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Agilent’s Dynamic Signal Analyser Review by Allan Linton-Smith 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. 82  Silicon Chip siliconchip.com.au 35670A T he 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. 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”. siliconchip.com.au What other instrument can analyse a bridge beam or automobile chassis, analyse for metal fatigue or troubleshoot 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.) It has a 16-bit ADC (90dB dynamic June 2012  83 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. terfall displays, frequency response using both Fast Fourier Transform (FFT) or swept sine and it also has an optional arbitrary waveform generator. This latter option is also capable of generating repetitive waveforms which have been previously stored. Various averaging modes let you further refine spectrum analysis measurements. Time averaging extracts repetitive signals out of the noise while RMS averaging reduces the noise to its mean value. The instrument also has “exponential” averaging for both time and RMS volts. This is useful for reducing the noise while following changing signals, such as tracking the resonance shifts in a fatiguing structure; when metal fatigue happens the natural resonance changes drastically and is easily observed with this instrument. Other features 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. You can match your spectrum measurement mode to the signal being tested and use the linear spectrum analysis to measure BOTH the amplitude and phase of periodic signals such as the spectra of rotating machinery. Power spectrum analysis is provided for averaging non-repetitive signals. In addition to all this, it can timecapture waveforms, measure phase distortion, side-band power, noise power, display spectral maps, wa- 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% 84  Silicon Chip The 35670A is a standalone instrument requiring no peripheral computer for general operation, although we used a small laptop PC to record traces and to add captions and colours. The standard instrument allows you to look at signals in the frequency, time and amplitude domain and there are several options which are available to either add new measurements or enhance all measurement modes. Options: AY6 adds two channels (four total) IDO computed order tracking Fig.2: a spectrum waterfall of mains harmonics gathered at 160 specified time intervals. A base suppression of 24% was used to eliminate noise. The lower chart is a slice of activity between counts 146-160 where the red marker is positioned at 1.326kHz showing around 1 microvolt. Our mains harmonic distortion looks like a veritable graveyard! siliconchip.com.au Fig.3: a comparison of the HYQ-5 microphone vs the Behringer ECM 8000 for our loudspeaker frequency response article (SC Dec 11) using the back/front display mode with the 1/3 octave real time option. Pink noise was used and fed to a loudspeaker and simultaneously picked up by both mics positioned closely. It can be seen that at 1kHz the HYQ-5 is only about 5dB less sensitive than the much more expensive calibrated ECM 8000 and is very close in response too! ID1 real-time octave measurement. UK4 microphone adapter and power supply ID2 swept sine measurements (has a dynamic range of 130dB) ID3 curve fit and synthesis ID4 arbitrary waveform source IC2 Instrument Basic Our 2-channel instrument had all of the options fitted except Instrument basic which allows you to develop a custom user-interface, integrate several instruments and peripherals into a system using the 35670A as the system controller and to automate measurements. These can be added retrospectively as can all the other options if desired. This option would be really useful because there are a lot of buttons to press for each setup and it is easy to make a mistake. There are large volumes of instrument and service manuals and application notes provided by Agilent and very good explanations of the theories Fig.5 mains harmonics are displayed to 25kHz and a peak of 16.38kHz is displayed. THD figure is incorrect because of “aliasing” errors and the “zoom” should be used for accuracy, nevertheless we included this trace to show how much rubbish is on our powerlines! siliconchip.com.au Fig.4: the blue trace shows harmonic distortion is 0.0063% from a very good sine wave generator. The green trace is 0.1034% from a cheaper unit. The number of harmonics set for measurement was 20 although 200 is possible! Bottom traces are the sine wave time record simultaneously obtained from both units behind all the more complicated tests. The online HELP menu is particularly useful as it gives full screen instructions of the functions of the last button press. It’s a rugged unit able to withstand extremes of temperature (0-55°C), altitude (4600 metres) and shock (up to 10G!!). Although it weighs in at 15kg it remains a very compact and portable unit well suited for applications in the field, especially since it can operate Fig.6 when the “help” key is pressed directly after the “harmonic marker” key you get to view an excellent set of instructions so you will never get lost! June 2012  85 Fig.7 THD can be saved to a table which shows the RMS voltage of each harmonic. In this case we were looking at an improved “Champ” LM386 chip which has mainly second harmonic distortion. 