I made another trip to American Sa’moa (KH8 for the radio amateurs in the audience) to deploy instrumentation. It was a tight timeframe but the instruments seem to work and I managed to make a few ham radio contacts as well.
Posts Tagged ‘SDR’
Teaching Evan the basics of radar signal processing with this baby-block 7-bit Barker code and its matched filter.
This is a pretty simple modification that converts a SoftRock Ensemble RXTX SDR RF interface board from “common-IF” (RX and TX share the same antenna port) to “split-IF” (RX and TX have separate ports). Split-IF is the standard for high-performance transverters on the 50-, (70-), 144-, 222-, and 432-MHz amateur bands.
The purple dots were the first option. Unfortunately, these locations on the actual printed circuit board were not easily accessible to miniature coax and this process would involve significant surgery to perform and restore the modification. The second alternative I considered was the red Xs…jumpering over the BS170 PA transistors. By this time, I was looking for a way to avoid butchering the original circuit too much. So, I elected for option three, which was to install a second T/R switch at the antenna jack (golden circle). This had at least two advantages, the first of which was being minimally invasive. The big advantage, though, is that the radio could be operated at its design output and run through a fixed or step attenuator on the TX side to the transverter. This meant that I would be assured of having a reasonably clean signal to work with if I configured the radio correctly. It also meant that I could operate the radio as designed if I simply disabled the the second T/R switch and connected the antenna to the default port.
So, here is the modification that I came up with using parts I had in my junk box. KB9YIG ships the Ensemble RXTX with a couple of spare BS170 FETs that can be used in place of the 2N7000. I just had a lot of 2N7000s and thought I’d save the BS170s in case I ever burnt one in the RXTX.
The interface to the transverter is through a DB9 connector. PTT for this transverter is +12 volts (as done with the TS-930S) on pin 6. Pin 1 is ground. Pins 2 and 3 are shorted together in the transverter cable connector to enable the modification in the SoftRock. The right portion of the schematic with the relay driver and Omron G6Y relay is based on the T/R switch from W1GHZ’s “Multiband Transverters for the Rover” that I decided not use on the microwaves when I found a cheap source of SMA coaxial relays. An I2C decoder would be great to put band data on the other pins to select a transverter (or transverter cascade for the microwaves).
The whole mess works great from a switching standpoint. When I key the PTT on Rocky or PowerSDR, it switches the TX line and keys the transverter. I’m a little ambivalent about the quality of transmitted signal that’s actually coming out. I think it’s good enough if you live in a sparsely-populated area, but I have a lot of VHF neighbors that I’d rather not upset. So, I need to do some more testing on this—a lot of it is getting the RXTX and PowerSDR configured correctly. It appears that the I/Q channels are flipped on my sound card (Audigy 2 ZS) between transmit and receive. I don’t know if that’s a wiring error in my breakout box or whether it’s normal. Spectra to come at some point…
A few photographs of my latest tinkering—a SoftRock Ensemble RXTX. This unit provides all of the hardware necessary to have a software-defined radio (SDR) transceiver that uses a computer soundcard for ADC and DAC. It is available here as a kit. They are kitted in batches of 20-100 every few weeks and usually sell out within 24-48 hours.
I originally had little interest in doing a full SDR transceiver (preferring receivers only) until it occurred to me that I could modify the RXTX as an IF strip for VHF/UHF transverters like many have done with the FlexRadio Systems rigs. So, I have developed a modification that is minimally invasive to the operation of the RXTX. At build time, you choose a nearly octave-wide “super band” for operation. This one was built for the 20-30 MHz band, covering the 15-, 12-, and 10-meter Amateur bands, as well as the 25-28 MHz IFs that I (will) use with present and planned transverters for the 6-, 2-, 1.25-, and 0.70-meter bands.
I’m not sure I have the enthusiasm to build another SoftRock. There are a lot of trifilar transformers to wind. The final result looks pretty good and seems to work. I’m going to put it on the spectrum analyzer soon to see how clean the output is and how I should balance the drive for the transverters. Then, I will also include a write-up of how the modifications are done. (I promise that writeup is the same place as the control circuit schematic for the 50-MHz transverter!)
Here, you can see the W1GHZ relay board for doing split-IF T/R switching. If the transverter control cable (DB-9) is disconnected, the SoftRock switches to common-RF for regular 15-/12-/10-meter operation.
Last night, as I have been doing lately in both the evenings and mornings, I was trawling the bands with a SoftRock (a dual-band v6.0 built for 40 and 80 meters—my first SoftRock) and Rocky. Without fail, the waterfall enables me to see something interesting, which brings me to the point that I really need to integrate these receivers into my station. But, I digress. The really tantalizing, fascinating signals are the weak ones, especially when they are weak and unusual.
