Understanding the Differences between RF Vector and CW Signal Generators

Publish Date: Oct 06, 2009 | 11 Ratings | 3.36 out of 5 | Print | Submit your review


The following whitepaper describes the differences between National Instruments’ more general purpose RF signal generators and continuous-wave (CW) sources (PXI-565x) and NI’s RF vector signal generator family (PXI-567x). Although some of the differences are apparent in their form factor and slot count, the critical difference comes down to their functional operation. This paper will outline these differences and delve into the architectural and functional differences as well as the business and application differences. Table 1 and 2 outline some of the key differences in NI’s RF signal generation portfolio.

Table of Contents

  1. PXI-565x and PXI-567x Architectures
  2. PXI-565x and PXI-567x Functional Operation
  3. Applications
  4. Conclusions
  5. Resources

1. PXI-565x and PXI-567x Architectures

Before going into the functional operation of the two signal generator families, it is useful to understand their underlying architecture.

PXI-565x RF Signal Generator and CW Source

The National Instruments PXI-565x RF and microwave signal generators are DDS-modulated continuous-wave sources.

Figure 1: NI PXI-565x Block Diagram

The system block diagram in Figure 1 illustrates how the system control unit manages the control signals and data transferred between the circuit blocks. The system control unit also contains the temperature monitor, modulation bit patterns and PRBS generator.

The clocking circuitry, shown in the upper left-hand corner of the diagram, contains the system clock reference and the direct digital synthesizer (DDS). The system clock is a 200 MHz voltage-controlled crystal oscillator (VCXO) that you can program to phase-lock to an external 10 MHz clock signal at the REF IN/OUT connector.

The DDS clocked by the 200 MHz reference provides signals of up to 50 MHz, with very fine frequency steps. RF signals below 50 MHz come directly from the DDS.

Above 50 MHz, signal generation takes place inside the main synthesizer circuit. The high-frequency synthesizer is phase-locked to the DDS output signal as a reference. This enables the synthesized signal to be modulated by the DDS signal for FM and FSK modulation. This DDS reference signal delivers the necessary fine-tuning steps of the synthesizer; the synthesized RF frequency steps are typically 1 Hz or less. This architecture delivers great clock stability and better phase noise than the PXI-567x family.

Following the main synthesizer are dividers and multipliers to scale the frequency over the range of 50 MHz to the upper frequency limit of PXI-565x signal generators.

The automatic leveling control (ALC) loop performs fine amplitude control. The ALC has a broad frequency response, typically from a few kilohertz to more than 6.6 GHz. A temperature-stable voltage digital-to-analog converter (DAC) sets the reference for the ALC. Additional temperature calibration performed during the manufacturing process makes the ALC very stable over the specified operating temperature. Attenuators (final block of the system) perform coarse amplitude control. These attenuators have a typical range of 100 dB and are responsible for the on-off keying (OOK) capabilities of the device.

The NI-RFSG instrument driver uses calibration data stored in the EEPROM to correctly set up the hardware for signal generation. The instrument driver uses calibration data to compensate for non-ideal components and temperature variation.

PXI-567x RF Vector Signal Generators

The National Instruments PXI-567x RF vector signal generators are capable of generating arbitrary signals up to 20MHz in width at up to 2.7GHz.

Figure 2: PXI-5671 Block Diagram

As show in Figure 2, the RF vector signal generator hardware consists of two parts: the NI PXI-54xx arbitrary waveform generator (AWG) module and the NI PXI-5610 RF upconverter module. The NI 54xx generates a signal centered at an intermediate frequency (IF) between 15 and 35 MHz. The IF signal is input to the NI 5610, which frequency-translates it to center between 250 kHz and 2.7 GHz. We will cover the functional operation of the RF VSG later on in the paper.

The onboard memory block serves two purposes: it stores the waveform data as well as the generation instructions that you load into the device. It obtains waveform data through the PCI bus interface block and is capable of operating in streaming mode to obtain waveforms real-time from the PC or controller.

The clocking circuit is responsible for clocking all blocks in the system. It allows you to create your sample clock and reference clock. It obtains the reference clock from the PXI-5610 downconverter when used as a vector signal generator.

The waveform generation engine retrieves the waveform data and instructions from the onboard memory using the sample clock. The waveform generation engine also uses this clock to retrieve triggers from the trigger and event control block.

The output from the waveform generation engine is sent to the DAC device and the digital data and control connector on the front panel after any digital gain or onboard signal processing is applied. The onboard signal processing block shown in Figure 3 has several components but most notable is the quadrature upconversion sub-block which allows digital upconversion of waveform signals, saving onboard waveform memory space and greatly reducing the transfer time required for the waveform. The other components include:

  • IQ Rate Component
  • Pre-Filter Gain and Pre-Filter Offset
  • FIR (Finite Impulse Response) Filters
  • CIC (Cascaded Integrator-Comb) Filters
  • NCO (Numerically Controlled Oscillator)
  • IQ Combiner

The DAC also contains a selectable digital filter, which interpolates and filters the waveform data. The waveform data is sent from the DAC to the analog output path where the waveform data is filtered and amplified.

Figure 3: Onboard Signal Processing Block Diagram

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2. PXI-565x and PXI-567x Functional Operation

The architecture differences in the two families of signal generators translate into functionality differences.  The principle difference being that vectors signal generators are able to output complex, user-defined waveforms with bandwidths upto 20 MHz while the RF signal generators are continuous wave sources with limited pattern, DDS-based frequency modulation.

