…Written by Frustrationi cum Noisare
In Part 1 and Part 2 of the 3-part blog series on noise troubleshooting, we looked at noise theory and some general principles of causes of electrical noise and general good-practice guidelines for low-noise recordings. Part 3 below gets down to some practical strategies for finding the source of the noise, with some recommendations on tools and hacks.
1. How to approach the stalk – some thoughts on strategy
1.1. Know your prey – define the problem properly
Your approach will be partly determined by whether it’s a new rig, or whether it’s a working setup which develops noise suddenly. But as a general principle, avoid the temptation to barge in and make changes… Gather as much information as possible before you start ringing the changes and WRITE EVERYTHING DOWN. Make notes, take pictures, keep record of traces. Approach it like you’d approach a new and complex experiment. Be very methodical:
Ask yourself some questions, depending on the setup’s history –
• What exactly are you looking at on the trace? Are there other ways of interpreting it?
• What has changed in the environment around your setup? Has anyone else been using the same rig?
• Are there any other unusual/concerning/suspicious issues on the rig? Has something that presented itself just before the noise surfaced?
1.2. Being obsessed with buffalo when you could easily go after rabbits – What would an easy solution look like?
For this, my suggestions would be to look out for some basic user-errors resulting in noisy traces. These could be:
• Bad recording electrode – your electrode could either be dirty, not coated properly, or the connection with the pin may be unstable
• Bad connection on the reference electrode, or the reference electrode not being coated properly
• Bodged pipette solution – for example, not enough chloride
• Wet or dirty pipette holder – Basic error, but someone using your rig (not you, of course not!) may be overfilling the pipette, resulting in the pipette solution being pushed out into the pipette holder. You only need 2-4mm of the electrode length to be immersed in pipette solution. If wet, dry it with clean air. If dirty, you’ll have to disassemble the pipette holder, remove the silver electrode and the rubber o-rings, rinse with ethanol and then distilled water several times and allow to dry. If you have a sonicator it may be useful to put the components through the process to remove small particles and salt residue
• Could it be that someone’s changed some of the cabling and/or the settings on your software? Incorrect scaling or gain settings may lead you to think you have more noise when you immerse the pipette in the bath
• Spillages and dried salt around the bath may lead to noise from an electrochemical junction. This appears as an unstable, fluctuating noise on a time scale of milliseconds to seconds. It may be caused by salt solution that is spilled, for example near the chamber, that creates a battery between dissimilar metals, or that bridges different ground connections
• Have you recently changed the levels of your perfusate in the bath? Perfusate levels which are too high or too low may lead to increases in noise
• A dirty perfusion system could further impact on noise levels. Small particles and salt residue can accumulate over time. Daily rinsing is the bare minimum you can do for the system. A regular thorough cleanse should be marked on your calendar and parts that are in contact with solutions should be exchanged for new ones regularly
• Are all your electrical rig components up to date? An outdated mercury lamp for example, can produce noise on top of being hazardous
• Headstage pin maintenance – make sure the pin is clean. You may need alcohol on a cotton swab for this, but be careful not to have alcohol flow into the sensitive headstage electronics
• Is your pneumatic antivibration table floating properly? An unusual one, but I’ve seen this being a noise source before
• Make sure the Faraday cage is also properly grounded to the central ground
- 2. Your weapons arsenal – useful tools
If none of these basic errors seem to be the cause of the issue, you’ll need to systematically go through the rest of the setup. Here’s a list of useful tools for the troubleshooting process. These are inexpensive, and useful to have in any electrophysiology lab toolkit:
• A good multimeter with audio output signal – for testing whether cables are intact and looking for voltage differences between points
• Ground bus – always useful. You can use a brass or copper bar and drill and tap some holes yourself. Mount it onto the airtable. Connect to the signal ground of the amp
• Grounding cables of varying lengths – remember, you don’t want any cables to be unnecessarily long
• Connectors – Crocodile clips, banana plugs and terminal rings.
• Aluminium foil – the ultimate sophistication!
• Conductive tape – for wrapping cables and sealing gaps in equipment housing where noisy signals may escape from
• Model cell for checking the amplifier – a model cell is usually included with an amplifier, but some manufacturers sell them separately as well
• Mini Faraday cages – Cardboard sheets covered in foil can be useful to shield potential fields from the headstage. Same with a coffee or soft drink can. Of course, this should always be connected to the ground with a grounding cable.
