The endless stream of signal generators can effectively save test time

All electronic circuits and devices take input signals and process them into new and different output signals. Where do engineers get these input signals when designing and testing circuits and devices? One possibility is to build your own signal source for a particular application, but this is not necessary.

That's because no matter what type of device is being designed or is waiting to be tested, a readily available signal generator can be used to generate the appropriate input signal. Signal generators are as common as oscilloscopes, multimeters, and power supplies on the workbench. Whether they are analog or digital, signal generators can be used to save design and test time and ensure that the product works properly (Figure 1).

Figure 1: Multifunction card reader interface design based on expensive high-pin count MCUs.

Function Generator

Basic function generators can generate sine, square, and triangle wave signals with frequencies ranging from about 0.2Hz to about 20MHz. Some generators can also provide linear ramps, positive or negative polarity pulse signals. These generators are mainly used for basic audio, ultrasonic frequency, and low-frequency RF testing. The pulse output is TTL/CMOS level, and the linear output voltage can reach about ±20Vp-p.

Low-cost generators are generally implemented using analog circuits and can produce continuously variable frequencies and output voltages. Although some low-cost analog function generators are still available, most current function generators use digital signal generation methods and frequency synthesis techniques.

In fact, most engineers prefer digital types of function generators, which are often called arbitrary function generators (AFGs) or arbitrary waveform generators (AWGs), both of which are generally referred to as ARBs (Figure 2).

Figure 2: External multiplexer and glue logic for a multifunction card reader interface design based on a low-pin MCU.

The AFG is the simpler of the two generators and is used to generate only the most commonly used signals, such as sine, triangle, sawtooth, or square waves. The AWG, on the other hand, can be used to generate virtually any type of signal. Most AFGs use direct digital synthesis techniques and waveform memory with standard waveforms and DAC outputs (see "DDS Basics" by DrillDeeper19147 at www.electronicdesign.com).

The output signal is stored in RAM or ROM as a series of binary samples of the set waveform. These data are output to the digital-to-analog converter (DAC) to generate the target output signal of the step-like approximation type. Some AFGs can generate sine and other waveforms with frequencies up to 300MHz.

The AFG has all the standard waveforms pre-stored in memory and can be selected via the front panel controls. The AWG can also generate standard waveforms, but the user can input any desired waveform into RAM or use external software to create a binary file representing the desired waveform.

The frequency synthesizer provides incremental addresses to the RAM, which then provides waveform samples to the DAC. In addition, an analog low-pass filter can be used to remove residual digital noise. The output level control function can be used to set the desired signal amplitude.

Some function generators also offer basic modulation types, including amplitude modulation (AM), amplitude shift keying (ASK), on-off keying (OOK), frequency modulation (FM), frequency shift keying (FSK), phase modulation (PM), phase shift keying (PSK), and some digital modulation types.

For example, the AWG5000 from Tektronix. This device uses a standard N-fold phase-locked loop (PLL) synthesizer (Figure 3) and has two output channels, each of which can be set to single-ended or differential output. Its DAC sampling rate is as high as 1.2Gsamples/s, and it can generate output waveforms with a maximum frequency of 600MHz. Due to this high-frequency characteristic, the AWG5000 can be used for RF testing in certain applications.

Figure 3: Multi-function card reader interface design based on MCU with flexible I/O imaging. [page]

The key specification of the AWG5000 is the dynamic range determined by the 14-bit DAC resolution. The maximum waveform storage capacity is 32M samples. The two outputs are set up to provide both I and Q signals for digital modulation testing.

Because the AWG5000 has a wide frequency range and waveform programmability, it has great flexibility and can implement virtually any form of digital modulation. Coupled with its excellent bit resolution, the AWG5000 is ideal for DAC and analog-to-digital converter (ADC) testing. For DAC testing, the 14-bit parallel digital word output from the waveform memory to the internal DAC can be directly output.