PXI-565x RF Signal Generator and CW Source
The PXI-565X RF Signal Generator is a continuous-wave signal generator with modulation capability in a single 3U PXI slot (see Figure 4).  The modulation capability is provided by the onboard DDS circuit in which the main synthesizer locks on to using a phase-lock loop. 

Signal generation is initiated by tuning both the DDS circuit and the main synthesizer.  The NI-RFSG driver optimizes frequency tuning by computing the most accurate combination of both the DDS and synthesizer.  For small frequency ranges, the DDS can perform phase-continuous frequency sweeping.  Sweeping across larger ranges requires re-tuning both the DDS and the synthesizer and occurs in frequency steps with an average tuning speed less than 2ms.

Figure 4: NI PXI-5652 Front Panel

The PXI-565x RF signal generator is limited to three types of modulated signals:

  • Frequency shift keying (FSK) signals using either a 1020-bit user defined bit stream, or pseudorandom bit sequence (PRBS)
  • On-off Keying (OOK) signals using PRBS
  • Frequency modulated (FM) signals using square, sine, triangle waveform

Unlike the PXI-567x, the PXI-565x RF signal generators can not generate user-defined waveforms or user-defined baseband or IQ signals.

PXI-567x Vector Signal Generators
As outlined above, the RF vector signal generator hardware consists of two parts (see Figure 5): the PXI-54xx arbitrary waveform generator (AWG) module and the PXI-5610 RF upconverter module.  The role of the PXI-5610 RF upconverter is to take the 20 MHz wide signal at the IF input and frequency-translate it to center between 250 kHz and 2.7 GHz.  The arbitrary waveform generator with an output sample rate of 100MS/s has a Niquist bandwidth between 40-50MHz (usually 1/2 the sampling rate) and can be used to generate user-defined complex IQ baseband signals.  Using the NI-RFSG driver, RF signals are centered at 25 MHz and are usually no wider than 20 MHz.  Therefore wide, spread spectrum, phase coherent signals, such as communication signals, can be generated to serve almost any RF and communication application.

Figure 5: Interconnected PXI-567x Front Panel

The PXI-5610 accomplishes frequency shifting by mixing the IF signal with local oscillators in the PXI-5610 signal chain circuit.

The NI-RFSG driver software operates both hardware modules as a single instrument by handling all module programming and interaction. When the user specifies a carrier frequency and power for the RF output signal, NI-RFSG selects the appropriate IF frequency and power level settings for the PXI-54xx AWG module, and the frequency shift, gain, or attenuation settings for the PXI-5610 upconverter module.

The power and carrier frequency of the IF signal generated by the PXI-54xx are determined by the NI-RFSG software. To optimize dynamic range, NI-RFSG attempts to use the full scale of the NI 54xx digital-to-analog converter (DAC), and adjust output signal power by means of the NI 5610 attenuators.

Figure 6:  NI RF Signal Generator Architecture

The RF vector signal generators have three available generation modes, which are specified using the Generation Mode property:

  • CW mode — NI-RFSG software internally generates a continuous-wave signal (a sine tone). Frequency and power settings relate to the frequency and power of the sine tone.
  • Arb waveform mode — user-provided complex IQ baseband signals are upconverted to IF. Phase continuity, signal bandwidth, frequency tolerance, IQ rate, and digital IF equalization settings are configurable. Output frequency and power settings relate to the center frequency and average power of the specified arbitrary waveform.
  • Script mode — scripts specify dynamic waveform generation operations. For example, a script could configure the device to generate waveform A, then wait for the Script trigger, then generate waveform B.

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    3. Applications

Although the PXI-567x vector signal generators are capable of performing the same functions the PXI-565x RF signal generators can perform, there are several applications that favor the PXI-565x devices. These applications either have different requirements on their specifications or the extra functionality provided by the vector signal generators is not required.

PXI-565x RF Signal Generators
Because of the versatility of the PXI-565x continuous-wave generators, they can be used in many signal generation and test applications. Also, because of the better continuous-wave performance of these generators over the PXI-567x VSGs they are ideal in certain applications.

The most common use of the RF signal generators will be for stimulus response and tracking generator applications in conjunction with the PXI-566x RF vector signal analyzers. However listed below are several more applications ideally suited for PXI-565x RF signal generators.

Because of the faster tuning speeds and better phase noise, PXI-565x devices are excellent for signal sweeping applications and device characterization including:

  • Antenna resonant frequency measurement and testing
  • Amplifier linearity
  • Amplifier frequency response
  • Selectivity measurement
  • SINAD sensitivity test
  • On-site effective sensitivity test
  • Cable fault detection
  • Filter tuning

With the DDS-enabled modulation and the OOK, the PXI-565x is ideal for simple communications equipment testing such as:

  • Antenna response to FM
  • Keyless entry using 2-FSK
  • Radar testing with burst signals using OOK
  • PRBS for bit-error rate testing (BERT) to test all symbols
  • Frequency hopping

Also, because of the excellent phase noise and low jitter of this device, it is also ideal for tone generation and clocking applications such as:

  • Single tone generation
  • Common tunable LO for RF devices
  • Arbitrary IQ and sample rates for waveform generators (clocking WCDMA signals)

PXI-567x RF Vector Signal Generators

The PXI-567x RF vector signal generators can be used in almost every RF and communication application including the applications listed above for the RF signal generators. However they are ideal for complex user-defined, arbitrary waveform signal generation including IQ baseband signals or multitone generation.

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4. Conclusions

The choice between using an RF signal generator or an RF vector signal generator depends on your specific application, the flexibility and versatility requirements and the particular RF and microwave specifications.

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5. Resources

Visit RF for more information

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