Signal and Waveform Generator Products from TTi - Thurlby Thandar Instruments

 Function Generators - An Explanation of Technologies and Terms - Analogue and Digital

Function Generator
 

The function generator -
is one of the most versatile pieces of test & measurement instrumentation available.

It can generate a variety of precision wave shapes over a range of frequencies from mHz to MHz.

It can provide a wide range of controlled amplitudes from a low-impedance source, and maintain constant amplitude as the frequency is varied.

Voltage control of frequency enables a source of swept frequency to be generated for frequency response testing. AM and FM modulation facilities can also be utilised.

 

Analogue or digital - which to choose ?

Function generators fall into two basic categories, analogue and digital. Analogue generators use a voltage controlled oscillator (VCO) to generate a triangular waveform of variable frequency. Sinusoidal waveforms and square waves are generated from this.

Digital generators use a digital to analogue converter (DAC) to generate a wave shape from values stored in memory. Normally such generators only offer sine and square waves up to the maximum generator frequency. Triangle waves and other waveforms are limited to a much lower frequency.

A third type of generator uses digital techniques to control an analogue VCO. TTi no longer manufactures a fully digitally controlled analogue generator. However, a simplified form of digital frequency control, known as Frequency Locking, is employed in the TG550.

Advantages of analogue generators

Analogue function generators offer several advantages:

1. They provide simple and instantaneous control of frequency and amplitude.

2. They do not have the high frequency limitations on non-sinusoidal waveforms such as triangles and ramps that digital generators do.

3. The starting price for an analogue generator is considerably lower than for a digital generator.

 

The TG120, TG210 and TG300 are pure analogue generators. The TG550 is an analogue generator with digital frequency locking.

Advantages of digital generators

Digital generators normally derive the waveform frequency from a crystal clock using a digital technique. Consequently the frequency accuracy and stability will usually be higher than can be obtained from an analogue generator. Digital generators may be able to generate a much greater number of standard waveforms than analogue generators. 

A variety of techniques may be used of which the most versatile is direct digital synthesis (D.D.S.). 

DDS uses a phase accumulator, a look-up table and a DAC. It offers not only exceptional accuracy and stability but also high spectral purity, low phase noise and excellent frequency agility.

A DDS generator can be swept over a much wider frequency range than an analogue generator and can perform phase continuous frequency hopping.

The TG2000, TG1010A and TG4001 are all DDS based based digital function generators.

 

Disadvantages of digital generators

The maximum frequency for triangles and other non-sinusoidal waveforms is limited to a small fraction of the upper frequency for sinewaves. This is related to the maximum clock rate combined with the filter characteristics.

Rectangular waveforms can be generated from the sinewave using analogue comparators and can therefore avoid this restriction, but performance limits may apply to pulse waveforms.

Digital generators are more complex to use. This can be a drawback in simple or traditional test environments. 

Arbitrary waveforms

Digital function generators have the potential for generating arbitrary waveforms.

However, the architecture of a DDS based function generator differs from that of a true arbitrary generator with consequent limitations to the arbitrary capabilities. 

Nevertheless, DDS function generators with an arbitrary capability can generate non-standard and custom waveforms which may be sufficient for many applications.

Universal Waveform Generators such as the TGA1240 and TGA12100 series combine a DDS function generator with a true variable clock arbitrary generator.

The TG4001 also uses this architecture, although it has less sophisticated arbitrary capabilities.

 

The TG1010A is a DDS function generator with DDS based arbitrary waveform capability.

To learn about the differences between DDS based an variable-clock based arbitrary generation, use this link: Arbitrary Waveform Generation Techniques.

Thurlby Thandar Instruments Limited
Glebe Road, Huntingdon, Cambridgeshire PE29 7DR United Kingdom
Tel: +44 1480 412451  Fax: +44 1480 450409  Web: www.tti-test.com  Email: sales@tti-test.com