QDX arrives - tomorrows Digital Transceiver, today

 QDX Digital Transceiver

I have blogged before, just down a bit, about the remarkable QRP Labs design for a digital transceiver for 80-40-30 & 20m. This design is a conventional SDR receiver, including digital sythesised frequency control, IQ audio to digital I2S codec, software filtering & decoding of USB signals and finally a "sound card" USB interface to your PC or Mac. [more below]

It is the transmit side which is very innovative. The digital USB audio from your PC has it's frequency measured, very fast and very accurately, this digital timing then sets the output frequency square wave of the synthesiser. The resulting RF frequency is amplified digitally and in a class "D" output amplifier. Giving an output of up to 5W across HF bands.

Also included is a CAT serial port implemented on the same USB socket, so your PC sees two ports, audio in/out and serial in/out. These can be directly selected in WSJT-X for FT8, WSPR or other digital modes.

My kit has arrived

QRP Labs were very kind to put me on the early release shipment list for the next batch of QDX kits and mine arrived yesterday. This is it

QDX kit, that I have started to build.

It is sooo small. Fits in the palm of your hand

The build

1. I read the documentation twice. Very detailed very clear. Seems most of the build is winding toroids. Anyway I have started as instructed to mount the very small capacitors. And I have had a lot of difficulty reading the tiny values printed on them. I mounted 7 and have now taken a break.

2. Finally finished the caps, by a process of elimination as I simply could not read the writing on final value. Hope OK...

3. Moved on and mounted the RFCs & diodes - one diode was cracked at the end where the wire comes out I hope it performs OK.  I then mounted the PA transistors and bolted them down, easy.

4. Now the coils. I have wound a few toroids in my time, but I hate it and feel like a novice every time I start. I have chosen the 12V operating power supply so wound the PA transformer 3:2. And I lost count so many times when winding the 19-30-36-41 turn input BPF inductor. But finally arrived here:

First components fitted and

started on the coils

5. So on with more and more toroids. I do wonder if QRP Labs could find a source of pre-wound toroids as this is the most tedious part of the kit construction. But then may be it would be too simple. Seven toroids to go... six for the transmit LPFs 80-40-30/20m and one for the balanced RF input

6. At last got all those pesky LPF coils wound and fitted. I am always scared at this point that I have got bad soldered joints and/or wrong number of turns on the coils. 

LPF coils mounted, did I get the turns right?

7. Seems very little 0.33mm wire left for the last transformer T2. Just starting measuring it now. Well I did not by a long way have enough 0.33mm (AWG 28) wire left, 3 x 20cm... so I had to use some 0.28mm (AWG 32) which I had in my components draw. What a faf to twist these three wires, then wind 10 turns, then a faf to identify the correct connections. At last to insert them, finally to solder them. They are very very close tother components so great care is needed. But I must praise the documentation for its calm and clear instructions, without which  I would have gone mad.

8. And now to finish up. USB, 12V power and Antenna sockets mounted. Finished. And here it is

I am proud of myself. A neat tiny 5W transceiver!

9. Now to try it out. Think I will box it first as this is easy and straightforward, and I can easily unmount it if anything is wrong, So I did

Boxed QDX, case ground connection not shown 

here (see comment below)

Testing

First let's get it connected. Plug in the antenna (an AL-705 loop and Mat-125 ATU). Whoops, the op manual says, "DCPower: use a regulated well- smoothed DC supply of not more than 12V". My shack is all running on a 13.8V supply but I think I have a home grown 12V supply somewhere in a dark pullout draw, I look it out. Yes, here it is, check the voltage, OK 11.8V. Last use this fancy new, posh, white  USB B to USB C cable that I bought for the purpose to connect the QDX to my MacBook Air. 

Software - MacOS

No drivers to install as the Mac has drivers for all main uses, digital audio and serial comms included. I have the latest version of WSJT-X installed which I normally use for FT8 with my ELAD FDM-DUO transceiver.

WSJT-X - preferences

1. Radio setup: I chose Kenwood TS-480 for the correct CAT commands and the QDX serial port (mine was called "/dev/tty.USBmodem101") should be set at at 8-1-N, the baud rate can be left at 9600 as the USB virtual port does not need this to be set to a particular value. PTT should be set to use CAT commands.

