LU1COP - Hernán "Capi" Sánchez


uSDX is optimized and modified based on the open source work uSDX/QCX-SSB

This is a shortwave QRP SSB/CW transceiver. The ultra-small size is easy to carry to outdoor use, using 1602N LCD screen, built-in speakers! Support to connect to PC and use FT8, JS8, FT4 and other digital mode software control, support CW automatic decoding.


+ 8 band coverage 80m/60m/40m/30m/20m/17m/15m/10m
+ Excellent PCB layout design in line with high-frequency circuit design principles to ensure excellent performance
+ Can reach 3-5W power in 8 bands and reserve TO-220 package pads required for upgrades
+ High emission efficiency, 80m/60m/40m/30m/20m efficiency is higher than 85%, 17m/15m efficiency is higher than 80%, 10m efficiency is higher than 70%
+ High-precision KDS brand TCXO, frequency accuracy is better than 1PPM, frequency stability is better than 0.5PPM
+ Independent core board and RF board to avoid own signal interference.
+ Ultra-small size: 82.8*28.8*110mm (excluding protruding parts)
+ Rich interfaces (CAT, PTT OUT, MIC/KEY, K-head microphone)
+ All LPF parts use 100V C0G/NP0 capacitors
+ Use original genuine Omron magnetic latching relay
+ Use original genuine LM4562 or LM833 op amp chip
+ Built-in speaker with cavity
+ BNC antenna interface

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, 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 noise gate, 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 160m-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 and semi-QSK operation) or assist in RX/TX switching for operating digital modes (no CAT or PTT interface required), external PTT output/PA control with TX-delay
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 (optionally): 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,
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, Straight/Iambic-A/B keyer
VFO A/B + RIT and Split, and corresponding relay band-filter switching via I2C CAT support (TS480 subset)


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

Menu Item



1.1 Volume

Audio level (0..13) & power-off/on (turn left)

E +turn

1.2 Mode

Modulation (LSB, USB, CW, AM, FM)



1.3 Filter BW

Audio passband (Full, 300..3000, 300..2400, 300..1800, 500, 200, 100, 50 Hz), this also controls the SSB TX BW.


R double

1.4 Band

Band-switch to pre-defined CW/FT8 freqs (80,60,40,30,20,17,15,12,10,6m)

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 VFO Mode

Selects different VFO, or RX/TX split-VFO (A, B, Split)

2x R long

1.7 RIT

RX in transit (ON, OFF)

R long

1.8 AGC

Automatic Gain Control (ON, OFF)


1.9 NR

Noise-reduction level (0-8), load-pass & smooth


1.10 ATT

Analog Attenuator (0, -13, -20, -33, -40, -53, -60, -73 dB)


1.11 ATT2

Digital Attenuator in CIC-stage (0-16) in steps of 6dB


1.12 S-meter

Type of S-Meter (OFF, dBm, S, S-bar)


2.1      CW


Enable/disable CW Decoder (ON, OFF)


2.4 Semi QSK

On TX silents RX on CW sign and word spaces


2.5    Keyer speed


CW Keyer speed in Paris-WPM (1..35)


2.6    Keyer mode


Type of keyer (Iambic-A, -B, Straight)


Menu Item



1.1 Volume

Audio level (0..13) & power-off/on (turn left)

E +turn

2.7    Keyer swap


to swap keyer DIH, DAH inputs (ON, OFF)


2.8 Practice

to disable TX for practice purposes (ON, OFF)


3.1 VOX

Voice Operated Xmit (ON, OFF)


3.2 Noise Gate

Audio threshold for SSB TX and VOX (0-255)


3.3 TX Drive

Transmit audio gain (0-8) in steps of 6dB, 8=constant amplitude for SSB


3.4 TX Delay

Delays TX to allow PA relay to be fully switched on before TX (0-255 ms)


4.1      CQ


Idle time in seconds before new CQ Message is given (0-60)


4.2      CQ


CQ Message text, pressing left-button in menu will start sending



8.1  PA  Bias min


PA amplitude PWM level (0-255) for representing 0% RF output


8.2  PA  Bias max


PA amplitude PWM level (0-255) for representing 100% RF output


8.3 Ref freq

Actual si5351 crystal frequency, used for frequency-calibration


8.4 IQ Phase

RX I/Q phase offset in degrees (0..180 degrees)


10.1 Backlight

Display backlight (ON, OFF)



Reset to factory settings

E long


Tune frequency (20kHz..99MHz)



Quick menu

L +turn


Menu enter



RIT back



Menu back


Connector definition:


RF efficiency test:

Note: The correct efficiency test should only measure the current consumed by the PA board, not including the current consumption of other components! PA Bias max is not the same for all machines, you need to find the best value based on testing.


