Tuesday, April 18, 2017

Mastr 3 UHF Low Pass Filter Testing

Last year, I got a good deal ($50) on a pallet of Mastr 3 repeaters. They were all T band (490MHz) and more or less clapped out, but I figured it would be a good learning experience (and I got a nice tear-down video out of it).  Being way off the amateur band and wrought with problems, most of the electronics have been of little use to me and has been little more than a learning opportunity, but one piece I see myself being able to use elsewhere is the final low-pass filters from the power amplifiers (Part number 19D902856G9).
I've built low pass filters before, but wouldn't trust my own constructions to actually being put in use for where you need these filters the most on VHF/UHF, which is on repeater systems. Repeaters tend to be key-down for long periods of time, so power handling in the filters is important, and since they're remote if something goes wrong there's no one there to notice and getting to it to service it is generally inconvenient.  Since these low-pass filters off the Mastr 3s are already designed for high power (90W) continuous duty, they're perfect for my repeater applications (Usually 10W-25W) with the one exception that they're tuned for 470-512MHz, where all the repeaters I tend to build are in the 440-470MHz range.

That being said, 440-470 is lower than 470-512, so it's conceivable that the impedance in the pass-band might still be acceptable, and then the only downside is that I wouldn't enjoy quite as much filtering of the second harmonic than if the filter was really designed for 440-470MHz.
One thing I found interesting about the filter design was that it isn't symmetric! It's a 7th order LC filter, but the capacitors to ground only reduce in value on one side and not the other! My best guess as to why this is the case is that the engineers designing this filter knew that the output of the power amplifier wasn't perfectly 50ohms resistive, so they added a little reactance here to better match the amplifier.

To muddy the waters, the schematic and the physical unit I took apart don't agree on which port has the lower capacitance, and I wasn't careful enough when disassembling the amplifiers to say which port was actually connected where. I also enjoy that this model of the filter has black boxes instead of the inductance values filled in like on the other LPFs in the spec sheet; I'm not the only one fudging the documentation at times!

In any case, I just want to check the pass-band behavior of these in the lower 440-470MHz range, and see if these are something worth keeping or not. To the ungodly expensive vector network analyzer!
So on the top we've got the S21 plot going through the filter, so you can see the flat, low insertion loss, region on the left for the desired signals, and then the filter starts rolling off as you go higher in frequency, to the point where, on the right side of the plot, 900MHz is attenuated 45dB. The two markers 1 and 2 are at the two limits of my frequencies of interest, so you can see how the filter stays flat for much longer than I need before starting to roll off, so that 900MHz attenuation number could be better.

On the bottom, we see smith charts for the J1 and J2 ports (way zoomed in to see the detail around the center), so there's certainly an asymmetry to the filter, but both ports are decently close to 50 ohms so this isn't a game stopper. In an ideal world these filters would be a dot at the middle of the smith charts, which would be 50Ω + 0jΩ (50 ohms resistance plus zero ohms reactance), but they start wandering off into the wilderness as you go higher in frequency, which makes sense since the whole point of a low pass filter is that the insertion loss goes up with frequency, so the impedance usually does weird things (you can design flat impedance diplex filters if you care, but we really don't here)
So now lets zoom into the pass band we're interested in and check the SWR to sanity check that this filter is an acceptable match for my transmitters at my lower frequencies. Annnnd they are! This filter actually happens to bottom out its SWR curve right at 470MHz, so this filter is going to be great for my part 90 stuff, and still find for amateur stuff with an SWR of <1 .20="" 90="" a="" amateurs="" for="" gear="" have="" lower="" nbsp="" of="" p="" part="" radio="" standard="" than="" thumb:="" ule="">
Notice that this check was pretty valid, since the SWR starts sliding upwards at lower frequencies. I probably should have done a sweep further down in frequency to demonstrate how bad it can get once you wander outside the band the filter was designed for, but these measurements were being taken on borrowed time on the company VNA.

Great. In any case, now lets take it apart!
Ten screws reveal about what is expected, four inductors and three capacitors to ground.
Of course, at UHF the needed inductance is about one turn, so these are pretty minimalist inductors! The capacitors are a little odd; I haven't seen these sorts of clad mica capacitor before, but they're designed to handle the high RF current seen in a filter like this without needing to be huge for the power dissipation from using a normal termination instead of these heavy wrap-around metal bits. The rated working voltage for these capacitors is just 100V, which was a bit surprising to me, but makes sense once you consider that this is a 50Ω 90W system.
My favorite part of the die-cast shield is the little cutouts in the walls leaving a gap where the strip line passes underneath the dividers between stages. I don't know if those are designed for a critical dimension or just "tiny gaps."

In any case, I've now got a pile of these great LPFs for various repeater projects. They're usable as-is, but I can always go in there and tweak inductor/capacitor values if I want to use the chassis for something else (or actually tweak them down to my frequencies of interest).

