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Test Equipment

Here I give my personal thoughts and recommendations on test equipment and other electronic tools I use either at work or at home.


In test equipment circles there are many terms which are either misunderstood or used incorrectly. Consequently there are many webpages explaining, in varying degrees of detail, what these terms mean, so I am not going to do that here other than to provide a brief summary for completeness. For example, Wikipedia has an entry explaining the two words (note: accuracy and precision can also be applied to the written word).

How close to reality a reading is.
How repeatable the reading is.
A measure of the smallest change in input that can be detected.




The AVO 8 is probably the most widely recognised analogue multimeter ever made. Yes it is old. Really old. Ancient. Built like the proverbial brick **** house. And still working fine today. Even in these modern times it still makes useful measurements, especially in the DC current ranges with its lowest range of only 50uA full scale (which on that scale is a lot!).


G & E Bradley CT471

Here's a weird one that I owned for many years, mostly while at school. As far as I can tell this was a meter designed for the Ministry of Defence (MOD) and used by servicemen in the field. I say field, as mine spent its time on HMS Illustrious before being sold off through one of those government auctions.

It was a weird meter, with a weird scale, and build like a tank! It was a very capable electronic analogue multimeter, covering all the usual measurements but with a spec that looks like it was designed by a committee of civil servants who had never used a multimeter in their life! I'll always remember the vivid yellow label on the front cover: "HAVE YOU SWITCHED OFF"

CT 471

Avometer 2001 Multimeter

AVO.... now there's a name. This rare little beauty was the first DMM I used in 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 the company.
Outline spec:
Function Range Accuracy
AC Volts 200mV - 1000V ±1%
AC Current200μA - 10A ±1.5%
DC Volts 200mV - 1000V ±0.25%
DC Current200μA - 10A ±0.75%
Resistance200Ω - 20MΩ 
Digits 3½ ("1.999") 
Avometer 2001 DMM

As you can see it's 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 Multimeter

As 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.

Outline spec:
Function Range
AC Volts 200mV - 750V
AC Current200μA - 2A
DC Volts 200mV - 1000V
DC Current200μA - 2A
Resistance200Ω - 3GΩ
Frequency 20Hz - 200kHz (auto-ranging)
Digits 4½ ("1.9999")

Discontinued. Replaced by model 289.

Original list price: $499.

Fluke 8060A

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).

A discussion on Dave Jones' EEVBLOG introduced the designer of the 8060A - Dave Taylor - and some interesting insights:

  • Another feature of the 8060 I was proud of was that it was the first DMM to have truly fast continuity. A separate hi speed comparator was added to the A/D silicon expressly for this purpose. It doesn't depend on a conversion. I rate almost all DMMs by hitting the probes together as fast as I can. A fast continuity will beep. I'm surprised how many DMMs designers still don't get how important this is in troubleshooting.
  • I have a list somewhere of 8060 tricks, but one I remember off the top of my head, because I spent a lot of time designing that switch array to do it... If you pop out the two bottom switches, the input is connected directly to the A/D and the 10M divider network is disconnected. Therefore, like far more expensive DMMs, you get near infinite input impedance. Works only on the 200mVdc and 2Vdc ranges of course. But useful for those times when you don't want to load the circuit with 10M ohms. The calibration shifts slightly, but relative accuracy it great with absolutely no circuit loading. (Neil's note: this is similar to the HP 3478A below, which has a very high input resistance in the low voltage ranges).

Hewlett Packard HP 3478A Bench Multimeter

Hand-held DMMs are great for general measurements both at the bench and when out and about, or even just head-down inside a mixer. However, for taking important measurements or perhaps a long-term experiment needs monitoring then a bench DMM is the better solution.

There are many good examples of bench DMMs on the used market these days. The obvious contenders are there: HP/Agilent and Fluke, with some credible offering from Keithley and Schlumberger. From the HP stable comes the venerable 3478A 5½ digit DMM.

HP 3478A

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.

If you have one and are looking to get it calibrated I can personally recommend Absolute Calibration. They calibrated my 3478A for a very reasonable fee, and that included their own van collecting and returning it to me.

