Tuesday, October 31, 2017

A Little Audio

This is not "high" tech, but finding a solution took a little while.  The AA6E station, for today's purpose, consists of a Windows 10 PC (the Intel NUC), a FlexRadio Flex-6500, and two compact bookshelf loudspeakers.

The usual solution these days for audio setups is to use "powered speakers" for computer audio and possibly for transceiver audio.  But these have problems for me:
  1. For HF Amateur Radio, powered speakers generally are too sensitive to the RF environment and require ferrite suppression.
  2. Powered speakers generally only offer a few feet of cabling between speakers, which is fine for your desktop but not so good if you want to fill a room with audio.
  3. It's awkward to support independent speaker systems for the radio and for the computer.  They take up space, and they make for lots of cabling.
  4. Higher end powered speakers do a fair job with audio fidelity, but not as good as good conventional speaker components.
In my case, I'm starting with these nice inherited speakers (thanks to son Eric!) and I'm working backward.  You need a basic audio amplifier.  These aren't as common as they once were.  There are some interesting very cheap "Class D" switching amplifiers.  I tried one (the Lepy LP-2020A).  It did OK with the audio, but it lacked multiple inputs, and, worse, it produced very strong VHF emissions that wiped out 2 meters for me, even after adding chokes.

I sat on this project for a while, until I ran across a family of simple audio amplifiers by AudioSource.  They offer the AMP100VS, which gives 50 W per stereo channel.  That's more than I need, but you only live once.  Amazon has it for $110.

The system (diagrammed above) provides several unexpected features:
  • Auto On-Off.  The amp can be set to power on whenever an audio input signal appears.  There's about a 3 second delay for turn-on.  Power shuts down after 5 minutes with no input. This feature saves us having to provide switched AC power. In the standby mode, the amp draws 8 W from the line, compared to about 17 W for power on idling. You can argue whether that's a worthwhile saving! (Power can alternatively be controlled by a 12 V DC signal.)
  • Audio Interrupt.  This amp is apparently designed for the commercial public address market, where you might have background music that is occasionally interrupted by "Attention K-Mart Shoppers".  In my case, it is convenient to allow the radio's output to interrupt the computer's output, so I don't need an audio switch or mixer.
This is low tech, as promised, but it sounds fine and solves a knotty little problem.

Monday, October 02, 2017

A little NUC on my desk

When my 8 year old computer, home built with a Core i7-920 processor, began freezing up randomly, a new generation computer was in order.  The only application I normally use that takes significant computing power is FlexRadio's Smart SDR for Windows that needs to control the Flex 6500 SDR transceiver.

Recent SSDR versions are much less demanding than they used to be, so maybe I could make do with a "downgrade" to a Core i5 system.  Intel processors divide broadly between "i3" (dual core), "i5 (dual core, with hypterthreading yielding 4 threads), and "i7" (quad core, 8 threads).  Of course, the later chips ("generations") in each category will be a lot more powerful than the earlier ones.

After some debate, I selected a Intel NUC (next unit of computing) tiny computer configuration in "kit" form.  You need to supply your own SSD (solid state disk) or hard drive and your own DDR4 RAM.  The NUC is available in quite a few versions, but I ended up with the Intel BOXNUC7I5BNH kit, which is a "7th generation" i5 box with room to add a 2.5 in. SSD or HD.

I installed a Samsung 960 EVO Series - 250GB PCIe NVMe internal SSD, which uses the (relatively) new M.2 interface and leaves the 2.5 in. bay free for the future.  Two 4GB DDR4 RAM chips complete the kit.  Assembly is trivial, if you're at all familiar with computer innards. Installing Windows 10 Home from a USB memory stick was quick. Transferring data and software from the old system was simplified by staging files onto an external USB hard drive.

After all that, we have a very fast little computer.  The Passmark benchmark comes out at 3,644, which is roughly 60% of the score of the old i7-920 with GT640 graphics -- but with half the cores and only on-chip graphics.  The NUC is happy to drive my two HDMI displays, although the second display requires a Thunderbolt/USB-C to HDMI adapter cable.  SSD I/O performance is blazing!