2. Audio setup: For input and output I chose the "QDX Transceiver"

I set the WSJT-X PWR slider to max as the QDX converts any input to square waves and the output is not dependant on the audio input level, only the power supply of 12V to the switching Class D PA.

Now the big moment, connect the 12V supply. Docs say the front red LED will

1. Quick flash for 5 sec on power up. Then go steady red and be in receive mode.

2. When in receive mode, steady red, for one second or

     a) 3 quick flashes - transmit mode, 

     b) 2 quick flashes - computer sending audio to QDX but volume too low to detect

     c) 1 quick flash - PTT activated but no audio detected.

Now to operate. I have RF output on 20 & 30m, but none on 40m. Oh. So I guess I have a poor connection somewhere in the 40m LPF, or may be it is that broken 1N4007 "pin"diode I found during construction... have to look into this.

Later: fixed the problem, poor soldered joints on LPF coils. Great care must be taken to burn off the coating on the wire used to wind them, do as docs say, hold your iron on for the full 10 sec!

I chose first 30m as I often use this band. I connect an antenna and my auto ATU, I hit WSJT-X  "Tune" to tune the ATU (<1.5 SWR indicated), then switch back to receive and FT8 signals magically appear. First contact!

First contact

Round up

Whoopee. By the way I get 3-4W output 20-80m, not a full 5W. I also noticed that the case QRP Labs supply is not connected to Ground of the circuits! I have screwed a small solder tab on the inside back panel and wired this to the ground side of the 12V connector.

So what is this QDX wonder?

It is an example, very well conceived, of the transformation of amateur radio to the digital world. So little of this transceiver is analog. At the front end there are BPFs which are digitally switched (no relays here). After that a base band detector, where the incoming sideband signals are shifted, or mixed, down to I & Q audio frequencies (actually 12kHz away from zero to allow the digital decoding to happen at an audio of 12-15kHz providing better signal filtering). And then we go to digital audio, with a high end analog/digital sound convertor, the output of which is standard PCM stereo audio signals along a standard I2S bus to the heart of the system, a microcomputer and DSP.

Microcomputer

The microcomputer handles a lot of things. It DSP filters and decodes the I & Q audio I2S inputs to mix them down to cancel one sideband and output Upper Side Band signals. It generates USB output digital audio signals to send to your PC, where applications such as WSJT-X decodes the spectrum and interpret the received audio tones of FT8 or other "digital" modes. So far so good, and fairly conventional as SDR receivers go. But innovative to use a dedicated software in a microcomputer to do this.

Transmit

The heart of the QDX innovation lies in the transmit chain. Applications such as WSJT generate audio tones, for example for FT8 a series of pulses of audio frequencies over a 15sec or so time to encode data in FSK. And these are sent out by your PC over the USB connection as digital audio. It is what the QDX does with these signals that is innovative. The incoming audio to the microcomputer is a digitised sine wave, and the period of this is accurately measured. This then gives the frequency of the tone.

But what we need is to transmit a Upper Side Band RF signal with this audio as modulation, in other word translate the audio back up to RF. This is done in the QDX by an Si5351 digital signal generator capable of generating RF square wave outputs up to 200MHz from any of its 3 outputs. If we chose to be on, say, the 40m band then this synthesiser will be programmed to output the FT8 frequency of 7074kHz plus the instantaneous audio tone frequency we have detected. Thus generating an FSK transmission, with no suppressed carrier and no wrong sidebands at all.

The huge advantage of this digitalisation, rather than analog mixing, is that two of the Si5351 outputs  can be programmed to output the digital signals 0/180deg out of phase. Why? Because these can be used to drive a push-pull power amplifier stage in class D operation. This has very high efficiency and thus very low heat dissipation. The PA generates 5W of RF from four small TO92 MOSFET transistors mounted on the PCB which is sufficient to provide a heat sink! Of course square waves have harmonic content, though by using push-pull amplifiers there is little 2nd harmonic and the first one to filter out is the 3rd at a much higher frequency. So conventional 5 element Low Pass Filters are all that is needed to meet legal harmonic output levels, and in practice much better.

