Sunday, December 23, 2012

Helical Duplexer Teardown

While cleaning out the CPARC shack last month, we happened upon a helical duplexer set, in additional to various other pieces of equipment, which seem to indicate that someone in the club in the 1990s was collecting parts for a homebrew 2m repeater.

Duplexers are an interesting part of a repeater. Repeaters are a common radio installation where many low-power mobile radios transmit on a single frequency, which is received by a strategically placed receiver.  This received signal is then re-transmitted at higher power, and usually a fixed frequency offset, so that every other mobile unit can hear each other, as long as they're within the coverage area of the repeater.

The problem with repeaters that you need duplexers to solve is the fact that you are trying to receive and transmit on two unusually close frequencies at the same time from the same place. For example, a 2 meter (146MHz) repeater will only have a 600kHz offset between the input and output.  Even using two separate antennas, when you're trying to transmit 15 watts and still receive a 2μV signal only 600kHz away from it, things can prove problematic. Duplexers solve this puzzle by being a pair of ridiculously good notch filters; one to filter out the input frequency from the transmitted signal, and a second to filter out the transmitted frequency from the receiver.

Notch filters are traditionally built from inductors and capacitors, but each pole of a passive filter only gets you 20dB of attenuation per decade of frequency change. A good rule of thumb for the needed isolation between the two sides of a duplexer is 90dB, and instead of being a decade apart (i.e. 14MHz vs 140MHz), a typical VHF repeater only has 0.00178 decades of separation between the input and output frequencies (146.160MHz vs 146.760MHz). Using traditional passive filters, you would need to build a 2,500 pole filter to get the needed 90dB, which is comically large.  Duplexers instead operate on the principle of using a resonant chamber as the notch filter, at which point getting a Q in the thousands with only a few stages becomes possible.
These resonant chambers are built to roughly the correct size, with a fine tuning rod held in place by a lock nut.
Due to the needed high Q of the notch filters, the tuning elements, and often the entire resonance chamber, are silver plated to reduce resistive loss.
The best duplexers are made with full size 1/4-wave cavities, but at 146MHz this works out to be almost two feet long, and 4-6" in diameter, per stage.  Helical duplexers trade slightly lower isolation and efficiency for being significantly smaller; slightly more than 2.5" long for 2m.  This is done by using two helical conductors to lower the cavity resonance to the frequency of interest, which can been seen in the two photos above.
Of course, one of the advantages of now being an EE grad student at Cal Poly SLO is that I have access to a fully equipped microwave lab, where I can use a vector network analyzer to feed test signals into these filters and see exactly how much they filter out in a specified frequency range.
As a first pass, I swept both filters from 10MHz up to 1GHz. The response of the one labeled "Reject High" came out about as expected; a very deep notch at the top of the 2m band, spurious notches at all the odd harmonics (440MHz, 740MHz, etc), and high attenuation as we approach DC, since high notch filters use capacitive intra-stage coupling (where low notch filters use inductive intra-stage coupling).
Performing the same sweep for the "Reject Low" filter gives somewhat less encouraging results.  As expected, there is a fundamental notch at 146MHz with higher notches at odd harmonics, and the attenuation goes to zero towards DC due to inductive coupling, but the huge notch at 211MHz is unexpected.
Re-calibrating the VNA to just focus in on the 2m band, the "Reject High" filter yet again gives the expected results.  The four markers are the input and output ranges for the lower repeater channels.  The filter's notch likely goes quite a bit lower than shown, but that's where the VNA hit the noise floor.  Ideally, the slope between ~0dB and lots of dB attenuation would be as far apart as the markers 1 and 3 or 2 and 4, but that just isn't possible without resonant length cavities.
Interestingly, the "Reject Low" filter seems to have a better Q to it, which is probably just because whomever last tuned these did a better job on this one.  Notice how both filters have very different responses above and below their notch frequencies. This is due to how each set of filters order their resonant and anti-resonant modes, to give the duplexer a steeper response in the critical 600kHz between the input and output. A good explanation of this notch-shifting concept can be found on VK6UU's article about traditional duplexers.
I decided to have some fun while I had these hooked up to the VNA and practice tuning them. Shown here is the reject high filter with all three stages randomly tuned across the 2m band.  It's hard to explain with just the plots, but it's really interesting to screw one tuning rod in or out and watch how it shifts the other two poles.
Moving the highest node closer to the other two shows how the three stages can quickly create a very deep notch, once they're all properly aligned to the desired frequency.
Unfortunately, due to the particularly low efficiency of helical filters, even when properly tuned, 600kHz splits mean that the attenuation at the pass band is still uncomfortably high, for a given notch frequency. Since the trade-off is to either used a non-standard split larger than 600kHz between repeater input and output, or throw 20+ dB of your power away in each passband of the duplexer, helical filters are far from ideal for repeater service, and should only be used if size constraints leave no other choice. The apparently more common solution when a full-size duplexer isn't possible is to use a cross-band repeater, where the input and output frequencies are on entirely different bands (pick two of: 50MHz, 144MHz, 220MHz, 440MHz, 900MHz, etc).