PA Bias




RF power(W)


























































Warning: The hardware version is updated and optimized without notice!!!

This machine is optimized and modified according to open source settings, and its performance and functions are fully compatible with the original design. You can refer to the following introduction from the open source website.

Introduction to qcx-ssb/usdx (organized from GitHub):
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), steep roll-off (-45dB/decade) combined with an oversampling and decimating ADC are responsible for a processing gain, dynamic range and alias rejection sufficient to handle weak and strong signal conditions (e.g. contests or listening on 40m just next to broadcasting band).
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: The original forum discussion on the topic here: QRPLabs Forum
Update: A new group with collaborative interest has started to build a compact, portable (SOTA) rig for the summer, based on the ideas presented here. More information can be found in this: Forum.

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; a double press on right button narrows the receiver filter bandwidth, the bandwidth is reset every time mode is changed. The volume is changed by turning the rotary encoder while pressed.
There is a menu available that can be accessed by a short left press. With the encoder it is possible to navigate through this menu. When you want to change a menu parameter, a press with left button allows you to change the parameter with the encoder. With the right button it is possible to exit the menu any time. A fast access to the menu and parameter can be achieved by pressing the left button while turning the encoder, once you lift the left button you can immediately change the parameter by turning the encoder.
For receive, by default an AGC is enabled. This increases the volume when there are weak signals and decreases for strong signals. This is good for SSB signals but can be annoying for CW operation. The AGC can be turned off in the menu, this makes the receiver less noisy but require more manual volume change. To further reduce the noise, a noise-reduction function can be enabled in the menu with the NR parameter. To use the available dynamic range optimally, you can attenuate incoming signal by enabling a front-end attenuator with "ATT" parameter. Especially on frequencies 3.5-7 MHz the atmospheric noise levels are much higher, so you can increase the receiver performance by adding attenuation (e.g 13dB) such that the noise-floor is still audible. To calibrate the transceiver frequency, you can tune to a calibrated signal source (e.g. WWV on 10 MHz) and zero-beat the signal by changing "Ref freq" parameter; alternatively you can measure the XTal frequency with a counter and set the parameter. A S-meter of choice (dBm, S, S-bar) can be selected with the S-meter parameter. Selecting an S-bar, shows a signal-strength bar where each tick represents a S-point (6dB).
For SSB voice operation, connect a microphone to the paddle jack, a PTT or onboard "key" press will bring the trasnceiver into transmit. With the "TX Drive" parameter, it is possible to set the mdulation depth or PA drive, it is default set to 4 increasing it gives a bit more punch (compression for SSB). Setting it to a value 8 in SSB means that the SSB modulation is transmitted with a constant amplitude (possibly reducing RFI but at the cost of audio quality). To monitor your own modulation, you can temporarily increase MOX parameter. Setting menu item "VOX" to ON, enters the transceiver in Voice-On-Xmit operation (in TX mode as soon audio is detected), the VOX sensitivity can be configured in the menu with "VOX threshold" parameter. The PA Bias min and max parameters sets the working range of the PWM envelope signal, a range of 0-255 is the full range which is fine if you use a
key-shaping circuit for envelope control, but when you directly bias the PA MOSFETs (note 3) with the PWM signal
then you specifiy the optimal working range from just above the MOSFET threshold level to the maximum peak power you would like to use (0-180 are good values on my QCX).
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 right button to enter VOX mode. Adjust the volume to a minimum and start your favorite FT8 application (JTDX for instance). The sensitivity of the VOX can be set in the "VOX threshold" parameter.
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. It also discovers the capabilties of the transceiver depending on the mods made. The following capabilities are detected and shown on the display: "QCX" for a QCX without mods; "QCX-SSB" for a QCX with SSB mod; "QCX-DSP" for a QCX with SIDETONE disconnected and connected to a speaker (through decoupling capacitor); "QCX-SDR" for a QCX with SDR mod. Please check if the this capability matches with the mods.





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