But what about changing bands? As we have seen on the receive input digital CMOS switches change to different BPFs, and here again on the output RF side the switching of the LPFs is done digitally by "pin diodes" under control of the microcomputer.

Lastly, CAT

The final task of the microcomputer is to provide a digital serial USB port which uses the same cable connection as the audio, to provide CAT control. The program emulates the Kenwood TS440 or 480 transceiver - using ASCII text strings for communication - a much better solution than the complex ICOM hex data coding...

Thus applications such as WSJT-X can set the band and provide CAT transmit/receive PTT control. And there is but a single USB cable from transceiver to PC carrying both digital audio and serial data.

Wonderful

Thus we have an almost entirely digital transceiver which fits in an incredibly small box that you can hold in the palm of your hand. The receive chain has an option to be used as a simple IQ audio detector and have the microcomputer send these IQ signals directly to your PC, where applications such as Cubic SDR or HDSDR on a PC can be used to display a spectrum and waterfall of the received signals. But of course it cannot transmit analog speech SSB as it is only a digital transmit system. 

Do we need a transceiver - or just a TX

 How about just a transmitter?

Let's think about this. Many people working 160-80-40 & 20m very often receive using WEB SDRs which are extremely popular considering the huge increase in noise at most QTHs reaching S7-9 levels.

So there has to be a case to separate TX & RX not to build transceivers at all. The Ham shack would then be a PC & Internet connection, browser and web SDR plus your own transmitter which you tune to the same frequency as reception.

So what should be the spec for a transmitter only

- Audio/MIC input with EQ & compression 

- digital; audio input for digital transmissions, and/or audio line input, no EQ or Comp

- Key input, straight and paddle

- SSB & CW 1.8-30MHz, probably SDR as much easier to get wide coverage, use ready made modules?

- 100W say, PA with ALC 5-100W selection, LPFs for all bands, relay switched

- a low level 0dBm output for transverter?

- Auto ATU with 50R coax, balanced and long wire outputs

- built-in power supply - SMPS for low dissipation.

- Front panel or remote operation. PTT & CAT commands

That's quite a list but it is do-able, ain't it? Got to define a way to switch TR between transmitter and reception on a PC...

All in all, it could save us a lot of money to have such a product. It might also stimulate better and more advanced web SDRs to be developed.

Computer and Radio interfacing - chaos

 The mess that is computer control of your rig. 

Connections!

On our computers we have almost universally today just USB input/outputs. These can handle digital audio input/output and/or serial data communications. Often two USB A connection are used for audio and serial separately, on others a single USB C handles both audio and serial. And this works, with the coveat that some OSs demand careful driver and levels setups. I am using a MacBook Air and frankly it just works, no driver installations, no fiddling around with sound panels!

Interface computer and radio

We need to send audio in and get it out, which needs an A to D and D to A convertors, running at any of the standard PCM audio bit rates from 8kHz to 192kHz! 48kHz is common.

The biggest issue is the serial data control data from and to our transceivers. There are two main camps, one using sequences of hexadecimal numbers (ICOM, Yaesu not the same), the other using much simpler ASCII character strings (Kenwood). Add to this the signal for PTT, which can be a special sequence of serial CAT signals, or a hardware serial interface RTS or DTR lines, of the right polarity.

And to interface the computer USB serial data to separate TX and RX digital lines, which can be 5V or 3V3 volt levels and speeds of anywhere from 9600 to 38400 baud (possibly lower or higher), the logic must be correct for example positive logic where +5V = logic ‘1’ and 0V = logic ‘0’, or it could be negative logic with +5V = logic ‘0’ and 0V = logic ‘1’. The serial data must also have the correct format, for example one start bit, 8 data bits and one stop bit, a total of 10 binary bits.

For ICOM radios, to make things worse, these signals are carried on a single wire C-IV, not two separate connections TX & RX, requiring special I/O circuitry between the computer and the radio.