Now what to do with a really nice set of helical filters?

QST Letter to the Editor

I just sent the following letter to the editor about the latest issue of QST. I felt that some of my readers may also find it of value or interest.  I wonder how much the Internet and blogs like my own have become the preferred venue of technical writers, and caused this lack of quality in traditional periodicals.

As an avid builder of electronics, I look forward to the January DIY issues of QST. Imagine my dismay when I took a break to skim through it and managed to find typos in nearly every electrical schematic included in the issue. I hope that the ARRL's Harold Kramer "Inside HQ" promise of "working on a more structured approach to kits that will provide positive outcomes" in 2013 includes proof-reading the technical illustrations.
  • "Cheap and Easy SDR" pg 34, Figure 10 - While the parts list and article indicate using an LM7805 voltage regulator, the schematic has it labeled the subtly different 78L05, which unfortunately has the opposite pinout. Pin 1 should be the output and pin 3 the input.
  • "A Sampling Down Converter for Low Frequency Oscilloscopes" pg 40, Figure 2 - 7 turns on L1 is likely deficient to form a 22μH inductor. Luckily, the text clarified the actual number as 77.
  • "A Sampling Down Converter for Low Frequency Oscilloscopes", pg 41, Figure 3 - The LM340-12 is shown with its input and output reversed. Apparently, it isn't a good month to be a voltage regulator.
  • "Hand-On Radio" pg 64, Figure 3B - As shown, this would be very counter-productive to protecting your equipment. The NC and NO contacts on the relay should be reversed.
Kenneth Finnegan, W6KWF

Saturday, December 22, 2012

How Active Devices Work CPARC Talk

Last month, I gave a relatively long talk at the biweekly CPARC meeting on the principle of operation of diodes and three-five terminal devices. This included bipolar transistors, field effect transistors, and vacuum tubes ranging from triodes to pentodes.

Semiconductors, part 1 of 2:

Vacuum tubes, part 2 of 2:

Needless to say, there are numerous technical flubs made during this two hour long talk without notes. One that particularly stood out was that I managed to get the polarity of electrons wrong when I was explaining the suppressor grid of a pentode.  Damn you Benjamin Franklin!

If you catch any other glaring flubs or mistakes, please feel free to post in the comments.

Repairing a Broken Panel Amp Meter

I have always been a sucker for vintage aesthetic. Unfortunately, this means that many things I want are either prohibitively expensive on a college budget, or in one way or another broken.  Analog panel meters are one of these things.
A good, classy panel meter will typically run you something on the order of $10-$25. For one component of a larger project, I dislike spending that much.  Fortunately, while providing one of my friends moral support for spending $2,200 on a service monitor at Halted, I found this wonderful GE 1mA meter on the dollar rack. The problem is that it is missing the positive signal lug.
DC amp meters such as this one are relatively simple mechanisms.  A spring holds a needle and a coil of wire at the zero reading on the meter.  The coil of wire is sitting between the two poles of a fixed magnet such that when current flows through the meter, the coil generates an opposing magnetic field which causes the needle to deflect.  In my diagram, I show the return spring as a linear spring, but they are usually coil springs, like you see in mechanical wrist watches.

