Using a Motorola PageTrac and a VHF Antenna Tuner in an APRS iGate

My APRS iGate has been using the old Kenwood TR-2500 that you see in the picture for a few months. The radio was connected to an old commertial VHF antenna. This antenna is not resonant on 144.800MHz, but it is sturdy and high, and it receives very well. So I used it in receive-only mode; the iGate relays APRS messages from RF to the internet, but not the other way around. But when I got two old Motorola VHF radios, I decided to replace the TR-2500 by one of them, a Motorola PageTrac. This was supposed to bring two advantages. First, the Motorola radio would start up on 144.800 after a power loss, whereas the TR-2500 had to be programmed manually for 144.800 after a power loss because its memory battery has long died. Second, the PageTrac puts out 45W whereas the TR-2500 puts out only 2.5W, so if I start transmitting messages, the PageTrac would have a much better range.

The plan proved more difficult to execute than I had thought.

A Matching Network

The existing antenna is sturdy, high, and mounted on an old mast that I had no desire to climb. So I decided to keep using it. To transmit using this antenna, I needed a matching network: a single-purpose antenna tuner. I borrowed an MFJ-269 antenna analyzer from Nir Israeli and measured the impedance of the antenna and coax at the radio-side connector (at 144.800 MHz). I then designed an L match circuit using an online calculator.

I didn’t completely trust the analyzer and I was not sure that the air inductor I prepared had the correct inductance. So I used a variable capacitor in the L match, so that I could tune it. This worked out well. By adjusting the variable capacitor I was able to bring the SWR to a low value that is good enough for transmitting (I think it was 1.2:1 but I didn’t record it). The L match is narrow band and the SWR is reasonable only across about 0.5 MHz, but this is of no significance for APRS, which uses a single frequency.

The online calculator gave me not one appropriate L match but two. The other solution had a tiny series inductance and a capacitor across the transmission line. I tried it (with no inductance at all) and could not get the antenna to match. I am not sure why this matching network did not work.

Antenna tuners of this sort are almost never used at VHF, because it is pretty easy to build resonant antennas. But in my situation, with a good antenna that works well even though it is not resonant, this special-purpose antenna tuner is a good solution.

Power Supply Troubles

The two Motorola radios I got are a DeskTrac, which is a desktop version of the more common mobile MaxTrac, and a PageTrac. The DeskTrac is documented very well on the web. I didn’t find any useful documentation on the PageTrac, but it is a very simple radio, so I didn’t really need much documentation. Both radios worked when I got them, but I quickly discovered that the power supply of the PageTrac was failing. Two large electrolytic capacitors were burned, and they actually charred the PCB beneath them. They were buzzing and burning when the radio was turned on. I tried to replace them, but I didn’t find replacement 15,000μF capacitors. I decided to replace the entire power supply. It was rated at 13.8V and 10A, so I needed a power supply with similar specs.

The first candidate was a power supply from a dead computer. Its 12V output was rated at 16A, so I thought it would work. I spent a bit of effort trying to raise the output from 12V to 13.8V, but this caused buzzing in the PC power supply, so I left it alone. I assumed that running the radio at 12V rather than 13.8V would reduce output a bit, but this was acceptable to me.

The radio worked with the computer power supply, but whenever I tried to transmit, the power supply shut down. I am not sure why it shuts down, but my guess is that the roughly 10A that the radio draws from the supply on transmit causes the switching regulator in the supply to generate wider pulses. This maintains regulation on the 12V line, but it raises all the other outputs that have almost no load on them (5V, 3.3V, etc.; all are generated from the same switching waveform). The over-voltage protection circuit of one of these outputs might be what is shutting down the supply.

Next, I tried an old 12V/11A Lambda switching supply that I thought at a surplus store for about $5. I adjusted it to 13.8V. It works without a problem, and it is small enough to fit inside the PageTrac enclosure. It just sits in the enclosure without being bolted to it, but since the radio just sits on a shelf, this does not cause problems.

