QMX - New 5 Band TRX from QRPlabs (Part 1)


Unfortunately only up to 20m …

The 60m band is nice… the 80m band didn’t have to be.

I would have preferred higher bands - especially with the coming sunspot maximum.

73 Armin


That is a very keen price, compared with the MountainTopper 5 band radios, which are no longer available. Of course the MTR-5B does have 17m and 15m (in addition to 40, 30 and 20m), so maybe more in line with your desire for higher bands?
This reminds me I need to start my morse practice again and get my currently idle MTR-5b on the air…!!

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Right … Unfortunately, the MTR-5b has not been available for some time.

I can imagine that there will soon be modifications for the QMX and then higher bands will also be feasible. Even if not with the same performance.

Let’s wait and see what happens.

If that’s not a reason! … Alternatively, you could also leave it to me. :wink:

73 Armin


Tks Armin for head up :wink: :+1:

From last post in QRPLabs@groups.io by Hans @G0UPL


QMX new transceiver announcement
From: Hans Summers
Date: Fri, 19 May 2023 02:52:16 UTC

Hello QRP Labbers

I’m pleased to announce a major new QRP Labs product, called QMX. The letters could stand for QRP Labs Multimode Xcvr but the M could stand for other things too… Marriage, Merger (of QMX with QDX), Multimode Multiband, Magnificent, Marvelous, you name it.

QMX physically looks almost exactly the same as QCX-mini. The enclosure dimensions are identical. It has LCD, two buttons, two rotary controls. The left rotary control on a QCX-mini is an audio gain potentiometer. On QMX it is a rotary encoder which also has the function of audio gain; the shaft button operates three functions:

Short press: mode change

Double press: band change

Long press: radio on/off

On the left panel are three connectors: DC in, audio out, and paddle: exactly as QCX-mini. On the right panel are three connectors: BNC RF, PTT out, and instead of a 3.5mm stereo jack for CAT as on QCX-mini, here we have a USB-C connector. USB-C is used because there is no space for a full-sized USB-B as QDX has; USB-micro is somewhat delicate and unreliable, and USB-mini isn’t easy to find. Note that the USB-C connector is used only for actual USB data, not for the provision of DC power to the radio. As on QDX, the QMX appears as a high performance 24-bit stereo USB soundcard as well as Virtual COM Serial port for CAT. The microcontroller has more USB endpoints so it should be possible to implement a second independent Virtual COM ports to connect a terminal at the same time. Note that the PTT output is QDX style (positive going, and grounded, configurable by band) NOT QCX-mini style (positive going output, and +5V to supply external devices such as GPS).

Internally the mechanical design is also similar to QCX-mini, with a main board, controls board and LCD board. The LCD board fits into the enclosure rails and the main board is suspended below it on four 11mm nylon spacers.

Electrically, the design of QMX is largely based on QDX. It is again an embedded SDR with the same BPF and LPF design, same push-pull PA, and PIN diode band switching, same PCM1804 stereo ADC chip. There are multiple changes to the design, incremental additions to QDX, as follows:

  1. Processor change.

  2. Audio headphones output

  3. User interface (buttons, LCD, rotary encoders)

  4. Switching power supplies

  5. RF envelope shaping

  6. SWR bridge.

QDX processor is an STM32F401 running at 72 MHz. Very powerful. With 128K Flash and 64K RAM. An 8K EEPROM is used for parameter storage. QMX has an STM32F446 running at 168 MHz so a considerable step up. It has 512K Flash and 128K RAM. A 128K EEPROM us used for parameter storage etc. The increase in processing capabilities should provide ample opportunity for lots of exciting future enhancements.

The audio output uses a CS4334 24-bit 48ksps stereo I2S DAC during a dual op-amp (NE5532). The output drive is therefore twice as powerful as QCX-mini. The 24-bit output resolution allows digital implementation of the volume control.

A QCX-mini uses 72mA. QCX+ is over 100mA. QDX 150mA. Now QMX with it’s more powerful processor and additional circuits wants to consume 220mA. But for portable operations people care about receive current. Therefore I implemented three buck converters, for the PIN diode forward bias on transmit (as on QDX), and for the 5V and 3.3V supply rails. The resulting receive current consumption of QMX is 80mA (display backlight off - increases 6mA with display backlight on). It may be possible to further improve the current consumption of QMX later with some firmware changes.