But what about Flex SSDR?  That's the primary app for this computer -- when I'm not using the Flex Maestro controller.  Here are some results:

Panadaptors |
window size |
CPU utilization |
Network Mb/s
(* maximize spectrum frames per second and waterfall rate)

The worst case CPU load (44% across the 4 i5 threads) seems to be a comfortable number.  The Flex 6700, on the other hand, with its maximum 8 panadapters might have trouble, if that's your operating style.

The NUC handles the required load with capacity to spare.  It uses much less power to run and has only about 1/60 the volume of the old system, fitting easily on the desk.

Friday, September 08, 2017

Don't believe everything you read (Kp=5)

(tnx N3KL, NASA)

The news is all about the "Major Solar (Geomagnetic) Storm" we are having.  Points:
  • No aurora visible last night (lat 41 deg), but Moon was very bright.
  • Kp (Planetary K index) is supposed to be 5 now (quite high), and it was up to 8 last night. But the 20 meter ham band is hopping.  I just bagged my first digital T77 (San Marino).
  • If it weren't for the news, I'd say 20 meter (14 MHz) conditions are fairly normal, but 15 and 17 meters (21 and 18 MHz) are largely dead.
Sometimes it's better not to know too much.

Wednesday, September 06, 2017

End of the world, coming into view?

Solar cataclysm (for shortwave radio folks) is happening right now.

The red shows the absorption of radio waves around the globe, with the center in the current sub-solar point (where it is noon).

We have "R3" radio blackout conditions.  (All from http://www.swpc.noaa.gov)

In addition to this, we have a "CME" (coronal mass ejection) of high-speed charged particles generated by a previous flare event that is supposed to hit us today, causing a geomagnetic storm that should further screw up the HF airwaves.  You probably won't lose your electric power, however, but you might get to see the aurora borealis tonight -- if it's not raining cats and dogs.

And, there are big thunderstorms outside.

And, there are two big hurricanes out there somewhere -- not in New England (yet) fortunately for us.

And lots of trouble coming from DC and some other world capitals.

Thursday, August 24, 2017

Flex Remote @ ARRL HQ

After a false start or two, we finally have Flex Radio's Version 2 SSDR software running.  It enables easy internet remote operation from wherever, using my Flex 6500 radio installation at home and the Maestro controller.

My first successful test was using an "outside" test internet connection via my Nexus 5X cell phone in "tethering" mode, while I was still at home.  It was convenient to have both local and remote sides of the connection on my operating desk.  (With my data plan, I estimate the hourly running cost for remote service is about $6/hour with standard settings and not minimizing bandwidth.)

Today, we are set up in the ARRL headquarters site in Newington CT, about 40 miles from home as the crow flies.  Everything seems to be working well.  "59" reports from Switzerland and Spain.

At home, I have the Flex 6500 controlling the tuning of my SteppIR 3-element beam (fixed on Europe), so I can remotely operate from 20 meters down to 6 meters.  Unfortunately, there is no remote pointing control -- yet.  Also, no integration with digital modes or my logging system.  One step at a time.

Wednesday, August 09, 2017

What we were doing in the 80's (VLBI)

Arthur Niell, Marshall Cohen, David Rogstad, and Martin Ewing, ca 1982

See full photo album.

PDP-11s were hot.  Helical scan video recording was just the thing for recording high-speed radio interferometer (VLBI) data.  In a Caltech-JPL collaboration, we built what was then the largest correlator to process data streams from up to 5 telescopes at once -- providing correlations (fringes) between all 10 pairs.   Getting any fringes at all required delicate synchronization of the telescopes (Loran C timing and Hydrogen maser frequency standards).  Geometric corrections were tricky, too. You had to know the baselines between telescopes accurately, giving the delay and doppler frequency offsets.   To manage all this, we had MSI-TTL logic, core memories, a PDP-11/40, and Forth software.  The hardest part -- keeping those tape machines running!  They were modified IVC and Ampex helical scan recorders, semi-professional.  A later version of the system used standard consumer VHS recorders, which were just becoming available.

