Tektronix TDS220

Overview

After Inaki donated a Logic Analyzer to me, I decided that I really needed an oscilloscope to add to my, admittedly small, range of test equipment.

Oscilloscopes come in two main forms - analogue and digital, the first 'scopes were analogue when the measured signal was displayed immediately on a CRT display and was a faithful representation of the measured signal. Modern 'scopes tend to be Digital Storage Oscilloscopes (DSO) where the input signal is digitally sampled and stored for display on, for example, a TFT display. A DSO uses an analogue to digital converter (ADC) to change the analogue signal into a digital representation. To avoid aliasing, the sampling rate of the ADC must be at least twice that of the required signal bandwidth, and for best results, should be 5-10 times faster. Depending on the available memory in the DSO and the frequency of the input signal, varying amounts of data from the sampled digital waveform can be stored/displayed.

The cost of a DSO are mainly governed by the required sampling frequency and the amount of data storage required.

When I read that a Digitizing Oscilloscope Module was available for my TLA714 Logic Analyser, I initially thought that installing one of those would be a good way to add an oscilloscope to my test kit. However, Inaki has advised me that the TLA 'scope modules are very basic storage scopes and not really a replacement for a good quality oscilloscope. Just as well really, as even used ones on ebay currently range from £600 for a two channel TLA7D1 to over £1000 for a four channel TLA7E2.

It is possible to buy low cost, mini portable, DSOs from ebay for under £100. These ARM based devices look to be quite useful for limited applications, but they typically have low resolution (8 bit) ADCs, a relatively low analogue bandwidth (<30 MHz) and limited storage capacity (4K points/channel). My first thoughts were to buy one of these to get me started, but I decided that I needed (wanted?) something better!

Update: OK - I've done it! I spent a little more than I had intended, but having been recommended the Tektronix TDS220, I jumped in and purchased one off ebay. The TDS200 series are digital real-time oscilloscopes, the specifications for the TDS220 are shown at the bottom of the page.

[NB: In 1998, Tektronix issued a Recall Notice (RN) in relation to TDS210/220 'scopes with serial numbers in falling in certain ranges - the 'scope that I bought was not affected, but you may want to check the serial number against the Recall Notice if you are considering purchasing a TDS210/220. The RN describes the issue as "certain incorrect use of these products could cause the ground connection to fail, potentially exposing the user to risk of serious personal injury or death."]      

Data Capture

When I bought the 'scope, I was aware that it did not have facilities to archive or export data in the same way that the cheapest, more modern, DSOs do using USB etc. I chose the 'scope on the basis of its bandwidth and sampling rate for use in "real time" diagnostics, rather than its ability to export the data for other uses. I am still happy with my choice, but have subsequently found that being able to export the data is a "nice to have" feature and would give much better results than the poor iPhone photo shown above.

The TDS200 range has an option slot that can be used to additional functionality to the 'scope, including a number of different communications options. You can download the Tektronix instruction manual for the TDS200 series extension modules from the documents page
The most basic option is the TDS2HM, this adds a parallel printer interface which can be used to generate hardcopy from the display. In addition to the printer interface, the TDC2CM communications module adds RS232 and GPIB / IEEE488 interfaces.
Based on the limited information in the extension module manual, I didn't think that the parallel option was going to satisfy my needs, so I tried to find a TDS2CM. These modules are obsolete and did not appear to be readily available. The cheapest (used) one that I could find in the UK was £185+ VAT (£37.00). I did manage to buy one from ebay.com, but with P&P, tax and duties, it still cost ~£135. The use that I will get from this module definitely does not justify the cost, but it was annual bonus time :-) The next problem was being able to interpret the data coming from the RS232 or GPIB port on a PC. The extension module manual is not very helpful in this area and I thought that I would need an GPIB to USB interface to make sense of the data. I guess because the market for GPIB equipment is quite specialised, although there are lots of suitable devices from the likes of HP and National Instruments for sale on ebay, they are generally very expensive, starting from around £100 and quickly getting to many hundreds. The cheapest device that I could find was from Australia (ebay.com.au) and cost ~£40, with tax and duty, that would have been ~£56.00 and the device did not seem to be fully compatible with IEEE 488.1/.2 and had had only a limited amount of testing.
I eventually found this module from Prologix in the US. List price for these is $149.95, with shipping & taxes, that would have been close to £150.00. However, the manual showed that this device was going to be suitable for my purposes and along with some open source software+, should have been capable of archiving data, reproducing 'scope traces etc. (+ The HP 7470A plotter emulator from the KE5FX GPIB Toolkit)
Only after I had managed to find and buy a used Prologix controller from ebay.uk, did I find out that this was not actually necessary. Tektronix have a couple of free software packages that can control and extract data from various Tek devices, including the TDS220, using the RS232 or GPIB port. Openchoice Desktop Wavestar Openchoice Desktop version 2.4 was released in March 2014 and is the latest supported program available for Microsoft Windows. Wavestar is obsolete and no longer supported, but it is still available for download and includes a couple of features not available in Openchoice. [A comparison of the features of Openchoice and Wavestar is available on the Tektronix Website.]
The Openchoice Desktop desktop - once my TDC2CM arrives, I'll be able to try it !

Tektronix TDS220 - Product Specifications