The Computer Interface

To use the radio as an APRS iGate, I needed to connect it to the soundcard of the computer running the iGate software. The interface that I used with the TR-2500 was not really an interface at all: just a cable connecting the TR-2500′s earphone output to the computer’s microphone input. But with the PageTrac, I needed some kind of interface, if only to activate the transmitter. I built a simple interface with (1) DC blocking and level adjustment on both the audio input and audio output directions, and (2) a PTT activation circuit. The PTT circuit is activated by the RTS signal of a serial port, and is isolated from the radio by an optocoupler. But the audio lines are not isolated, so the radio and computer are not really isolated. So far this did not cause any problems.

Initially, the interface did not work. When I would connect it, the radio would go into transmit mode, even if only audio cables were connected, not a serial cable. It took me a long time to debug, but eventually I discovered that I made a mistake in the wiring of the cable connecting the interface to the radio’s front speaker-mic connector. Once I fixed the mistake, the interface started working.

Limitations of the PageTrac

The PageTrac has some limitations. One is the fact that the only audio/PTT connector available is the RJ45 speaker-mic connector on the front. It has an RJ11 connector in the back, but it is undocumented. In contrast, the backThe DeskTrac has a DB-25 jack on the back designed for connecting it for to computers and other equipment. The second is that it has no “monitor” button to turn off the squelch. APRS works a bit better without a squelch, and the DeskTrac has a button that allows you to turn it off. I also think that unsquelched audio is available from the DeskTrac’s DB-25 connector. But even through the squelched speaker-mic connection, the PageTrac works fine. Another limitation of both radios is that the speaker is never completely muted (even at the lowest volume level). To prevent the radio from sounding the APRS packets, I simply disconnected one of the speaker’s wire.

Software Modem Issues

Soundmodem, the soundcard-modem that I have been using with the TR-2500, did not decode packets received by the PageTrac. I thought of adding a high-pass filter to the audio interface, to compensate for a possibly too-aggressive de-emphasis, but eventually wrote a new software modem that decodes packets from both radios without a problem. But this is a topic for another post.

After resolving all of these issues, the new iGate configuration is up and running. The iGate beacons on both the internet and on 144.800 MHz, and it relays text messages and other packets from the internet to mobile stations. In the screenshot below you can see me exchanging text messages with a mobile station (which uses a Kenwood D700). You can see both stations on the map, the text-message window of APRSIS32, and the actual packets that are received and transmitted by the iGate, some via RF and others via the internet.

Finishing Up the Satellite Yagi

Over the past week I finished the satellite Yagi. I added a VHF Yagi and a band splitter to the UHF Yagi that I constructed first.

Kent Britain, the designer of the antenna, wrote that you can obtain good results with a 2-element 145MHz antenna, but I decided to stay on the safe side with a 3-element antenna, like the one described by Richard Crow. I noticed that Crow’s 3-element antenna was a little different than Britain’s, and decided to build to Britain’s measurements. I first cut the elements out of the same stiff copper wire that I used for the 435MHz Yagi. I was not sure, however, how to combine the two antennas into a single structure. Britain suggested to mount the VHF antenna behind the UHF antenna, on the same boom and on the same plane. This makes it easy to transport and store the antenna, since it is flat, but the boom is pretty long. Crow mounts the antennas side by side; this is easy with his very lightweight construction, but would be harder with wood booms. Shamai Opfer suggested to mount them on separate booms connected by hinges, so I can store them together flat but flip them into a crossed configuration for operating.

I eventually decided to start by mounting them one behind the other on the same boom, and to saw them apart later if I want to try a different configuration. I initially mounted the VHF Yagi 6 inches behind the UHF one, but when I re-read Kent’s article I realized that h wrote that they must be spaced only 3″ apart. So I removed the VHF elements, drilled new holes in the boom, and mounted them in the correct places. I soldered a connector to the VHF Yagi and verified that it was a good match to the transceiver. I was; no tweaking was necessary. I was also able to receive stations communicating using Morse code through VO-52, a satellite with a linear transponder whose downlink is on 145MHz.