Here's a video of Dave doing a teardown on a 3478A: EEVBlog

Hewlett Packard HP 3456A Bench Multimeter

The 3478A is a great general purpose DMM for general bench duties. For greater precision and accuracy you need something with at least one more digit. While modern offerings from HP/Agilent/Keysight are the 34401A the venerable 3456A (and its cousin the 3457A) are still worthy machines to have:

HP 3456A

Agilent 34970A Bench Data Acquisition System

The 34970A from the-company-formerly-known-as-HP (TCFKAHP) is a versatile unit, comprising a three-slot chassis with RS232 and HPIB control, an internal 6½-digit DMM (the guts of the famous 34401A - see below), and a range of plugin interfaces for routing inputs to the DMM and other switching functions. For example, the 34901A provides 20 fully-switched DMM volt/ohm/thermocouple/counter (reciprocal, yay!) inputs, and two current measuring inputs. Together with a breakout box on an umbilical cable it makes for a versatile measurement and logging system.

HP 34970A

Hewlett Packard 34401A Bench Multimeter

Thanks to a good friend of mine I have recently (December 2019) added the classic 6½-digit 34401A to my bench DMM setup. This is a well-known and highly-regarded bench DMM used throughout industry. Newer models have since been released, but this instrumentsstill todays holds up well.

HP 34401A

Hewlett Packard HP 400 Series AC Voltmeters

DMMs are great for taking measurements, but when you just want to get a feel for how a circuit is behaving there's nothing quite like a proper analogue meter needle to show you - not so much the actual value, but how the circuit under test responds when you adjust something like a trimmer. For audio circuits I find the classic HP 400 series bench AC voltmeters are ideal: great sensitivity, huge meter movement, and wide range. The 400F has a built-in 100kHz low pass filter, so ideal for audio, and is the most sensitive, going down to 100µV FSD on its most sensitive range. The 400EL, while being less sensitive (minimum 1mV FSD), has the advantage of a linear-in-dB scale.

Both meters conveniently use 1mW into 600Ω as the dB scale, so you can read off directly in dBu. One thing to note is that while the meter shows "RMS Volts" what they really mean is average reading but scale adjusted for a sine wave - anything more involved than a sine wave and the readings will not be quite so true.

HP 400

AVO B183 LCR Meter

A dedicated LCR meter is a useful addition to the electronics workshop. While the DMMs described above are able to measure resistance, they do not measure inductance or capacitance. And while there are plenty of oriental DMMs that will also measure L and C for you, I prefer instruments to do a few things well rather than many things not so well.

The B183 comes from the same stable as the quirky 2001 DMM shown above. It shares the same industrial design (case, LCD, switches, 4mm sockets) and dates from the same period when AVO was part of Thorn.

With it's 3½ digit display and range switching it can measure up to 200µF, 20MΩ and 200H. It uses either a 100Hz or 1kHz internally-generated sine wave, and applies very little stress on the component under test (typically 150mV or 100-150µA). That should help keep self-heating effects to a minimum.

Overall a nice little meter in a quirky/funky case!

AVO B183


Tektronix TDS340A Oscilloscope

The 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.

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 Probes

The 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 70MHz. 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 Data

One 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.

Agilent 54622D Oscilloscope

My day gig involves both analogue and digital circuitry. For quick probing around a misbehaving circuit, capturing serial comms, monitoring power circuits, and so on, having a mixed-signal scope is really handy. Probably the earliest best of these is the Agilent (or whatever they're called this week) 54622D. It has two 100MHz analogue channels and 16 digital channels, and one of the earliest scopes to come equiped with the MegaZoom function, capturing up to 4 million samples so you can zoom into a captured signal long after the event.


Tektronix TDS540C Oscilloscope

As circuits increase in complexity and speed you need a bigger and faster scope to see what those circuits are doing. For example, on a recent design I had three inputs and an output (audio input, control voltage, carrier signal, output) to work with, and being able to monitor all of them on the same screen would have been very illuminating.

On the used test equipment market the venerable TDS series digital scopes from Tektronix are excellent buys. The TDS540C is one such beast, with four 500MHz input channels, up to 2GS/s sampling (up to 100GS/s with ET sampling), and various options including the highly-useful Centronics printer port (see the above comment about printers).


Tektronix DPO2024B Oscilloscope

Modern circuits, modern times, and a absolute bargain on eBay. The DPO 2024B is a modern 4-channel 200MHz digital scope from Tektronix. With USB and suitable software on a PC you can easily download screenshots, data dumps, etc. It is also much smaller and lighter than the TDS340A it replaced.

