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Review: Agilent's 35670A Dynamic Signal Analyser

The Agilent 35670A has been around for many years and has become virtually the industry standard for sound and vibration engineers. As well as carrying out audio analysis, it is equally at home with measurement applications in aeronautical, structural, mechanical, civil, automotive and electronics engineering. It can be used for everything from analysing microphones to earthquakes, from examining bridges and beams for metal fatigue to vibration in motors.

By Allan Linton-Smith

The 35670A has been in production for quite a few years and has not undergone any significant updates in that time, so it still looks and feels like a 1990s instrument.

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The rear panel houses sockets for the GPIB interface, keyboard, serial and parallel ports plus power supply inputs (AC and DC) and power switching options. We used the blue GPIB-USB device to connect to our computer to download coloured traces.

On the other hand, it is a real workhorse in the field, built to withstand tough conditions and able to be operated from mains or battery. But it is also a very accurate bench top instrument with many useful features not readily found elsewhere. So in effect, the Agilent 35670A is “an oldie but a goody”.

What other instrument can analyse a bridge beam or automobile chassis, analyse for metal fatigue or trouble-shoot engine problems?

One of the photos in this review shows a typical automotive test for analysing road and engine noise. Sensors can be placed at various locations around the vehicle and a connection made to the tachometer input for determination of noise vs RPM.

Closer to the interest of those in the SILICON CHIP offices, it measures THD and does spectrum analysis (via FFT) simultaneously on two channels. You can also “save to table” and observe or print out the value of each harmonic in the spectrum analysis.

It is extremely sensitive and can accurately measure RMS voltages down to the nanovolt region, which is important when using accelerometers and sensors. (Most audio analysers have trouble analysing RMS levels less than 10mV.)

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Fig.1: the trace shows 10 averages the spectrum of 50Hz mains harmonics up to 1.63kHz. The signal has been significantly attenuated to prevent instrument overload. Each harmonic is marked and a THD figure is calculated shown in red. In this case 31 harmonics can be read and the THD is 2%

It has a 16-bit ADC (90dB dynamic range) and a real time bandwidth of 0-25.6kHz so you can be sure nothing will be missed. In the swept sine mode, the dynamic range increases to a whopping 130dB.

You can resolve signals using 100-1600 lines or for really close-in analysis, use the frequency zoom to resolve signals with up to 61µHz resolution. (Even very good spectrum analysers can only manage about 1Hz resolution!).

There is a facility for time or RPM arming to develop waterfalls of sequential vibration spectra for trend analysis, or for an overview of device vibration.

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