FDIM 2012: HamOS

A few weeks ago, I went to the Dayton Hamvention and the accompanying QRP event, Four Days In May (FDIM). QRP-ARCI always puts on a good program for FDIM, with a full day conference on Thursday, the day before Hamvention opens, and evening events Thursday, Friday, and Saturday.

Among the evening events are a show-and-tell night and a building contest night. Show-and-tell is open to anyone to bring a QRP-related project to share, whether finished or work-in-progress. The building contest has had varying rules over the years, but this year it had several categories, including kit-built radios, scratch-built radios, and station accessories. Judging was by audience ballot.

I forgot about show-and-tell night and regretted it. The FPGA morse code keyer would have fit right in.

I took some photos and notes from the evening events and will be sharing them over the next few weeks.

HamOS demonstration

First up is HamOS. Rich Gordon KD0BJT and Brady AC0XR were demonstrating this Linux-based operating system and handing out CD-R’s as fast as they could burn them. HamOS is a GNU/Linux distribution focusing on ham radio, with a variety of radio applications pre-installed. I can’t find a download link for HamOS, but hopefully that will be rectified soon.

Rich was lucky enough to get the “BJT” (Bipolar Junction Transistor) acronym in his call — and it looks like it is sequentially assigned, not vanity!

Rich and Brady also produce the lowSWR podcast, to which I have just subscribed.

SMT Soldering – It’s Easier Than You Think

Front page of "SMT Soldering: It's Easier Than You Think"Soldering Is Easy was a great starter, and now here’s a sequel: SMT Soldering – It’s Easier Than You Think.  Greg Peek and Dave Roberts put together a great little comic book that teaches the art of soldering tiny SMT components. After reading it, you will be ready to tackle chip resistors, SOT-23’s, and SOIC’s with confidence.  (You might also like to take a look at Skywired’s own tutorial on soldering SMT ICs.)

The comic book is licensed CC-BY-SA (Creative Commons Attribution Share Alike), so everyone is free to pass it on.

(Via MightyOhm.)

Getting parts organized for the Dayton Hamvention

The Dayton Hamvention is coming fast, and for me this year, that means it’s time to get organized.  As anyone who works or plays with electronics soon learns, parts organization is a problem. In addition to not always remembering what I had, finding things that I know I have can be hard. This has especially been a problem in the last year, as I got back into the hobby after a few years away. I have a good memory (at least sometimes!), but no one can remember hundreds of part numbers, values, and locations for years on end.

This caused me some concern as I look forward to my first Hamvention trip in several years. Hamvention has a giant flea market, full of wonderful junk (junque?) to buy. Without knowing what I have, how will I know what to buy?

On top of that, as I’ve worked on projects recently, my frustration has grown with not being able to find parts that I know I have… somewhere.

Disorganized bin of resistors
This is one of the more organized bins in my workshop...

Though I’ve tried at least four different ways to organize my parts over the years, none has worked particularly well. This time, though, I have an edge on the chaos: My dear wife, Lyn, who has spent years in inventory management and running warehouses. With plenty of her help, I think I’ve finally hit on an effective system.

The first step is little bags. Each baggie holds one unique kind of part. Sometimes it’s a single part number, but for commodity components, I will group parts with different manufacturers or part numbers as long as the specifications are identical. In other words, all of my 100 nF 50 V X7R monolithic ceramic capacitors can go in a single bag, but 100 V versions go into their own bag, the Z5U version gets its own bag, and the ceramic disc version gets its own.

Each bag gets a unique number. I started at 0001 and worked my way up. I used four digits because I suspect I might hit 1000 bags by the time I’m done.

Capacitors in numbered bags

The cheapest place for good-quality 3×5″ poly bags that I’ve found, once shipping costs are taken into account, is my local Michael’s store. (Shopping there for bags was another one of Lyn’s suggestions.)

For the semiconductors, I splurged and bought a pack of static-shielding bags from Mouser. Rather than judge the relative static sensitivity of different components, I decided to put all of the transistors, chips, and diodes in static-shield bags, even if robust parts like 2N2222‘s probably would survive fine without them.

Transistors in anti-static bags

Next, the small baggies are stored in 1-gallon bags. Several 1-gallon bags go in a bin.

Gallon bags in a bin

The bin is numbered, and the 1-gallon bags within the bin are lettered. That means every baggie can be identified and found by combining the bin and bag codes and the baggie number. “1-B-0067”, for example, would send me to “bin 1, bag B, baggie 0067”.