Another Tektronix AWG7000 provides a basic DAC sampling rate of 10Gsamples/s. After using interleaving technology on two channels, it can reach 20Gsamples/s, thus achieving a 10GHz waveform. The output resolution is 10 bits.

AWG7000 is mainly used to test high-speed serial devices with interfaces such as PCI Express (PCIe), SATA, Rapid-IO and Ethernet. Its programmable features allow users to create waveforms containing noise and other impairments to achieve more robust testing.

AWG5000 and AWG7000 both have corresponding signal generation software. RFExpress is used to create digital modulation waveforms, and SerialExpress is used to create waveforms to test high-speed serial interfaces and devices. You can also use software such as Matlab or LabVIEW.

RF Generator

To test wireless devices, engineers often turn to radio frequency (RF) generators. These generators can produce signals from 10 MHz to over 30 GHz. There are two basic types of RF generators, continuous wave (CW) and vector signal generator (VSG), both of which have some form of modulation capability (Figure 4). Digital signal generation is the most common, but some analog models are still in use.

RF generators are often used in place of local oscillators (LOs). Highly stable and accurate reference crystals can be used to drive PLL synthesizers. N-times dividers provide frequency selection via front panel controls. The PLL output is fed to an automatic level control (ALC) circuit that maintains a constant output signal. A power amplifier and variable attenuator form the output circuit. Other circuits provide modulation.

Figure 4: Virtual device initialization tool.

The RF generator implements FM and PM by using some related circuits to change the frequency or phase of the voltage controlled oscillator (VCO) through the modulation signal. AM is achieved by using another variable attenuator in the output circuit.

Sometimes engineers also add additional upconversion stages consisting of a mixer and a high-frequency LO to increase the output range. For example, an yttrium iron garnet (YIG) VCO is a common LO used to convert signals to the near-ten-gigahertz range.

The PXI-5652 modular RF generator from National Instruments (NI) can be plugged into a PXI chassis and work with software such as LabVIEW in a virtual instrument environment (Figure 5). Its frequency range is 500kHz to 6.6GHz. Other models have upper frequency limits of 1.3 or 3.3GHz. The frequency limit is 6.6GHz with a step size of 4Hz and the upper limit is 1.3 or 3.3GHz with a step size of 1 or 2Hz. The 10MHz internal frequency standard provides a maximum accuracy of ±3ppm. The output impedance is a standard 50Ω.

Figure 5: NI's PXI-5652RFCW generator. [page]

Key indicators of RF generators include spectrum purity, harmonics, output amplitude and modulation function. The spectrum purity of PXI-5652 is -90dBc/Hz phase noise, 50fs jitter at 2.488GHz, 5kHz to 15MHz jitter width, and less than 1.5Hz residual FM at 2.4GHz. Harmonic output measurement value from 3.3 to 6.6GHz is -20dBc.

The output power varies from about -100 to 10 dBm at frequencies below 3.3 GHz, and 0 dBm at 6.6 GHz, with a step size of 0.1 dB. The PXI-5652 generator also has internal modulation functions such as FM, FSK, and OOK. The maximum FM deviation in the range of 3.3 to 6.6 GHz is 8 MHz. The FSK symbol rate ranges from 763 Hz to 100 kHz, and the OOK symbol rate ranges from 153 Hz to 100 kHz.

Newer CW RF generators use a DDS synthesizer to create a basic sine wave signal. The signal is filtered to remove the harmonic components generated by the step approximation process and then sent to the power amplifier and attenuator. In addition, a heterodyne up-conversion circuit can be used to increase the output frequency to a higher range.

The most popular RF generator is the VSG, which can be used to create the most common RF signals used in testing digital wireless products. Virtually all digital modulation schemes use an IQ signal generation mechanism (Figure 6). Most VSGs also have built-in AWG baseband circuits that implement digital modulation using software and then output to the VSG modulator shown via a DAC. In some generators, the LO can provide the desired frequency, so no subsequent up-conversion circuitry is required.

Figure 6: Basic vector signal generator circuit.

