Multifunction synthesizer for building complex waveforms - HP 8904A Multifunction Synthesizer - includes related article on the mechanical design of the HP 8904A - technical

Hewlett-Packard Journal, Feb, 1989 by Fred H. Ives

Multifunction Synthesizer for Building Complex Waveforms

MODERN COMMUNICATIONS SYSTEMS employ complex modulation formats, sometimes using subcarriers or time varying signals to increase their capacity and usability. The divergent testing requirements imposed by these systems have been typically solved with one-of-a-kind custom solutions. This is a costly and sometimes unreliable solution. The HP 8904A Multifunction Synthesizer (Fig. 1) was developed to provide a low-cost, high-performance alternative to these application-specific complex waveform synthesis solutions.

The HP 8904A uses VLSI technology to create complex waveforms from six fundamental waveforms: sine, square, triangle, ramp, Gaussian white noise, and dc. Sine waves are provided in the range from 0.1 Hz to 600 kHz with 0.1-Hz resolution. The square, ramp, and triangle waveforms are available from 0.1 Hz to 50 kHz. All waveform values in the HP 8904A are digitally calculated in real time by HP's digital waveform synthesis IC (DWSIC). The use of this IC results in signals that have well-defined accuracy and exact repeatability.

The standard HP 8904A routes all signals to a single output, and with Option 002 a second floating output can be added, providing a separate signal for two-channel applications. Option 001 adds three more identical synthesizers (channels) which can either modulate the first synthesizer or be summed to the output (see Fig. 2). Frequency, amplitude, waveform, phase, and destination can be independently set for each synthesizer. Available modulation types for channel A include AM, FM, [phi]M, DSBSC (double-sideband suppressed carrier), and pulse modulation. The HP 8904A provides fast hop in frequency, phase, and/or amplitude with Option 003. This feature can be used to create digital modulation formats. By hopping in frequency, FSK (frequency shift keying) and other frequency switching modulation formats with up to 16 frequencies can be generated. BPSK, QPSK, and other phase shifting formats can be made by hopping in phase.

The HP 8904A can be used in applications such as navigation, commercial electronics, audio testing, and communications signaling. For navigation applications the HP 8904A, used as a modulation source in combination with an RF signal generator, can generate VOR (VHF omnirange) and ILS (instrument landing system) signals (see Fig. 3). These signals are used by modern aircraft for navigation. For VOR, channel B is used to frequency modulate channel A, while channel C is summed with the modulated channel A. The bearing angle can be changed by altering the relative phase of channel C. The minimum bearing angle resolution is 0.1 degree. Since the entire VOR composite waveform is calculated in real time by the digital waveform synthesis IC, the HP 8904A Option 001 can deliver typical bearing accuracy on the order of [plus-or-minus]0.05 degree. High accuracy can also be achieved in the generation of the instrument landing system localizer signal, which determines the center of the runway, and the glide slope signal, which determines the angle of the aircraft descent. These modulation signals are formed very precisely by the simple addition of 90-Hz and 150-Hz tones.

In conjunction with an RF signal generator, the HP 8904A can generate the signals required to test commercial FM stereo receivers. FM stereo multiplex composite signals require exceptional phase and amplitude linearity to maintain good stereo separation (see Fig. 4). The HP 8904A can generate stereo composite signals that typically maintain greater than 65 dB separation over the full 20-Hz-to-15-kHz audio bandwidth. Synthesizer accuracy gives an exact relationship between the 19-kHz pilot tone and the 38-kHz DSBSC subcarrier. Since the DSBSC modulation is calculated in the HP 8904A by the DWSIC, subcarrier suppression is greater than 70 dB. Unlike dedicated stereo encoders, the HP 8904A can vary frequency, phase, and amplitude for parametric testing. For example, the pilot tone amplitude can be varied to test phase-locked loop stereo decoder chips for lock range.

The HP 8904A can generate many types of signals used in audio applications. By summing or modulating with the four internal channels, the HP 8904A can generate signals that conform to international standards such as CCITT (International Consultative Committee for Telephone and Telegraph) twin tone, and SMPTE (Society of Motion Picture and Television Engineers) intermodulation test signals (see Fig. 5). Typical residual intermodulation distortion is less than -70 dB. Other useful audio signals that can be generated by the HP 8904A include the IHFM (Institute of High Fidelity Manufacturers) dynamic headroom test signal and a phase continuous linear sweep signal.

In addition to three internal channels, Option 001 also adds three sequence modes to the HP 8904A: tone sequence mode, dual-tone multifrequency (DTMF) sequence mode, and digital sequence mode. These sequence modes can be used to generate signals for paging radios, mobile telephones, and two-way radios. The tone sequencing mode allows entry of 16 different sine wave tones, each with an on time and an off time. From these 16 tones, sequences can be built up to a length of 250 tones. The minimum on or off time is 800 [mu]s with 10-[mu]s resolution, while the maximum value is 655.35 ms. Timing accuracy is better than [plus-or-minus]20 [mu]s.