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FlexRadio Systems SDR-1000 HF+VHF Software Defined Radio Redux

October 3, 2005

By Lindquist, Rick

In product review terms it could become a serious challenge to keep up with the ever-evolving FlexRadio Systems SDR-1000-the first commercially available software defined Amateur Radio transceiver. This marks our second look at the SDR-1000. Must reading is our first snapshot, “SDR-1000 Software-Defined HF/VHF Transceiver,” by Steve Ford, WB8IMY, in the April 2005 QST “Product Review.” Steve noted some limitations and setup problems, and FlexRadio has addressed them. It now offers a fully integrated package including a PC to sidestep the problems of getting everything set up and configured to play together.

A Work in Progress

The improvements since our last look, involving both hardware and software, have been significant, and even more arrived or were in the offing as this review went to press-including another software increment. To avoid getting caught in an endless loop, we froze changes at PowerSDR software version 1.4.3.

The architects and developers of the current DSP code used in the PowerSDR software, Frank Brickie, AB2KT, and Bob McGwier, N4HY, worked closely during the development process with FlexRadio’s Gerald Youngblood, K5SDR, and Eric Wachsmann, KE5DTO. The FlexRadio Web site, www.flex-radio.com, includes all software downloads, the Operating Manual and any documentation you’d ever need. There’s also an excellent reflector and-at least so far-they’re able to keep up with user inquiries and troubleshooting issues.

With cutting-edge technology like this, it’s sure nice to know you’re not out there alone!

A Marriage of Convenience

With the SDR-1000, the union of Amateur Radio gear and the personal computer has transitioned to the point that a transceiver’s functions can reside in a virtual world we can’t really touch or see. With the advent of the SDR-1000, the physical radio box has become a “peripheral” in the genre of software-generated data. As Steve Ford’s earlier review noted, software defines this transceiver’s functionality, so each PowerSDR revision can, in effect, represent a new-or at least an improved or different- transceiver.

Download and install, and you’re there! The only thing you really need in the shack plus the SDR-1000, mic and key is a fast PC, and they now offer one.

The possibilities become almost limitless. There’s still that pesky radio hardware box, the PC and a sound card and perhaps an accessory or two to contend with. The physical components of what we might call the SDR-1000 “system” do impose some real restrictions on what the transceiver is capable of now or in the future. The open- source software can accommodate a wide range of upgrades that might otherwise only be possible through hardware or firmware upgrades. There are no DSP chips to become obsolete in the SDR-1000, for example.

“The Radio that Keeps Getting Better”

That’s what FlexRadio Systems calls its product, and the characterization is right on target. Since we first looked at the SDR-1000 earlier this year, its manufacturers wisely decided to market the full-blown transceiver in a form that’s as close as possible to a turnkey system. This way, you don’t require a software and RF engineering background to get up and running. The move also seems to have been an effort to appeal to a wide audience-to capitalize on the title of Gerald Youngblood’s series, “A Software Defined Radio for the Masses,” which won him the 2002 Doug DeMaw, W1FB, Technical Excellence Award. The articles appear in the July/ August and September/October 2002 issues of QEX and are available on their Web site.

For most, the SDR-1000 system will be only marginally more difficult to set up than the average PC with a printer and another accessory or two. It more likely will require more tinkering-and maybe some hand-holding from FlexRadio-to tweak the system to suit your needs. While the full complement of equipment fills a small table or desktop (see lead photo), in most stations most of the equipment can be beneath the desk.

In addition to the SDR-1000 “black box,” the SDR-1000 “packaged system” now can include a Dell PC with a professional-quality Delta 44 sound card installed and configured, a Delta 44 “break-out box” (see Figure 1) for audio I/O connections, computer speakers, cables and an accessory connector kit to interface with the SDR-1000 and a parallel control cable. The PC with our system had a Celeron 2.4 GHz processor and 512 MB of RAM, and the limitations of the Celeron versus an Intel Pentium 4 (P4) processor were noticeable, especially for CW. FlexRadio now offers the 3.2 GHz P4 computer (the Dell Optiplex 170L) for the SDR-1000 package. For now, the PowerSDR software will only run under Windows-in our case, XP Home with Service Pack 2.

