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Test EquipmentHere I give my personal thoughts and recommendations on test equipment and other electronic tools I use either at work or at home. Accuracy and PrecisionAccuracy and Precision are two words bandied around when there is talk of test equipment. Often the two will be used incorrectly, either at random or the wrong way round. Wikipedia has a nice little explanation of the two words. A brief summary:
ContentsMultimetersAvometer 2001 MultimeterAVO.... now there's a name. This rare little beauty was the first DMM I used in a my professional career as a design engineer at Thorn EMI Electronics. And I only just recently found out that this was in fact made by Thorn EMI Instruments Ltd when Thorn EMI owned company.
As you can see its a bit of an odd design: a large case with a small LCD at the bottom and the three 4mm sockets at the top. There are two slide switches: one for mode (off, ac, dc/diode, resistance/continuity), and one for range. I can attest that it is a solidly built unit, and being through-hole construction it is a very nice instrument to service. And in practice the LCD is very clear and responsive, far better than those cheap DMMs you can get for peanuts these days. Fluke 8060A MultimeterAs well as a basic multimeter it is good to have a second meter to both give you a second display and to provide more advanced features that you do not always need for general bench and field work. The 8060A is a classic DMM, with excellent true RMS AC reading to well above the audio range. And you can usually pick them up for very little on eBay these days.
This meter is great for audio work. The dB ranges are normalised to 600Ω so can read dBu directly. And in Relative mode you can measure dB directly, both in DC and AC modes. The frequency counter is useful for spot checks (although I'd recommend a proper frequency counter for anything more involved). Hewlett Packard HP 3478A Bench MultimeterHand-held DMMs a great for general measurements both at the bench and when out and about, or even just head-down inside a mixer. However, once in a while you need higher accuracy than a hand-held, or perhaps a long-term experiment needs monitoring that would otherwise exhaust the batteries in a hand-held.There are many good examples of bench DMMs on the used market these days. The obvious contenders are there: HP/Agilent and Fluke. From the HP stable comes the venerable 3478A 5½ digit DMM. While it only does AC/DC volts, AC/DC current, and 2- and 4-wire resistance (it doesn't do continuity, diode test, frequency, temperature, capacitance, transistor hfe, and so on), it makes measurements to a high precision and, when calibrated, to a very high accuracy. The worst-case DC volts accuracy is ±(0.044% + 41) on the 30mV range after 1 year and within ±5°C of calibration temperature. More typically, if it says "1.00000V" on the 3V range then the worst-case inaccuracy is ±(0.019% + 2), or ±210μV. Another nice feature of high-end DMMs is the input resistance. The HP3478A (and its later incarnations) have very high (>10GΩ) input resistances on the low voltage ranges. Useful for work on high-impedance or otherwise sensitive circuits. And of course it has a GPIB connector on the back for hooking up to a computer. OscilloscopesTektronix TDS340A OscilloscopeThe old saying goes: "There are only three names to look for in oscilloscopes: Tektronix, Tektronix, Tektronix". Personally I'd modify that to "Tektronix, Tektronix, LeCroy" but only because I spent the first few years of my professional life with a LeCroy 9420.(I've never really got on with HP/Agilent scopes -- I guess its a LoveIt-HateIt thing) For home use these days I've upgraded from an old Tek 468 to a newer digital TDS340A. Same 100MHz bandwidth, but it has some clever triggering modes, can do FFTs for a bit of rough spectral analysis (it is not a spectrum analyser!), and I can dump plots to a printer or save them to disk for later documentation or analysis.
