uSDX is an open source, arduino based SDR all mode HF QRP transceiver . This version of the pcb is meant for a compact size transceiver. And most of the components are smd now .so that PCB assembly can be automated for the most part.t 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 signals after amplification and band-limiting by a dual operational amplifier, are fed directly to the inputs of the ATmega328P microcontroller analog-to-digital converter multiplexer, which is used in the Arduino Nano module here. Further processing of the signal takes place only in the digital domain, which is quite an achievement, considering the limited capabilities of the 8-bit microcontroller. The ATmega28P samples the ADC input at a 62kHz sample-rate, and decimates this high-samplerate to a lower samplerate, performs a phase-shift by means of a Hilbert-transform, summing the result to obtain side-band rejection; it subsequently applies a low-pass filtering, AGC and noise-reduction functions. 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. Three embedded attenuators are available for optimally using dynamic range; the first attenuator is the RX FET switch Q2 responsible for 20dB attenuation, the second attenuator is ADC range (1.1V or 5V) selected by the ATMEGA ADC analog reference (AREF) logic and is responsible for 13dB attenation, the third attenuator is a pull-down of an analog input on the ATMEGA with a GPIO port responsible for 53dB attenation. Combining the three attenuators provides the attenation steps 0dB, -13dB, -20dB, -33dB, -53dB, -60dB, -73dB. The receiver circuit does not have an analog headphone amplifier – a digital output with PWM pulse width modulation is used. The audio signal is strong enough, even a small speaker with an impedance of 8 ohms can be used.The SSB transmit -stage is entirely impliminted in software.The heart of this SDR is an ATMEGA328p ,which is sampling input audio and reconstructing the SSB signal manipulating SI5351 PLL phase(over 800kbit/s I2C) and PA power (through PWM on the key-shaping circuit). The PWM driven Class-E desgin keeps the transciver simple and power efficient. 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). List of features:EER Class-E driven SSB transmit-stageApproximately 5W PEP SSB output from 13.8V supplyAll-Mode support: USB, LSB, CW, AM, FMDSP filters: 4000, 2500, 1700, 500, 200, 100, 50 Hz passbandDSP 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 -50dBcMultiband 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 codeSoftware-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-delayLightweight 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 themThree 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-frequencyBlocking dynamic range: 20kHz offset 123dB, 2kHz offset 78dBCW decoder, Straight/Iambic-A/B keyerOperation:Currently, the following functions have been assigned to shortcut buttons (L=left, E=encoder, R=right) and menu-items:perating 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.Technical Description: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, an decimates this high-samplerate to a lower samplerate, performs a phase-shift by means of a Hilbert-transform, summing the result to obtain side-band rejection; it subsequently applies a low-pass filtering, AGC and noise-reduction functions. Since the original QCX phase-shifting network and analog CW filter are not used, about half of the original QCX 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. Three embedded attenuators are available for optimally using dynamic range; the first attenuator is the RX MOSFET switch Q5 responsible for 20dB attenuation, the second attenuator is ADC range (1.1V or 5V) selected by the ATMEGA ADC analog reference (AREF) logic and is responsible for 13dB attenation, the third attenuator is a pull-down of an analog input on the ATMEGA with a GPIO port responsible for 53dB attenation. Combining the three attenuators provides the attenation steps 0dB, -13dB, -20dB, -33dB, -53dB, -60dB, -73dB.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 4x4800 samples/s, performs a Hilbert transformation and determines the phase and amplitude of the complex-signal; the phase-changes are restrictednote 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. Instead of using a key-shaping circuit for evelope control, it is possible to directly bias the PA MOSFETs with the (filtered) PWM signal. This has the advantage of less losses and simplifies at the cost of linearity which result in more compression for an SSB signal (which is actually a good thing).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.Please note: Some of the images and content are taken from different sources . Please find them here 1 2 3
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