Options include a Dell flat panel display (monitor)-a must unless you happen to have a spare sitting around, an automatic antenna tuning unit with tuning capability to 10:1 SWR, a low power Down East Microwave DEMI144-28FRS 2 meter transverter that can fit inside the black box, a Contour ShuttlePm 2 (see Figure 2) or Griffin PowerMate controller and a USBto-parallel port converter.

Hardware Upgrades

A few changes have been made in the black box since our initial outing. “We improved the Quadrature Sampling Detector (QSD) out-of- band rejection by adding two resistors to properly terminate the filters,” Youngblood explains. FlexRadio also swapped in an RF preamplifier with higher dynamic range.

The latter change could help to explain why the dynamic range numbers we measured with the preamplifier enabled were better on this version of the SDR-1000 than those taken with the preamp off. See Table 1. Note that the near-in third order IMD dynamic range is right up there with the best radios we’ve ever measured, and extends at least in to 2 kHz.

FlexRadio also corrected an anomaly noted in our original review- in which the box drew more current on 160 meters than on other HF bands-by changing the value of two resistors on the power amplifier.

The beauty part here is that all SDR-1000s, starting with the very first, can be upgraded to match the capabilities and performance of the latest unit off the line. In fact, our black box is the very same unit we used for the first QST review with upgrades added at FlexRadio. Youngblood says FlexRadio plans to offer similar upgrade service soon to existing SDR-1000 owners via a third party.

Getting Things Under Control

For all intents and purposes, you can control the radio using the PowerSDR “console”-which is what FlexRadio calls its user interface- the PC keyboard and a wheel mouse. Steve’s earlier review describes this.

Figure 1-The break-out box for the optional M-Audio Delta 44 professional sound card allows making the connections without crawling behind the PC.

Figure 2-The optional Shuttle-Pro controller.

The optional controllers are another way. Steve checked out the PowerMate for his review. We got the ShuttlePro 2 for this one. It’s more than just a knob to tune the radio. It also has 13 buttons (9 are user-programmable, and labels to go under the button caps are provided). FlexRadio provides an Excel document that describes the assignable functions, which you record like macros in other software packages. Using the ShuttlePro or the mouse-or both-may leave your keyboard collecting dust, although it’s handy for direct frequency entry and even for programming CW memories or a keyboard buffer.

Like the tuning knob on many conventional ham radio transceivers, the ShuttlePro’s has a dimple (actually three of them). It also has a spring-loaded “shuttle” ring surrounding the tuning knob that simplifies quick frequency excursions. Move it a little and the frequency changes more slowly. Crank it over all the way (either right or left), and the frequency may change too fast for you to keep up. Both controls can be assigned to other functions.

I found it a pretty handy accessory although it’s almost as easy to tune the radio using a wheel mouse. The ShuttlePro also requires some programming to assign the buttons. As an added bonus, however, it may be programmed for use with other applications by changing its “application” setting. This may be the only radio that can do your taxes when the band is dead!

CW Operation

As McGwier, a CW operator, told me, “This is definitely not a QSK radio.” That’s quite an understatement. As Steve Ford noted in his April review, CW operation was not yet the forte of the SDR-1000. It is now much closer to a serious CW radio-albeit only in semi-break- in mode. The processing power of the Celeron chip seemed to be a limiting factor in the case of our SDR-1000 system, and FlexRadio now recommends a P4 computer for high-speed CW operation. Intel P4 processors support a high-performance motherboard timer; the Celeron does not. CW operation is now far better than with the version described in the earlier review,.

The real issue is something called latency. This is essentially the time it takes for the PC to process a command once it’s been issued; for example, sensing that the key or paddle has been pressed, switching into transmit and telling the black box to generate the necessary RF. This takes a finite period of time- almost instantaneous in human terms but lengthy in computer terms. In hisreview Steve called it “maddening delay.”