The buck converters are implemented using discrete components and based on the QDX PIN diode bias supply. The processor implements the control loop, generating PWM and measuring the output voltage using an ADC input. Of course this is very noisy, harmonics go up into the RF spectrum where we want to hear weak signals. However I have designed out this noise. Firstly a normal switching chip normally has a free-running RC oscillator, these are drifty and have large phase noise; it results in wide bands of noise at the harmonic frequency that also drifts around the band. In QMX everything is referenced to the single 25MHz TCXO reference. A very low drift, low phase noise crystal oscillator. The second very important step is this. The processor generates the Pulse Width Modulated drive signal for the switch transistor at a frequency it chooses. It also knows the operating frequency. It can calculate the harmonics of the buck converter switching frequency and choose to move frequency, if it would interfere with reception on the radio operating frequency. This idea of mine works very well in practice and the superb receiver performance of QDX is maintained. At the moment I implemented just two filters: a 300Hz wide, sharp CW filter the 3.2kHz filter used in QDX for digital operations.

Note that the processor being a software control loop for the three buck converter switching regulators creates a chicken and egg situation with regard to the 3.3V supply. The processor is the control loop but it can’t do that job until it has a 3.3V supply to operate it. Accordingly I have included a 78M33 linear regulator which is used for approximately the first 0.25 seconds on power-up. During this time a 47-ohm resistor is connected to the 3.3V SMPS as a dummy load; when the 3.3V supply is ready, a processor output pin causes a switch to select the SMPS and disconnect the 78M33 linear regulator and disconnect the dummy load.

Furthermore the power supply circuits include a soft on/off switch which is activated by a long press on the left rotary encoder. It also includes an AOD403 P-channel MOSFET reverse polarity protection circuit.

There are two small power supply boards which plug into the main QMX board using pin headers.

RF envelope shaping is applied using the same method as in the QRP Labs 5W PA kit. In QMX the modulating element is a P-channel MOSFET AOD403. A control circuit is driven by one of the processor’s 12-bit DAC outputs. Practically speaking it provides 37dB of high resolution RF amplitude control range with very high linearity.

An SWR bridge is implemented using two transformers wound onto a single BN43-1502 binocular ferrite core. It is permanently in line between the BNC connector and the LPF bank. The Fwd and Rev measurement outputs of the bridge are connected to processor ADC inputs.

QMX includes a lot of potential for detecting error conditions and preventing damage. The PA DC supply voltage (AFTER the RF envelope shaping) is measured by the processor. It therefore knows what voltage it has commanded (on the DAC output) and what voltage it got; in the event if a significant mismatch such as would occur if a BS170 was broken, or the output transformer installed incorrectly, the processor can therefore take protective action, shutting down the transmtiter in this case. QMX will be able to continuously monitor SWR and reduce power in the event of high SWR; as well as select a lower power level output for operating an auto-tuner to avoid temporary high SWR. QMX also continuously monitors battery voltage and could be set to reduce power if more than 12V supply was used for example. The performance envelope of the SMPS buck regulators is modeled in the firmware so that if there are any faults, PWM duty cycle would be prevented from getting excessively high that would cause over voltage or over current damage. There are also zener diodes on both the 3.3V and 5V rails to limit any potential for temporary over voltage.

QMX also includes a microphone so that SSB transmission can be implemented in the EER style used by Guido PE1NNZ in uSDX that originally derived from QCX. It supports a microphone and PTT input to the paddle connector as well as an internal electret microphone on the controls board. How good the SSB is remains to be seen. However it should be noted that QMX has significant hardware advantages compared to uSDX. Specifically a 168MHz 32-bit ARM Cortex M4 DSP processor (compared to uSDX 20MHz 8-bit AVR); 512K Flash (compared to 32K); 128K RAM (compared to 2K); and 37dB of very linear amplitude control range (compared to 20dB from applying 10-bit PWM DC bias modulation to the BS170 gates). Due to these hardware advantages I am optimistic that QMX will be able to provide a respectable SSB quality but this remains to be seen, has not been implemented yet in the firmware, and therefore is not a hard promise at this stage.