On the downside, the plots that it saves are not very pretty. SO I am writing a tool (in Tcl/Tk with gnuplot) to read in CSV files from the scope and plotting them in a nicer, clearer form.


Analogue Scopes

While modern DSOs (like the TDS340A) have lots of super-whizzo features, there is still a need for a good analogue scope in the lab. They are much faster to use (no faffing around in menus); they usually don't have fans (quieter operation); they don't lie to you -- what you see on screen is a direct view of the voltage at the input terminals (no need to think about Nyquist when setting the timebase); and a quicker turn-on time.

I own, or have owned, a variety of analogue scopes, including:

  • Tektronix 2215A - fanless 60MHz dual channel
  • Tektronix 465M - fanless 100MHz dual channel
  • Tektronix 468 - 100MHz dual channel with early sampling system and very noisy fan
  • Hameg HM203-6 (really nice beginner scope)
If you're on the lookout for a good scope to learn on then I would strongly recommend the Hameg.

Frequency/Time Counters

Hewlett Packard HP 5334A Universal Counter

While 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).

Hewlett Packard HP 5315A Universal Counter

While the larger 5334A is suitable for detailed measurements the smaller 5315A is an ideal frequency counter when you want a quick measurement of a single signal. And being a reciprocal counter it is quick to use for measuring low-frequency audio signals.


Also, if you can, get one with the TCXO option (001) installed as that provides a more stable reference oscillator. Only after opening the case did I discover that mine has that option.

Hewlett Packard HP 53131A Universal Counter

The HP 53131A is one of the newer frequency counters/timers, and nicely integrates into a GPIB setup. The two channels are very configurable, and with internal routing can measure very long periods (low frequencies).


Power Supplies

As 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 6236B

Probably 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:

HP 6236B

Thurlby PL320

These 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!

Thurlby PL320

Hewlett Packard 6624A

Automated testing often requires several supplies at different voltages. Or, in the case of designing analogue synthesizers, you also want several CVs for controlling VCOs, VCAs, VCFs, etc. Together with an HPIB computer you can quickly put together an automated system to power and/or drive your circuit.

The 6624A is a 4-channel power supply unit which is ideal for this. It can supply more than enough power for smaller op-amp circuits, and it also features programmable over-voltage and over-current protection just in case your test program sends the wrong voltage setting. One role it is very good at is powering mixing desks - the four channels can provide the two op-amp rails (typically ±15V but can be up to ±18V), phantom power (+48V), and a fourth output for desks that have a separate logic supply (typically +5V).

HP 6624A

Hewlett Packard 6612C

Sitting roughly half-way between the 6236 (small, versatile) and the 6624 (GPIB, flexible protection) is this versatile little chap. Up to 20V and up to 2A makes it perfect for powering smaller embedded systems designed to be powered from wall-warts. And having RS232 (as well as the ubiquitous GPIB) makes it a doddle to hook up to a PC for some quick automated testing.

HP 6612C

Agilent E3640A

While building a VCO characterisation system I needed a GPIB-controlled PSU to act as a CV source. An E3640A happened to pop up on eBay at the time, so grabbed it. A useful little box, single output, GPIB-programmable.

Agilent E3640A

Spectrum Analysers

Oscilloscopes 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 (Note: the exeception that proves this rule is the HP 3580A, which covers the range 5Hz to 50kHz).

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.
Note: this is theory. Actual machines may add processing time, or require the sample record to be full before processing, etc.

Hewlett Packard HP 35660A Dynamic Signal Analyzer

The first spectrum analyser I owned was an 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.

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.

HP 35660A

Here's a video of Dave fixing up a 35660A: EEVBlog

Hewlett Packard HP 8594E Spectrum Analyzer

Analogue electronics can be a tricky thing at times. As the old saying goes: "amplifiers oscillate, oscillators don't". It is remarkably easy to make a seemingly simple analogue circuit oscillate at a high frequency, maybe 100s of kHz or even up in the MHz range. All it takes is a bit of capacitive coupling in the wrong place, a lead too long (higher inductance), or some nefarious feedback path you hadn't thought of, and suddenly you're an AM transmitter! So having some equipment that can detect signals up to very high frequencies is very useful even when designing audio-band circuits. Especially useful for on-the-bench sniffing around a circuit or module to check for any unwanted emissions as part of EMC pre-compliance checking (radiated emissions for synthesizer modules is likely to extend up to 1GHz, unless you have an internal clock over 108MHz in which case you could be looking at up to 6GHz).