Our PowerSDR version incorporates a “new keyer” design that, while it lacked some important features of the “old keyer,” makes it possible to transmit serious CW. I’ve been operating CW for many years now, and I found with a little practice I got to the point where I was able to send passable CW at a fairly good clip-on the order of 30 WPM or greater-once I got used to the delay. Setting the Delta 44′s buffer size to 512 (that is, 512 samples per audio buffer) while using the new keyer made key closure and the appearance of the corresponding sidetone in my headset very close to instantaneous. Although I could not altogether eliminate the delay, I was able to train my brain to work with it.

The “old keyer,” which the user “selects” by unchecking the NEW KEYER in the software, includes CW memories into which you can type text, plus the ability to send text from a keyboard buffer at speeds up to the keyer’s limit. If you’re so inclined, you can send CW using the left and right mouse buttons or by via the “.” and “/” characters on the keyboard.

On the plus side, the software keyer permits a wide range of adjustment in weighting, rise time and debouncing.

FlexRadio has developed a hardware workaround to make CW operation more like “normal,” although QSK operation is down the road. The workaround involves hard-wiring a keyer paddle to three pins on a serial port, but our software revision did not support this feature. For now, an external keyer is one option to obtain excellent CW results from the SDR-1000.

Listening on CW is superb. As Steve noted in his review, the radio’s ability to generate binaural audio (provided you have a stereo headset) gives the bands-on both CW and phone-a new presence. The software filtering is fantastic. You can click your choice of preset filters or shape your own. Being able to narrow the bandwidth to 25 Hz essentially obviated the need for a manual notch, but there is an automatic notch. You can readily switch from the lower side to the upper side of a signal.

When changing from one mode to another, the filter shifts to the setting selected the last time you were in the new mode. There are no default filter settings for a given mode. This offers a level of flexibility not found in many conventional radios, particularly those that depend on crystal filters.

There’s a passband shift slider, too (the SDR-1000′s version of an IF shift). Very helpful. Don’t like the default AGC settings? Set your own via the menu.

SSB Operation

SSB operation is a bit more routine, but, just as there’s no QSK for CW, the SDR-1000 doesn’t yet have provision for VOX (voice operated transmit). This is another feature on the wish list to the developers.

Nonetheless, I was able to easily interface my Heil ProSet Plus headset to the Delta 44 sound card’s break-out box, although it did require some adapters. For the microphone input, a readily available 3.5 mm to inch adapter plug into one input jack did the trick (it’s also possible to configure a four-pin microphone connector to the jack on the front panel of the black box). For stereo audio output, I jiggered up my own adapter using two mono plugs to a inch stereo jack.

Although the front-panel microphone input allows for push-to- talk, I was not able to hook up my Heil headset without going directly into the sound card-unless I wanted to devise my own adapter. Otherwise, you must either click the MOX (manually operated transmit) box on the console or hard wire some kind of foot (or hand) switch. If you’re using the ShuttlePro 2, you can program one of its buttons to enable MOX. Holding down the “dit” side of the keyer paddle also switches to transmit, I discovered.

Table 1

FlexRadio SDR-1000

Table 1

FlexRadio SDR-1000

The audio reports I received were uniformly positive. When I opened up the 12-octave graphic equalizer (see Figure 6) to further polish the transmit audio, the results invariably drew additional praise.

To improve transmitted audio efficiency, the SDR-1000 lets users set something called feed forward compression. Unlike the typical audio or RF level audio compressor, the feed-forward design anticipates the compression level required rather than, as the manual put it, “simply following the signal around.” Upping the value gives higher average power output without peaks, pumping or popping.

Additionally, version 1.4.3 of PowerSDR incorporates AGC leveling and a limiter to replace the clipper in earlier revisions. While you’re transmitting and using the panadaptor setting to view the received (see Figure 7) and transmitted signal, you can view your signal’s waveform, noting its overall bandwidth and its audio emphasis right there on the screen as shown in Figure 8. You can also view a signal histogram as shown in Figure 9.