Since baggies have unique numbers in the whole system, I can move them around from bag to bag or bin to bin as needed, and I can reuse baggies for different kinds of components as my stocks are depleted.

Tracking these numbers in my head would be no better than where I was, so the final piece of the system is a spreadsheet. With columns for the number and location, I can find things fast. Additional columns have the type and subtype of component (such as transistor/n-JFET, or capacitor/tantalum), the component’s package, manufacturer, part number, and value, and a description field.

Screenshot of inventory spreadsheet

I try to be as specific as I can, and I try to be consistent with nomenclature, to make searching easier later on. For example, I use units of μF, nF, and pF for capacitors, and I do not use decimal μF for capacitances below 1 μF. In other words, all of my 0.1 μF capacitors are listed as 100 nF, and the 0.01 μF capacitors are listed as 10 nF.

I thought about using one of the web-based inventory programs that are available, such as PartKeepr, but there is something to be said for keeping it simple, and besides, migrating a spreadsheet to newer software over the years will be easier than migrating a database.

The big success for this system may also be its biggest failing. As I work through my existing parts collection, I’m finding less and less that I want to buy at Hamvention. I already have all the common capacitor values I am likely to need for several years, and I found PN2907 PNP transistors cached in three different places, together amounting to what will probably be a lifetime supply. The oddball project-specific stuff I’ll just buy from Digi-Key or Mouser when I need it.

The cool part will be putting the spreadsheet on my phone, so I can check my inventory on the spot in the flea market. That should keep me from buying another 100 PN2907’s!

How have you organized your parts? Do you have any tricks to share?

KK7B R2 receiver: lifted pads, a scorched board, and it works anyway

I’m slowly making progress on my KK7B R2/T2 transceiver project. At my last report, I was waiting for replacement capacitors to arrive. They did, and I pulled out my ancient solder-removal iron, a Radio Shack unit from who knows how long ago.

Either my unsoldering skills are rusty or I was too impatient, or both. I managed to lift four pads, completely demolishing one. The other three were salvageable. I’d like the board to be perfect, but having it work is an acceptable substitute.

The new caps went in easily, with only a little fiddling to wire around the ruined pad. Better yet, the excessive bias current I saw before the replacement is gone! The board is supposed to be adjusted to 100 mA current. It used to start at 120 mA, with the bias adjustment set to its minimum, and drift its way up to more and more current from there (going as high as 200 mA before I’ve lost my nerve and switched it off). Now it starts at 84 mA and… drifts its way up from there to 130 mA and more.

OK, so one problem was solved. I can always increase current with the bias adjustment potentiometer.

After scratching my head a bit, I finally noticed one small sentence in KK7B’s articles on the R1 and R2. The audio output transistors need a heat sink, do they? Oops! I dug around a bit in my junk box but couldn’t find anything that would fit. The articles say the audio amp will drive a pair of headphones fine without the output transistors, so I decided to take them out.

I recently got a Sparkfun hot-air soldering station (a Sparkfun Free Day prize!) and thought I’d give it a shot. Sure, hot air is usually for surface-mount parts, but solder is solder, right?  Not being sure how to set the airflow and temperature settings, I set both on the high side, put some flux on the output transistors’ pads, and went for it. The result wasn’t pretty:

Oops... A scorched R2 PCB

Yes, I scorched the board. Oops. Between this and the lifted pads, I think I need to work on my unsoldering skills.

Since the transistors are 50 cent parts (TIP29/TIP30), I unsoldered them the easy way: I cut the leads, removed the leads from the board with my conventional iron, then cleaned out the holes with my solder sucker. I didn’t damage any pads this time!  (The picture above was taken after all of these steps.)

A little more soldering to hook up a BNC and some other goodies, and the board was alive!

R2 on the bench, surrounded by test equipment

That’s a Tek 191 signal generator as the VFO (variable frequency oscillator), with a frequency counter as the readout. That’s an MFJ QRP antenna tuner in the foreground.

I didn’t build a phasing network yet, so I drove only one VFO input. That causes the R2 to function like a conventional direct-conversion receiver,receiving both sidebands simultaneously. That said, it works. I was able to hear signals, though frankly many were quite hard to copy. I’m not sure what else might be wrong.

The frequency counter spits out a lot of digital noise. I learned to flip it on only to spot-check my frequency. It’s a lot quieter in standby mode.

Did I mention that IT WORKS?