HP/Agilent have a range of portable spectrum analysers in the 8500-series. I picked up this 8594E from the usual place for a great price, and while it may not have the resolution bandwidth of a larger non-portable lab-grade machine (it only goes down to 1kHz, bigger machines can go down to 3Hz) it is certainly good enough for probing around a circuit to see what it might be doing, good or bad.

HP 8594E

Audio Analysers

Developing 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 spectrum analysers, 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:

  • older-generation audio analyzers,
  • home-brew apparatus
  • PC soundcard and software

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 software that can analyse the data (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 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.

Hewlett Packard HP 8903B Audio Analyzer

The 8903B is the first audio analyser I acquired, 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.

HP 8903B

Audio Precision SYS-22A

Where I think the HP 8903B excels at benchwork, with its front panel and built-in computer, the standard reference audio analyser is, of course, the Audio Precision analyser.

The first machine they produced after spinning out of Tektronix was the System One. While no longer supported and ancient by modern standards, it still achieves a respectable level of performance, easily a factor of ten better than the 8903B (my 8903B in self-test gets down to around 0.0034%, the SYS-22A down to around 0.00035% - notice the extra '0'). In the "22" configuration it has two analogue channels (two generator output channels and two analyser input channels) but lacks the DSP option of the "222" or "322".


Other the years I have been slowly expanding my AP system. The main unit has the wow-and-flutter board, the burst board, and the CCIR, A-weighted, D50F, and RCR filters. I also have a DCX-127 and a SWR-122F.

The downside of the AP systems is that they require an external PC to operate them. The original software requires either DOS (to run S1.EXE) or early Windows (an application called ApWin), so keeping an old PC going is something to think about. But I think that is a small price to pay for owning these machines. The connection to the PC was originally through a custom interface card, which in the first versions was a something (but not exactly) like an ECP/EPP printer port on a non-standard I/O address long before the days of ECP/EPP ports. Today there are at least three four options:

  1. Design and build your own
  2. Acquire an original APIB card - around £150 to £250 on eBay
  3. APIB-LPT software and cable - about $150
  4. diyaudio USB-APIB adaptor - about $150
Note that the newer AP-designed USB-APIB adaptors do not support the System One.

After a bit of ferreting around and asking for some help from the obvious place I have pieced together the latest/last S1 and the two companion post-processor tools POST and PLOT. Download them (zip folder, 280kB). I have used these tools to take the output of the analyser program and convert the plots into EPS files for import into Scribus. Works a treat.

Marconi 893B Audio Power Meter

When developing small power amplifiers for driving headphones and speakers, it is good to both have a representative load that can be set to a range of different impedances, together with a power meter. The Marconi 893B conveniently satisfies both requirements.

Marconi 893B

I find it particularly useful in developing headphone amps, as I can set it to a range of impedances to see how the amplifier will behave when driving different headphones (I typically test at 8Ω, 32Ω and 600Ω) where I'm typically only outputting a watt or so.

Signal Generators

Hewlett Packard HP 3311A Function Generator

The 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.

HP 3311A

Hewlett Packard HP 3325B Synthesizer/Function Generator

A 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.

HP 3325B

Hewlett Packard HP 8165A Programmable Signal Source

The 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 of an 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!

HP 8165A

Hewlett Packard HP 3314A Function Generator

With a similar feature to the 8165A - albeit in a package half the size - sits the 3314A. While it only goes up to 20MHz, it adds a more flexible sweeping mode, arbitrary waveform mode (with a scope you can "draw" the waveshape), and a half-cycle mode. It has a second internal oscillator for triggering sweeps and bursts, which is actually really useful to be able to set it up for repeated bursts or sweeps into a circuit.

For general audio and electronic music circuit development this is a perfect bit of kit. Granted the sine wave is not pure enough for distortion checking, but that's what the audio analyser is for.

HP 3314A

Hewlett Packard HP 8112A Pulse Generator

Signal and function generators are all well and good for analogue development, but when it comes to a bit of digital work you need signals that are high or low rather than sine or triangle. While most of the signal sources can generate square or rectangular waveforms, they don't provide the kind of control that you really need.

Enter the pulse generator. In this case the HP 8112A. All it does is generate pulses. Single pulses. Bursts of pulses. Continuous streams of pulses. With variable rise and fall times and slopes. And all manner of other parameters to define the stream of pulses you need to stimulate your circuit. Very useful in developing digital circuits.

