usdx/README.md

QCX-SSB: SSB with your QCX transceiver

This is a simple and experimental modification that transforms a QCX into a (Class-E driven) SSB transceiver. It can be used to make QRP SSB contacts, or (in combination with a PC) used for the digital modes such as FT8, JS8, FT4. It can be fully-continuous tuned through bands 80m-10m in the LSB/USB-modes with a 2400Hz bandwidth has up to 5W PEP SSB output and features a software-based full Break-In VOX for fast RX/TX switching in voice and digital operations.

The SSB transmit-stage is implemented entirely in digital and software-based manner: at the heart the ATMEGA328P is sampling the input-audio and reconstructing a SSB-signal by controlling the SI5351 PLL phase (through tiny frequency changes over 800kbit/s I2C) and controlling the PA Power (through PWM on the key-shaping circuit). In this way a highly power-efficient class-E driven SSB-signal can be realized; a PWM driven class-E design keeps the SSB transceiver simple, tiny, cool, power-efficient and low-cost (ie. no need for power-inefficient and complex linear amplifier with bulky heat-sink as often is seen in SSB transceivers).

For the receiver, a large portion of the original QCX circuit has been removed and implemented in digital manner (software): the ATMEGA328P is now implementing the 90 degree phase shift circuit, the (CW/SSB) filter circuit and the audio amplifier circuit (now a class-D amplifier). This has simplified the QCX circuit a lot (50% less components needed), and there are a number of advantages and features: there is no longer a need for an alignment procedure due to the very accurate 90 degree Hilbert phase shifter; and there are now adjustable IF DSP filters for CW and SSB; and there is an AGC and there is a noise-reducing DSP signal conditioning function and there are three indepent built-in attenuators in the analog front-end which helps utilizing the full dynamic range. The speaker is directly connected and driven by the ATMEGA. A digital mixer with narrow low-pass window (2 kHz) and steep roll-off (-45dB/decade) with ADC oversampling are offering enough alias rejection, processing-gain and dynamic-range for handling weak (0.12μV) and strong-signals (S9+60dB).

This experiment is created to try out what is can be achieved with minimal hardware while moving complexity towards software; here the approach followed is to simplify the design where possible while keep a reasonable performance. The result is a cheap, easy to build, versatile QRP SSB transceiver that actually is quite suitable for making QSOs (even in contest situations), however due to the experimental nature some parts are still in progress and hence limited. Feel free to try it out or to experiment with this sketch, let me know your thoughts or contribute here: https://github.com/threeme3/QCX-SSB There is a forum discussion on the topic here: QRPLabs Forum

73, Guido pe1nnz@amsat.org

List of features:

• Simple, fun and versatile QRP SSB HF transceiver with embedded DSP and SDR functions;
• EER Class-E driven SSB transmit-stage
• Approximately 5W PEP SSB output from 13.8V supply
• All-Mode support: USB, LSB, experimental modes: CW, AM, FM
• DSP filters: 4000, 2500, 1700, 500, 200, 100, 50 Hz passband
• DSP features: Automatic Gain Control (AGC), Noise-reduction (NR), Voice-triggered Xmit (VOX), RX Attentuators (ATT), TX drive control, Volume control, dBm/S-meter.
• SSB opposite side-band/carrier supression Transmit: better than -45dBc, IMD3 (two-tone) -33dBc, Receive: better than -50dBc
• Multiband support, continuously tunable through bands 80m-10m (and from 20kHz..99MHz with loss in performance)
• Open source firmware, built with Arduino IDE; allows experimentation, new features can be added, contributions can be shared via Github, software-complexity: 2000 lines of code
• Software-based VOX that can be used as fast Full Break-In (QSK operation) or assist in RX/TX switching for operating digital modes (no CAT or PTT interface required)
• Simple to install modification with 8 component changes and 8 wires
• Lightweight and low-cost transceiver design: because of the EER-transmitter class-E stage it is highly power-efficient (no bulky heatsinks required), and has a simple design (no complex balanced linear power amplifier required)
• Fully digital and software-based SSB transmit-stage: samples microphone-input and reconstruct a SSB-signal by controlling the phase of the SI5351 PLL (through tiny frequency changes over 800kbits/s I2C) and the amplitude of the PA (through PWM of the PA key-shaping circuit)
• Fully digital and software-based SDR receiver-stages: samples I/Q (complex) signal from Quadrature Sampling Detector digital mixer, and performs a 90-degree phase-shift mathematically in software (Hilbert-transform) and cancels out one side-band by adding them
• Three independent switchable analog front-end receiver attenuators (0dB, -13dB, -20dB, -33dB, -53dB, -60dB, -73dB)
• Receiver Noise floor MDS: –135 dBm at 28MHz (in 200Hz BW)
• Receiver Front-end selectivity: steep -45dB/decade roll-off +/-2kHz from tuned-frequency
• Blocking dynamic range: 20kHz offset 123dB, 2kHz offset 78dB
• CW decoder (experimental).
• Probably the most cost effective and easy to build standalone SDR/SSB transceiver that you can find. Very much simplifies the original QCX circuit (i.e. 50% less components to install, no complex transformer windings, no alignment procedure) and more versatile in use.