10 averages were used for this result although you can have 200 averages if you wish! from 12-28V DC as well as from 90264VAC. At the back of the instrument there is a BNC socket for the source signals, external trigger and tachometer input, capable of handling 42V peak, so you can read RPM without any signal conditioning (such as a micro switch or solenoid) There is also the power select switch which switches between AC and DC power sources without interrupting operation and is protected from accidental switching. There is a parallel port and a serial port for printers (sorry, no USB) and also a keyboard socket to accommodate a standard PC keyboard, which allows you to add captions and control the instrument. There is an HPIB (Hewlett-Packard Interface Bus) connector which can be used for controlling and programming the instrument using an appropriate interface or linking it to other instrument equipped with HPIB (or GPIB – General Purpose Interface Bus). There has been quite a bit of discussion on the internet about saving traces to files but we used the National Instruments GPIB-USB converter to download traces into a notebook computer using John Miles’s excellent (free) HP7470A emulator software. This allows you to download traces in colour, change colours and add captions and save then print or save as PDF files. It’s a much better option than printing from the serial port which is only in monochrome. If you need to do this, go to the website www.ke5fx.com/gpib/7470. htm and you will find that you can do 86  Silicon Chip this for a whole raft of HP, Tektronix & Rohde & Schwarz spectrum analysers The front of the instrument has 21 “hard keys” which are fairly self explanatory and eight “soft keys” labelled preset, help, basic, HPIB/local, utility, plot/print and save/recall. Our instrument has two BNC inputs and one output for source signals (duplicated at the back). The system uses a 3.5 inch floppy which stores instrument states, programs, time captured data, waterfall data, trace data, limits, math functions, data tables, and curve fit/synthesis tables. Supported disk formats are HP-LIF and MS-DOS. Internal RAM on our device was 8Mb, which appears to be more than enough for most purposes. One of the nice features is the superquiet fan which you can only hear in extremely quiet environments. There is provision to turn it off for short pe- riods while the instrument is running, so as not to interfere with sensitive loudspeaker tests. But at a quoted output of <45dB it is almost impossible to hear the difference! It is without doubt a very quiet instrument. Now we come to one of the most unpopular features of this device – the external monitor output socket. This is a DE9 9-pin socket which does not fit any modern external monitor. Not only that but even if you use a converter to a 15-pin socket, the monitor will not work, because it requires multi-sync monitor which may now be difficult to obtain. Agilent has devoted a fair amount of time on this issue and details for selecting and fitting suitable monitors can be found at: www.home.agilent.com/agilent/ editorial.jspx?cc=AU&lc=eng&ckey =490434&nid=-11143.0.00&id =490434&pselect=SR.GENERAL Measurements There are five basic measurement types that can be performed: 1: Measuring Rotating Machinery    This involves connecting & calibrating transducers and measuring vibration power, characterising tachometer signals and measuring an order spectrum.    An RPM stepped order map can be set up to observe, for example, what happens as a motor increases its speed.    Using proximity sensors and orbital diagrams, it is also possible to identify problems such as bent shafts or eccentric rotation. Key Specifications (1 channel) 195.3mHz to 102.4kHz (2 channel) 97.7mHz to 51.2kHz (4 channel) 48.8mHz to 25.6kHz Lines of resolution: 100, 200, 400, 800 and 1600 lines of resolution Dynamic range: 90dB (130dB in swept-sine mode) Accuracy: ±0.15dB Maximum input: 42V peak Source: Random, Burst Random, Periodic Chirp, Burst Chirp, Pink Noise, Sine, Arbitrary Waveform Maximum output: ±5V peak (AC) Measurements: Linear, Cross, and Power Spectrum, Power Spectral Density, Frequency Response, Coherence, THD to 0.0015%, Phase Distortion, Harmonic Power, Time Waveform, Auto-correlation, Cross-Correlation, Histogram, Polar Display, Octave analysis with triggered waterfall display Tachometer input and order tracking with orbit diagram Engineering units: g, m/s2, m/s, m, in/s2, in/s, in, mil, kg, dyne, lb, N, and pascals Frequency range: siliconchip.com.au One of the major advantages of the 35670A is that it can operate from AC and DC, making it highly versatile and one of the few instruments that can swap from the laboratory to the field (including mobile use) with ease. Here it is being used in a motor vehicle where a range of parameters can be recorded for later analysis. 