Proper tools are half the job…
… is a Dutch saying. A bit cliché but true non the less. This is a quick post to acknowledge two pieces of vital equipment I acquired recently that have made my DIY life much easier.
Rigol DS2072 digital oscilloscope
My old handed down Kenwood CS-1040 scope slowly died after being in service since 1982. Contacts and switches gave up and measurements became unreliable, so an update was in order. Lots of really good second-hand gear out there, but since real estate on my desk is limited, I looked into digital scopes (not to be confused with the CRT scopes with digital functionality). They’re sometimes frowned upon when used for analog audio design, but I really don’t understand why. Especially with the newer generation with very fast screen updates, high accuracy and tons of useful added functionality.
Rigol was one of the first to produce a digital scope for DIY’ers on a budget, the DS1052. A 50 MHz scope that could be hacked into its 100MHz bigger brother. The screen is a bit tiny and low resolution, refresh rate is limited, it has noisy fan and many other limitations for the more demanding user, but you get what you pay for. For the price tag, you get a hell of a lot of scope! After reading a lot of reviews, comparing specifications and the Rigol brand in the back of my head, I bit the bullet on Rigol’s new line of scopes, the DS2072. A two-channel 70 MHz digital oscilloscope, with a sample rate of 2 GSa/s (two giga-samples per second), an eight inch LCD display with 800×480 resolution and all the functionality you could wish for and more. For Dutch readers, I bought mine at AR Benelux. They were very quick and helpful in their communications with very fast actual delivery. I can strongly recommend them.
The first thing I fell in love with were the permanent on-screen measured values. No more counting divisions! Just a personal selection of peak-to-peak values, frequency, you name it. Second was the ability to make the waveform persistent on-screen. When dealing with once-in-a-while peak values (like peaks in the signal of slap bass guitar), capturing them on an analog scope can be hard. Setting the screen to infinite persistency permanently shows ALL past waveforms with the biggest of course easily identified. Third (I hate to admit), the auto key. Volts/div, time/div, trigger and screen position, all done in the blink of an eye. Fourth and final, the ability to capture measurements to a PC. The scope can be connected by USB or network. Over on the EEVBlog forum, there’s a user developed program for capturing and controlling measurements. All credit goes to user Marmad, developer of this RIUU (Rigol Ultravision Utilities) software, and for being on the forefront of what is now a very large and helpful community around the Rigol DS2000/4000 series. The RIUU software has added A LOT of functionality to the DS2072. The latest version even let’s you make 3D plots. Good stuff. Cursors, zoom windows, waveform recording, RS232 decoding et cetera et cetera are among the very extensive toolset of this scope, but I will not be going into too much detail or do an in-depth review. Others have done that already, better than I can. Below are some reviews by two guru’s I hold in high regard.
Philips/Fluke PM5136 function generator
I bought this one second-hand for a great price. There’s not a whole lot of info to be found online about this awesome machine. Maybe because it’s not very exciting at first glance. All it does is put a certain waveform at a certain amplitude. But the ease of use through the clear menu driven control is just great. All basics are there; sine, square and sawtooth in multiple ways, ranging from 0.1 mHz (yes, milli-hertz) up to 5 MHz with a rise- and falltime of less than 20 nanoseconds @ +500kHz . Sweep, DC, AM, FM and pulse output. Separate in- and outputs for TTL level out, input for external frequency modulation or sweep to name but a few. A full featured device that does everything it advertises very well.
A function generator is an essential piece of gear when working with audio for measurements on amplification, frequency response and distortion artifacts. A near perfect base signal (sine) is mandatory. A lesser known test is to send a square wave of around 10 kHz through an amp to look for high frequency oscillation or ‘ringing’.
Another awesome use for a function generator I found online is to determine the length of an open-ended piece of cable. Useful in situations where long lengths are tucked away inside walls or ceilings or to track down and roughly pinpoint the location of breaks. A fast risetime, like the PM5136’s, is essential here. This is how it works. Every cable suffers from reflection when not terminated with the same resistance as the cable’s characteristic impedance. You can see this reflection on a proper scope when sending e.g. a 100 kHz square wave through an un-terminated cable. For the amount of time the signal travels through the cable, it only sees the cable’s impedance, lets say 75 ohms. The scope shows the amplitude that goes with this impedance-relation. Until it hits the open end which of course is very high impedance, and is reflected back to the signal generator. A high impedance input gives a much higher amplitude (almost no attenuation), again shown on the oscilloscope. This ‘step’ in amplitude is the time it takes the signal to travel through the cable, hit the open end and back again. Signals don’t travel at the speed of light (29,97 cm/ns) through cables with non-air insulators between the mantle and core. For e.g. a RG59 cable with PE insulator, the speed is reduced by a factor of around 0.66 (66%), called the velocity factor. The length of the cable can then be calculated by dividing the length in time of the ‘low impedance step’ by two, times the speed of light times the velocity factor:
L = (T / 2) * (c * VF)
I tested this procedure on a known length of cable and it was accurate to about 2%. There’s dedicated calibrated equipment that uses the same basic principle but with more accuracy. Below is a video demonstrating the procedure.