So in theory, this should be a very simple repair.  Take out the set screws holding the meter in its Bakelite shell, desolder the remnants of the old lug, solder a new 8-32 bolt in its place, and reassemble.
Very simple, until you manage to fumble your pair of tweezers, which then stick to the permanent magnet, and utterly mangle the return spring in the process.  That mess of brass ribbon in there used to be a very carefully wound spiral...

Well damn.  I can sit there and untangle the spring somewhat with a set of sharpened toothpicks, but I need some way to test the meter as I work on it.
I want you to understand how difficult it is working on electronics when you are visiting your parents and left all your test equipment in San Luis Obispo, spare for your trusty Fluke 27 and an SLA battery.
Yes. I improvised a 50kΩ variable resistor with a piece of paper and a graphite pencil.  This is what happens when you leave all of your resistors in SLO. This was not particularly how I wanted this project to go, but I had already invested $1, half an hour, and a shred of my dignity taking the dang thing apart, so I sat there and kept poking at the return spring until I got it respectably close to its original 1mA full scale calibration.
In the end, it's questionable if it really was worth my time and effort to save this meter, considering that you can get plastic white meters from China on eBay for $4. The only problem with them is that they just.... look like they're from China.  I'll need to get this meter on my bench current reference before I'm willing to build it into my planned project for it, but I'm optimistic.

Friday, December 21, 2012

CPARC Work Week After Finals

My regular readers might have picked up on my pattern of ramping up the number of posts during finals weeks and school holidays, thanks to the additional free time afforded. You might wonder then why I have dropped off the face of the Earth again for a few weeks...

The answer would be W6BHZ, the Cal Poly Amateur Radio Club.  After we finished finals, my buddy Marcel, AI6MS, organized an epic nine day long work week where we finish up the new tower he got donated to the club and converted the old tower to a VHF/UHF/Microwave array with full azimuth/elevation capability.  This will be useful because several members of CPARC are interested in trying to work some of the amateur satellites in orbit.

The first matter of business on the old tower was to remove the old HF antennas, which we were then able to sell to a local ham. Marcel ran a trolley line out to a tree on the other side of the EE building, then we had two people on the ground slowly let out line to lower the antenna along the first line.

Unfortunately, things tended to always take longer than we had hoped, and there was a storm rolling into SLO, so putting the new array up on the tower before everyone went home for break ran well into the night. We used our portable tower as an anchor point to lift the new mast into place.

Once the rain rolled in, we retreated into the shack for some badly needed sorting and organizing.  By the end of the week, we had the shack in probably the best order it's been in decades.
After the rain rolled out, so had most of our work crew.  For the last few days, Marcel, myself, and Nolan were about all there was.  At one point Marcel and I took a break to give the EE department chair a tour of our repeater site on the top of the admin building.
I took the chance to attempt a dramatic reenactment of the CPARC logo, which was mostly hindered by the fact that Jeff put the two peaks too dang close to each other for me to be able to frame it without walking off the back of the admin building.
One interesting fact I learned while on the top of the admin building is that, apparently, the university president has a Voice of God system at their disposal in case of emergencies.
In the end, CPARC now has a top-notch Az/El array, and we all had a pretty good time doing it.  I'm interested in seeing what the rest of the club members do with it, and maybe I'll even manage to convince one or two of them to write about it in the blogosphere.

Saturday, December 1, 2012

High Impedance "Ruby" Audio Amplifier

One of the enjoyable parts of the electronics hobby is the fact that, once you get the most basic set of tools, you're able to start building your own specialized equipment, depending on what direction you are then interested in taking the hobby.  A perfect recent example of this is my rubidium frequency standard that I built for <$150 to test oscillator drift characteristics.

This quarter, I've been going on a "finally learn analog or bust" stint (which is one of the reasons why this blog has been so quiet), so one circuit that I found myself repeatedly breadboarding up has been a basic audio amplifier.

