Tag Archives: uSDX

Early days with the Chinese uSDR / uSDX reveals weaknesses

A couple days ago I finally took delivery of the uSDX/uSDR transceiver I ordered in late October from this seller on eBay. I’ve been tinkering with it in the shack since then and have started to form some initial impressions.

The uSDX is a super cheap transceiver and, to be clear, my expectations were (spoiler: thankfully) very low.

If you’ve been here for long, you’ll know that I don’t normally test or review super cheap transceiver varieties found on eBay, AliExpress, etc. So many people asked me to check out the uSDX, however, that I decided I would try to give it a shake out.

I’ve yet to take this little radio to the field, but I have made a couple dozen contacts from the shack, all in CW and I’ve done a lot of listening. Yesterday afternoon, I even hooked it up to an oscilloscope.

It’s still early days and I’m sharing the following observations and notes with the hope that uSDX owners might be able to guide me if I need to make menu adjustments to sort out a few issues.

More specifically, there are two big cons with my uSDX:

Con #1: The Audio

Trying to be diplomatic and kind with my words here, but let’s just say the uSDX audio leaves much to be desired.

I think it’s great this little transceiver has a built-in top-mounted speaker, but it produces some of the harshest, most spluttery audio I’ve ever heard in a radio.

On top of this, the volume control (which requires going into an embedded menu item to adjust) is just…strange. It’s hard to explain, but the audio feedback isn’t what I would expect from a volume control or AF gain. It seems like each volume control step (starting at “-1” and going up to “+16”) is a mix of both AF and RF gain values. It doesn’t have a fluid amplification progression like I would normally expect.

In fact, I can’t really turn the volume up to +16 because on many bands around level +14 or +15, it starts to emit a really loud squeal.

Although very minor compared with the issues above, the audio amplification chain also has an ever-present hiss.

In addition, even with the volume turned to “-1” I can still hear splattering and even garbled whispers of CW signals if connected to an antenna.

I hope I’m missing something here and the audio can be tailored for better listening. Perhaps there’s a combination of adjustments I can make in the menu options to help?

Please comment if you own a uSDX and can provide some feedback.

Con #2: The Receiver

Again, unless there’s a magic combination of adjustments I can make in menu items, I find the receiver of the uSDX to be incredibly anemic.

I took the uSDX to my buddy Vlado (N3CZ) yesterday and we hooked it up to an analog oscilloscope and signal generator.

We concluded that the uSDX is very sensitive, but the front end seems to be as wide open as a barn door.

This confirmed my on-the-air observations made over the past two days: even with the 500 Hz DSP filter engaged, CW signals as far away as +1.7 kHz could easily bleed through. In fact, quite often when I tune to a POTA or SOTA station operating CW, I could even hear FT8/FT4 stations bleeding through from far across the band.

I couldn’t help but think if I had taken the uSDX on my recent Mt Mitchell SOTA activation instead of the QRP Labs QCX-Mini, there’s no possible way I could have handled the pileup. The uSDX receiver would have completely fallen apart because it shows no ability to handle tightly spaced signals.

Again, if you’re a uSDX owner and can provide some insight here, I would very much appreciate it.

Still testing the uSDX TX

One of my main goals with purchasing the uSDX was to test the transmit signal to see how clean it might be and if there were spurs in any harmonics.

Vlado hooked up the uSDX to his oscilloscope and we discovered that it did produce spurious emissions in harmonics of the 40M band. The spurs were negligible and we both assume it might possibly be within FCC guidelines.

With that said, Vlado didn’t completely trust this particular analog oscilloscope because there appeared to be a slight fault in its BNC input port. We ran enough tests–and even compared the uSDX to my KX2–to know that there are definitely faint spurious emissions and that the CW transmit signal isn’t nearly as clean as the KX2.

Here’s a 2 second video clip showing the uSDX transmitting CW on 7 MHz into the scope:

Click here to view on Vimeo.

I plan to hook the uSDX to a digital oscilloscope to get more accurate results in the near future.

Not all cons

The uSDX does have some positive attributes.

For example, the QSK is quiet and even full break-in. This little radio is also chock-full of features. I’ve even found that though it’s advertised as an 8 band radio, mine will transmit on 10 bands; everything from 160-10 meters.

I think if I planned to operate the uSDX on SSB when the bands were relatively quiet, it might do quite well for casual contacts.

Time will tell…

To be clear, though, the issues above can be deal-killers for me.

I want my transmitted signal to be clean enough to at least meet FCC requirements–I like being a good neighbor on the bands.  This might require some modifications on the output, but let’s see what a digital oscilloscope might reveal.

Of course, if the uSDX can’t handle multiple CW signals being thrown at me at once, I can’t see how this would possibly work as a SOTA or POTA field radio.

I can already tell that the ergonomics of the particular uSDX model I purchased will likely lead to–as Spock put it in Star Trek IV–some “colorful metaphors.”  Especially when I reach for the volume control buried behind a menu item. (I mean, seriously?)

Again, if you own a similar uSDX, I’d love to hear your comments and suggestions!

This turned out to be an expensive week!

Yesterday, it was pouring rain but I had a hankering for some radio therapy, so I popped by Tuttle Educational State Forest (K-4861) knowing that they have a large covered shelter where I could set up the Elecraft KX1 and AX1 antenna and play POTA.

It being a rainy, cool day, I was the only visitor at the park.

As I walked into the shelter and sorted out which table might serve best as my field shack, a random thought popped up in my head: OM, this has been an expensive week for you!