Documentation is easy to find on the net. Here are some files I've found and saved here for posterity: A Word on ProbesThe probe and the scope form a complete tip-to-trace test system. There is no point spending good money on a decent scope only to spend pennies on rubbish probes. So after hunting around the net for a make and model of scope probe that would befit this fine scope (other than Tektronix probes) I've bought a couple of brand new Probemaster 4905-2RA 250MHz probes. While they may not meet the highest specs of Tektronix probes they are perfectly good enough for my home use.Why 250MHz probes? Why not 100MHz to match the scope? To understand why you need to understand what it means to say a scope is "100MHz". In simple terms, a scope's bandwidth is the -3dB point of its vertical channels. So a 100MHz scope measuring a 100MHz sinewave will display it with an amplitude of 0.707 times the actual amplitude. Now, that does not include the probe. The probe also has a -3dB point, so the complete system of scope plus probe will have a -3dB point lower than the individual components. For example, a 100MHz scope with a 100MHz probe will have a -3dB point of around 60MHz. So it is worth splashing out that little bit extra on higher bandwidth probes so you don't limit the bandwidth of the tip-to-trace system so much. Recording DataOne nice thing about the TDS340A is that it has a Centronics printer port and it talks DeskJet, LaserJet and Epson protocols. I recently picked up an old DeskJet 500 printer for next to nothing (thanks Ben!), and with a fresh ink cartridge I can hit the HARDCOPY button and get a printout while I'm working at the bench. Certainly easier than scribbling down some measurements on a scrap of paper.I scanned in a couple of scope plots during the development of my dualfo. And having a hard paper copy is great for making annotations or adding other notes. Frequency/Time CountersHewlett Packard HP 5334A Universal CounterWhile a scope can be used to make frequency or period measurements of a cycle or two, a proper standalone counter makes the job much easier and provides more precise results and more ways to measure the signals. It also frees up the scope for probing around the circuit looking at waveforms while the counter keeps an eye on frequency.HP (now Agilent) produce some really good counters. This one I picked up on eBay a while ago:
One important feature of this counter over cheaper/simpler models is that it uses the reciprocal technique. Simple counters measure frequency by counting the number of cycles of the input signal in a given window of time. With simple decimal scaling you get the result in Hz, kHz, MHz, etc (my old Racal Dana 9902A did this). Unfortunately this means that measuring low frequencies with any degree of precision can take some time: to measure a 1Hz signal to three significant digits requires a gate time of at least 1000 seconds. Yes, I know, I could measure the period and then calculate the frequency from that with a pocket calculator. But dammit that's what test equipment is for!!! The reciprocal technique takes the opposite approach: the input signal determines how long the gate is open to count pulses from a stable reference oscillator (*). A CPU then does the maths to work out frequency. All it needs is one complete cycle of the input to measure the frequency to a very high degree of precision. In the case of the 5334A, to nine significant digits. Taking the earlier example, with the reciprocal technique it will take a little over one second to measure a 1Hz waveform. Very useful when developing low frequency oscillators for example. (* - for greater accuracy use an ovened oscillator, or a rubidium oscillator, or a GPS-locked oscillator, depending on how much you're prepared to pay). Power SuppliesAs the saying goes: "Power corrupts. Absolute power... is kinda neat." Bench power supplies are so important in the electronics lab, and yet so often they are hidden away, never discussed; the family cousins that everyone acknowledges exist but doesn't mention in polite company. I've designed, built, and used many power supplies over the years. Expensive ones. Cheap ones. High voltage ones and high current ones (sometimes both at the same time). Small ones and heavy ones. Simple ones and complex ones. Most of the time though you can get by with a basic dual bench power supply. Which I guess is why they're so popular. Hewlett Packard HP 6236BProbably one of the nicest little bench PSUs for op-amp development. Two outputs provide tracking bipolar rails up to ±20V limited to 500mA. Plenty for op-amp circuits. The third output provides a higher current 0-6V up to 2A, which can be used for a digital rail or perhaps a control voltage. Two meters provide voltage and current monitoring, switchable between the three outputs. All in all it is a nicely packaged little bench PSU: Thurlby PL320These single (and their dual cousins) bench supplies are a common sight in UK R&D labs. They're everywhere! In schools, universities, and labs, they have provided stable supplies for many years. The basic single output model (pictured below) can supply up to 30V at 2A, and can operate in constant-voltage with current limit, or constant-current modes. Very useful for bringing up new circuits with the current limit turned right down! Signal AnalysersHewlett Packard HP 35660A Dynamic Signal AnalyzerOscilloscopes are great for showing you what is happening to a signal in the time domain. But often times you want to know what is happening in the frequency domain. This is what spectrum analysers provide. They can measure cutoff frequencies, resonant frequencies, and filter responses amongst other things. Spectrum analysers fall into two general types: swept filter and FFT. The former is the classic analogue spectrum analyser, sweeping a filter over the required range, then measuring the signal level coming out of the filter. Great for radio frequencies, but due to filter response times not so good for audio use. The first spectrum analyser I owned was a venerable old HP 141T, with a range of plugins, including one that covered the audio range. That was a beast indeed! One problem with swept filter spectrum analysers is that as you reduce the video width to get more detail in the spectrum, because of the delays introduced by the high-Q filters, sweep times could be measured in tens of seconds. Tedious, especially for audio work where you might want a video bandwidth of a few Hertz. With the advent of cheap computing power came the FFT-based spectrum
analysers (or dynamic signal analysers (DSA) as they tend to be called).
These
sample the signal as-is, without any filtering, then apply the power of FFT maths
to extract and display the spectral content. This provides data much faster as
you only need to wait as long as one cycle of the smallest frequency you are
interested in. For example to resolve to 1Hz requires a 1 second measurement
period. More typically, to cover the audio range with a 25kHz span would take
about 15ms per acquisition, or 64 updates per second, i.e., realtime.
