AnaPico APLC high-frequency analog signal generators: how to solve the trade-off between low phase noise, high switching speed and high signal purity?

Figure 1 shows a simplified block diagram of the device. In order to meet the demanding specifications for frequency switching speed while maintaining low phase noise, a voltage controlled oscillator (VCO) and direct digital synthesis (DDS) were chosen as the basis for frequency synthesis. The primary reference signal for the entire system is a high frequency clock directly derived from an internal low noise 100 MHz temperature stabilized crystal oscillator (OXCO).

To effectively suppress the interfering harmonic components, we used a technique called variable reference synthesis. The APLC phase-locked loop (PLL) system offers two degrees of freedom to adjust either the DDS synthesizer frequency or the REF1 reference frequency in small steps. By using both methods along with a well-designed frequency plan, we can minimize unwanted components.

The "DDS Upconversion Expansion" block consists of various multipliers and dividers that adjust the REF1 and REF2 frequencies as needed. The output blocks, which consist of multipliers, sub-divisors, gain stages, and a tunable very low frequency (VLF) filter, represent a classical approach to keeping harmonic levels low over a wide frequency range while maintaining high signal performance.

As a critical parameter, phase noise often determines how well a signal generator is suited for a given application. Ensuring good phase noise over all frequency ranges while maintaining switching speed has been a major goal of our APLC series.

In addition to the standard 100 MHz reference, the APLC can optionally have a more stable low drift OCXO-based 10 MHz reference (LN and LN+ options). The 100 MHz oscillator reference significantly reduces the phase noise level (0 to 100 Hz). The annual aging rate improves to a level of 20 ppb.

The excellent phase noise characteristics of the 100 MHz reference, combined with innovative frequency synthesis circuitry, provide remarkably low phase noise levels in the mid-offset region.

In the high offset range, no YIG-like filtering mechanism has been intentionally used, as is the case with other AnaPico signal sources, allowing for fast signal source switching.

Figure 2 shows the phase noise performance of the APLC signal generators equipped with the optional LN module in single sideband (SSB). Excellent phase noise performance is achieved over the entire offset range and over a range of signal frequencies up to 54 GHz. As can be seen from the measured results, at a carrier frequency of 10 GHz, the APLC achieved values of -90 and -130 dBc/Hz at offset frequencies of 10 Hz and 20 kHz.

Another fundamental aspect related to signal quality is signal purity, characterized by harmonics, subharmonics and interference levels. Low levels of all types of unwanted signals are necessary to minimize signal distortion, to produce fewer and lower level interfering signals, and thus to improve the accuracy and sensitivity of the overall system.

Through a combination of circuit design approach, filtering and frequency plan optimization, APLC signal sources feature superior performance for all levels of harmonic and non-harmonic signals. For example, at 10 GHz, harmonic and inharmonic signal suppression reaches -50 and -85 dBc, respectively.

In the microwave frequency band, achieving high output power is crucial. In order to address the losses occurring at higher frequencies, we decided to install multiple output amplifiers covering the entire frequency range from kHz to 54 GHz. With this architecture, the output power reaches approximately 22 dBm in the 30 to 40 GHz frequency range, and still exceeds 15 dBm in the 40 to 54 GHz range. This enhancement provides sufficient power levels over a wide range of frequencies with good harmonic suppression to meet the demanding requirements of microwave applications.

In applications such as radar testing, the quality of continuous wave (CW) pulsed signals is paramount for performance verification. The signal generator must be capable of generating pulses with narrow widths and fast rise and fall times at high repetition rates.

For pulse modulation, attenuators and switches are commonly used to achieve fast transition times. To achieve a high ON/OFF ratio while maintaining fast switching between states, multi-stage switches are used in the APLC series. We achieved an on/off ratio of 100 dB and a pulse width of 10 ns, with a typical rise/fall time of approximately 2 to 3 ns. Figure 4 shows two such pulses with a width of 30 ns and at frequencies of 10 and 50 GHz, respectively.