To communicate through satellites with the FT-857, I also needed a band splitter, a 3-port filter that would allow the single VHF/UHF connector of the radio to be connected to the two antennas (you don’t need it if you use separate UHF and VHF radios). Kent Britain’s article describes a simple splitter consisting of two 3-component filters, a high-pass for the UHF port and a low-pass for the VHF port. I did not have capacitors with the values in Britain’s article, so I designed my own splitter with Elsie, a filter design program.

Elsie has an option to design diplexers, which are band splitters that aim to keep the impedance at one port constant over a large frequency range. Diplexers are used following mixers, for example, where you want a proper termination for the image frequency and to various spurious frequencies that the mixer might output. You can use a diplexer as a band splitter, but if you don’t need constant-impedance termination you have much more freedom to design the low-pass and high-pass filters. I designed the two filters by telling Elsie to design a Chebychev filter and playing with the cutoff frequency and ripple settings until I got the capacitor value I wanted in combination with very little attenuation at in the 145 and 435MHz bands. My capacitors were one 12pF cap for the 145MHz low-pass filter and two 5pF units for the 435MHz high-pass. This generated two inductor values. I then used Elsie inductor calculator to come up with appropriate air inductor designs. I played around a bit to get a sense of what inductance different diameters and turn counts  gave. When I got close, I designed the actual filters by specifying the diameter and number of turns. For the UHF filter, I specified 3 turns and a 0.5cm diameter; a length of 0.9cm gave the proper inductance. For the VHF filter, I specified 6 turns and a 1cm diameter, which required a 1.9cm length. I wound the two VHF inductors from the same piece of wire and then bent it so that the inductors were at a 90° angle, to minimize coupling. The UHF inductor was mounted upright, also at a 90° angle from the VHF coils. The wire for all inductors came from a choke from a broken PC power supply.

I connected the antennas and the radio to the splitter and tested the match. I was almost perfect on both UHF and VHF. I could open repeaters on both 145MHz and 435Mhz.

I clamped the wooden boom to a camera tripod, as you can see in the picture at the top. The weight of the antenna is not balanced on the tripod. To relieve some of the force it applied to the tripod’s head, I tied the back of the boom to a leg of the tripod with a cord. It works, but I’ll need to find a better solution.

The next step was to try the antenna on an FM-repeater satellite. Yesterday two of them passed overhead at reasonable hours, allowing me to try satellite communication for the first time. The first was AO-27, a very old satellite (launched in 1993). It’s web site specifies a schedule, which suggested that only its telemetry beacon would be active when it was over me. I tried to receive the UHF signal nonetheless. When it came over the horizon, I could hear the quieting in the receiver, but then heard a voice station calling. I replied and was able to work that station and 3 more. The next repeater sat to pass over was AO-51, whose repeater is active all the time, and which I was able to hear well when I only had the UHF antenna. AO-51 was much more crowded, with stations transmitting at the same time (and causing severe interference), but I was still able to work a few stations. Tracking the Doppler shift on FM is not hard; I’ll explain the issues in another post. I tracked the location of the satellites by manually rotating the tripod a couple of times during each pass.

I am very pleased that the antenna allowed me to communicate through the satellites. The antenna is sitting in my balcony. As you can see in the picture, it is essentially indoors, facing a large window. There’s a ceiling above it; the walls around it and the ceiling are all reinforced concrete. From the balcony, about 120° of the sky is visible, towards the east and north. When satellites are to the west or south of me, I can’t see them at all. But in spite of all of these limitations, I was able to communicate through the sats.

Repairing a Deaf Softrock Transceiver

My Softrock Ensemble RXTX transceiver has gone deaf a while ago. It was working very well, allowing me to to experiment with WSPR and PSK31. But at some point it gradually got deaf, until it could receive nothing. It could still transmit fine.

I tried to fix it, trying the easy things. I checked that the Si570 oscillator was working and changing frequencies; it was fine. I also inspected the board visually for bad solder joints, but didn’t see anything obviously wrong. I don’t have a signal generator, so I could not easily check where the input signal is lost in the receive path.

