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.