One bit sampling was the choice for best signal-to-noise ratio on a broadband "continuum" radio source.

All this came to light again when I started digging into some old files.

When all else fails...

My Keuffel & Esser Decilon 10, model 68-1100 slide rule.  There are lots of K&E sites. For example, this one.

Saturday, July 08, 2017

Things that can't be understood

My "fancy" homebrew computer (not pictured here) has been running nicely for about 8 years. It was big and powerful in 2009, using the new Intel Core i7-920 chip, with lots of RAM and lots of cooling.  Nowadays, Gimli (as I call it) runs mostly Windows 10, with a touch of Linux.

It runs up to 8 background threads simultaneously, running up some nice statistics with BOINC -- the science "@home" brokering site. Maybe we've discovered a pulsar or two, who knows?

This was alongside my "real" work which has been operating my ham radio station -- running SDR radios, logging, digital mode operation, etc.

That was until a few weeks ago, when Gimli started failing -- stopping suddenly with no blue screen of death or software logging, just a blank screen.  I thought maybe my checkered past as an overclocking experimenter was catching up with me.  I tried going back to the absolute minimal BIOS settings just in case.

I starting checking out what I might need to buy to upgrade this "ancient" machine through Newegg or Amazon -- and the discussions I'd need to have with the XYL.  Yes, there are many new options on the market.  The biggest one might be the advent of SSD technology (Solid State Disk).

As a last stand, I thought I'd check out the computer as best I could.  It's practically impossible to find a fault of the kind I was having.  You'd just have to replace key components one by one until it started working again.

Opening the box and blowing out the dust, I started wiggling the cables and re-seating the connectors. I pulled out the two RAM modules and "cleaned" the contacts (wiping with fingers), and I plugged them back swapping their positions.

Naturally when I put things all back together, Gimli came to life and hasn't crashed for a week or so now.  Maybe we're go for another 7 years?

Or maybe we're running on fumes.

Sunday, July 02, 2017

Sad day at AA6E

My FlexRadio Systems model 6500 transceiver has gotten a serious bug, dropping frequency lock periodically.  It may arise from operating with the GPS Disciplined Oscillator -- and summertime high ambient operating temperatures.  Back to factory it will go.

The following crude video shows how this looks when listening to WWV (15 MHz).  Problem tends to develop after high duty-cycle transmission (JT65 - 50 W) for a few minutes.  It sometimes goes away if the radio is left in receive for a long while (10+ minutes).

Saturday, February 25, 2017

Whatever became of Eimac?

In my recent post, I reported some measurements on the power tubes (Eimac 3-500Zs) in my SB-220 amplifier to see if they still were serviceable after ~45 years.  (Mostly, yes.)  But this started me to wonder about the "Eimac" name, which, though once the gold standard, has largely disappeared in the Amateur Radio market.

With the help of DuckDuckGo, I was able to trace a long corporate history, starting with the original Eitel-McCullough company (1934), which became Eimac.  Then, a merger with Varian Associates, ending up as a part of Communications and Power Industries (CPI).

The good news is that, within CPI, the Eimac name still exists, and they still market power transmitting tubes.  Alas, the classic glass envelope / internal anode tubes (as shown above), including the 3-500Z, are no longer offered by Eimac.  (Some are still widely available as imports and marketed by firms such as RF Parts.)

Eimac is found in high-end markets for commercial, industrial, and military transmitters.  Glass has given way to ceramic for insulating seals, while external directly cooled anodes are the choice for efficiency and performance.  "Low end" Eimac tubes (triodes and tetrodes that are effective for our 1.5 kW power levels) may still be produced in low volumes, but they will be very expensive compared with glass envelope imports for our ICAS operations.

Over recent years, the old vacuum tube technology has been replaced by solid state designs, especially for commercial service.  So there is little demand for the kW size tubes.  Still, it's much cheaper to keep an SB-220 going with new imported tubes if needed, than to upgrade to the latest transistor amps.

Your reward for reading this far!