Yesterday I decided to give it another go. I used a 1-transistor crystal oscillator that I put together as a signal generator. It had a 10.116MHz crystal. I hooked its output to an oscilloscope, verified that it was oscillating, and that the signal was not too large (it was around 200mV peak-to-peak). I hooked its output to the input of the Softrock and started tracing with the scope’s probe.

I could still see the signal at the antenna side of T4. So the low-pass filter was fine (the signal was also much cleaner at that point, because the low-pass filter removed the harmonics from the oscillator’s output). Next, I checked resistors R54 and R53, which feed the RF signal to the detector. No signal. This narrowed down the search considerably. Testing both sides of inductor L4 showed that it was blocking the signal. From the top side of the board, the joints did not look good; there was no solder creeping up the wires. I unsoldered it and discovered that one wire was not tinned properly. I scraped the enamel off the wire, tinned it, and soldered it back in place. Now I was able to see the audio-frequency signal at the output of the receiver on the scope, indicating that the receiver was now working. I’m using it right now on 14MHz WSPR.

Robby’s building instructions tell you to remove the enamel very carefully off enameled wire, and that not doing this leads to many of the problems with the kits. I did not heed the warning carefully enough, I guess. I still find the evolution of this fault interesting, in that the radio was working fine for a few months before the soldering fault showed up.

Building Chavdar Levkov’s Active Wideband Loop

A little while ago I discovered on the web yet another design for an active wideband receiving loop antenna, by Chavdar Levkov. Chavdar clearly analyzed both the requirements that the amplifier needs to satisfy and the circuit that he built very carefully. Therefore, I hoped that this antenna would outperform my existing active wideband loop, which uses a different amplifier, designed by John Hawes. I use my wideband a lot; it is sensitive (much more than the active whip) and does not require tuning (unlike the tuned receiving loop and the transmitting loop). Therefore, I really welcomed the possibility of a more sensitive receiving antenna that would still not overload.

The basic difference between Chavdar’s amplifier and John Hawes’ is that Chavdar’s input stage is a grounded base amplifier with a very low input impedance, which matches the impedance of the loop better than Hawes’ common emitter amplifier. The grounded base amplifier drives a common emitter stage, so Chavdar’s amplifier is also a bit more complicated to build, but not by much. When I wrote to Chavdar with some questions on his design, he compared it to Hawes’ and wrote back that he thought that his own design has higher dynamic range and higher gain at lower frequencies, but perhaps also more noise at high frequencies. I decided to give it a try (particularly since I wanted another antenna of this type anyway, because the existing one is tied all the time to a WSPR spotting activity).

One interesting aspect of Chavdar’s design is the use of networking cable rather than a coax. The CAT5 or CAT5e cable carries the power to the amplifier and the signal back on two separate twisted pairs; in my earlier active antennas I used the conventional method of putting the DC and RF on the same coax and separating them with bias tees. I was a bit hesitant to use networking cables, mainly because I did not want to deal with the 8-pin sockets, which won’t fit into a 0.1″ breadboard hole pattern. Chavdar encouraged me to use networking cables and sockets, because they are cheap, deliver high performance, and they eliminate the need for bias tees. I eventually realized that if I mount the sockets upside down and wire them ugly style, I could probably use them. It worked. (I later discovered that this is exactly how Chavdar mounted the sockets; I didn’t study the pictures in his article carefully enough.)

Before I could put the sockets into the circuit, I had to figure out how to ground the cable shield on the receiver side but not on the antenna side. Until then, I never paid any attention to shielding in these cables. I looked at one plastic connector connector and couldn’t figure out where the shield connection was. It was not there at all, because some of these cables are unshielded (they are marked UTP) and have plastic unshielded connectors. Shielded cables, marked STP or FTP, use connectors with a metal shield that’s connected to the cable’s shield. After I figured this out, I mounted the two sockets; on the receiver side, I mounted a metal-shielded socket by soldering it to a scrap PCB; on the antenna side, I super-glued a plastic socket to the PCB.

