Function Generator Vs Signal Generator

Function Generator Vs Signal Generator

Function Generator Vs Signal Generator

Function Generator Questions And Tips

Many function generators are designed with a 50 O output impedance. Common RF connections cables, such as RG-58/U, have 50 ohms. Some generators have adjustable output impedance. The amplitude of these generators is correct when the generator’s output is terminated in a load that matches the output impedance. If you terminate the generator’s output first with a proper feed-through terminor, the voltage will be the same as its setting. You’ll get approximately double the generator output if you connect a 50 O generator to a high-impedance input such as an oscilloscope having a 1 MO input. A quick test is to put a non-reactive resistor (e.g., a carbon film resistor) across the output terminals of the suspected output impedance of the generator and check the voltage output across the termination resistor. Be careful not to exceed the resistor’s power rating.

An impedance mismatch is a possibility. Another symptom is that the measured amplitude on the scope won’t match the generator’s setting (see the previous question).

When you input a signal using coaxial cable into an oscilloscope with a high input impedance, you should terminate the coaxial cable with a 50 O feedthrough termination at the oscilloscope’s input. You will notice ringing at fast rise times edges if you don’t terminate the coaxial cable or place it on its output.

A 50 O N BNC feedthrough terminal is not required. However, you can use a 50 Ohm carbon film resistor.

Some DDS function generators supply pulses as a standard feature. These generators often let you control the pulse width, pulse repetition rate, and rise and fall times. Some older analog function generators have pulse capabilities; a symmetry adjust knob would change the pulse duty cycle.

If your generator has arbitrary waveform generation capabilities, you can program it with the pulse shape you want. This is more difficult than any of the methods, but it allows you to create the exact pulse shape that you desire.

If your generator is capable of burst operation this can be done easily. You just need to set the waveform cycle count in the burst, and then trigger the external, internal, or manually. It’s a little more work if your generator doesn’t support burst mode, but supports gated operation.

To get the correct number of pulses, use a proper-length periodic wave as your gate signal. An analog function generator allows you adjust the trigger level. This is possible when combined with a sinewave for the gate signal. More modern function generators may require a TTL-level signal to “open” and “close” the gate.

As mentioned in the question for generating pulses, you can also use the arbitrary waveform generation capabilities if your generator supports them. Although it is more difficult to generate the waveforms, they can be enhanced with special features such as short glitches and rounded edges.

Function generators can supply an input, usually on the back panel. This input can be a frequency reference (often 10 MHz). These generators can also output the 10 MHz signal and thus be the frequency reference to a group of other generators. These generators can be locked to a reference signal and their output phase can often be adjusted to match the reference frequency. Some laboratories provide a 10 MHz reference signal that is available to all labs.

If you have to lock two or more generators together that don’t have the ability to input a common reference signal, it still can be done: you can trigger one or more generators from one reference generator. However, this technique may take a bit of care, as improper triggering can lead to improper output, such as unintentionally gated signals or a signal with a small but noticeable distortion. It may not be possible to adjust the phase relative between signals. If you do use this method, it is recommended that you check the output of each generator with an oscilloscope to verify you’re getting the correct signals. You may also want to build some circuitry for shaping and distributing the triggering signal.

The two most common sweep types for generators with a sweep function are logarithmic and linear. These describe how frequency changes over time. More complicated sweeping can be done using the frequency modulation capability. You can frequency modulate the signals with a quicker ramp if your sweep is faster than the built in sweep speed (often 10 ms). The frequency sweep will be completed within one millisecond of the FM modulation frequency specifications (which is usually between 10 and 20 kHz).

Although a ramp voltage can give you a linear sweep profile, it is possible to create arbitrarily complicated sweep profiles. Complex frequency hopping is possible with an arbitrary generator or a custom waveform.

Don’t confuse the frequency specification for frequency modulation to mean that is the maximum frequency deviation you can get from the nominal generator frequency. It is the maximum signal frequency you can input to the modulation input. The modulation voltage is usually specified to be up to +-5 volts. The generator may allow for a voltage between +5 and +5 volts to increase the output frequency by a factor up to five times depending on what the generator is. The output frequency will be decreased by a negative voltage, which is usually in the same proportions. A quick check with your generator, a DC power supply, and an oscilloscope will tell you what these ratios are.

Let’s say that the +-5 voltage causes output ratios between f/4 and 4f. Where f is the frequency setting. Let’s say that f is 40 kHz. This means -5 volts causes an output of 10 kHz and +5 volts causes 160 kHz. The stop frequency of sweeping is 16 times that of the ramp from -5 to +5 Volts.