Then comes the problem of the rig audio levels. What does it need for input and output? These could be standard line levels (about 1V rms) or most likely the direct Microphone (10-100mV) and LS output (0-5V). With various input impedances of maybe 100R to 10k and rig output impedance suitable for driving an 8R loudspeaker.

Finally some radios have moved completely away from audio and data interfaces to use WLAN and communicate with your computer over LAN and via your home router. Giving the advantage of remote operation. But here again we have a new set of communication protocols and signalling.

Xieigu G1M example

A couple of blog posts back I describe the fiddling around I had to do to get my Xiegu G1M to work FT8

Better solution in the ELAD FDM-DUO

On the other hand my FDM-DUO is simplicity itself. It has separte USB B connectors for CAT and TX/RX digital audio (standard printer cables can be used). These can be connected through a simple 2 x USB A to USB C Hub to my MacBook Air where they appear as a digital "device" and an audio "device" which can be selected directly in WSJT-X. Which also has a built-in "hamlib" control configuration for the FDM-DUO. And it all just works. 

QRP Labs is the best

Even better solution from QRP Labs on their Digital Transceiver QDX. A single USB connection carrying both the CAT serial data and the digital audio from a built-in "sound card". One wire, just plug it in. Excellent.

Follow me

The rest of the radio industry needs to follow ELAD and QRP Labs QDX and provide a simple USB connection direct to our computers, with implemented CAT signalling - preferably using ASCII text commands.

Plugs confusion

 Plugs, plugs, plugs

How many of us have this problem? Main stream amateur equipment has no standard plug. The choices seem to be PL259, N Type, BNC or SMA. OK they all have their advantages and disadvantages. The tendency is to use PL259 for higher powers up to 1GHz, though the N Type is often preferred. Then BNC are frequent for smaller equipments and lower powers, and finally SMA which are used for all frequencies as "mount on the PCB" interfaces like most board SDRs.

But this confusion does not help in the shack, does it. And you end up with tons of convertors like this

PL259s, SMAs, BNCs joiners and convertors

And if you play around a lot with the configurations in your shack, as I do, you find yourself forever searching for cables and convertors and screwing and unscrewing SMAs.

So enough is enough. I have decided to standardise on BCN, because its quick and easy to plug and unplug. I have bought the right cables and/or convertors for my QRP Labs U3S I use for WSPR, my ELAD FDM-DUO the main SSB/DIgital modes transceiver, and my new (not so sure about) Xiegu G1M transceiver. Then of course there is my NanoVNA with SMA which I use to tune up my loop antenna. I have included an SMA output from my antenna switcher just for this and use an SMA-SMA cable... was this wise, I don't know.

Antenna switcher

This is a very simple switch, far from coaxial! Just a 4-way rotary in a small box.

Antenna out and transceivers in BNC

SMA out to NanoVNA

End up

Now I need to buy a bunch more BNC/BNC cables. On the common web sites these come is very limited lengths, so take care. 

Checking for RFI in the shack

Are you sitting in a puddle - of RF?

One thing you can do when suspecting RFI in the shack is to just see how much RF is around. I do this with a sensitive RF meter based on an Arduino, OLED display and a AD8307 log detector module. See this previous blog posting for details.

The RF METER reading from -80 to +20 dBm input

See this blog September 2018 for design details

The circuit is very simple. It is an Arduino Nano with connections

* A4 & A5 (I2C bus) to the OLED screen

* A3 as a DC input from the AD8307 module

* AREF for the A/D converter from a 3V3 voltage regulator

Power is from a 12V input feeding the Nano and the 3V3 regulator. Nano 5V output to AD8307 & OLED display.

The new Arduino sketch code for this project SIMPLE_WATT is here (note you must load the Oled.h header and the U8g2 library for the screen display (download libraries.zip and put them in your Arduino libraries folder). Unplug the 12V supply, connect the Nano to the IDE on your PC and upload the sketch, then plug in the 12V to establish the 3V3 A/D reference.

An extending aerial is connected to the SMA (50R) input connector. 

Note: if a 2K2 resistor is placed in series with the input then the meter will read -30dB down, or up to 20W (= 20mW) input across a 50R antenna or dummy load.