Audio amplifiers generally fall into two categories; power amplifiers, which just make a signal louder, and "preamplifiers," which are designed to only match impedance of a signal instead of power gain, to bring very small and low power signals from microphones or radio detectors up to a more manageable level.  Since both of these types of amplifier are useful, I decided to build myself a small amplifier module based on the RunOffGroove Ruby design.
Schematic courtesy of

I made no significant changes to their design, other than substituting a fixed 1μF capacitor for the 1k gain pot, since I generally only need volume control, so I picked a single gain value (since the difference between gain and volume control is... nuanced...).  You also normally power this circuit with a 9V battery, but I instead use a 2.1mm barrel jack since 2.1mmx5mm 12V is my standard project power connector.
Getting all of this to fit in a 60x35x25mm project box was challenging and a little tight, but possible.

The amplifier acts in two stages; the MPF102 JFET acts as a very high impedance voltage follower, to ensure that the amplifier doesn't load down whatever circuit it's amplifying (i.e. a small microphone). The JFET performs no voltage amplification, but only transforms the impedance to a lower value for the 10k volume pot and LM386 op-amp.

The LM386 makes up the second stage, which is where most of the power gain of this amplifier is had. The LM386 is a really convenient op-amp to use, because it is specifically designed for audio amplification.  It has an internally set gain, which can be variably increased another order of magnitude by coupling together pins 1 and 8. On the output, the 220μF capacitor blocks DC from flowing through the speaker, and the series 10Ω resistor and 47nF capacitor to ground are a lag compensator to help stabilize the LM386 (which is notorious for "motorboating," where it breaks into oscillation) and remove higher pitch whine from whatever signal you're amplifying.

I've found this high impedance amplifier useful in a few projects already, and I think it's a nice little module to have in your toolbox.  I also use it quite a bit for driving a large speaker from an MP3 player, using a 12.6V lead acid battery.

Friday, November 30, 2012

Rubidium Frequency Standard Enclosure

At the beginning of the year, Dave Jones did a very interesting tear down video of a FE-5680A Rubidium frequency standard. Rubidium standards are a type of atomic clock, which are able to keep very accurate and stable time.  While not as good as cesium clocks, rubidium references are still able to keep time on the order of 2x10-10, which works out to be 6ms of drift per year.

Normally, rubidium standards are very expensive, but it is possible to find these modules on eBay for $50-$100 as they come out of decommissioned equipment (such as cell phone towers, which need this level of time stability). On a hobbyist bench like my own, these standards are useful as a very good time base to calibrate other oscillators against.  They're also useful because 10MHz is the standard time base used by bench test equipment, so this standard can be used to drive frequency counters and spectrum analyzers via their time base ports so that they are more precise than with their stock internal 10MHz oscillators.

When you buy the module on eBay, it comes as a sealed package with a single DB-9 connector on the side for power in and signal out.

  1. 15V-18V, 2A
  2. GND
  3. Signal Lock (active low, high impedance)
  4. 5V, 100mA
  5. GND
  6. 1 μs long pulse per second output (TTL compatible)
  7. 10MHz 1Vp-p
  8. RS-232 Rx
  9. RS-232 Tx
After accidentally blowing up one of these standards when fly-wiring these power connections (The FE-5680A does not like 15V applied to the 5V pin), I decided to build an enclosure around it, so that powering it up now only consists of plugging in a 16V laptop power supply, flipping a switch, and waiting for it to heat up enough for the PLL LED to light.
I glued threaded standoffs to the inside of the case to mount the FE-5680A on, but glued the 7805 linear voltage regulator for the TTL supply directly down to the case, since its less likely I'll ever want to scavenge the 50 cent regulator than the $75 frequency standard module.
I used an interesting trick that one of my amateur radio friends taught me this summer about how to get good thermal contact between a power package and a metal case without drilling into the case.  Apply thermal paste across the majority of the case (A TO-220, in the case of a 7805), but carefully leave two opposite ends free of thermal compound. Apply two small dabs of epoxy on the free corners to fix the package down, without needing any sort of special thermally conductive cement to maintain good thermal contact.

