, they will have the same phase offset ϕ. The APMS can also be programmed to phase match the outputs (ϕ = 0 degrees). Programming one channel does not affect the signal from the other channels; only the channel being programmed has a phase discontinuity. With phase memory, whenever a channel hops frequency, then goes back to a previous frequency, it behaves as if it had always been running at the first frequency. All these features can be extended beyond four channels by cascading and synchronising multiple APMS units. Table 1 summaries the key specifications of the APMS40G.
AnaPico’s APMS multi-channel signal generators support the requirements of a wide range of applications, such as testing phased arrays, beam forming antennas, satellite payloads and the implementation of quantum computing. With a unique design, the signal generators provide outstanding channel-to-channel phase coherence and are scalable to virtually any number of channels. The PHS option adds phase coherent switching, phase memory and phase matching features.