Despite the success, I’ve been struck by a bit of paralysis in moving forward. There are so many choices to make for integrating the radio.

  • What kind of VFO should I use? Should I design my own or buy a kit?
  • Which modes should I include?
  • How much power output do I want?
  • What power amp should I use? Should I design my own or buy a kit?
  • Which case should all of this go in?
  • What microphone, and microphone connector, should I plan for?

Keep in mind that this is supposed to me my fast route to getting on the air, so I’m thinking kits for both the VFO and amp. Besides, with as busy as it has been at work lately, it is nice to sit back and just build something.

It’s not a pretty project any more, but it works! Hooray!

Ouch! Watch out for power supply memory buttons

What is it about power supplies?  They are so very boring, until suddenly they bring excitement. It’s always the wrong kind of excitement, too…

It was a quiet day at work. I was using a GW Instek PST-3201 bench power supply, a nice unit with three 30 volt/1 amp outputs. It was purring along at 12V and a couple dozen milliamps, powering a rare and expensive prototype while I did firmware development. Then, when I reached over to grab something, I bumped the “RECALL ↑” button. Bang! The supply’s relays clacked, and suddenly all three outputs were configured for maximum output. The supply pushed a full 30 V, with a 1 A limit, into my circuit.

Needless to say, I switched the output off as fast as I realized what happened. Thank heavens for the clatter of those relays, which told me something unexpected had happened. With the output off, it was time to check for damage.

To my surprise, there was none. The power supply wasn’t plugged directly into the prototype — instead it went through a test board that interfaced a PC to the device under test. By very fortunate coincidence, the wimpiest chips on that board were rated for a hefty 36V, and some were rated for as high as 42V. (Those “wimpy” chips weren’t wimpy in any real sense — they were some LT1970 high-power op amps, more than capable of hefty output of their own.)  The interface board was undamaged. Better yet, none of the high voltage made it to that precious prototype.

As I breathed a sigh of relief, I looked for the cause of the accident. It turns out this supply came from the box with half of its memories set for 30 V / 1 A and the other half set for 0 V / 0 A. Hitting “RECALL ↑” took it to one of the 30 V memories.

This could have been much worse. It was sheer luck that all of the parts I picked for that interface board had maximum ratings higher than 30 V.

Needless to say, if you happen to use GW Instek PST-3201 supplies or the closely related PST-3202, it would be wise to check the memories and set them all to zero volts. That’s what I did as soon as I realized I was not going to have to tell my boss I had blown the prototype.

Updated 3/24/12: Corrected the name of the button at fault. It is “RECALL ↑”, not “MEMORY ↑”.

Cool chip: LT1970 power op amp with current limit

LT1970 power op amp configured for a gain of two and a variable current limit inputThis week I want to share a new favorite chip of mine, the LT1970 power op amp from Linear Technology. This nifty device runs from a 36 V supply and can source or sink 500 mA. That’s not too unusual, but how about programmable current limiting, outputs to indicate over-temperature and over-current, and more? Think of it as a four-quadrant power supply on a chip, or as the source part of a source measurement unit.

The first nice feature of the chip is the programmable current limit. The chip has two inputs, one for sourced current and one for sunk current, that accept a voltage from 0 V to 5 V. It compares these voltages against the current measured across an external sense resistor and limits its output when the measured current exceeds 1/10 the voltage on the relevant control pin. Differential input pins, independent of the output pin, are provided for the sense resistor.

Another great feature is that the output stage has its own power pins. One of the neater “trick” circuits out there is to boost the current capability of an op amp by using the power supply pins to drive external transistors. An ordinary op amp without separate power pins can also be used this way, but the bias current of the external transistors will be proportional to the op amp’s quiescent current, which means one will probably need a low quiescent current model. By pinning out the output stage’s power pins, the LT1970 makes it easier.

This circuit ends up running the output transistors in class AB. The bias current in the transistors is controlled, in part, by the quiescent current of the op amp. When the op amp tries to increase the voltage of its output, it will deliver more current to the output, pulling that current from its power supply. That, in turn, changes the voltage on the base of the upper external MOSFET, causing it to source more current as well. The equivalent happens when the op amp needs to sink current.

Another advantage of choosing the LT1970 for this configuration, instead of an ordinary op amp, is that all stages except the output stage will see a stable power supply, keeping the PSRR up where it belongs. As for the last stage, when combined with the external transistors, it resembles two Sziklai pairs (also called “complementary Darlingtons”) biased into class AB.