A couple things to note about the rest of the build which are important:
  • The FE-5680A gets hot. This is on purpose. Many people mount it to a heat sink when they enclose it, but don't realize that the FE-5680A is thermostatically heated, so any heat sink you attach to it will only cause it to draw more power until it has reached the same temperature.
  • The loop lock indicator (pin 3) is both an input and a output. It goes low when the standard has achieved phase lock with it's rubidium lamp, but will stay unlocked as long as you externally hold the pin high.  This means that you can't use this pin to directly drive an LED, but need to buffer the LED with a transistor. I used a 2N3906 PNP transistor, and a 10k resistor from the transistor's base to pin 3.
Some other useful links for the FE-5680A:

Thursday, November 29, 2012

The Slowest LC Tank You've Ever Seen

I recently came into possession of two "boat anchors," which is slang for vacuum tube powered radio receivers (since they weigh a LOT). Both of them were pulled out of an e-waste pile, and were both well beyond saving.  I pulled them to strip them for parts before returning the carcass.  This has yielded several interesting components (a CRT tube, tube transformers, an assortment of vacuum tubes and matching sockets, etc), but one of particular interest is the two chokes from one of the power supplies.

After rectifying 60Hz wall power to be DC, it still has large amounts of ripple in it which needs to be filtered out.  This is usually done with a capacitor, but when you're working with several hundred volts, getting enough capacitance to filter out all the ripple can get unreasonable, so in addition to the capacitors, inductors are used.  The interesting part is when you spend enough time cleaning dirt and grease off the labels to figure out that these are EIGHT HENRY inductors.

Eight Henrys is huge. Typical coils of wire are measured in μH, or some of the large power inductors in mH, but these monsters are 8H.
In response to me posting a picture of these on Twitter, Tony Long pondered what the self-resonant frequency of an 8H choke is. I was curious as well, so I hooked one of these inductors up to my oscilloscope and gave it a 15V pulse.  Every inductor has a frequency at which it by itself will tend to ring, but while most inductors have this frequency in the MHz, this one rang at 5kHz.  The scope capture above is at 200μs and 50V per division.

Yes. 50V per vertical division. Just giving this inductor a 15V pulse, it rang with 500V peak-to-peak.
This of course then begs the question: how low can you go? I wired one of the 8H inductors in parallel with a 10μF 600V oil-filled capacitor pulled from the same rig.  In parallel, these form a resonant LC tank, with a resonant frequency that's a function of the square root of the product of the two.
0.1 seconds / 50 Volts per division

This LC pair resonates at 12Hz, which is crazy. Usually, getting an LC circuit to even resonate as low as audio frequencies is challenging, but here we've got one which resonates at infrasonic frequencies, and with plenty of arcie-sparkie to go with it.

Moral: Always dig through e-waste piles, and make sure you always have a friend with a pickup truck.

Wednesday, November 7, 2012

Cal Poly Amateur Radio Club

As any of my regular readers have probably noticed, things have been fairly quiet from me online lately.  This isn't because nothing has been happening, but because there has been so MUCH going on I haven't had anywhere near enough time to work on personal projects and document them on here as usual.

One of the more exciting things I've been working on this quarter is my involvement with the Cal Poly Amateur Radio Club. I originally got my amateur radio license (W6KWF) while attending UC Davis for my bachelors degree in mechanical engineering.  There was essentially no ham radio presence on campus, so I remained relatively inactive at the time, and did little more than operate for field day every year.

Now that I'm attending Cal Poly, I'm thoroughly enjoying participating in this very active and well-run club. CPARC not only has a 2m/440 repeater on-campus, but our own club room in the electrical engineering building outfitted with two HF rigs, and a newly built 70' tower attached to the building.

The ham shack has been a nice place to relax between classes and shoot the breeze, but what caught my eye in the shack was the corner covered with tiny drawers full of components.  I started digging through the component collection to see what an amateur radio club stocks for it's member's projects, but started finding some strangely old components in the drawers... and almost none of what you'd expect hams to need (op-amps, voltage regulators, RF connectors, perf board, etc).

After talking with several of the club's officers, it appears that no one has touched the shack's components collection in at least a decade. That would explained why the most advanced microcontroller I found in the collection was a Zilog Z80, and that the collection included several sets of 1970's style manufacturing reject transistors and the like.

Clearly, something needs to be done about this neglected collection of drawers. An amateur radio club that doesn't promote the building electronics component of the hobby is missing out on what I feel is a significant and educational part of the hobby, and an EE degree.