I used an HP 3561A in my first job building high power passive audio filters, hand-winding coils and building fan-cooled capacitor banks, tuning them with the aide of the 3561A and its internal noise source to get the right cut-off frequency and slope. Nowadays I use an HP 35660A DSA at home for audio development. It has the advantage of two input channels, so together with its built-in signal source it can also be used as a network analyser to show both amplitude and phase response of audio filter networks. Hewlett Packard HP 8903B Audio AnalyzerDeveloping and testing audio electronics (for example, mixers and synthesizer modules) often requires the measurement of signal noise and distortion to a degree that cannot be easily achieved with oscilloscopes or multimeters. Measuring these characteristics is best done with dedicated test equipment, such as the industry-standard Audio Precision series. However they are rather expensive, so the hobbyist has three options:
The PC soundcard option can be the cheapest option. I say "can be" since to get worthwhile results you need a very good soundcard, ideally external to the PC rather than built-in to minimise the noise floor, and decent software that can analyse the data properly (e.g., TrueRTA). To measure distortion products it is also a good idea to build or buy an analogue notch filter so that you can remove the fundamental test tone and get the most from the soundcard's dynamic range. You also need a low-distortion signal generator to minimise any masking effects that a noisy generator would introduce (a PC soundcard might be good enough). And then you need to set it up in a way that minimises noise-pickup from the surrounding environment. Building your own THD Analyser can be very rewarding if you enjoy the challenge and have the abilities to build it to the required high standard. It most certainly is not the cheapest option, especially if you include your time. But, when finished, you end up with equipment that you can tailor to your specific needs. Finally, by far the quickest overall option is to buy a used audio analyser from a dealer or elsewhere. Names to look out for include Audio Precision, HP, Boonton, and Tektronix. The 8903B is one such machine, incorporating a low-distortion sinewave oscillator, a tunable notch filter, a frequency counter, and a sensitive RMS AC voltmeter. Each of them on their own are very useful, but having them together in one instrument with GPIB control makes for a versatile test setup. The photo below shows my 8903B driving itself at about 1kHz. The displayed distortion+noise is -87.43dB, or about 0.0043%, with a 30kHz low-pass filter.
Signal GeneratorsHewlett Packard HP 3311A Function GeneratorThe 3311A is a strange little curiosity. I believe it was an attempt by HP to produce a low-cost signal generator to compete at the lower end of the price spectrum, probably aimed at schools and colleges and basic function generator duties at the bench. Which is a fine idea, but on closer inspection you can clearly see that HP, at the time, just didn't understand how to make something cheap: high-quality Allan Bradley pots, gold-plated double-sided PCB, a sturdy diecast clam-shell case making servicing easy, and high-quality panel furniture (knobs, binding posts, etc). It sports a decent spec as well, covering the range 0.1Hz to 1MHz, three waveforms (sine, triangle, square), a separate TTL-compatible pulse output, fully-adjustable amplitude and offset, and even has connections on the back to allow modulation of the internal VCO. Hewlett Packard HP 3325B Synthesizer/Function GeneratorA decent lab needs a decent signal generator. While the 8903B has a low-distortion sine oscillator suitable for audio measurements it is not very precise in frequency and only covers the range 20Hz to 100kHz. The 3311A has a wider range (0.1Hz to 1MHz), has a choice of waveforms, and output amplitude and offset control, but it is only suitable for rough measurements as the frequency is liable to drift with time, and the dial setting is not very precise. So this is where a machine like the 3325B comes it. It is a synthesized function generator, giving 11-digit frequency precision, referenced to a quartz crystal, covers the range 1μHz to 21MHz sine (less for square, triangle and ramp, or up to 60MHz for TTL-compatible pulses), has a separate modulation oscillator for AM or PM duties, both linear and log sweep modes, GPIB, and so on. There are options to fit an ovened crystal for better stability (like mine has), or use an external reference clock for greatest precision, such as a GPS-disciplined 10MHz oscillator. Hewlett Packard HP 8165A Programmable Signal SourceThe 8165A sits between the 3325B and 3311A. It is certainly superior to the basic 3311A, although one could argue that the 3U case, the noisy fan, and the weight of the 8165A make it a tiresome beast compared to the lightweight 3311A. And when you need a simple source to tickle a circuit, the 3311A is right there in moments, rather than faffing around with the 8165A's parameter punching. At the other end of the spectrum, the 3325B is visibly superior when it comes to frequency control, with seven more digits of precision than the 8165A, over a wider range (down to μHz), and the option (as in mine) of a built-in ovened crystal for greater stability. The 3325B also has superior sweeping facilities over the full frequency range, as well as a built-in modulation signal generator, and can provide a TTL clock up to 60MHz. And yet... I find the 8165A sits in a sweet-spot all of its own. In many ways it is inferior to the 3325B, and yet in several ways it is actually superior!!!. Its sine, triangle and square output waveforms cover the full frequency range of 1mHz to 50MHz (the 3325B restricts the square to 11MHz and the triangle and ramps to 11kHz). With Option 002 the 8165A can also do logarithmic frequency sweeps, and while the sweep profiles are greatly restricted compared to the 3325B, what it does have is very usable for electronic music instrument development. And where the 3325B has a built-in modulation generator (a capable signal generator in its own right) the 8165A can generate counted bursts -- very useful for testing the response of VU meter drivers for example. In many ways, unless you need the higher precision or extensive frequency sweeping capabilities of the 3325B, then the 8165A is very close to the ideal bench signal generator for the general electronics lab. And the really crazy thing is how little they go for on ebay! |
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Copyright © 2001-2013 Neil Johnson |
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