While you can see your signal, listening to it by enabling the onboard monitor is not advisable, at least if you’re using a 2.4 GHz Celeron or lesser PC. Remember the latency issue? The same thing occurs in voice modes, so when you speak, it takes a few milliseconds for the audio to appear in your headset. This “echo box” effect can be very disconcerting. Again, this is a PC issue, not really a radio issue, per se; sometimes fiddling with buffer settings yields better results. Faster computers and superior processors will conquer these latency issues. It made us again wish we had selected the faster P4 machine.

Digital Mode Operation

The SDR-1000 does not include out-of-the-box, integrated capability to operate on digital modes-something on Steve’s wish list last April. FlexRadio says you should be able to use almost any soundcard-based digital application with the unit, but at this stage of development, you’ll need a second sound card.

In fact, the Dell PC already has one that’s not really being used. Since the Delta 44 handles the needs of the SDR-1000, the Dell sound card is available to serve digital-mode software. You’ll also need a third-party vCOM virtual serial port driver developed by Phil Covington, N8VB, to create a nocable-needed COM port for communication between the digital mode software and the PowerSDR software..

A virtual sound card that FlexSystems says is just around the bend will eliminate the need for a “real” (second) sound card to use digital mode software.

Some Feature Notes

Like almost any modern transceiver, the SDR-1000 offers memories- their number undoubtedly only limited by the size of your hard drive. These let you save frequency, mode, filter setting, tuning step size, call sign (if appropriate), squelch setting and AGC setting. Checking a box makes that memory available for scanning purposes.

The easy-to-read frequency readouts for the two VFOs display out to six decimal places, which some users might consider overkill. It was not possible to alter this to, say, five decimal places, with our software version.

Frequencies (and corresponding information) are saved in a table that you bring up by clicking the RECALL button. Click on a frequency you’ve saved and the SDR1000 takes you right to it. Ooops! It doesn’t close the table at the same time!

There are several band-stacking registers for each band. These save frequency, filter and mode settings. Very nice!

The sky’s the limit as filter settings are concerned, both for transmit and receive. If you set the transmit bandwidth beyond 3 kHz, however, a little dialog box pops up: Warning: Transmit Bandwidth. The transmit bandwidth is being increased beyond 3 kHz. As the control operator, you are responsible for compliance with current rules and good operating practice. The FCC’s Riley Hollingsworth couldn’t have said it any better.

Figure 3-Worst-case spectral display of the SDR-1000 transmitter during two-tone intermodulation distortion (IMD) testing on HF. The worst-case HF third-order product is approximately 26 dB below PEP output, and the worst-case fifthorder is approximately 37 dB down. The transmitter was being operated at 100 W output at 1.85 MHz.

Figure 4-CW keying waveform for the SDR-1000 showing the first two dits in semi break-in mode using external keying. Equivalent keying speed is 60 WPM. The upper trace is the actual key closure (first closure starting at left edge of figure); the lower trace is the RF envelope. Horizontal divisions are 20 ms. The transceiver was being operated at 100 W output on 14.02 MHz.

Figure 5-Worst-case tested HF spectral display of the SDR-1000 transmitter output during composite-noise testing at 14.02 MHz. Power output is 100 W. The carrier, off the left edge of the plot, is not shown. This plot shows composite transmitted noise 2 to 22 kHz from the carrier. Note the numerous spurs that are not part of the noise output.

Figure 6-The PowerSDR Transmit setup tab permits adjustments to the 12 octave transmit audio equalizer, transmit compression, transmit filter and the “tune” power level.

Figure 7-Receiving a 20 dB over S9 SSB signal on 75 meters.

Click OK and the SDR-1000 lets you set the transmit bandwidth as wide as you’d like anyway. This might be great for tailoring AM audio or for “enhanced SSB” experimenters.