Tuesday, March 7, 2017

Podcast Interviews on APRS

When it comes to amateur radio, APRS is pretty near and dear to my heart. I did my masters thesis on it, and I'm now the maintainer of Aprx, which is one of the most popular digipeater software packages.

I recently started talking to Cale, K4CDN, and have recorded a pair of podcast episodes for HamRadio360 on APRS. Enjoy:

  1. All About APRS with W6KWF
  2. APRS Follow-up (Listener Q&A)

Tuesday, January 17, 2017

APRS Symbol Look Up Table

Click to Enlarge
In APRS, symbols on the map are encoded as two ASCII characters. The first character selects the table (primary or secondary) and the second character selects the symbol from that table. (The table selector can also be used as a alphanumberic overlay on top of the secondary table)

The symbol list is available online here, but I've always wanted an easy-to-use graphic with the symbol codes and the icons right next to each other. I finally broke down this week and created the image you see above based on Hessu's great APRS vector symbol set. Click on the table to see it in full resolution.

To use this table, simply find the icon you want to use, read off the symbol code above it, and use "/[symbol code]" if it's in the middle square and "\[symbol code]" if it's in the bottom square.

Sunday, January 1, 2017

2016 Retrospective

It's the end of the year, so I figured I'd do a general life recap and wax on what I'm looking at in 2017.

First and foremost, I changed jobs this year. I'm now an electrical engineer at Lam Research in the etch engineering group, where I'm working on their wafer plasma etchers used in semiconductor fabrication. This is particularly fun because I got this job through one of my Cal Poly Amateur Radio Club cohorts, so I get to see him nearly every day (thanks Sean!). It's an interesting group to work in since wafer processing spans a wide breath of fields including power systems, vacuum tech, RF power, embedded controls, etc. Most of my workload is related to my experience in RF systems, so designing RF filters, but the Lam Etch Engineering group is organized in such a way that I enjoy a high level of diversity in the problems I get assigned to, so I'm not concerned about ever getting pigeon-holed into just working on RF.

My group at Lam has been growing very quickly this year. Even just since I started we've hired three more EEs and looking for a few more. If you're in the bay area and looking for a job feel free to shoot me an email if you're interested.

My personal hobbies have continued to be involved and time consuming. My previous new year's resolution was to get back into public speaking and give four talks to radio clubs. I only ended up developing two new talks this year, but there was enough interest in them to take them on tour and give the same talks several times so I think it mostly counts as a success:
I've been having fun building and deploying radio repeaters. On the bench side, I've been putting together a collection of videos on repeater building [1][2][3][4]. Online information on repeaters is tough to come by, and I don't feel like I've even made a dent yet, so plenty more repeater videos are yet to come. On the repeater deployment side, I got to spin up several of my personal repeaters at events like the Wildflower Triathlon under my commercial radio license and at the Bay Area Maker Faire on the amateur repeater test pair.

In addition to working communications for the Wildflower Triathlon for TriCalifornia, I got talked into volunteering for several other of TriCal's events. A dedicated communications team isn't needed for their other events, so I instead got to work on the course team placing cones and traffic control around the race courses for the Pacific Grove Triathlon, the SF Triathlon at Alcatraz, and the Giant Race. Getting to drop traffic cones off the back of a moving box truck is a kick.

Sadly, TriCalifornia recently announced that they're not hosting any of their own events in 2017, so my event support in 2017 is going to need to be more diverse.

I've been having fun climbing towers for the Salinas Valley Repeater Group this year. They're a good group of guys with an impressive repeater network set up covering the bay area down to Fresno. I love tower climbing since it's a good day of exercise on top of a great excuse to get tours of radio sites.

In the category of "crazy adventures I never got around to writing about," one of my friends this year called me up at the last minute in the end of June:
"Hey Kenneth, I know you like to go on crazy adventures. You want to help one of my friends put on a commercial fireworks in Ft Bragg?"
Yes. Yes I do.

It ended up being a full day experience of get up at 5am, drive up to Ft Bragg, spend the afternoon unloading 650lb of fireworks and the corresponding gun racks out of a box truck, and then firing the show before crashing in a motel. The fun part about these "hand-fired" shows is that we don't use a mile and a half of electronic matches to light off the shells but I get handed a 20 min road flare at the beginning of the show and get to run around using the road flare to light off the fireworks.


So now for the question on what I want to try and do in 2017. My tradition of trying to pick up a new hobby every year started getting more difficult recently mainly because my hobbies (repeaters, beer brewing, tower climbing, etc) all tend to be so equipment intensive that my apartment is starting to get ridiculous with all my stuff.

I think I was on to something good with the 2016 "public speaking" objective, so I think I'm going to go on a similar vein this year and try and get four magazine articles out the door. I've been complaining about how disappointing the amateur radio magazines have been long enough that I just need to put up and start delivering the content I want to read. The ARRL also pays pretty decent money for accepted content too, so that's a nice perk that this year's hobby might actually be cash positive for once. We'll see at the end of the year as to how I do.

Here's to another year, and I hope all of your objectives and achievements find you in good health as well!