The rest of the construction is pretty similar to the way I built Hawes’ amplifier, both in terms of circuit construction and in terms of the enclosure and loop connection. Chavdar’s design used a 10V regulator on the antenna side, powered by an 12V input supply. Chavdar used a 7810 fixed-voltage regulator which I did not have, so I used an adjustable LM317 regulator. This caused me a some problems. I initially put a REG1117 regulator in the circuit. It promptly died. I replaced it and the replacement also died almost immediately. At that point I read the data sheet more carefully, and realized that the regulator needs 2 protection diodes if it is used with large-value capacitors. I used a 10μ tantalum cap on the input terminal and 22μ tantalum caps on the output and adjustment terminals; these caps can easily provide enough current to destroy the regulator when you power the unit down, and they did. I then put in the diodes, replaced the dead REG1117, and put in a new regulator (this time an LM317, but I could have used a REG1117). Now the regulator section works fine.

On the receiver side, you need a unit with an RJ45 socket, BNC to connect the receiver, and DC power input (and some filtering for both, which I did not yet put in). I initially mounted all 3 sockets on a piece of PCB, for lack of an appropriate enclosure. A little later, I realized that since the amplifier unit contains a voltage regulator, there is no need to power the amplifier using a regulated 12V supply; an unregulated one would work just fine. I therefore added a little mains transformer, diode bridge, and filtering (a pi network with two electrolytic caps and an inductor on the receiver side. There’s still no enclosure, but it’s more convenient than before. It looks weird, but it works.

I did not compare the antenna carefully to the one with Hawes’ amplifier. The loop element I used in both antennas is exactly the same, so comparing the antennas should show any differences between the amplifier circuits. But I have not done that. In casual listening, the antenna performed very well. During the SSB context over the weekend, I was able to hear many US SSB stations; I never heard any with the Hawes loop at the same location (ouside 1st floor balcony in an urban area about 10,000km from the US east coast) . This does not say much, because the contest brought many strong stations to the air, and propagation conditions were good over the weekend. But this definitely made me happy. The antenna performed fine from 3.5MHz to 24MHz (28MHz was closed when I was listening). It also receives the airband (VHF AM) and 144MHz and 430MHz, but on these frequencies it’s a little less sensitive than a 144MHz dipole. It works on MF, but not as well as I hoped; I can receive BBC World on 1323kHz, but not as well as in my car.

I now agree with Chavdar that using networking cables and connectors for receive antennas is a good technique. I plan to replace the cabling in my tuned-loop amplifier, because it needs not only RF and power, but also tuning voltage for the varactor diodes; on a CAT5e cable, I can just use a third pair for the control voltage.

I am indebted to Chavdar for both putting the design and the detailed analysis on the web and answering all my questions via email; thanks!

Adding PTT Activation to the Serial and Sound-Card Interface

One of the things I’d like to try soon is the JT65 digital mode. Like WSPR, it works even with very weak signals, but unlike WSPR, it allows two-way communication rather than beaconing. When trying out WSTJ, the program that runs JT65, I discovered that it switches the transceiver from receive to transmit modes using the DTR or RTS signals on a serial port, not using CAT commands (multi-byte commands through the serial port). JT65-HF, another program that supports JT65, does the same.

I therefore decided to add receive-transmit switching (PTT) to the serial/sound-card interface I hacked together for the FT-857D. I checked the DTR and RTS signals on the FT232R breakout board and discovered that both were active high, meaning that they are high most of the time, but when a program like WSPR or WSJT wants to transmit, it activates DTR or RTS, which brings them low, to near 0V. The PTT connection in the transceiver should be driven by an open-collector transistor (e.g., an NPN transistor with the collector connected to the PTT line and emitter to ground), and it’s easier to drive the transistor with an active-high logic signal. Fortunately, the maker of the FT232R, FTDI, makes available a utility called MProg that can configure these signals to active-high. I ran it, configured the chip, and the signals indeed got inverted. I added the NPN transistor to the strip board, it’s base connected to RTS via a 2.2kΩ transistor (PN2222). Now these programs can switch the transceiver to transmit using the RTS signal.

I didn’t have time to try JT65 yet (there’s a learning curve there), but I’m now ready, hardware wise.