You can pick the preset filters: 6.0, 4.0, 2.6, 2.1 and 1.0 kHz and 500, 250, 100, 50 and 25 Hz, plus two user-settable filters, VAR 1 and VAR 2, that you can adjust using the WIDTH slider. The results can be astounding.

Youngblood notes that there’s no analog AGC ahead of the final “brick wall” receiver filter. “This means that it is not possible for signals outside the passband to modulate the AGC system,” he says. The same filters are the final step in the transmit audio chain as well.

Figure 8-The PowerSDR console during an SSB transmission into a dummy load. The audio waveform is clearly visible within the green column representing the signal bandwidth, 2.6 kHz in this case. The “Out of Band” legend can be seen on VFO B.

Figure 9-The “histogram” display on 75 meters. Setting the buffers at too low a value whi\le using this display can chop “holes” in the received signal. This is an apparent computer processing issue.

There are two noise blankers in the SDR-1000. NB2 is not a noise blanker in the traditional sense. Instead of chopping noise out of the signal-which can lead to distortion-Youngblood said it replaces noise impulses with something called a “sliding rank order median” estimate of the original signal.

It’s possible to customize the console colors and even some of the design features. Don’t like the default yellow frequency readout numerals? Change them to red oras I did-to cyan. There’s a palette of choices.

By the way, there’s a label below each Amateur Radio band frequency that tells you where you are in terms of the ARRL band plan. For example, 40M EXT/ADV SSB if you’re listening between 7.150 and 7.225 MHz, or 75M AM CALLING FREQUENCY if you’re on 3.885 MHz. On general coverage, you’ll usually get a hard-tomiss OUT OF BAND message on a bright red background. This jarring alert made me feel I was doing something I shouldn’t be whenever I tuned through the HF shortwave broadcasting bands.

Obtaining audio from the SDR-1000 still means hooking up a headset or a set of amplified speakers to the sound card’s break- out box. You certainly could use the amplified speakers that come with the Dell, although you’ll need an adapter to connect to the Delta 44 break-out box. Under most circumstances, though, there’s plenty of audio at least to drive a headset. Again, as Steve suggested back in April, FlexRadio might consider some means of being able to drive a regular speaker or speaker pair.

Some Performance Notes: Thinking Outside the Box

The professional-quality Delta 44 sound card has become the standard for the SDR-1000 because of its vastly superior dynamic range compared to the consumer-quality unit in use during the original review. This move is especially important when you consider that once the black box provides the 11.025 kHz signal to the computer, the PowerSDR software does the rest at that intermediate frequency. So the sound card is paramount to good performance, and this shows in the numbers we measured in the ARRL Laboratory (see Table 1).

Two-tone, third-order (TT3O) intermodulation distortion (IMD) dynamic range is a great receiver performance benchmark. With the setup Steve Ford reviewed in April, this figure was in the modest mid-80s on 14 MHz-whether the preamp was on or off and whether the spacing was 20 kHz or the much more critical 5 kHz.

For this review, using the “medium” preamplifier setting as the factory default, the TT3O IMD dynamic range was 99 dB on 14 MHz, about as good as we’ve seen, and ever so slightly worse with the preamp on its highest setting-which actually was the point where the receiver’s minimum discernable signal (MDS), or sensitivity, at 14 MHz was comparable to that of most conventional HF transceivers- that is, in the vicinity of-130 dBm.

Another popular benchmark is thirdorder intercept (TOI), which is derived from the receiver’s sensitivity and its two-tone, third- order IMD dynamic range. At 14 MHz and using the medium preamplifier setting, we calculated the TOI for the SDR-1000 at a very respectable +26 dBm (it was +17 dBm at the high preamp setting).

Here’s the thing, though. The ARRL Laboratory found that two- tone, thirdorder IMD dynamic range was essentially the same at 2 kHz as it was at 20 kHz and beyond. With a conventional receiver, you’d expect dynamic range at 50 kHz spacing, for example, to be much better, especially since it’s well outside the passband of the typical 20 kHz roofing filter.