With its sophisticated design, also optimized for frequency changes, the APLC achieves best-in-class switching speeds (analog signal generators) of typically 15 μs. This switching speed is particularly important in automated test equipment (ATE) systems, where faster switching can significantly speed up the verification process. The APLC series comes as the first model that overcomes the trade-off between phase noise level, interference and switching speed, offering unmatched performance in both aspects simultaneously.

Another important application for fast frequency switching generators is the generation of rapidly changing radar pulses or fast frequency jumps. In addition, APLC multichannel signal generators with custom phase coherence and optional phase memory features can be cost-effectively used to generate multichannel signals for antenna arrays used for radar testing and beamforming.

APLC-X multi-channel signal generators feature strong phase coherence and unique phase coherent switching. While the basis of phase coherence is the use of a common stable and accurate frequency reference for digital signal synthesis across all channels, the careful thermo-mechanical design ensures very similar conditions for all channels and results in similar channel drift behavior, further enhancing the level of phase coherence. In addition, thanks to its proprietary high-frequency synchronization mechanism between multiple devices, AnaPico extends the phase coherence properties to multi-channel systems.

Figure 5 shows the measured phase coherence of the APLC-X. With two APLC-X channels set to the same 38 GHz frequency, the relative RMS phase difference deviation can be measured for 10 hours. When measuring the same on two channels from two different and synchronized APLC-X devices, very similar phase coherence behavior can be observed.


In addition to the phase coherence feature built into the basic APLC-X devices, AnaPico has also implemented a unique phase coherence switching feature on the APLC-X platform. With this optional feature (optional PHS function), the APLC-X can remember relative phase constellations between multiple channels depending on signal frequencies. The memory function is still effective even when the signal channels are switched off and on again. In Figure 6, it can be seen that the relative phase constellation between channels at a certain frequency is still the same, even after changing the frequency of the channels. In other words, phase coherent switching or phase memory ensures deterministic and reproducible phase relationships between multiple channels.

Phase coherence and phase coherent switching are properties that enable use in many applications. In radar-based antenna arrays and test systems, intelligent beamforming allows systems to remember important angular information. In instrumented quantum computing systems, these properties are essential for generating consistent qubit states, generating phase-matched qubit manipulation signals, and also reducing system calibration requirements, thereby reducing system latency.

Compared to other commercially available analog signal generators on the market, the described performance parameters of the APLC series, such as phase noise, spectral purity, pulse on/off ratio, switching speed, etc., are characteristics of higher price category devices. Developing and delivering high performance yet cost effective T&M instruments has always been AnaPico's mission. With this in mind, we often avoid the use of expensive and novelty components, preferring to use sophisticated circuit design, maintain modular product architecture and deliver highly efficient instruments.

Due to the above described features and overall performance, the APLC series can be used in many applications:

• RF and microwave component testing
• LO (local oscillator) replacement
• receiver testing
• ADC characterization
• instrument and system calibration

APLC multi-channel phase coherent signal generators are particularly suitable for:

• multichannel and/or MIMO receiver testing
• Highly sensitive intermodulation testing
• signal generation for qubit manipulation in quantum computing
• automated high throughput testing
• smart antenna and beamforming testing
• Radar and EW testing

AnaPico's APLC series analog signal generators are the first signal sources on the market with a remarkable combination of features: very low phase noise, fast frequency and amplitude switching, very low noise, clean pulse modulation, compact design, low power consumption, yet at an affordable price. Thanks to the innovative design, the device can be operated either from the main power supply or in the field from an external power bank. AnaPico is the first manufacturer to make this level of generator capability available for use outside the laboratory.

The APLC-X series offers a compact and high-performance solution for generating multi-channel phase coherent signals. By synchronizing multiple APLC-X generators via AnaPico's proprietary high-frequency clock mechanism, users can tailor their setup to specific needs in terms of number of channels or RF and microwave frequency bands, while maintaining strong phase coherence and unique phase memory.