Wireless test sets are used for formal testing of mobile phones and radio equipment for compliance with specific standards. The RF generators can generate all modulation types such as Global System for Mobile Communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Code Division Multiple Access (CDMA), High Speed ​​Downlink Packet Access (HSDPA) or Orthogonal Frequency Division Multiplexing (OFDM) used by Long Term Evolution (LTE) and WiMAX.

Keithley's 2910 is a modern VSG that can generate signals from 10 MHz to 2.5 GHz (Figure 7). The model 2920 has an upper frequency limit of 6 GHz and can accommodate 25, 40, and 80 MHz steps. A built-in AWG is used to generate modulated waveforms. In addition, its software-defined wireless architecture has great flexibility and can be programmed to generate signals that comply with almost any digital modulation or wireless standard.

Figure 7: Keithley’s 2910RF vector signal generator.

The desired modulation/standard can be created using Keithley's SignalMeister software or software such as Matlab or LabVIEW, and then stored in the huge 100M sample RAM. There is a 500MHz DSP inside the 2910 to process the waveform data. There is also an FPGA and digital up/down converter to further process the resulting signal, which is then sent to the DAC and vector modulator with DDS synthesizer. In addition, the output amplifier and attenuator provide variable output power from -130 to +13dBm, depending on the signal frequency.

In terms of spectrum purity, the single sideband (SSB) phase noise is -101dBc at 6GHz, 20kHz offset, and the typical value of harmonics at 6GHz is -40dBc. The phase noise is -104dBc at 6GHz, 100kHz offset. Available modulation types include AM, GSMGPRS-EDGE, cdmaOne and cdma2000, wideband CDMA (WCDMA) and GPS. Newer modulation types such as HSDPA, LTE and WiMAX can be generated using software. Multiple 2920s can be used with 28?5 synchronization units to generate multiple input and multiple output (MIMO) test signals.

There are two famous RF generators in Agilent's MXG series. Among them, the N5183A microwave analog signal generator is a high-end RF CW generator. The output frequency range of this series of generators is from 100kHz to 20, 31.8 or 40GHz, and the output power is as high as +18dBm at 20GHz. Its high-speed frequency switching speed is less than 900?s, and the typical value is less than 600?s. The measured value of phase noise at 20kHz offset is less than -98dBc/Hz.

Although the N5183A is mainly used for production testing of antennas and microwave devices, it can also provide modulation functions such as AM, FM, PM and pulse. Its pulse rise/fall time is less than 10ns and the minimum pulse width is 20ns. The N5183A can work in two modes: digital frequency-by-frequency and continuous frequency sweep.

Agilent's N5182AMXGVSG has two versions, one with a frequency range of 250kHz to 3GHz and the other with a frequency upper limit of up to 6GHz. The output power is up to +13dBm at 1GHz, and the phase noise is less than -121dBc/Hz at 1GHz and 20kHz offset. The modulation mode is the same as the N5183A, and another key feature is the internal baseband generation and modulation signal creation function. The internal AWG baseband circuit can provide a signal of up to 125M samples/s to the DAC with a bandwidth of 100MHz. The internal 16-bit DAC gives the N5182A an excellent dynamic range and supports almost all tests.

The waveform playback memory can hold up to 6 M samples, while the storage memory can hold up to 100 M samples. With this feature and support from software such as Agilent SignalStudio, users can generate many standard wireless signals, such as Wi-Fi wireless LAN (WLAN), WiMAX, WCDMA, cdma2000, GSM, Time Division Synchronous Code Multiple Access (TD-SCDMA), and even more advanced 3G and 4G wireless standards, such as HSDPA and LTE. Both MXG generators have general purpose interface bus (GPIB), USB2.0, and Ethernet 100Base-T interfaces, and comply with LXIC class specifications. [page]

The upgraded analog N5161A and N5162AVSGMXGATE of these generators are targeted at cost-sensitive automatic test equipment (ATE) applications. Improved performance includes: higher output power, up to +23dBm; improved distortion specifications; and phase coherence support for MIMO and beamforming antenna applications. In addition, these models do not feature a standard front panel and all connections are moved to the back (Figure 8).

