$1,000 Signal Opportunities

Figure 1. GFG-3015 Analog Function Generator
Courtesy of GW Instek

GW Instek’s Model GFG-3015 Analog Function Generator (Figure 1) uses a frequency feedback method to ensure accuracy. The GFG-3015 truly is an analog generator with a DAC-controlled VCO and a high-precision frequency counter within the control loop. Any error between the set and measured frequencies is corrected by the loop. Sine, square, triangle, ramp, and pulse waveforms are supported.

The counter time base ±20-ppm accuracy limits the overall performance, but it is important to note the basic difference between this generator and DDS- or Arb-based sources that have a filtered DAC output. The GFG-3015 output is a continuous analog waveform created by an oscillator without clock-related glitches. The sine wave distortion up to 100 kHz is -46 dBc, which compares to the TTi TG1000/2000 with -45 dBc to 300 kHz.

The generator displays eight ranges of frequency from 10 mHz to 15 MHz with six-digit precision and four ranges of four-digit amplitude from 10 mV to 10 V with a 50-O load. A DC offset up to ±5V can be applied as well as AM or FM modulation from a built-in or external source and burst or gate control.

The B8000FD Series of function generators from Protek Test and Measurement is a DDS-based design boasting two outputs. Each provides the full selection of sine, square, pulse, triangle, ramp, sine (X)/X, exponential up and down, and noise waveforms, but the second output runs at rates from 0.1 Hz to 20 kHz compared to the main output with a range from 1-µHz to 3, 10, 20, or 40 MHz depending on the model.

Linear and logarithmic sweep and AM, FM, FSK, ASK, PSK, and burst modes of operation are standard features. A 100-MHz counter is included as is an RS-232 interface, which supports complete programmability. When in the single trigger mode, every time the trigger button is pressed or a pulse applied to the external trigger input, the generator will output one sweep, FSK, PSK, ASK, or burst waveform.

Agilent Technologies’ DDS-based U2761A USB Modular Function Generator falls outside the $1,000 price limit at $1,686 but is close enough to include for comparison. This instrument generally matches the functionality of lower cost products but improves upon the signal integrity. For example, small signal sine wave harmonic distortion is specified at -65 dBc up to 100 kHz. Spurious contributions up to 1 MHz also are listed at -65 dBc.

Internal modulation ranges typically are wide although there could be an advantage to supporting external modulation, which this instrument does not. The company’s Measurement Manager software is included, which provides an arbitrary waveform editor for waveform generation customization.

PC-Based Sources

Although the PicoScope 2200 and 3200B Oscilloscopes from Pico Technology are not primarily signal sources, they do contain an Arb function capable of generating sine, square, triangle, ramp, sin(X)/X, Gaussian, and half-sine waveforms—all with optional frequency sweep. In addition, you can download data captured by the scope portion of the instrument or separately create truly arbitrary waveforms.

Figure 2. PicoScope 3204B
Courtesy of Pico Technology

Output resolution is 12 bits, and memory length is 8-kS for the 3204B (Figure 2) and 3205B models and 16-kS for the 3206B. For all 3200B Series models, bandwidth is >1 MHz, rise time <100 ns, and update rate = 20 MHz. The 2200 Series scopes all have 4-kS Arb memory, 8-b resolution, a 2-MHz clock frequency, and 100-kHz bandwidth. Both series generate the same range of standard output signals, but 3200B scopes also can produce white Gaussian noise and pseudo-random bit sequences (PRBS).

According to Jeff Bronks, a spokesman for the company, “These scopes/signal sources are all USB 2.0-powered so their power consumption is limited to 500 mA at a nominal 5 V. This has to supply both the oscilloscope and signal source. Using denser FPGAs has allowed us to increase the Arb buffer size with each new product range and offer more complex standard waveforms such as white noise and PRBS.”

The Network BitScope 325 from BitScope Designs has a U.S. list price of $795 and, like the PicoScope products, a signal-generator function. The two 100-MHz analog scope inputs have independent ADCs sampled at up to 40 MS/s. Signals acquired on these input channels or those acquired via two additional lower speed analog inputs or arbitrary waveforms loaded via the eight digital I/O lines can be output to one of the BNC connectors.

In contrast to many other low-cost instruments, the signal generator portion of the Model 325 is well integrated with the overall design. This means that both the digital I/O buffer and the separate analog trace buffers are 512 kS long, allowing very complex waveforms. Signal quality is enhanced by the full metal case. A host PC and separate 12-V supply are required although the BitScope DSO software application is included.

National Instruments’ (NI’s) low-cost signal sources are implemented as the analog output sections of data acquisition (DAQ) devices. The USB-6211, as the model designation suggests, is powered and programmed via USB from a PC. There are two 16-b analog output channels, each updated at 250-kS/s with a shared 8-k FIFO. The specification is detailed and complete, even including reference to a power-on glitch of up to 1 V for 200 ms and output DAC glitchs up to 100 mV for 2.6 µs. The company’s PCIe-6321 also has two output channels, but they update at 840 kS/s, and this module requires a PCIe chassis for use.

Because the analog outputs accompany DAQ capabilities, stimulus-response testing is well supported. NI’s Sam Freed, DAQ product manager, elaborated. “Engineers use analog outputs to apply one or more signals to the DUT, which might be an electrical circuit or a mechanical device, and then read back the response using a synchronized analog input channel. For example, you might use an analog output pattern to drive an industrial motor and then read back the voltage and current to determine the instantaneous power characteristics of the motor over a certain ramp-up/ramp-down profile.”

Of course, if you can support the programming these devices require, the benefits can be substantial. In addition to creating simple continuous waveforms, you can generate complex arbitrary waveforms and stream them continuously. And, if you find that more channels are required, there are similar products with four output channels that will run the same test application programs.

Figure 3. DT9853/4 Analog Output Module
Courtesy of Data Translation

Data Translation’s DT9853 and DT9854 Analog Output modules (Figure 3) only have a 1-kS/s update rate but feature industrial-strength isolation. True galvanic isolation is a big deal, especially for industrial applications that may involve large currents and high voltages. The ±300-V isolation for these modules protects the attached PC from damage should the module be connected to a reference voltage well away from ground. Because there is no direct connection, any ground loops are eliminated by true galvanic isolation. In addition, these modules meet the EN50082-1 specification for 8-kV ESD protection. Any pin can receive an 8-kV transient, and the overall operation of the module will not be disrupted.

The DT9853 has four glitchless analog output channels, and the DT9854 has eight. An M version available for both modules provides a current output in tandem with each voltage output. This feature is particularly suited to industrial 4- to 20-mA current-driven process control. The plastic module housing is loaded with conductive stainless steel fiber to ensure EMI/RFI/ESD protection, and screw terminals are used for all I/O connections.

Summary

There is no shortage of low-cost signal sources, but you may need to re-evaluate your test approach to gain the greatest advantage from their use. Some requirements simply aren’t compatible with the performance available in a $1,000 instrument, such as switching at microwave frequencies or professional audio-quality distortion at lower frequencies.

However, if you’re working at frequencies up to a few megahertz, several instruments are available with a wide range of features. You won’t get the absolutely lowest possible distortion or jitter, but the signal from a low-cost source could be more than adequate for your purpose. At RF and microwave frequencies, you still may find suitable low-cost sources but additional features will be missing. Many generators are CW only, not even supporting modulation.