Revision History:

Rev.	Date	Features
R1.02 (current)	2020-04-12	Integrated SDR receiver, CW decoder, DSP filters, AGC, NR, ATT, experimental modes CW, AM, FM, quick menu, persistent settings, improved SSB TX quality.
R1.01d	2019-05-05	Q6 now digitally switched (remove C31) - improving stability and IMD. Improved signal processing, audio quality, increased bandwidth, cosmetic changes and reduced RF feedback, reduced s-meter RFI, S-meter readings, self-test on startup. Receiver I/Q calibration, (experimental) amplitude pre-distortion and calibration. See here original QCX-SSB modification (it is also supported by current firmware!)
R1.00	2019-01-29	Initial release of SSB transceiver prototype.

Schematic:

Below the schematic after the modification is applied, unused components are left out and changed components are marked in red (click to zoom & download) (link to original schematic):

Installation:

For the modification, you need to remove 10 and change 8 components, install 8 wires, upload firmware and connect a microphone. On a newly to be build QCX, 46 components can be left out.

Please note that if you apply the mod on a newer than QCX than Rev 4 (see the number on PCB in bottom right corner), you have in addition to convert back to the original Rev4 circuit (e.g. restore R49, R50, C39, R53, (C21, C22 excluded) on a Rev 5 PCB) See PCB Revision section and QCX Assembly instruction manuals for more info about the changes.

Change the following component values (and type of component in some cases), and wire the following component pins/pads:

1. To implement the SDR receiver: R11,12,14,15,17,27,29,59,IC10 (remove); IC6-10,R11-40,R59-60,C9-24,C52-53,D5,Q7 (optionally omit on new builds); change R7,10(82k); C4,C7(1nF); wire IC2(pin15) to IC10(pin1); disconnect R50(5V side) and R52(5V side) and both wire to IC2(pin21); wire C39(R27/R29 side) to IC5(pin1); wire C40(IC10 side) to IC5(pin7). Rationale: This will feed the amplified I/Q signals to the ADC0, ADC1 input, biased at 0.5*VAREF(=1.1 or 2.5V), the rest of the receiver will be handled in software and audio output is realised on PB1. To implement the original (analog) QCX receiver, you can skip this step and just brigde the CW-filter as was done in the original QCX-SSB modification.
2. To implement the SSB transmitter: C31 (remove); change D4,R56 (10k); R58 (.22uF); C32 (10uF); wire IC2(pin21) to pin R57(to DVM-pin3); wire IC2(pin20) to DVM(pin2); wire IC2(pin18) to junction D4-C42-R58. Rationale: This will bias the mic input (at DAH line) with 5V and pass the audio to ADC2, biased at AREF/2 V; the key-shaping circuit is digitally switching the voltage supply to the PA (or alternatively directly controlled via PA bias^{note 3}).
3. Optionally, to implement multiband support: C1,C5,C8,T1,R64 (remove); at T1 landing pattern (see QCX Assembly instruction page 53) install R (1K) over 6-8; R (1K) over 3-4; C (10nF) over 4-8; C30 (30pF); L4 (1uH/16t); replace C25-28,L1-L3 with different LPFs as you wish (get some inspiration by QRPLabs Low Pass Filter kit). Rationale: The resonant elements and the transformer are replaced with a pass-through capacitor.
4. Upload the hex firmware-file to original or new ATMEGA328/328P chip (here is latest released hex file and click on "Assets" below the description). The standard QCX firmware upload procedure can be followed (for details ^{note 1}). The fuse settings should be E=FD H=D1 L=F7. After uploading, and the mods are applied correclt the display should show QCX-SDR (or QCX-SSB for the analog RX implementation).
5. Connect an electret microphone pins (+) to tip and (-) to sleeve of paddle-jack; PTT-switch pins to ring and sleeve (see X1M-mic).