2: Measuring Structures    All structures have natural frequencies of resonance – as some very large bridge builders have found to their horror.   The frequency response function measures the input excitation and output response simultaneously.   To find natural frequencies, an impulse response measurement can be performed on the structure and the Agilent 35670A can be used to compute the frequency response.   An instrumented hammer impacts the structure and an accelerometer measures the response.   The impact hammer has a load cell that measures the level of force during the impact.   Basically, bigger hammers are required for bigger structures and various methods can be used to finely tune the instrument to obtain reliable results.   Because it is such a transient test, it is best to first look at a time trace of the excitation (like an oscilloscope record) and setting the instrument amplitude ranges to avoid clipping during the frequency response evaluation.   This method would be useful in evaluating speaker enclosures and room vibrations too. 3: Measuring Sound   Using the microphone adapter which screws on to the bottom of the 35670A, with a 4-channel instrument it is possible to attach and calibrate up to four microphones and also provide them with phansiliconchip.com.au tom power up to 200V DC. will allow measurements of sound pressure levels (SPLs) and impulses and real time 1/3 or 1/12 octave measurements. You can also view frequency response and impulses over time with the waterfall function and obtain a time record.   This 4: Measuring Spectra and Networks    This involves measuring wideband and narrowband spectra, frequency and amplitude, noise power, harmonic distortion and sideband power.    Frequency response can be measured using FFT (includes phase measurement) or swept sine. The latter is more accurate for frequencies lower than 100Hz.   Spectral maps can also be generated, phase distortion calculated and displayed as microsecond delay vs frequency. 5: Measuring Control Systems   Performance, step response, stability, loop response, gain and phase margins can all be measured with this instrument.   Also of interest is the ability to generate Nyquist diagrams for evaluating various control systems such as servo systems.   The control loop may be composed of mechanical devices and/ or analog, digital or electrical elements. Drawbacks There is no doubt the 35670A has already become an industry standard for those applications mentioned above. In fact, many industry and even government department and organisation tenders and contracts specifically call for the 35670A as part of their validation, quality control and testing procedures. As such, it has become de rigeur in many standards – to replace it would call for massive re-writes (and therefore costs). However, the popularity of the device should be reason enough for a long-overdue upgrade, such as faster processing times, better and more user-friendly programming, USB connectivity, external monitoring, pre-programmed setups and better displays. This should be relatively easy for Agilent to undertake. Perhaps because the instrument has such a monopoly on low and ultra low frequency analysis that it has little to compete against and has generated a culture of “its good enough!” Conclusion With the rare ability to accurately evaluate low frequencies, the instrument is at the opposite end of the ever-growing high frequency range of spectrum analysers where there are many manufacturers fighting to demonstrate they have the best devices for upwards of 100GHz. But, in a way this has kept the Agilent 35670A in the doldrums of development. The fact that it has survived for so long demonstrates that it is an excellent and well-respected instrument. Current users are largely happy to put up with the drawbacks because they feel familiar with its controls and all their standards are based on it. It will probably continue to be manufactured for some years to come but it would be nice to see some of those long-overdue improvements. SC Where from? The Agilent 35670A Dynamic Signal Analyser and its extensive range of options/accessories is available from Agilent Technologies Australia Pty Ltd, 679 Springvale Road, Mulgrave Vic 3170; Tel (03) 9560-7133, Fax: (03) 9560-7950. The company’s international website is www.agilent.com, from where you can specify your country. June 2012  87