The subject weak and unusual signal is shown in the figure headlining this post. Rocky’s cursor shows the approximate bandwidth of the CW filter (250 Hz, if I recall correctly) and the center frequency (7026.25 MHz). I tuned it in and caught a CQ from “AA1T…” Recalling that I head read about Mike, AA1TJ‘s, Das DereLicht transmitter built mostly from parts scavenged from a dead CFL, I suspected it was him. After a moment the QSB came up and I caught the ‘J.’ Thanks to the fact that the SDR is not fully integrated into the station, I scrambled to plug my 15-year-old Small Wonder SW-40 in and get it online. Mid-scramble, Sarah handed Evan off to me for a diaper change, but I did manage to snap the screenshot above. By the time I returned to the shack awhile later to close things down, AA1TJ had disappeared.
Through the (ubiquitous) magic of the Internet, I sent Mike an apologetic e-mail QSL, which yielded a nice response. It turns out that he was trying a 40-meter version of the first transistor transmitter described in the amateur literature. The design for 146 MHz by K2AH appears in March 1953 QST. If you are an ARRL member, pull up those old QSTs in the archives and read about it. K2AH is on the cover of February 1953 QST with the same transmitter. Mike figures he was making 20 mW, or 20650 miles/Watt on the path from his station to mine. The more remarkable thing is that he was using a 1956-era point-contact transistor (the same type as in the K2AH article). Fascinating stuff! Hopefully, we’ll connect for a real over-the-air QSO sometime soon.
Thanks, Mike, for making my day by doing something interesting and sharing it on the air!
As I mentioned previously, the receive capability in my 50-MHz transverter seemed to be a little bit anemic. So, I decided to add a little bit of gain. Looking at the block diagram for the transverter (shown below in updated form—jumper across the SGA-4586Z to understand what the circuit looked like initially—by the way, full-sized PDF versions of all of my notes and schematics will eventually be available here), I reasoned that I had the following losses and gains: +10 dB for the LNA (this might be as much as +15 dB, but 10 seems more realistic, if not generous), -3 dB for the bandpass filter, -7 dB for the mixer conversion loss, and -3 dB in the diplexer, leaving me with -3 dB overall conversion gain.
So, I endeavored to find some more gain in the form of a MMIC (which is really what all gain blocks in this transverter would be if I were to do it again). I have a nice (but dwindling) supply of SGA-4586Z’s that produce about 20 dB gain with a 1-dB compression point around 16 dBm, which is probably a good part for this location. The idea is to overcome the losses in following stages with robust gain in earlier stages (in order to keep system NF low). So, I ripped out the existing two-pole BPF and replaced it with this:
I was relatively satisfied that I had everything working with the cover of the transverter off to peak the BPF on the W3APL beacon. So, I replaced the cover and surprise! To borrow a poetic description of RF circuit doom from N3UM, the MMIC “amplifier burst into song.” Bursting into song is a bit of a charitable description for something that sounded more like S9 power line noise in the IF receiver. I did not actually know that it was the MMIC oscillating at first, but I started wiggling cables and finally found that if I touched the 12RX (+13.8 volts on RX) line, the problem disappeared. So, I added the 0.01-uF bypass capacitor to ground on the supply side of 180-ohm bias resistor and the problem was solved.
With the completion of the HF SoftRock with Si570 LO, I now have a tunable IF to play with the transverter. We had a nice opening to the Southeast and Gulf this morning and I even heard my first DX with the transverter and SoftRock combination—CO2WF. With the appropriate software, I can configure a the SoftRock as a panadapter with the TS-930S as the transmitter. More on this in the future. Best of friends:
The real next step in making the transverter useful is building a 20-watt PA stage. This should be good for driving a Mirage or TE Systems brick or even something bigger like a 3CX800 or 50-volt solid-state amp…
Got the SoftRock v9.0 Lite+USB Xtall working last night. I’m not 100% sure what the problem was, but I think it may have been due to me programming an ATTiny85 with the latest SoftRock firmware (V15.14), rather than using the supplied ATTiny45 with V15.4. So, I’m sticking with old version for now. Not sure if it was cockpit error on my part programming (more likely) or an incompatibility with the new firmware which is designed to work with the Ensemble series of SoftRocks.
As shown in the screenshot from Rocky above, there appears to be a spur that repeats every 1 kHz (see left side of the waterfall). The right side of the waterfall is with the USB cable unplugged from the computer. I’m 99% certain this is something internal to the SoftRock because I wrapped a few turns of the USB cable around a big type-31 ferrite toroid and the spurs are still there. So, need to play with that.