This is what arbitrary waveform generators (AWG) are made for. See the examples beginning on page 15.

Many modern DDS generators have complex waveforms built in. For example, the following diagram shows some of the special waveforms available in the B&K Precision 408X generator series: Although DDS generator models 4084-4087 do not provide a user programmable arbitrarily waveform memory, it provides users with the ability to output complex waveforms which is typically the domain of AWGs.

12

Staircase (ten voltage levels at the steps).

13

Codified pulse

14

Full-wave rectified signal

15

Half-wave rectified signal
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16

Clipped sine wave

17

Vertically-cut sine wave (represents, for example, a waveform from a lamp dimmer)

18

Square wave with sinusoidally modulated sine waves

19

Logarithmic

20

Exponential

21

Rounded-half

22

Sin(x)/x, i.e. the sinc function

23

Square root

24

Tangent

25

Cardiac signal

26

Alert for Earthquake

27

Combination signal (10 voltage levels in the steps)

Note: The names in the above table don’t necessarily correspond with what the generator uses for the waveform’s name.

Realize that the generator may limit the output frequency of these special waveforms. This is necessary to maintain signal fidelity (the reason should be clear if you read the DDS Theory of Operation section above). Some people might believe that any function generator that has a frequency of 50 MHz displayed on its front panel is capable of producing all possible waveforms. This maximum frequency is usually limited to sine waves and perhaps square waves. The other waveforms that the generator can supply are at reduced frequencies.

These waveforms are very useful for stimulus-response test. For example, in testing a logarithmic amplifier, the exponential function can be input and a ramp waveform output would be expected. Deviations from linearity of the ramp would tell you how well the amplifier is working.

This can be a complex question and is, of course, driven by your needs. It can be daunting to look through all of the datasheet specifications, particularly if they are not clear. Here are some things to consider since most AWGs and function generators can be used for any purpose.

Amplitude

If you’re doing frequency response testing, amplitude flatness may be of interest to you. It is the specification of how the amplitude changes with frequency.

Frequency

The frequency put on the instrument is almost always the highest sine wave frequency the instrument is capable of delivering. DDS generators have higher maximum frequencies for different waveforms than they do with sine waves.

You may also want to pay attention to the low frequency end of the specification. Specialized applications may need very low frequency variations, so frequencies to 1 mHz or lower may be required. DDS technology is well-suited for low frequency generation. It’s not unusual for DDS generators with a 1 Hz frequency resolution.

If you require a generator to produce a steady signal, frequency stability could be an important consideration. It is possible to draw conclusions from specific specifications like frequency accuracy and timebase accuracy.

Distortion

If signal purity is important to you, you will want to understand the generator’s distortion specifications. DDS signal generation is not known for generating high purity sine waves, so you may want to choose a different technology if very low distortion is needed. The distortion specification will also vary over the frequency range, generally being higher for higher frequencies.

Rise/fall times

Fast rise and fall times are important for pulses or square waves. The faster these are, the higher the frequency content of the waveform. Some function generators provide you with the ability to set the slope of the rising and falling portions of a pulse.

Sample rate

This fundamentally controls the raw capabilities of the generator. It is usually given in samples per second (Sa/s). The maximum frequency the generator can produce is controlled by this parameter.

Waveform number of points

This may be specified for the built-in waveforms, but is usually of interest when discussing AWGs. Larger waveform storage can store more complex or individual waveforms. You may also want to look at how quickly the waveforms can be downloaded to the instrument.

DC Offset
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A DC offset ability in the generator is very helpful if you intend to use it to troubleshoot electronic equipment. You can use this capability to convert square waves into unipolar impulses.

Fluxual output

Some function generators are supplied with BNC output terminals that are isolated from ground. This lets the generator be floating with respect to ground (read the manual for the maximum allowed voltage or contact the manufacturer). It can be used to provide a DC offset greater than what the generator’s internal controls allow.

The control of the output

It may be convenient in your application to be able to turn the generator’s output on and off without affecting any other settings. Look for a generator that has an output switch dedicated to this task (it can be either a dedicated key or a soft key).

If such a key isn’t available, you may still be able to accomplish the same thing by controlling the instrument via software and toggling the amplitude between zero and the desired value (or using other control commands).

If neither are available, a small box with BNC terminals can be made that has a switch to turn the output on and off. Double pole switches are recommended for floating BNC output terminals. The contact bounce of the switch may be relevant to your application.

