A Weston standard cell – an introduction to voltnuttery?

Weston standard cells are pretty, but can they be more than a broken antique? And yes, that is liquid mercury visible at the bottom of the left leg.

One Cell

One Cell

I remember using Weston cells at school during physics lessons to measure voltages more accurately that could be done with a standard analogue meter (DMMs being years in the future). The measurements are based on using a 1m ruler plus resistance wire plus an uncalibrated NiFe cell as a bridge, and use a Weston cell to calibrate the bridge. That technique (which is a good example of using simple materials, understanding and imagination – rather than merely throwing money at the problem) is outlined below.

Anyway, I have vaguely wanted a Weston cell ever since, a little more so since I got hold of an HP3468 and have wanted to check whether it is still “accurate”. I’ve wondered about buying one on ebay, but they are unjustifiably expensive and sufficiently fragile that I doubt they would survive shipping.

Unfortunately I’ve now found a cheap one, and I fear it might lead me down the expensive path of voltnuttery. Memo to self: do not become a voltnut.
Continue reading

Renovating a Tektronix P6013A 12kV 100kHz Scope Probe

I’ve recently had to debug several scope’s 2kV-3kV HV supply and the CRT’s Z-axis waveforms at 2.5kV. So far I’ve got away with using a homemade 1000:1 voltage divider and a multimeter. Since that is crude and not particularly safe, I’m not going to mention the details in order to avoid someone apeing me and hurting themselves.

Then, at a recent auction I managed to pick up:

  • a 40kV meter for measuring 17kV anode voltages, but which barely registers 2kV
  • a Tek P6013A 12kV 1000:1 100kHz scope probe

The probe was functional but missing part of the handle. While not strictly necessary, I wanted to have a little fun fabricating the missing part…

Continue reading

A Traditional TDR Cable Tester With <2cm Resolution

What can you see, test and measure with a traditional time domain reflectometer (TDR)? The answer is “more than you might expect”:

  • measure impedance variations in connectors/filters/antennas/PCBs
  • locate short/open circuits and damage in cables
  • locate intermittent faults in cables and connectors
  • locate connectors in cables

and can resolve discontinuities around 2cm apart. That resolution is at least 10 times better than can be obtained with the typical homebrew logic pulse + oscilloscope combination.

I recently bought a couple of cheap 1970s Tektronix 1502s in the hope that I could make a single working frankenmachine.  My initial assessment was depressing: one had a cracked and broken case (so I assumed the CRT was also broken), the other’s electrolytic caps had spewed acid across the PSU and had a faulty 2kV PSU, and both had defective NiCd batteries – and it won’t even start without a working battery. But eventually I managed to get both working: I recapped the PSUs, rewelded the case with methylene chloride, used my “new” 12kV scope probe and 40kV meter to repair the HV PSU, created a “NiCd emulator”, and the CRT wasn’t damaged after all. Later reading of a TekScope magazine indicates it isn’t surprising the CRT survived: it is mechanically completely isolated from the chassis to protect against up to 26 12″ drops.

So I am now the proud possessor of two nice little portable waterproof instruments, literally designed for field use – one of the service manuals indicates they were used with Patriot missile defence systems.

Tektronix 1502 TDR Cable Tester

Tektronix 1502 TDR Cable Tester

Continue reading

Rescuing a Broken Tektronix 465 CRT

A non-functional Tektronix 465 oscilloscope with a dented case and several serious faults was recently donated to my local HackSpace. Since they didn’t want another broken scope, I took it to see what was wrong. Eventually, after learning about CRT theory, with the help of very knowledgable people on the TekScopes forum, and gingerly measuring waveforms at -2450V, I came to the conclusion the CRT’s grid was faulty. Reluctantly removing the CRT, I found that it could not be repaired. Since parts of the CRT are rather beautiful, I decided to salvage those parts for display. This note outlines the original tube, the fault, and the process of turning it into something fit for a display cabinet.

Continue reading

SDR Utilities

I’ve been experimenting with an SDR dongle to see how it can be used as a 1.5GHz scalar network analyser, as a time domain reflectometer, and to estimate digital signal edge speeds. While doing that I’ve developed several general-purpose command-line utilities which capture and post-process power spectrums. These utilities are distributed with a MIT licence, and the code is at the bottom of this note.

  • rtlscan: scans, measures and displays a raw scalar power spectrum, and saves the power spectrum in a CSV file
  • rtlplot: displays one or more raw power spectrums
  • rtldiff: displays one or more calibrated power spectrums, i.e. the difference between multiple raw spectrums and a single reference or calibration spectrum
  • rtltdr: displays the impulse response implicit in a calibrated power spectrum

The graphs can have linear or logarithmic frequency axis, can be zoomed, panned and printed, and have text annotations added. The interaction with the SDR dongle is via Kyle Keen’s rtl_power program, so its capabilities and limitations are reflected in these utilities.

As is traditional, these are an unfinished work; I already know of the next change I would make, but I have not verified that it would be an improvement. Nonetheless, the utilities are usable.

Continue reading

Measuring Digital Signal Edge Speeds Without An Oscilloscope

This note shows that measuring a digital signal’s risetimes and falltimes does not require multi-GHz oscilloscopes; with imagination, very cheap test equipment is sufficient. Measurements show that even common-or-garden 74LVC gates can have 10%-90% transition times of around 625ps.

Continue reading

Measuring RF Filters with a Homebrew 1.5GHz Scalar Network Analyser Costing £32/$48

I already have useful low/medium frequency signal sources, spectrum analysers and oscilloscopes. Now I want to inexpensively measure RF filters and transmission line imperfections. This is possible for only £32/$48, as illustrated by the measured response of 300MHz and 460MHz high-pass filters and an open-stub transmission line filter:

Continue reading

Poor Man’s Homebrew TDR With 4cm Resolution – Part 1

The standard equipment for measuring impedance variations is a Time Domain Reflectometer, TDR. TDRs are very effective but resolving small elements requires a wide bandwidth, which implies the TDR will be very expensive. This note explores a £35/$55 alternative based on SDR dongles and noise sources, to see what can and cannot be achieved.

Although there are limitations, initial results are surprisingly good and useful. For example, Figure 1 shows reflections in two different transmission lines with an open-circuit stub 3.1m from the TDR.  The first stub is 19cm long, and the second is 29cm long.


Figure 1: 19cm & 29cm Stubs

The stubs’ differing lengths are clearly distinguishable.

Why do such impedance variations matter? Because with RF circuits and medium/high speed digital circuits, connections must be uniform-impedance correctly terminated transmission lines. Impedance discontinuities in RF circuits causes peaks and troughs in the frequency response, leading to poor performance and/or link failure. Impedance discontinuities in digital circuits cause signal integrity problems, leading to marginal operation and/or pattern-sensitive errors.

Continue reading