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A blog for electronics professionals, amateurs, hackers, and anyone interested in the world of electronics.

27 September 2010

A New Arduino (or Two)

Arduino have announced a couple of new boards, still based on the same Atmel family of controllers. The Arduino UNO and Arduino Mega 2560 are successors to the current Duemilanove and Mega. USB has moved from the ubiquitous FTDI chip to a (presumably cheaper) ATMega8U2.

Now, the Arduino has achieved huge popularity, partly down to its "open" architecture. But there are many many open design boards out there, and have been for years, I wonder how and why Arduino really became "viral" because there are many better processors and dev boards out there.

19 September 2010

Prospero - Part 2

Work progresses on my RF Network / Spectrum analyser project. Not really much to show with regards to the electronic design (other than lots of sheets of A4 with notes and diagrams) but I thought I would upload a CAD rendering I made if a possible front panel. It may well change between now and whenever I get the soldering iron out, but it was good fun to play around with 3D Studio again.

15 September 2010

Multimeter Review - Fluke 233

I have been using the Fluke 233 for a few weeks now, so it feels like a good time to publish an update. Overall, there is no doubt that the Fluke 233 is a really good piece of kit, but one or two minor annoyances have appeared. For all that Fluke price themselves near the top of the market, there are signs that accountants and marketing executives have had a say in things. More about that later.

To set the tone, I present a photograph of two Fluke hand held multimeters, produced around twenty years apart, and connected to a common bench power supply.
That is a difference of 0.1%. In twenty years. I can appreciate that a cheap meter purchased today may be trimmed to that level of accuracy, but I really don’t think they will be that close in five years time let alone twenty. The old Fluke 77 hasn’t had an easy life either. Overvolts, overcurrent, moisture, cold, heat, concrete floors. You name it. It is an interesting point to note that it still has the desk stand intact, despite being rather frail looking.

I could copy a table of measurement comparisons, but there is really no need to. The photo above sums it up really; volts, amps, ohms - all absolutely spot-on. I don’t have lab grade references, just a few 0.1% resistors, an Analog Devices voltage reference I hacked together, and of course other meters. For all these test restrictions, the real world accuracy of the Fluke 233 is clearly better than that specified in the sales literature and user guide.

It does seem disproportionate to spend such a small amount of space writing about how good a meter it is, but there you have it. A damn good reliable meter, designed not to be replaced after 2 years. It has already earned a permanent place on my desk.

It seems churlish to pick out the bad points from what is clearly such a good piece of kit. But I feel it is only fair to prospective purchasers that I cover certain limitations and problems.

First of all, as I mentioned in my initial review, charging £43 for a Chinese made soft woven nylon carry case is ridiculous. Heck, that is £2 more than a set of decent Fluke test probes. Which reminds me, the probes supplied are okay, but no more than that. I would expect probes like these to be supplied with a £100 meter. Testing them in the ‘fridge, they don’t appear to be low-temperature flexible ones with silicone insulation. Have some pride in your products Fluke! It is like buying a BMW M5, only to find they have supplied it with Korean ditchfinder tyres. Details really matter.

I guess Fluke rely on people buying accessories through department budgets. With the US and European economies in the state they are, people will be looking for better value from their products, and Fluke could do worse than to take a look at their bundled accessories.

AVO and Beyond

Beyond the usual amps / volts / ohms,  what do we have? Well, I am afraid this is where the marketing execs strike.

The Annoyances.

Diodes. The diode test reads up to 2 Volts, which is fine for diodes and most LEDs, but not for blue, UV or “white” hybrids currently replacing filament lamps by their million. The unit has 4.5V available to it directly from the batteries without any boost (and minus any lost from the constant current circuit), I’m sure they could and should do better than 2V.

Capacitance. Useful for doing some general checks, but a resolution of 1nF isn’t going to cut it with radio frequency, or even audio circuits, and a maximum of 10,000uF is way below the values used in many conventional linear power supplies. They seem to have forgotten that old technology still exists and needs repairing. I can understand not going down to the pF level in a meter like this, but it should measure to 100,000uF. As with all the ranges though, accuracy looks to be spot on.

Frequency measurement. A maximum stated frequency of 50kHz (although people have measured up to around 100kHz). Given that small switched mode PSUs often run up to 4MHz, 50kHz is going to limit the meter’s usability. Somewhat of a “tacked-on” facility I feel.

Batteries.  400 hours estimated life is excellent. That probably works out at 3 to 4 years usage for me. But, when they go, it does mean potentially 5 batteries have to be replaced. Would it not have been better to integrate a small rechargeable battery in the head unit, and charge it via two direct contacts? Sufficient capacity for 5 hours running should be plenty. Mounting stainless contacts in the runners would allow them to stay clean. Battery life should not be compromised as there would be no need to run an I/R sender.

Missing in Action:

Zeroing. Despite having a Max/Min/Average facility, there is no zeroing function! I’m sure it must be a relatively trivial job to add this to the firmware. It isn’t vital by any means, but adding it would need no more hardware. I wonder if marketing people made the engineers remove this facility?

Duty ratio. Again, this surely cannot be a difficult function to add, and it is one that is essential to anyone dealing with automotive development and repair. I guess they want you to buy a more specialised meter – but that one wouldn’t have the removable display, which is so useful in the auto market.