The following tools may be useful to invest in if you don’t manage to solve the issue quickly. Still, if you don’t have all of it at hand, you’re likely to find some if you were to ask around. A neighbouring lab may have it stashed away in a drawer:
• An adjustable clean lab power supply unit. Try this as an alternative to some supplies on the setup. This will require quite a bit of soldering to have the correct connectors, and you’ll need to be careful not to overload your expensive equipment with too much current or switch polarity.
• Test an alternative headstage or amplifier – this is when you’re quite desperate and unable to find the solution elsewhere
• Try finding the source using an oscilloscope with open-ended probe. It’s useful if your oscilloscope has an audio output channel. The “clinical” version of this approach is demonstrated nicely in a video by Neuralynx Inc. (skip in to approximately 1:30).
• Use a notch filter – sometimes these are built into the amplifier, but they’re available as separate units. If you don’t have one, NPI Electronic might be able to help
• Use a cancellation device, like the Humbug – of course, with both the Humbug and the notch filter, you’re running a risk of filtering or cancelling out some relevant part of your signal. Bear this in mind, but these devices are sometimes essential to enable you to continue with your experiments.
3. Looking through the scope – Further pointers of where you stand a good chance of finding noise sources
Once you’ve eliminated the easy errors, you want to methodically look for the cause by the process of systematic elimination. Remember, the remedy tends to be in repositioning, swapping or shielding cables and equipment.
Knowing the most likely causes, play around with it by methodically evaluating and ticking off the most likely causes on your rig. This may require many switch off-unplug-test reiterations.
Plan your approach carefully. With the knowledge you already have, you can trust your intuition
– but remember to do it systematically, with notes, pictures and screenshots. Decide beforehand what your test approach will be following each change, and keep to it: make the change, make notes describing this, take the necessary pictures and screenshots, and make sure you investigate the effect properly. For example, change the time axis on your trace to detect slow cycles induced by the change. Also zoom in to look for a change in the waveform shape, which may give you some clues.
So, let’s take stock: At this stage you have probably investigated all the suggestions and best practice solutions for getting rid of unwanted noise in your rig. Having also integrated the best practice suggestions above into your approach, some further priorities for your troubleshooting protocol should include other common causes (some of these have been alluded before):
• The lab’s air conditioning system
• Cheap power supplies, especially switching power supplies (e.g. AC/DC converters)
• Make sure you only have DC cables inside the cage – no AC
• Keep mobile phones away from the headstage – switch it off
• Be suspicious of electromechanical devices, including manipulators, valves and mechanical shutters
• Devices with system clocks and oscillators are likely sources
• Old/Damaged cables, that either lack conductivity or exclude shielding (this is especially important to investigate when any old power/data/ground cables have been repaired with improper DIY solutions
• Cables acting as antennae, electric or magnetic field sources and receivers more extensively discussed under section two above – “Good hunting ethics – what a low-noise rig looks like”. Be particularly suspicious of very long cables, coiled up cables and cables carrying heavy loads. Try repositioning cables and/or shield it with grounded shielding plates, conductive tape or foil.
• Electrostatic noise from devices with high loads, very often electrical stimulators and high current power supplies for light sources
• Hairline cracks in thermistors, resulting in electrical current leaking into perfusate
Any anodized components near the chamber are likely to be electrically disconnected from ground and may, therefore, need separate grounding. This may include:
- Micromanipulators
- Some microscope components. Microscopes often have dedicated grounding sockets – These may be useful, but some components are not bonded electrically to the grounding point.
- The experimenter’s body – this is easy to see if you touch the table and the noise drops…wear an earthing strap to resolve this
If you don’t see the noise before you start recordings, a bad seal is likely the cause. If this is the case, also consider an unstable recording electrode or a tissue slice that is moving around because of perfusion inlet/outlet which is not positioned properly.
4. On target – or not…
You’ve planned well, applied your intuition and logic and asked the right questions. Your approach has been systematic, and you’ve managed to interrupt the coupling and protect the receiver (see Part 1 to understand this reference). Congratulations!
But what if the process of switching, repositioning and unplugging has not delivered? You may want to approach it from the other side…switch everything in the room off. Unplug everything except the amplifier, DAQ and computer. Start from this skeleton setup and reconnect step by step, doing a reverse hunt.
WE WISH YOU GOOD LUCK! OH, AND REMEMBER TO WEAR YOUR LUCKY SOCKS.