A Simple Serial and Sound-Card Interface for the Yaesu FT-857D

I bought a new Yaesu FT-857D transceiver and needed a computer interface for it. Yesterday I put together a quick interface, which you can see in the picture. The serial interface to the transceiver (which is called a CAT interface) uses an FT232R USB-to-serial bridge chip on a breakout board (made by sparkfun). The audio connections are simply wired directly: the audio out and audio in lines of the transceiver are connected to two 3.5mm stereo jacks (but the audio is mono, not stereo). The jacks are salvaged ones from the motherboard of an old Apple eMac.

The stereo jacks have pins that won’t fit nicely on a prototyping board. They had a metal cover, so eventually I decided to solder their metal cans to a piece of scrap PCB. This held them in place, and I soldered the audio in/out wires to their pins, which now point to the sky.

The serial breakout board can be wired for either 3.3V or 5V logic. It was originally wired for 3.3V; I removed the solder bridge that configures it to 3.3V and soldered a wire between the VCCIO and VCC pins to configure it to 5V, which is the logic level of the FT-857D. I then secured it to the scrap PCB with screws and spacers.

Finding the 6-pin mini-DIN connector for the audio connection to the transceiver was easy; I just used the cable from an old PS/2 keyboard. The 8-pin mini-DIN connector for the serial CAT connection was harder to find. I ended up buying an antique serial cable made for an Apple IIc, which had the 8-pin mini-DIN on one side and a full size 5-pin DIN connector on the other. I cut out the 5-pin DIN connector and thought I was all set. It turned out that in both cables not all the pins of the mini-DIN connectors were connected. But luckily, all the connections I needed were wired. This includes the serial in and out lines, the audio in and out lines, and also the PTT line, which I have not yet used but I might. The 13.8V supply from the transceiver is also available, in case I want to power an accessory.

That’s about it. I soldered the wires coming from the transceiver to a piece of strip board that I soldered to the base PCB and connected the audio lines to the stereo jacks. I left the serial connections open, in case I made some mistake in figuring out which wire is which (13.8V applied to the FT232R would have killed it). I connected the transceiver and verified the voltages on coming out on the wires. The 13.8V was where it should have been, and the serial lines were pulled up to 5V. I connected the remaining wires, connected the laptop to the USB-to-serial bridge, and fired Ham-Radio Deluxe (HRD), the free and slick transceiver control program that I wanted to use. It worked! I then fired up DM780, the digital-modes program that comes with HRD and was pleased to see that the audio-in line indeed carried the signals I expected. DM780 decoded many PSK31 signals. The audio out line also worked fine.

It was pretty late in the evening, so I tried to contact stations using PSK31 on 7MHz, using the coax loop. The loop tunes fine on 7MHz, but is pretty inefficient there. One station heard me, but not well enough to make a contact. I switched to WSPR, received a few stations, and then tried to transmit. A station in Ukraine heard me immediately. This was the first time, I think, that this antenna transmitted on 7MHz.

This interface is not something that you would want to build if you had to buy the parts; the breakout board costs almost as much as a ready-made USB-to-serial cable with a connector specifically made for this transceiver. But I had all the parts at hand, and I wanted to connect the laptop to the transceiver as quickly as possible.

A final note: most of the sound-card interface designs I found on the web for both this transceiver and other ones used 1:1 audio isolation transformers. Many of the same designs make the serial connection without isolation, meaning that the radio ground is connected to the computer ground. Given that the grounds are already connected, I didn’t see any reason to use isolation on the audio paths, so I just wired the transceiver audio directly to the sound card. I also verified that my laptop ground is floating. That is, even when it is connected to the AC mains through its power supply, the DC connections of the power supply are isolated from the AC ground connector. An isolated interface is safer, but it would also require isolating the serial connection using opto isolators or RF isolators. Without the serial isolators, I think that the audio isolation is not useful.