Below the layout with components marked in red that needs to be changed; gray components must be installed and blank components may be omitted and some must be remove (see above):

Below the wires that needs to be installed on the bottom PCB. Note that a circle represents resistor R50/R52 that is in series with the wire and the pad at which the circle is drawn (the resistor is no longer connected to its original neighbour pad and its probably the best to place this wiring and resistor at the components side):

Operation:

Currently, the following functions have been assigned to shortcut buttons (L=left, E=encoder, R=right) and menu-items:

Menu Item	Function	Shortcut
1.1 Volume	Audio level (0..16) & power-off/on	E +turn
1.2 Mode	Modulation (LSB, USB, CW, AM, FM)	R
1.3 Filter BW	Audio passband (Full, 300..4000, 300..2500, 300..1700, 200, 100 Hz)	R double
1.4 Band	Band-switch to pre-defined FT8 freqs (80,60,40,30,20,17,15,12,10,6,4m)	E double
1.5 Tuning Rate	Tuning step size 10M, 1M, 0.5M, 100k, 10k, 1k, 0.5k, 100, 10, 1	E or E long
1.6 AGC	Automatic Gain Control (ON, OFF)
1.7 NR	Noise-reduction level (0-8), load-pass & smooth
1.8 ATT	Analog Attenuator (0, -13, -20, -33, -40, -53, -60, -73 dB)
1.9 ATT2	Digital Attenuator in CIC-stage (0-16) in steps of 6dB
1.10 S-meter	Type of S-Meter (OFF, dBm, S, S-bar)
2.1 CW Decoder	Enable/disable CW Decoder (ON, OFF)
3.1 VOX	Voice Operated Xmit (ON, OFF)	R long
3.2 VOX Level	Audio threshold of VOX (0-255)
3.3 MOX	Monitor on Xmit (audio unmuted during transmit)
3.4 TX Drive	Transmit audio gain (0-8) in steps of 6dB, 8=constant amplitude for SSB
8.1 Ref freq	Actual si5351 crystal frequency, used for frequency-calibration
8.2 PA Bias min	KEY_OUT PWM level (0-255) for representing 0% RF output
8.3 PA Bias max	KEY_OUT PWM level (0-255) for representing 100% RF output
9.1 Sample rate	for debugging, testing and experimental purpose
9.2 CPU load	for debugging, testing and experimental purpose
9.3 Param A	for debugging, testing and experimental purpose
9.4 Param B	for debugging, testing and experimental purpose
9.5 Param C	for debugging, testing and experimental purpose
main	Frequency (20kHz..99MHz)	turn
main	Quick menu	L +turn
main	Menu enter	L
menu	Menu back	R

Operating Instructions:

Tuning can be done by turning the rotary encoder. Its step size can be decreased or increased by a short or long press. A change of band can be done with a double press. The mode of operation is altered with a short press on the right button; this can be combined with changing S4 to change between wide-band (SSB) and small-band operation operation.