Consider, though, that the IF for what’s essentially a single- conversion receiver in the SDR-1000 is 11.025 kHz-still in the audible range for most of us. This is what the sound card sees from the black box. The superiority of the Delta 44 sound card compared to the sound card we used in the previous review seems to account for this seeming dynamic range incongruity. In short, the sound card’s inherent dynamic range becomes the receiver’s, so it exhibits essentially the same dynamic range across its entire passband, which can be considerable.

The first IF rejection figure of 114 dB on 14 MHz tops my ICOM IC- 756PROIII. The image rejection figure of 75 dB on the same band, due to the very low IF frequency no doubt, leaves a bit to be desired, however.

FM numbers were not as good as the typical FM radio. The 29 MHz FM adjacent channel rejection at 20 kHz channel spacing and using the high preamp setting was only 36 dB. FM two-tone, third-order IMD dynamic range was just 37 dB. Measurements on the order of twice these values would be more acceptable. Let’s just say that this nice radio was reaching a bit too far when it tried to do FM. When informed of this, FlexRadio identified the problem as a software issue that they would address in the next release.

I encountered some birdies at odd places across the spectrum, some in the vicinity of -95 dBm (S5). In once case, the birdie was almost on top of a station I was trying to copy (around 7.02858 MHz), but when I tuned, the birdie shifted as well, and I was able to tune in the station in the clear.

A User-Defined Radio

The FlexRadio folks and those who are helping to develop the open- source software that defines their radio already are hinting at what lies ahead for their particular piece of gear. They invite-indeed, encourage-developers and users to cast their own ideas into the pot via the FlexRadio reflector, weekly TeamSpeak VoIP community forums and the source code that’s the very heart (or brain) of the radio.

One might say that Amateur Radio SDR technology, as represented by the pioneering SDR-1000, is where SSB was in the years just after World War II. Then it was the subject of articles in technical journals and in QST, but not at a point where the average radio amateur was prepared-on a number of levels-to jump into the fray. Do you have to be a software engineer to own and operate an SDR-1000? No, but as Steve Ford suggested in his review last April, familiarity with the operation of personal computers and their potential to interface with radio equipment is a major plus.

FlexRadio encourages the more technically inclined to tinker with the open-source code. Wachsmann says the graphic user inlerface/ Windows interface code is written in C# (pronounced “C sharp”), while the DSP is written in C (shared source with a Linux version). The PowerSDR source code is available free from the FlexRadio Web site’s download page.

The Future is Now

Some plans already on the drawing board call for “a totally new look and feel with changeable ‘skins,’” Youngblood told us. “We will be working on integrated remote operation, quad receivers in the passband, SO2R (single operator, two radios), real-time spots on the spectrum display, virtual sound cards for digital modes, ad infinitum.” Youngblood says FlexRadio is in the process of adding support for the universal controller board that provides a 16×16 relay matrix for antenna and transverter control.

Manufacturer: FlexRadio Systems, 8900 Marybank Dr, Austin, TX 78750; tel 512-250-8595; sales@flex-radio.com; www.flex-radio.com. Price: Model SDR-ASM/TRA (fully assembled transceiver with 100 W amplifier and RF expansion board-no PC or sound card), $1375; SDR- ATU automatic antenna tuner module, $235; M-Audio Delta 44 professional sound card, $159; Shuttle-Pro controller, $99; Dell Optiplex 170L with software and hardware integrated and calibrated; 2.4 GHz Celeron, $769; 2.8 GHz Pentium 4, $899; 3.2 GHz Pentium 4, $1065; Dell E153FP, 15 inch flat panel display, $235.

Bottom Line

The newest and fully integrated SDR-1000 system avoids most of the earlier start-up issues, resulting in a top performing radio “out of the boxes.” New software updates will continue to add new features and further improve performance.

Joel R. Hailas, W1ZR * QST Technical Editor * w1zr@arrl.org

Rick Lindquist, N1RL

ARRL Senior News Editor

Copyright The American Radio Relay League, Incorporated Oct 2005




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