I’m listening to a Es opening to the south on 50 MHz right now using the SoftRock as the RX IF. Yes, that means that I got the RX on the transverter souped up a little hotter. I will post something about that later today and place a non-causal link in this post.
Brian, ND3F (aka N3IQ/R), gifted me a partially-complete SoftRock kit a few months ago on the condition that I put it on the air. I’m making some progress on that. I took this photograph to show the SparkFun USB break-out board installed in the Bud CU-124 enclosure. The whole thing is assembled now, but there is probably a solder bridge somewhere. This kit is going to be a lot of fun because it’s actually more flexible for experimenting than the present Ensemble II RX kit.
This is about building electronics, not making beer, at home; although, I am sure there are parallels. Three things brought me to writing this: 1. an eHam forum thread I responded to a few weeks ago; 2. the June 2011 issue of IEEE Microwave magazine (has articles by K2UYH, N2UO, and KK7B, perhaps others? thanks to W3KL via the PVRC reflector for bringing it to my attention since I let my IEEE/MTT membership lapse); and 3. a few minutes spent last night resuming a partially-completed Softrock kit gifted to me by a friend who decided to buy a FLEX-3000 instead.
Every once in a while, a thread appears on an amateur radio forum that goes a little bit like this, “Hi, I’m a new ham and I don’t have a lot of money to spend so I want to build an HF SSB station from scratch” or something similar. Somehow, somewhere, somebody has given the impression that it is less expensive to build your own amateur radio equipment than to buy it. That’s true in some circumstances, but certainly rarely for anything that is mature, mass-produced, and readily-available on the second-hand market. After all, there is nothing novel about a 100-watt superheterodyne HF SSB transceiver these days. The principal uncounted cost is the “engineering cost” associated with getting your first few projects working and keeping them working.
One of the first construction projects I undertook as a new ham was to build a Ramsey Electronics HR-20 (NE602-based) 20-meter receiver—$20 at a hamfest. It did actually work eventually—but this was a simple kit with maybe two dozen parts. Next, I built a ONER transmitter kit from now defunct 624 Kits. I think that was another $20. I never made any QSOs with that combination because I was always afraid of blowing out the receiver with the transmitter. The first thing that I built that I actually managed to make a QSO with was a Small Wonder Labs SW-40, which I still have. That set me back $55 and it did not work immediately. Suddenly, that’s over $100 by the time you include the money I spent on a soldering iron and solder. That’s one-third to half-way to a “real” used HF transceiver and I had two bands at 1 watt on CW only. Furthermore—these are all kits—they leverage economies of scale in purchasing parts from various vendors and they have instructions to help you along. And, I’d like to think that I was a relatively representative example of a recently-minted ham who had more ambition than money or skills…
As I soldered down 1206-size (easy ones) SMT capacitors last night, I was thinking of times that I rushed through a homebrew or kit project just to get it on the air. In those instances the process was often, as I have belabored above, about saving money, not about the act of creating something. Last night was about creating, not saving, and that is the joy of homebrew.
Finally got all of the SoftRock downconverters here enclosed and repaired. Turns out I managed to cross two of the wires on the input transformer of my 20-meter v6.2 Lite (“upgraded” variant). Once I found that, it sprung to life. The other project was getting the 144-MHz Ensemble II VHF into an enclosure. This has been a long-standing struggle since the nearest size diecast box is just a hair too small. So, I put it into an extruded aluminum enclosure I found at Dayton a few years ago. Unfortunately, I had to make my own front and rear panels. But, I had an old minibox that was perfect for the donor material.
I should have polished the edges of the front and rear panels, but it’s not too bad. KK7B opined in a QST article many years ago that after homebrewing “about 50 enclosures, they start to look respectable.”
Holes were punched with a hand punch from Harbor Freight. Given the cost of the Roper-Whitney equivalent and for as much as I will use it, this is perfectly acceptable. There’s nothing like having the right tool for the job! However, mine came with two 5/16-inch punches (no 1/4-inch), but one each 1/4-inch and 5/16-inch die. Ooops. VFBBYQC. So, I had to drill the 1/4-inch LED hole. A nibbling tool made quick work of the USB cut-out. I cut the panels with hand shears. So, I guess it should be clear what’s next on my sheetmetal shopping list…
Now, I need to actually play with some “real” SDR instead of just diddling around with Rocky (which is very capable). I’m running it under Windows XP on a 1.3 GHz Pentium IV with 1 GB of RAM. The sound card is a Creative Labs Audigy 2 ZS. There is a little latency when running a large waterfall and resizing windows, but it’s adequate for tinkering. Eventually, I’d like to try some of the GNU/Linux SDR engines. But, I’m just going to wait until a new (to me) computer falls into my lap before that happens.