QMX is implemented on a single 6-layer board which is broken out into LCD board, main board, controls board and the two power supply boards. Of course there are a large number of SMD components, all are pre-soldered.

The current firmware implements QDX digimodes and QCX CW mode functionality (all QCX radio features including VFO A/B/Split, message and frequency memories, iambic Keyer, CW decoder, CW and WSPR beacon, GPS interface). The current firmware isn’t well polished yet and should be considered beta; I anticipate frequent updates over the coming weeks and months to finesse the transceiver and implement new exciting functionality. Firmware update uses the same mechanism as on QDX: the radio appears as a USB Flash drive, and you copy in the new firmware file downloaded from QRP Labs website.

The band coverage of QMX is 80, 60, 40, 30 and 20m. Just as on QDX you can change component values in the filters if you want to configure it for other bands. And just as QDX, I do anticipate offering a 20-10m version at a later date.


$95 the kit

$20 the enclosure

$50 if you want an assembled/tested radio.

Compare $95 to $72 for QCX (with TCXO and AGC options, single band CW only) and $69 for QDX (digital only).

Order here QMX 5-band 5W Digi transceiver

Please be patient with shipping as demand will likely exceed our shipping capability and production capability in the early stages. Documentation will be added to the website in the coming days and couple of weeks. The assembly manual is done already, I will set up a page for it when I’m back from Dayton hamvention.

73 Hans G0UPL

73 Éric


@DL6GCA Tnx Armin for the information. I wonder the new trx would be available in Friedrichshafen? In the List I’ve found QRP Labs only with other offers and not the new one.

If no info is available here I will send Hans an email.

@F5JKK Tnx Éric for much more details.

73, Ludwig

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(Text from info sent by Hans G0UPL)

Which other bands, 17 m and higher? Does the hardware except the filters support these bands? Does anyone has infos about this?

73, Ludwig

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Hi Ludwig @DH8WN ,

You can read like me this :wink:

73 Éric


Of course Eric, I read this :wink:. This could mean

  • the later version is “only” a new software allowing 17 m and higher and the only hardware change is related to the filters or

  • the whole hardware is modified in more points.

So would it be enaugh to modify the filters to work on the higher bands, do I need aditional a newer Software or is more different?

73, Ludwig

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It goes together it seems to me, we will also have to wait for a new software which will allow the SSB… he said later :slight_smile:
Wait and see ! Are you registered on QRPLabs@groups.io ?

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Thank you Armin @DL6GCA for this update :+1:

73 Marcel DM3FAM

I see lots of reduced price QCX and QDX radios for sale on eBay etc. real soon now. A multi band CW radio would be nice, sorry data modes don’t (currently) float my boat. YMMV


Where is the long announced and awaited QSX ? :rofl:

I like the fact you can update the firmware through a USB socket. Updating the firmware on the QCX Mini is a pain.


Bonsoir Patrick,

Voilà la réponse faite par Hans à la question d’1 OM :wink:

Re: QMX new transceiver announcement
From: Hans Summers
Date: Tue, 23 May 2023 16:55:28 UTC

Hi Simon

  • I am wondering if the QSX is still on

It is…

73, Éric



5 ans après l’annonce tonitruante de cette merveille !
En tous cas, il a la bosse du commerce ! :rofl: :rofl:

Nice work!!! My interest would be greatest for a radio with 40 meters and up and excellent QSK. No interest in digital modes or SSB when portable on a SOTA. - KT5X


They were selling the crap (that means a lot) of them at Dayton this year. The table was always busy. I heard that with an upcoming firmware update phone will be available. (matters not to me as I am all CW). I was ready to buy one until I saw only up to 20 meters. I think they have a version that does 20-10, but again missing key bands.

Kent K9EZ


Ordered (assembled version) - 601st in line :sweat_smile:


Ordered (assembled version) - 583st in line


That’s right, the rig even includes a built-in microphone, similarly to tr(uSDX).