For SSB voice operation, adjust the amplitude drive by double-pressing right button to a level where voice peaks providing maximum power output (not more than that); this provides an acceptable IMD with good intelligability for local and normal distant operations. In cases where your signal is too weak, set the drive level to 8 to increase the average power output by using a constant amplitude-envelope; in some cases this might be just enough to put your signal above the noise-floor and make yourselve heard; note that this operation degrades the IMD considerably, but since this does not impact the intelligability and since these inter-modulation products are anyway below the noise-floor (and BW limited) they are not in the way, ie. they are not observable by the other station. For long duration QSOs on a specific frequency you can stop holding the PTT by enter (or leave) VOX mode with a long press on right button.

For FT8 (and any other digital) operation, select one of the pre-programmed FT8 bands by double press the rotary encoder, connect the headphone jack to sound card microphone jack, sound card speaker jack to microphone jack, and give a long press on left button to enter VOX mode. Adjust the sound card speaker volume to a minimum and start your favorite FT8 application (JTDX for instance).

To experiment with amplitude pre-distortion algorithm, double-press left button to train the PA amplitude characteristic. This sweeps the amplitude from maximum PWM to minimum PWM and measures the PA response through an internal receiver loopback and stores the values into volatile memory. Once trained, set the appropriate amplitude drive level for voice input. Pre-distorted amplitude response can be measured with a storage spectrum-analyser and a long-press of left button; it will sweep the pre-distorted amplitude from 0 to 100% in 255 steps, where each step has a 10Hz frequency offset.

The receiver side-band rejection can be measured and adjusted through a left single press button. To do so, turn down the volume, connect a dummy-load and enable the original CW-filter. After pressing the button, the I-Q balance, Lo Phase and High phase is measured; adjust R27, R24, R17 subsequently to its minimum side-band rejection value in dB.

On startup, the transceiver is performing a self-test. It is checking the supply and bias voltages, I2C communications and algorithmic performance. In case of deviations, the display will report an error during startup.

Technical Description:

The principle of operation (at least what is ging on in the ATMEGA) is a bit as in the following video-fragment: Opzij (in Dutch saying; "Sideways, sideways, sideways, Make room, make room, make room, We are in an incredible rush"; full lyrics) here... :-) jokes aside; below the block diagram of the QCX-SSB, SDR transceiver:

For SSB reception, the QCX analog phasing receiver stage is replaced with a digital SDR stage; this means that the phase shifting op-amp IC6 is changed into a regular amplifier and whereby the individual I and Q outputs are directly fed into the ATMEGA328P ADC inputs for signal processing. The ATMEGA328P (over-)samples the ADC input at a 62kHz sample-rate, down-samples this high-samplerate to a lower samplerate and performs a phase-shift by means of a Hilbert-transform, summing the result to obtain side-band rejection; it subsequently it performs a low-pass filtering and provide an AGC function. Since the original QCX phase-shifting network and analog CW filter are not used, about half of the components can be left out; by combining the function of IC7B into IC6A another op-amp can be saved. The ADC inputs are low-pass filtered (-40dB/decade roll-off at 1.5kHz cut-off) to prevent aliasing and input are biased with a 1.1V analog reference voltage to obtain additional sensitivity and dynamic range. With the 10-bit ADCs and a 4x over-sampling rate, a theoretical dynamic range of 72dB can be obtained in 2.4kHz SSB bandwidth. LSB/USB mode switching is done by changing the 90 degree phase shift on the CLK0/CLK1 signals of the SI5351 PLL.