Re-Crystaling a 10MHz Softrock Receiver

A few weeks ago I helped Nir Israeli diagnose a problem with his Softrock RXTX 6.2 transceiver. The transceiver is crystal controlled and covers portions of the 10MHz and 7MHz bands. While looking at the schematics I realized that Tony Park, the designer of the kit, used a 40.5MHz crystal to cover the 10MHz band. I had a partially built Softrock 6.2 receiver, and I decided to convert it to 10MHz using a 40.5MHz crystal that I ordered from GenesisRadio (along with 9 other crystals; they offer good frequency selection and low prices). The receiver is the “upgraded” version, which means it has lower noise and higher gain opamps than 6.2 receivers for lower frequencies. I only partially built it because I tried to use it with an Si570 synthesizer and a switched bandpass filter; this setup did not work so well.

I removed the components I added to the board for the experiment, wound the toroids for 10MHz, installed them, installed the oscillator circuit and the new crystal, and tried it out. It works very well with WSPR (it again took me a while to figure out the fiq setting, but I eventually got it right). I can now cover most of the 10MHz band with this unit. It joins the 7MHz receiver I used previously for WSPR, the 14MHz receiver (which unfortunately does not cover the WSPR sub-band), and the Si570-driven Ensemble II RX receiver.

The use of the 40.5MHz crystal should make the receiver more sensitive than the crystal that Tony Park ships with the kit, because the original 13.5MHz crystal relies on subharmonic sampling (it does not sample the radio signal at every cycle, but only once every 3 cycles). I did not compare them directly, however. I am not sure why Tony used the 40.5MHz crystals in transceivers but not in receivers.

A Driverless Ethernet Sound Card

External USB and Firewire sound cards suffer from several problems. One problem is limited support for high sampling-rates and for 24-bit samples. Release 1.0 of the USB Audio standard did not support high-end cards, which forces high-end cards to use card-specific drivers. This problem may disappear over time, since release 2.0 of the standard allows higher sampling rates and 24-bit samples, but operating system support for this version is still patchy. A second problem is that the USB driver transfers the samples to/from the operating system’s audio subsystem. The software interfaces (APIs) that the audio subsystem presents to applications vary widely between different operating systems. Some operating systems have multiple audio interfaces with different capabilities (e.g., DirectSound and ASIO in Windows). This makes it difficult to write portable audio applications, and in particular software-radio applications that rely on audio baseband sampling. (There is a good reason for this complexity: multimedia applications like games and video editing need low latency control of audio streams, but this is less important for SDR applications.)

Portable abstraction layers like portaudio are one way to address this problem, but this method does not address all the issues. Cards with obsolete drivers remain obsolete, and software installation and configuration remains difficult (actually, it tends to get more difficult, because of the need to deal with yet one more software component).

An Ethernet connection to the sound card is a more complete way to solve these problems. If the sound card sends and receives samples using UDP or TCP, it communicates directly with the audio (or software-radio) application without passing through the operating system’s audio subsystem. The data still passes through device drivers on the PC, but now these are the networking drivers, which are built into all operating systems. Furthermore, the software interfaces to networking functions are essentially the same across all operating systems, unlike the interfaces to the audio subsystem. This makes it easy to write portable audio applications.

The prototype. The green board is the EK-LM3S9B96 evaluation board. The CODEC is on the lower left. The board in the plastic enclosure is the programming and debugging dongle.

To test this approach, I built an Ethernet sound card using a simple microcontroller evaluation kit called EK-LM3S9B96. The board contains a Texas Instruments LM3S9B96 with a 100Mb/s Ethernet controller (both MAC and PHY; all you need to add is the isolation transformer and Ethernet jack). This microcontroller also has an interface to audio DACs, ADCs and CODECs.  I mounted the evaluation-kit board on a large prototyping board that also had space for the CODEC, a Texas Instruments TLV320AIC23B. The line-in port and headphones output ports of the CODEC are connected to standard ¼” stereo jacks.

Getting the firmware to support 24-bit stereo 192kHz sampling in both directions was a real challenge. I ended up using TCP rather than UDP, which I originally thought would work well. But at the end the card works very well.

An article in QEX tells the full story. It is also available on my university web site.