Figure 8: Agilent’s N5182AMXG AWGRF signal generator (above) and the newer N5162AMXGATEAWG generator assembly.

Digital Generator

Digital generators are used to generate binary signals or pulses. Basic pulse generators can generate pulses over a wide frequency range, with control over pulse properties such as rise/fall time, duty cycle, and jitter. The pulse format can be standard NRZ, RZ, or other formats, and can be positive and/or negative. The pulse amplitude can also be varied.

A data or pattern generator has RAM and/or ROM for storing digitized data. The data can be user-defined patterns for testing the device under test (DUT) or standard test patterns such as PN9 (109-1 points) or PN15 (1015-1 points) pseudo-random bit sequences (PRBS). Some generators can produce multiple output data streams. These generators generally also provide pulse input triggers. Users can also use the delay feature to generate delayed sequences to set up specific devices.

The Berkeley Nucleonics Model 575 digital delay/pulse generator is a two-in-one generator (Figure 9). The pulse generator section provides independent control of pulse rate, delay, and width, and includes an external trigger input. It provides 2, 4, or 8 output channels, each of which can be individually set to different pulse states.

Figure 9: Berkeley Nucleonics Model 575 Digital Delay/Pulse Generator.

The generator has a frequency range of 0.0002Hz to 20MHz, with a 40MHz option. Resolution is 5ns, with jitter less than 200ps. The standard output is transistor-transistor logic (TTL), but it can also be an adjustable 0-4V output with a typical rise time of 3ns, and there is an adjustable high-voltage output option. Each channel can have its own input trigger, or all channels can be triggered simultaneously.

The 575 has an independent delay generator that provides fine resolution with high delay and width accuracy. The basic resolution is 250ps with an accuracy of 1ppm. The rate is set to 10ns resolution based on the cycle selection. The 575 provides independent delay channel trigger inputs, or all channels can be triggered simultaneously.

In addition, the 575 has an optical output interface with an ST connector. The output LED operates at 820nm and 1310nm, with a rate of 5M baud and a resolution of 500ps. The maximum optical path distance is 1.5km. The 575 provides up to two optical inputs and can also be externally programmable through a standard RS-232 or USB port. An optional GPIB or Ethernet programming interface is available. In addition, the onboard memory can be used to save setup parameters. The 575 also provides a LabVIEW driver from National Instruments.

The Tektronix DTG5000 pulse generator is another data generator (Figure 10). Depending on which module is selected, the generator can generate pulses and data streams at speeds up to 3.35 Gbits/s on 1, 8, 16, 32, 6, or 96 channels. The user also has full control over all pulse parameters. Each channel has a pulse delay function with a resolution of 0.2 ps.

Different models offer different pulse generation frequencies, ranging from 50kHz to 750Mbits/s, 2.7Gbits/s or 3.35Gbits/s. Typical pulse formats are NRZ, RZ and R1. Pulse width, duty cycle and delay are fully variable. The pulse width resolution is 5ps. Random jitter of less than 3psrms can be added. DTG5000 also provides data pattern capabilities from PRBSPN15 to PN23. The pattern length of each channel can be 8, 32 or 6? Mbits, with different values ​​for different models.

DTG5000 also provides many pre-stored patterns, such as binary, Gray, Johnson or checkers code. In order to facilitate instrument control and data transfer, DTG5000 provides GPIB and Ethernet interfaces. Available I/O ports are: USB, RS232, RJ45 for 10/100M Ethernet and VGA output.

There are many other types of generators used for special tests. For example, video generators can produce signals for TV and video testing. In addition, noise generators can produce white or pink noise that can be added to the output of another signal generator to test the noise immunity or performance of amplifiers or other circuits. There are also special generators that produce signals that can be used to generate jitter in pulse generators or AWGs.