For SSB transmission the QCX DVM-circuitry is changed and used as an audio-input circuit. An electret-microphone (with PTT switch) is added to the Paddle jack connecting the DVM-circuitry, whereby the DOT input acts as the PTT and the DASH input acts as the audio-input. The electret microphone is biased with 5V through a 10K resistor. A 10nF blocking capacitor prevents RF leakage into the circuit. The audio is fed into ADC2 input of the ATMEGA328P microprocessor through a 220nF decoupling capacitor. The ADC2 input is biased at 0.55V via a divider network of 10K to a 1.1V analog reference voltage, with 10-bits ADC resolution this means the microphone-input sensitivity is about 1mV (1.1V/1024) which is just sufficient to process unamplified speech.

A new QCX-SSB firmware is uploaded to the ATMEGA328P, and facilitates a digital SSB generation technique in a completely software-based manner. A DSP algorithm samples the ADC2 audio-input at a rate of 4800 samples/s, performs a Hilbert transformation and determines the phase and amplitude of the complex-signal; the phase-changes are restricted^{note 2} and transformed into either positive (for USB) or negative (for LSB) phase changes which in turn transformed into temporary frequency changes which are sent 4800 times per second over 800kbit/s I2C towards the SI5351 PLL. This result in phase changes on the SSB carrier signal and delivers a SSB-signal with a bandwidth of 2400 Hz whereby spurious in the opposite side-band components is attenuated.

The amplitude of the complex-signal controls the supply-voltage of the PA, and thus the envelope of the SSB-signal. The key-shaping circuit is controlled with a 32kHz PWM signal, which can control the PA voltage from 0 to about 12V in 256 steps, providing a dynamic range of (log2(256) * 6 =) 48dB in the SSB signal. C31 is removed to ensure that Q6 is operating as a digital switch, this improves the efficiency, thermal stability, linearity, dynamic range and response-time. Though the amplitude information is not mandatory to make a SSB signal intelligable, adding amplitude information improves quality. The complex-amplitude is also used in VOX-mode to determine when RX and TX transitions are supposed to be made.

The IMD performance is related dependent on the quality of the system: the linearity (accuracy) of the amplitude and phase response and the precision (dynamic range) of these quantities. Especially the DSP bit-width, the precision used in the DSP algorithms, the PWM and key-shaping circuit that supplies the PA and the PA phase response are critical. Decreasing (or removing) C32 improves the IMD characteristics but at the cost of an increase of PWM products around the carrier.

Results

Here is a sample me calling CQ on 40m with my QCX-SSB at 5W and received back by the Hack Green websdr about 400km away, note that the audio quality has further improved since then.

Several OMs reported a successful QCX-SSB modification and were able to make SSB QRP DX contacts over thousands of kilometers on the 20m and 40m bands. During CQ WW contest I was able to make 34 random QSOs on 40m with 5W and an inverted-V over the house in just a few hours with CN3A as my furthest contact, I could observe the benefits of using SSB with constant-envelope in cases where my signal was weak; for FT8 a Raspberry Pi 3B+ with JTDX was used to make FT8 contacts all the way up to NA.

Measurements: The following performance measurements were made with QCX-SSB R1.01, a modified RTL-SDR, Spektrum-SVmod-v0.19, Sweex 5.0 USB Audio device and Audicity player. It is recognized that this measurement setup has its own limitations, hence the dynamic range of the measurements is somewhat limited by the RTL-SDR as this device goes easily into overload. Measurements were made with the following setttings: USB modulation, no pre-distortion, two-tone input 1000Hz/1200Hz where audio level is set just before the point where compression starts. Results:

• Intermodulation distortion products (two-tone; SSB with varying envelope) IMD3, IMD5, IMD7: respectively -33dBc; -36dBc; -39dBc
• Intermodulation distortion products (two-tone; SSB with constant envelope) IMD3, IMD5, IMD7: respectively -16dBc; -16dBc; -19dBc
• Opposite side-band rejection (two-tone): better than -45dBc
• Carrier rejection (two-tone): better than -45dBc
• Wide-band spurious (two-tone): better than -45dBc
• 3dB bandwidth (sweep): 0..2400Hz

Notes:

