SDR Transceiver QSD, QSE, LO, & GPS Board

January 22, 2023 Update

Investigation of the QSE carrier suppression continued following initial findings presented in the previous update.

QSE IQ Gain and Phase errors affect the unwanted sideband suppression, and have little effect on carrier suppression.

DC offset variations between the 0- and 180-degree phase shifted IQ signals affect the carrier suppression. Capacitive coupling of the 0- and 180-degree phase shifted IQ signals to the QSE should prevent a DC offset, resulting in a well suppressed carrier. However, initial measurements showed carrier suppression of ~20dB, far lower than expected. In A Software Defined Radio for the Masses, Part 4, QEX Mar/Apr 2003, Gerald Youngblood, AC5OG, reported 48dB carrier suppression relative to the wanted sideband with a similar QSE design.

Leakage around or through the QSE was investigated. PCB layout was initially suspected, but it now appears leakage is primarily through 3253 analog multiplexer; after all, these devices were never intended to be high frequency quadrature analog multiplexers.

With the Post Mixer Amplifier in the design, audio drive to the QSE had been backed off to avoid over driving the TX PA Driver. However, it was determined the QSE audio drive needs to be high enough to create dominance of the wanted sideband signal over the carrier signal leakage; as a result, the Post Mixer Amplifier was removed.

The following measurements were made with a Rigol DSA 815 Spectrum Analyzer with audio drive to the QSE set achieve 160mW output from the TX PA Driver, which is the required drive level for 10W PA output. Levels are relative to the wanted sideband.

BandCarrier Level (dB)Unwanted Sideband Level (dB)
QSE Carrier and Unwanted Sideband Levels Relative to the Wanted Sideband

Carrier “suppression” degrades by up to 10dB as the drive to the QSE is decreased to lower TX PA Driver output to ~20mW (1-2W PA output).

In the course of testing, spurious signals ~40dB below the wanted sideband signal level were found +/- 48-KHz of the wanted sideband signal frequency. A closer look at the audio output of the SGTL5000 DAC showed replications of the audio input signal at 48-KHz +/- audio signal frequency. Such replications of the input signal will occur at integer multiples of the Sampling Frequency (Fs). The SGTL5000 is a low cost audio codec intended for consumer audio products, and the TI datasheet provides little detail of the DAC or filtering performance. Therefore, it was decided to add a 2nd order Sallen-Key filter with cutoff frequency of ~11-KHz at the IQ inputs of the QSE to attenuate these spurious signals by an additional ~30dB.

QSD, QSE, LO, & GPS Board Version 3 Schematic.

December 30, 2022 Update.

IQ Calibration routines were added to the DSP receive and transmit software to allow adjustment of the Q signal amplitude and phase to correct for imbalance between the I&Q signals. In the QSD, amplitude and phase are adjusted to minimize the image signal which is 96-KHz above the desired signal; in the QSE, amplitude and phase are adjusted to suppress the opposite sideband.

In the case of QSE, it was possible to adjust amplitude and phase to suppress the opposite side band by ~50dB. However, the carrier suppression which was only ~20dB. After some investigation, it was determined that carrier leakage was occurring due to length and location of the tracks between the 74HC74, used to generate the I&Q LO signals (at the carrier frequency), and the SN74CBT3253C. A new board layout (version 2) is in process to move the 74HC74s to the rear of the board opposite the SN74CBT3253Cs on the front side of the board, thereby minimizing the length of the tracks carrying the I&Q LO signals. No change to the board schematic is contemplated.

October 2022 Original Post

This board contains the Quadrature Sampling Detector (QSD), Quadrature Sampling Exciter (QSE), Post Mixer Amplifier, Local Oscillator (LO), and GPS receiver.

The QSD design is based on the QRP Labs Receiver Module, the design is also used in the 2020 Transceiver receiver to convert 9-MHz IF to audio. In the SDR Transceiver, the QSD converts RF to 48-KHz IF In-phase (I) and Quadrature (Q) signals. The IQ signals pass through Sallen-Key low pass (anti-aliasing) filters with 66-KHz cutoff frequency before being connected to the ADC on the Teensy 4.1, Transmit and Receive Audio Board. With ADC sample rate of 192-KHz, the Nyquist Frequency is 96-KHz.

The QSE takes IQ audio from the Teensy 4.1, Transmit and Receive Audio Board and converts it to a single sideband RF signal in the selected band. Two DRV134 Audio Balanced Line Drivers, one for the I audio signal and one for the Q audio signal, are used to generate 0-degree and 180-degree phased shifted IQ audio signals for the SN74CBT3253C Dual 1-of-4 Multiplexer. Outputs from the dual 1-of-4 multiplexers are combined using a transformer and then fed to a post mixer amplifier.

The Post Mixer Amplifier uses a 2N5109 that drives a 6dB pad at the output to provide a broadband 50 Ohm termination for the filters that follow on the RF Band Pass Filter Board.

A Si5351 breakout board from Adafruit provides the LO signals for the QSD, QSE, and a 2.5-MHz signal used for GPS correction of the Si5351. The Si5351 Output Enable Control register is used to disable the transmit LO during receive and disable the receive LO during transmit.

At the time of writing, per the photo below, the Metro Mini (ATmega328) and GPS Receiver have not been installed; however, the same GPS correction scheme is currently in use in version 2 of the 2020 Transceiver.

QSD, QSE, LO, & GPS Board schematic.

In the photo below, the QSE and post mixer amplifier are located in the top left half of the board, and the QSD and anti-aliasing filters in the top right half. In the bottom third of the board from left to right are the Metro Mini, Si51531 breakout board, and GPS receiver.

QSD, QSE, LO, & GPS Board Front View

Power regulation and distribution is done on the backside of the board as shown below.

QSD, QSE, LO, & GPS Board Rear View

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