This is why I've now volunteered and been elected as the Projects Coordinator officer for CPARC.  As the Projects Coordinator, I've been cleaning out decades of collected trash in the electronics work bench area, and am putting together a number of useful and easy-to-build electronic projects for the other club members to help them get started in home-brew. I'm also putting together a number of educational presentations on building your own electronics, which I will be giving during our biweekly meetings throughout the year. Finally, I am working to restock the ham shack with a useful set of components so that members can find the parts they need.

This is where you, my loyal readers, can possibly pitch in if you would like to help.  Unlike other engineering clubs that enjoy the support of a large national organization and yearly membership dues, the CPARC club has no regular source of income, and the additional burden of maintaining an unusually large amount of equipment, for a school club (radios, antennas, the repeater system, etc).  As the projects coordinator, I'm going to need a few hundred dollars in order to restock a club's worth of components. I'm already working with a few different corporation's university programs to donate any of their products I need (op-amps, voltage regulators, etc), but this doesn't cover all of the other, unbranded, components like perf board, passives, enclosures,  etc.

If you have enjoyed what you've seen me do on this blog and found it useful, and are willing and able to help support me bring the same learning experience, in a very hands-on way, to the rest of the members of the Cal Poly Amateur Radio Club, I'd like to ask you to consider sending CPARC a cash donation to help me bootstrap the club's components fund, to then sell members their needed parts at-cost on-site. Unfortunately, the best way to send CPARC donations is via mailing us a physical check, since any electronic payment must be processed through the university, which retains an unbelievable 15% of any and all donations, for university advertising and promotion. Donations done by check sent to our campus PO box will be directly deposited in the club's account, and we will be able to use all of your donation to support CPARC.  Checks should be sent to:

Cal Poly Amateur Radio Club 
UU Box 53 
University Union Epicenter 
California Polytechnic State University 
San Luis Obispo, CA 93407-0675

In addition to a thank you letter for any size donation, I am also going to send (kickstarter-style) the first three cash donations of at least $100 to the CPARC components fund a thank you gift of a Motorola 1N4002 diode still in its original cellophane wrapper.  I found these while digging through the long neglected components drawers, and thought they were rather entertaining.  I hope this gives you an idea of how neglected this collection has been.

I look forward to any support you can provide us, and hope to put as much as possible that happens at CPARC back up on this site for your enjoyment. If you have any questions or comments, please feel free to send me an email.

Friday, October 19, 2012

A Cheap Source of LEDs and Solar Cells

I was recently shopping around on eBay for some li-ion batteries and solar cells for a project. Unfortunately, I was generally pretty dissatisfied with the prices for small coil lithium cells.  I then found an eBay seller selling  solar rechargeable key chain lights for $0.99.  I figured, since it's driving white LEDs, but less than $1, there's no way they have a boost converter in there, and Li-ion is the only common battery chemistry with a high enough voltage to drive white LEDs.  Therefore, there is most lightly a Li-ion cell in there, and three white LEDs and a small 5V solar panel are a nice bonus.
I ordered two as part of a larger order from him, and tore them apart. Tearing it apart, it seems I was mostly right...  The circuit is about as simple as it gets; a 5V 1mA solar cell, trickle charging a lithium coin cell through a diode, and then a push button to three white LEDs. There isn't even any current limiting here; it's depending on the high internal resistance of such a small battery.

Unfortunately, instead of it being a single rechargeable 2032 battery like I theorized, it ended up being two 2016 3V lithium cells heat-shrinked together. Once the initial 6V charge on these (not technically rechargeable) batteries dies, these things are probably pretty pitiful after being recharged by a 5V source...

On the bright side, three white LEDs and a 5V 1mA solar panel is a nice consolation prize for a failed attempt at out-smarting cheap electronics from China.  The solar panel is double-stick taped to the PCB, and you want to leave it stuck there for safety, and desolder everything else instead. Since it is a raw solar cell, the glass and terminals are entirely unprotected, so breaking the cell or lifting a pad is hard to avoid when you're handling it.  Leaving it on the PCB gives you hardier pads to solder to, and the surface mount diode is generally useful for solar projects.