CPI provides a library of some of the older Eimac applications support documents.  I found "Care and Feeding of Power Grid Tubes" especially interesting.  So here are some links to the PDF files.  The basics will be interesting to ham operators, especially those of a certain age.  The advanced sections cover some fascinating large and exotic transmitting tubes that most of us will never see.

Tuesday, February 14, 2017

SB-220 Amp, need new tubes?

My station has a trusty Heathkit SB-220 amplifier for those times when 100W is not quite enough.  I don't use it a lot, but I am considering whether it's time to get a new pair of 3-500Z tubes.

The amp was built by Paul Gallier (WQ1C, now SK) in 1972.  Paul upgraded the amplifier with many of the recommended mods, including new power caps, QSK switching, bias cutoff, etc.  But so far as I know, I still have the original Eimac 3-500Z tubes.

The amplifier seems to work well, except that it's hard to get full power output on the 10 and 15 M bands.  I had data from WQ1C that suggested that he was getting fairly constant power output across 80 through 10 M at the time I bought the amp, some 10 years ago.

Time to make some measurements of my own.  Since there are a number of settings that can be twiddled, it is hard to know exactly how to make a comparison.  You can adjust drive level (Flex 6500 output), amp tuning and loading.  I settled on CW mode, with the amp switched to its low voltage "CW/Tune" setting.  I looked for maximum output power (Po) with various drive and loading settings, at the maximum allowed grid current (Ig) of 200 mA.  Po is measured with an LP-100 RF Wattmeter feeding a Drake DL1000 dummy load.

The results are shown in the figure.  I plot the DC input power (Pdc = Ip x Ep), output power (Po) and efficiency (Po/Pdc).  Clearly, there is a problem with 10 and 15 M, which show low output power and efficiency.  At 10 M, the output power has fallen by 3.3 dB -- about 1/2 S-unit.

High frequency fall-off like this is said to be a typical problem in power tubes as they age. (See W8JI's amplifier site for example.) Presumably the gas ions, being much heavier than electrons, move relatively slowly and "gum things up" with their long transit times at higher frequencies.  (I would like to find a more technical treatment of the problem.)

It's good to have data.  Now the question is whether getting back that 1/2 S unit on one band is worth the cost of new tubes.  Short of that, there is the possibility of reducing gas by "gettering" the tube -- running with high dissipation (plates glowing red!) -- which activates  a special gas-absorbing getter material covering the plates.

Note added: Running with bright glowing anodes is dramatic, but I have to wonder if it is safe for the SB-220.  The cooling "system" (fan) is rather primitive.  It seems to be OK for normal ICAS operations (plates dim if red at all), but when dissipating power at the maximum spec. (500 W) for any length of time, you have to worry.  I understand that a particular issue is cooling of the anode and base connection pins, where a sustained overheating will compromise the glass seal, letting the vacuum out.  So aggressive gettering is probably not a good idea with the stock Heathkit.  It would be handy if we had a way to measure those critical seal temperatures.

Sunday, January 15, 2017

First Meteor Scatter Contact

Meteor in a Perseids shower, Wikipedia
It's not exactly ground-breaking in the Amateur Radio world, but I'm pleased to have made my first "meteor scatter" contact with another radio amateur.  Thanks to W4IMD, Peter in Georgia, who exchanged information with me over a path of about 1,100 miles on 50.28 MHz with about 50 Watts of power.  We used the software mode called "MSK144", developed by friend and colleague Joe Taylor (K1JT).

Meteors strike the Earth's atmosphere frequently, even in the absence of a big meteor "shower".  They generate a trail of ionization that lasts up to a minute or so (at 50 MHz), which is sufficient to exchange a bit of information between ground stations that have mutual visibility -- up to about 1,300 miles.

Strong meteor burst from N0TB in Minnesota displayed
on FlexRadio FLEX-6500 SDR transceiver
There are other worlds to conquer: working through Amateur Radio satellites or the ultimate in weak signals -- moon-bounce communication. All these have been done one way or another for decades, but they keep me learning new tricks!