Reading specifications is only one way to go if you’re looking for an AWG or function generator. It is valuable to get your hands on the instrument you’re considering and try it out in your environment. If you feel you will use the AWG software a lot for working with arbitrary waveforms, you will want to also try out the software.

References

M. Schwartz, Information Transmission, Modeulation and Noise, McGraw-Hill, 1970, ISBN 05-055761-6.

W. Beyer, CRC Standard Mathematical Tables, 26th ed., CRC Press, 1981.

Function Generator Vs Signal Generator

Introduction

Function and arbitrary waveform generators are among the most important and versatile pieces of electronic test equipment. A controllable signal is required to mimic normal circuit operation in electronic troubleshooting and design. The testing of physical systems and transducers often needs stable and reliable signals. Signal levels required range from microvolts up to several tens of thousands of volts.

Modern DDS (direct digital synthesis) function generators are able to provide a wide variety of signals. The basic unit can produce sine, square and triangle outputs ranging from less than 1Hz to more than 1MHz. They also have adjustable amplitudes and DC offsets. Generators can be equipped with additional features such as variable frequency, variable frequency, frequency sweep and AM/FM operation. More advanced models offer a variety of additional waveforms and Arbitrary Waveform Generators can supply user-defined periodic waveforms.

Function generators are used where stable and repeatable stimulus signals are needed. These are just a few of the common functions and users.

  • Research and development
  • Education institutions
  • Repair shops for electrical and electronic equipment
  • In-circuit signal injection, frequency response analysis, stimulation/response testing
  • Hobbyists in electronic technology
  • A basic knowledge of the controls and features of arbitrary waveform or function generators is necessary in order to use them at their best. This guidebook is useful to those with little knowledge of function generators, as well as the experienced technician or engineer who wishes to refresh his/her memory or explore new uses for function generators and more sophisticated arbitrary waveform generators.

    First, we will explain the controls of a typical function generator. The theory behind a DDS function generation will be next. The next section is on applications and contains the majority of the material in this guidebook. A final section discusses common questions. An appendix provides a glossary of terms related to function generators.

    A wide range of functions generators are available on the marketplace, with prices ranging from just a few hundred dollars up to thousands. There are many types of function generators, including dedicated instruments, which we’ll discuss in detail. Some can be plugged directly into a computer, while others are wired to instrumentation buss or to a parallel port. Others are programs that are run on a personal computer and generate waveforms via the sound card or on the parallel port. Hobbyists can also purchase inexpensive kits.

    Software-only generators are the most affordable and may be appealing to students or hobbyists who have a limited budget. They are also the most limited in frequency capabilities, often just spanning the audio range.

    Black boxes come next in price and offer the advantages of low cost, portability, and power. They are often intended to operate with laptop computers.

    Where space is limited, generators that can plug into multiple buses (e.g. PC, VXI), are ideal. a dedicated purpose.

    The dedicated benchtop generators can be used in isolation with all the controls and displays. The more expensive dedicated instruments add features and usually include one or more types of interface connections that allow computer control.

    Function Generator Vs Signal Generator

    What To Consider While Selecting A Signal Generator?

    When buying a signal generator, we have a lot of options available in the market. Signal generators come in all shapes and sizes with few models costing lower than $50 while some equipment costing more than 10,000s of USDs. So, before deciding a model we need to know our exact requirements, the type of signal we need, and other parameters such as frequency and accuracy. In the next section, I will briefly explain the parameters we need to consider before making the purchase.

    The most important parameter we need to consider before starting a signal generator search is the type of waveforms it can produce. As explained above, function generators are capable of producing simple signals like sinusoidal waveforms, step signals, saw tooth, etc, but if you require any advance or custom signals then you might have to go for an Arbitrary Signal Generator. Therefore, the first and most important parameter before selecting a function generator is to know the type of signal you need from it.

    Frequency range
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    Resolution

    Resolution is one of the most important parameters in measurement systems. It measures the smallest amplitude at which an electronic instrument can distinguish 2 points in a waveform. In simpler words, it is the lowest quantity our instrument can detect and display. The instrument can display smaller units of measurement unit if it has a higher resolution.

    Sampling rate

    The sampling rate is defined by the number of samples per second. Higher sampling rates are essential to achieve greater accuracy and resolution. The Nyquist principle states that a sampling rate greater than twice the frequency at which the signal is being processed can result in perfect signal reconstruction.

    Your signal generator must have phase locking capability to synchronize RF signals. When working with high frequency circuits, phase locking is often done with a clock or local oscillator.