The separate power pins also make it easy to use a lower-noise supply for the input stages and a high-current supply for the output. I used them for another purpose: My application needed a wide input range but drove an input sensitive to overvoltage. Running the input stages off of 24V and the output stage from a lower voltage ensures the output will not hit an unsafe value. Watch out, though, because V+ and V- must be within the VCC and VEE supplies to the op amp’s input stages.

Another neat feature is three open-collector outputs that indicate that the op amp has gone into thermal limiting, into current source limiting, or into current sink limiting. These pins can drive LEDs for an easy status display.

The biggest drawback of the LT1970 is its package, a 20-pin TSSOP with a thermal pad on the bottom. That thermal pad means you pretty much have to have either a hot air or reflow setup to solder it.

As you might expect for a part like this, there are ways to use it beyond the obvious. The current-limiting flag outputs can be used for snap-back current limiting, where the output current is sharply reduced after it enters limiting. The current sense pins can be used as voltage-limit inputs, enabling circuits like a symmetric, voltage-controlled limiter. These ideas are from the data sheet; I haven’t tried my creativity at coming up with other ways to (ab)use the chip.

No, I used the chip for a boring old power supply. It was a two-quadrant supply, though, meaning (in this case) that I needed a positive-output power supply that would either source or sink current to maintain its output voltage. The voltage was controlled by an analog input. What I needed was basically just an op amp with current limits and a few hundred milliamps output. The LT1970 was perfect. I fitted it with potentiometers to adjust the current limits, a set of LEDs for the status outputs, and as much PCB copper as I could manage for a heat spreader. I also included some compensation for capacitive loads. It worked great. I haven’t had any issues with oscillation, the LEDs have been handy to tell me when I mess up, and the current limit has saved my bacon at least once. Sure, I could have built the same supply from separate parts for a lower cost, but it would not have been nearly as easy or as quick. This is another case where it was worth spending a little money on a chip in exchange for saving a little engineering time.

The feature I miss most from the LT1970 is a current amplifier output. It does current measurement internally, for the current limit feature, but a voltage proportional to current is not brought out. For my power supply, I had to add an external difference amplifier to measure the output current. The difference amp used the same current sense resistor as the LT1970, but it would have been nice to have that function built in.

The quantity 1 price for the LT1970 is $9.88 from Digi-Key or $6.05 directly from Linear Technology.

Figures: Copyright 2002 Linear Technology Corporation. Used by permission.

R2 receiver update: Time for new electrolytic capacitors

Crunch time at work has been limiting my basement tinkering, but I recently found time to work on my R2 receiver a bit more. It is pulling excessive bias current, which had me scratching my head. The audio power amplifier bias is supposed to be adjusted so that the whole board pulls 100 mA, but it’s taking 120 mA even with the bias pot set to its minimum. After double-checking all of the component values and verifying all of the bias voltages on the board, I was left scratching my head. Then I remembered the age of the kit. Even while building it, I had doubts about the electrolytic capacitors…

This R2 kit includes 15 Panasonic Z-series electrolytic capacitors. Aluminum electrolytic caps are good at one thing: Lots of capacitance at a low cost. In nearly every regard, they have inferior performance, with high equivalent series resistance (ESR), inductance (ESL), and, yes, a short lifetime. I’m not sure exactly how to translate the lifetime specifications for electrolytics to room temperature storage, but the rule of thumb tends to be that they can tolerate 5 years on the shelf, after which they require a “reforming” procedure. Reforming them at that point can get them to last another 5 years or so. After that, figure that they are shot. Old electrolytic capacitors can have excessive leakage current. In extreme cases, this current is enough to heat them up excessively and they go bang! The capacitors on this board are old enough to vote. Maybe one or more of them are to blame for the extra milliamps.

I had a little trouble finding the perfect capacitors, but then I was looking for performance at least as good as the original. This was perhaps foolish, because I couldn’t find data on the Z-series caps. Panasonic discontinued them in 2000. In lieu of further information, I arbitrarily went for high-quality models from a favorite manufacturer. In some cases, these were nearly double the price of the cheapest options, a breathtaking 23 cents versus 12 cents, quantity one. Seriously, replacements for all of the caps cost all of $2.85, including a few spares for good measure.

My one mistake so far is to forget about the T2 transmitter. If the R2’s capacitors are bad, the T2’s surely are as well. Maybe the spares I bought will end up there.

Troubleshooting is half the fun of this stuff. Who needs sudoku when you can puzzle out a misbehaving circuit?