HP 8112A

Fluke 54200 TV Signal Generator

Anyone working wtih analogue TV (yes, it's still a thing in 2021!) needs a way of generating all sorts of test signals. The Fluke 54200 is one of the best devices of its kind: PAL, NTSC and SECAM, test patterns, NICAM, digital services, and fully remote controllable over RS232 or GPIB. It's a bit heavy, and it doesn't run embedded Windows, but it's a reliable workhorse for development and test.

Fluke 54200

Logic Analysers

Oscilloscopes show you how a voltage changes with time. Spectrum analysers show you the spectral content of a signal. Logic analysers move up the semantic ladder and show you a logic interpretation of a signal or group of signals, and can also interpret sequences of states as higher-level events (e.g., processor instructions, programs, etc).

Intronix LogicPort

I've had this little chap in my kit bag for years. It is small, portable, USB-powered, and has more than enough channels and speed for the majority of on-site debugging (ah, the life of the travelling apps engineer!). While the PC software may not be the very latest and funkiest with millions of data analyser plugins galore, what it does is good enough, and with a bit of thought can be made to do what you want, i.e., zoom in on the glitch or whatever and work out what is going wrong.


Agilent 16702B

Probably the zenith in Logic Analyzers was the Agilent 16702B. Running HP-UX (not Windows as later machines ran), a large colour touch screen, or keyboard and mouse, or remote X sessions, five slots for building a system for your specific needs, for the home lab these are the machines of choice:

Agilent 16702B

I am slowly pulling together software tools and libraries for this machine. So far I have:

  • 16702B Setup CD ISO (various versions)
  • HP10391B Inverse Assembler
The inverse assembler (IA) is useful when working on a CPU/micro or a bus.

Thanks to Glen for help in sourcing the CD images.

Controllers and Computers

Hewlett Packard HP 85

Ancient. Truly ancient. And yet.... so easy to use, quiet (no fan), instant turn-on (a couple of seconds and it's ready to go), with a built-in printer and tape drive storage, decent keyboard, and a 5" mono screen. It is, in my opinion, pretty close to the ideal instrument controller. And with the legendary HP maths abilities it is also rather good at analysing the data for you!

On a more detailed note, for anyone contemplating hunting one out for a HPIB controller make sure you get the HPIB interface card - unlike it's larger brother the HP 87 it does not come with HPIB as standard.

Truly a thing of beauty:

HP 85

The HP 85 can be expanded through a range of ROM modules and plugin cards. The ROM modules expand the built-in HP BASIC with new commands or features, while the plugin cards add memory and communications interfaces. My own system includes:

  • ROM modules:
    • Matrix
    • Advanced Programming
    • Input/Output
    • Plotter/Printer
    • Mass Storage
  • Plugin cards:
    • 82936A ROM drawer (for the aforementioned ROM modules)
    • 82903A 16kB memory
    • 82937A HPIB interface
    • 82939A Serial interface


I group calculators in with test equipment for the simple reason that they are another form of electronic device used in the design, test and debug of electronic and software systems. And just as for the kinds of test equipment shown above, quality matters.

Texas Instruments TI-33

This was the very first scientific calculator I ever owned. I think I bought it at a jumble sale and it served me well through school. Nowadays it is relegated to the "curiosity" pile.


Hewlett Packard HP 15C

The HP 15C is considered by many to be the perfect engineer's calculator. It is small, lightweight, runs for years from a set of watch batteries, and the quality of the maths algorithms is legendary. This is my home lab default calculator, and there is also a pretty good free software emulator (written in Tcl no less!) available.

HP 15C

Hewlett Packard HP 32S

The HP 32S is a later version of the HP 11C (the simpler cousin of the 15C). While it lacks many of high-end functions, it is a good solid engineering calculator. This is my work calculator - I trust the results, and being RPN it is less likely to be "borrowed" by my colleagues!

HP 32S

Hewlett Packard HP 48SX

Where the 15C is perfect for quick numerical answers that you can trust, the 48SX adds a large screen, expandability, and communications.


Emergency Backup Calculator

When I need to do calculations without the need for batteries I have my emergency backup calculator to help me out. I bought mine from a long-gone army surplus store (Malcolm Mitchell's in Hastings old town) many years ago.

No batteries!

Copyright © 2001-2024 Neil Johnson