Ignoring the fact that we did some repairs to the ActivationMobile (our Subaru),  and the fact we just committed to a pretty pricey arborist quote for tree work at the QTH (those guys do earn their keep), I also spent some of my hobby money. On what, you ask? Continue reading This turned out to be an expensive week!

An Introduction to the uSDX transceiver kit

Many thanks to Pete (WB9FLW) who shares the following article by Bob (KD8CGH) regarding the uSDX transceiver kit.

I reached out to Bob who has kindly given me permission to share his article on QRPer:


An Introduction to the uSDX

by Bob Benedict (KD8CGH)

There is a new open source, home brew multi band, multi mode QRP transceiver that grew out of the QRP Labs QCX. Through some serious wizardry  it retains an efficient class E RF amplifier for SSB and digital modes. It crams impressive SDR capabilities into an Arduino.

This has an interesting international development process conducted on  https://groups.io/g/ucx/topics with contributions by many, including the usual gang of suspects: Hans Summers G0UPL, Guido Ten Dolle PE1NN, Barbaros Asuroglu WB2CBA , Manuel Klaerig DL2MAN, Kees Talen K5BCQ, Allison Parent KB1GMX, Jean-Marie T’Jaeckx ON7EN, Ashhar Farhan VU2ESE,  and Miguel Angelo Bartie PY2OHH. I apologize to the many others whose names I didn’t list. A summary is in the WIKI https://groups.io/g/ucx/wiki.

The basic work uSDX appears to have been accomplished by Guido Ten Dolle PE1NNZ. It uses pulse width modulation of the PA supply voltage to transmit  modes other than CW while retaining class E efficiency and uses a direct conversion SDR receiver.

The basic idea behind Class E nonlinear amplifiers is that transistors have little loss when they are switched fully on or off. The losses occur when devices are limiting power flow in linear amplifiers. The idea behind a Class E amplifier is to use transistors in a switching mode to generate a square wave to drive a resonant circuit to generate RF power.

This method is used in the popular QCX QRP CW transceiver kit line developed by Hans Summers and sold through QRP Labs  https://qrp-labs.com/.  More than 10,000 of these great transceiver kits have been sold (I built one). There is a good discussion of the circuit and particularly of the class E amplifier in the excellent QCX documentation https://www.qrp-labs.com/images/qcx/assembly_A4-Rev-5e.pdf.

The QCX was the base for the QCX-SSB which starts with a QCX and modified the circuit and software to add SSB capabilities. The wizardry that  Guido accomplished uses pulse width modulation of the PA supply voltage to control the amplifier in an Envelope Elimination and Restoration (EER) technique  https://core.ac.uk/download/pdf/148657773.pdf. To generate SSB a DSP algorithm samples the  audio input and performs a Hilbert transformation to determine the phase and amplitude of the complex signal.  The phase changes are transformed into temporary frequency changes which are sent  to the  clock generator. This result in phase changes on the SSB carrier signal and delivers a SSB-signal with the opposite side-band components is attenuated.

On the receive side a direct conversion SDR receiver is used with the I and Q signal digitized and all further processing carrying out digitally. Attenuators are included to help not overload the ADC range.  Documentation is at  https://github.com/threeme3/QCX-SSB .  In addition to a good description of the theory and hardware mod there is also a good description of the software command menu.

From there development took off in several directions. One is by Barbaros Asuroglu WB2CBA  and Antrak that uses through hole components (mostly) and replaceable band boards that  hold the low pass filter and band dependent class E amplifier components (an inductor and capacitor). Barb also includes boards designed to be a case top and bottom, battery pack and a PA.

Another development track by Manuel Klaerig DL2MAN uses SMT components in a stacked board layout and has a relay switched band pass board https://groups.io/g/ucx/message/1596  and  https://groups.io/g/ucx/files/DL2MAN uSDX-Sandwich Files. A new revision has been released that uses serial resonance class E amp design and easier to obtain relays, https://groups.io/g/ucx/files/DL2MAN uSDX-Sandwich Files with new Serial Resonance Class E Multiband Circuit .

Other development streams include one by Kees Talen K5BCQ https://groups.io/g/ucx/files/K5BCQ uSDX Board Schematics and Jean-Marie T’Jaeckx ON7EN https://groups.io/g/ucx/files/QCXV4.zip.

I built the variant designed by Barbaros Asuroglu WB2CBA   and I’m pleased with it’s performance. I ordered 10 main boards and 40 LP filter band boards PCBs from PCBWAY, but now you can also purchase single boards sets from https://shop.offline.systems/.

I also designed and 3D printed a case for the transceiver and a small box to carry band boards. Info at https://www.thingiverse.com/thing:4582865 and at https://www.thingiverse.com/thing:4587868 and also in the files section https://groups.io/g/ucx/files/3D printed case for Barb WB2CBA V1.02.

In an example of hams collaborating at its finest, Hans Summers  announced on 9/11/2020 that his new QCX mini product, a QCX in a smaller package,  will include a daughter board that can be used to give the QCX mini a uSDX like SSB capability. The QCX mini has the same circuit as the QCX but uses SMD components packaged it into a two board stack that is less than half the volume of the original QCX. The mod is unsupported by QRP-LABS but may be supported by the uSDX group.

http://qrp-labs.com/qcxmini.html 

More information at https://groups.io/g/ucx/topics and don’t forget the WIKI https://groups.io/g/ucx/wiki.

73

Bob,  KD8CGH


Many thanks again, Bob, for sharing this excellent uSDX introduction. Thanks again for the tip, Pete!

Pete also notes that there is a very active uSDX experimenters discussion group on Groups.io with over 100 members: https://groups.io/g/ucx