Millivolts. I feel rather churlish mentioning it, but 0.1mV feels rather coarse for a modern meter.


I don’t want to mislead the reader, none of these are killer problems in any way, but most of them could have been removed or mitigated. You might think a meter with all my requested modifications would start to encroach on Fluke’s top line portable multimeters, but they have 20,000 and 50,000 count displays and a minimum of double the measurement accuracy. Surely that would be enough clear water. None of these problems stop the 233 being a seriously good multimeter that I will probably still be using in another 20 years.

Headless Meters Are The Future

I have the suspicion that, before long, most multimeter ranges are going to offer a ‘headless’ option. I wasn’t sure how useful I would find it, but in reality I have already used it several times in situations that would normally have involved messy extension wires.  Having said that, I am still a bit puzzled at who the Fluke 233 is targeted at. If you need a remote display meter then there is no choice. It has to be the 233. If you don’t, then go for Fluke’s 175 and save yourself £70. Personally I would pay a little more and go for the 287, which is what I think the marketing people want you to do. In my case, I already have a nice little space on my desk reserved for a 287. All I have to do is save up for it - hopefully it will not be 20 years this time!


9 September 2010

"I'm sorry Mike, you can't do that"

Just a short blog-ette. Maybe it is due to the economic circumstances, and companies are running down their stock levels to make their profitability look artificially better, but I do seem to be hitting "Current stock: 0" quite a lot recently, mostly with IC suppliers, and Analog Devices especially. I emailed AD yesterday asking for an estimated lead time for a couple of reasonably common chips and they haven't got back to me yet. I hope all is well inside AD, there are no alternatives for some of the stuff they make, particularly Direct Digital Synthesis chips.

3 September 2010

The Inner Geek

When I was a kid in the 1980s, I had a calculator watch. Not Just a calculator watch, but a scientific calculator watch. Prior to that, I had one you could play a game on. It was clear that this was the way forward for the 80s equivalent of geeks, techies and nerds. Portable data banks became big, and the home computer was taking the UK by storm, the next step was clearly to make it a programmable calculator. Only it wasn't. Things went all retro, with analogue dials back in favour. Chic, elegant, and very boring. I mean - all they do is tell the time, and maybe the date!

Now fast forward to 2010 and Apple have just announced their new updated iPod range. Yes, the iPod touch has been updated with a new higher res screen and camera, but the thing that caught my eye was the Nano.

It now has a tiny full colour, high res, full multi-touch screen. Oh lordy, just how much do I want someone in need of a modern geek watch to approach me with a sackful of money! I imagine it would need a full custom ASIC to drive, but with the right case, and the right OS, how sweeeeeet would that be? Just one thing though, I don't want it to be Apple branded. I'm afraid the whole Fanboy thing is getting a bit out of hand.

Here's hoping someone does a teardown soon. Who knows - they might be hackable!

2 September 2010

Filter Design (or, A Man With Two Watches Never Knows The Time)

There is an old saying; "A man with two watches never knows the time". Because they will never agree with each other. I feel like that today.

Theoretical 7-Pole Elliptic Low Pass Filter design, Fpass=450MHz, Fcutoff=500MHz

I have been designing a Low Pass Filter for a DDS. In order to get as sharp a cut-off as possible, I really need to use an elliptic (otherwise known as a Cauer) filter topology. There is always a tradeoff in electronics design, and the elliptic has considerable ripple in the pass-band, and relatively high peaks in the stop-band, due to the use of series capacitors. The ripple is fairly easy to remove with calibration. Even so, 4.6dB ripple for a 60dB stop-band attenuation is getting a bit 'iffy'.

However, when I come to model the filter with LTSpice, or the Altium DXP integrated Spice modeller, it seems to degrade the stop-band to around 40dB. Oddly, changing the component Q seems to make little difference.I wonder what it is doing differently to the simulators built into the filter design software.

Moving from the cheap (free) 'n' cheerful Elsie to Nuhertz Filter Solutions generates a very similar set of component values (as you would expect). It too claims 60dB stop band attenuation. I wonder what I am missing. Maybe Spice has a configuration option that I have missed.

*Scratches chin*

I'm not getting enough stop-band depth, so I may have to move up from a 7-pole filter to a 9-pole, or use a hybrid filter topology that gives me a reasonably fast cutoff with deeper stop-band attenuation.

In theory, with a limit of 450MHz, the DDS shouldn't be producing spurs in the 500-550MHz region anyway (for a clock of Fc=1GHz) so if I move the stop-band to start at 550MHz rather than 500MHz, that relaxes the cutoff speed constraint and so improves stop-band depth and ripple. Will 30dB cutoff at Fc/2 be enough though.

Or I could do the sensible thing and say "yeah that's fine".Of course, if I did that, it would come back to bite me...

It strikes me that all this filter software looks amazingly old-fashioned. Like it was originally written for Windows 3.1. I'm sure there must be something better for Mathcad. If that proves to be expensive, I might have a go at writing my own.

1 September 2010

The Amp Hour Podcast - Number 6

Dave Jones and Chris Gammell have a great regular electronics podcast called The Amp Hour, and edition number 6 is now out. Always a great hours engineering entertainment! I can't list all the stuff they talk about here, there's just so much in it.

The Amp Hour