1. Firmware upload variations:

• AVRDudess tool or avrdude CLI (avrdude -c avrisp -b 19200 -P /dev/ttyACM0 (or: /dev/ttyUSB0) -p m328p -e -U efuse:w:0xfd:m -U hfuse:w:0xC1:m -U lfuse:w:0xF7:m -U flash:w:firmware.hex) can be used for uploading the firmware via the ISP connector on the QCX. Follow Arduino as ISP instructions if you have a Arduino UNO board available (tip: use female-to-male breadboard cables to connect Arduino to QCX ISP jumper); or USBasp instructions if you have a USBasp programmer, alternatively use USPasp ExtremeBurner; but many other ISP programmers can be used in similar manner such as USBtiny or AVRisp mkII. During ISP, mic should be disconnected, power supply should be connected; in tool do not erase, program EEPROM or set fuse settings (they are by default ok: E=FD H=D1 L=F7).
• Alternatively, in case you have an ATMEGA328P chip with Arduino bootloader, you can place the chip in an Arduino UNO board and upload directly (without the need for a ISP cable and QCX) by specifying 'arduino' programmer and baudrate 115200.
• Alternatively, in case you have an Arduino 1.8.9 environment installed, you can upload the QCX-SSB Sketch directly from the Arduino environment (without using AVRDudess and firmware file); make sure "Tools > Board > Arduino/Genuino Uno", "Tools > Port > /dev/ttyUSB0 or ttyACM0", and then "Sketch > Upload" is selected, while the ATMEGA328P chip is placed in the Arduino UNO socket. It is also possible to use Arduino as ISP method: upload this variation of ArduinoISP to the Arduino board and select "Tools > Programmer > Arduino as ISP", and "Sketch > Upload Using Programmer".

2. The occupied SSB bandwidth can be further reduced by restricting the maximum phase change (set MAX_DP to half a unit-circle _UA/2 (equivalent to 180 degrees)). The sensitivity of the VOX switching can be set with parameter VOX_THRESHOLD. Audio-input can be attenuated by increasing parameter MIC_ATTEN (6dB per step).
3. Alternatively, the PA MOSFETs can be directly biased by the PWM envelope signal, basically making the key-shaping circuit redundant. To do so, Q6,Q4,R41,R42,C32,C31 can be removed entirely, whereby C-E pads of Q6 are wired, and where a 100nF capacitor is inserted at IC3A-pin3 and G of Q1-3, and where a 10k resistor is placed at G-D pads of Q4, a 10nF capacitor between S-D pads of Q4, and where a 10k resistor is placed between D of Q4 and G of Q1-3.

Credits:

QCX (QRP Labs CW Xcvr) is a kit designed by Hans Summers (G0UPL), originally built for RSGB's YOTA summer camp 2017, a high performance, image rejecting DC transceiver; basically a simplified implementation of the NorCal 2030 by Dan Tayloe (N7VE) designed in 2004 combined with a Hi-Per-Mite Active Audio CW Filter by David Cripe (NMØS), Low Pass Filters from Ed (W3NQN) 1983 Articles, a key-shaping circuit by Donald Huff (W6JL), a BS170 switched CMOS driven MOSFET PA architecture as used in the ATS designs by Steven Weber (KD1JV) (originating from the Power MOSFET revolution in the mid 70s), and combined with popular components such as Atmel ATMEGA328P microprocessor, a Hitachi HD44780 LCD display and a Silicon Labs SI5351 Clock Generator (and using a phase shift in the SI5351 clocks). The QCX-SSB transmitter and QCX-SDR receiver stage both running on a ATMEGA328P, including its multiband front-end and direct PA biasing/envelope-generation technique; its concept, circuit, code and modification to run on a QCX are a design by Guido (PE1NNZ); the software-based SSB transmit stage is a derivate of earlier experiments with a digital SSB generation technique on a Raspberry Pi. Many thanks to all of you who got interested in this project and took the challege and effort to try out QCX-SSB. Without your valuable feedback the project could not have kept moving on, improving and challenging new ideas!