Bias Tees and Other Inline Gadgets

To feed the active antennas, you need bias tees, which feed DC to the coax near the receiver and extract it at the antenna amplifier. I normally build the amplifier-side bias tee into the amplifier, but on the receiver side I use a small box with two BNC connectors and a DC connector, the bottom one on the picture on the right. The gadget on the top is a resistive SWR bridge with an LED indicator. The design of the bridge is from Steve Weber’s low-power L tuner; it works very well.

Both are built into boxes that used to house Tektronix test-equipment inline gadgets called input RC normalizers, which consist of a resistor, a capacitor, and a trimmer capacitor. The box has a little hole with a plastic cover to let the user adjust the trimmer. To use the box as an enclosure for something else, you remove of course the original contents. In the bias tee I enlarged the hole a bit and used it for a panel DC socket. In the SWR bridge I drilled the plastic cover to make room for the LED. A third normalizer became a 50Ω terminator.

I have a few more of these boxes that I bought at a swapfest, along with two slightly different ones (a slightly bigger inline Pomona box, and one with 3 BNCs). Very handy for building little inline or 3-port devices.

Bias tees are very simple devices, consisting of just an inductor and a capacitor. The inductor lets DC pass to the antenna but prevents RF from going into the DC power supply; the capacitor prevents DC from going into the receiver but allows RF to pass.

In spite of the simplicity, it is possible to build them incorrectly. The mistake I made with the first bias tee I built for active antennas was to use an inductor with too much resistance. The series resistance caused it to heat up and burn (I think it happened when I used it to feed the active whip). In the pictures below you can see on the left the packaged bias tee (it’s huge for no good reason); on the right you can see the inside, with the burned inductor, which is now just an open circuit. I now use in my bias tees high-current inductors salvaged from broken switching power supplies.

Obviously, if the inductor has too little inductance or the capacitor too little capacitance (or a voltage rating that’s too low), that’s also not good. But there are more surprising failures to bias tees. Phil Salas described in QST in July 2004 a pair of relatively complex tees, with reverse voltage protection and with a fuse. He discovered that the 100μH inductor that he used had enough parasitic capacitance to resonate at 11.5MHz, which means that signals around that frequency will pass through the inductor and into the power supply (he modified it to remove the resonance). Another interesting article on bias tees is by Tom Cefalo Jr., who described in QEX in May/June 2002 bias tees optimized for 144MHz or 432MHz.

Amplifier Update: Now Using an IRF630

I couldn’t figure out exactly what went wrong with the amplifier, so eventually I decided to try to replace the MOSFET with a more resilient one and hope it won’t explode like the first one. I know that I need to add SWR protection, and perhaps to improve the T/R switching, but I don’t have time to work on these right now. I hope that the more resilient MOSFET would be able to survive the conditions that caused the IRF510 to explode.

When I took the amplifier apart, I saw that the MOSFET blew up in a spectacular way.

I replaced the IRF510 with an IRF630. The IRF630 has a 200V and 9A rating, so it will be hard to destroy with a laptop 24V supply. The trade off in power MOSFETs is between improved rating (voltage, current, and on resistance) and input capacitance. The IRF510, with a 100V 5.6A rating, has 195pF input capacitance; the IRF630 has 600pF. This makes the IRF630 harder to drive. I had experience with another amplifier, an experimental class E amplifier, in which the driver chip could drive an IRF510 but not an IRF630. I was not sure that the 1W of the Softrock Ensemble would be able to drive the IRF630.

I soldered the IRF630 into the circuit anyway, connected the amplifier to the Softrock and a dummy load, and tried it out. Success! The IRF630 delivered about the same amount of power that the IRF510 did.

I then connected the amplifier to the “unusual” loop and ran WSPR on 14MHz. As expected, there was a significant difference between the signal reports I got with and without the amplifier (and also in the number of stations that heard me). I left the setup running for most of Friday. At the end of the day, the amplifier’s signal was received all the way from Sydny, Australia, through Singapore, India, and Europe, to the eastern United States. Not bad at all. And so far it has not exploded