SDR Cube Transceiver

Online Assembly Guide

Detailed construction notes for building and testing each of the SDR Cube kit modules

    Home  Bill of Materials    I/O Board    Controls Board    DSP Board      Softrock SR-Base     Softrock TX/PA  
RXAMP  Multi-Rx  X-LPF    Internal Cable Set    External Cable Set 
Start Here for Quick Kits ==>    Main Enclosure    Digital Subassembly Test    Final Assembly    RF Functional Test   


RF Functional Test                       ... (Section version 1.0a)


The purpose of this section is to test the SDR Cube with the RF Front End (e.g., with the Softrock RXTX 6.3: SR-Base, TXPA, RXAMP and X-LPF modules).


Step 1:  The Starting Point ...

Here's what you should be starting with, as a result of going through Final Assembly ...



Or, if you didn't intend on using the main Cube Enclosure, here are some views of the completed and connected boards out in the open ...




Step 2:  Apply power ...


Step 3:   Receive I-Q Balancing

Adjusting the receive-side I and Q audio paths for gain and phase is important to ensure maximum sideband rejection.  In this step, you will adjust either the RX Gain I or RX Gain Q settings, and the RX Gain X setting to set the received audio of the opposite sideband to a minimum level.

Typical Rx Gain settings are I: 30000, Q: 26300, X: -3700


Step 4:    Transmit I & Q Balancing

Adjusting the transmit-side I and Q audio paths for gain and phase is important to ensure maximum sideband suppression when transmitting.  In this step, you will adjust either the TX Gain I or TX Gain Q, and TX Gain X controls to set the transmit audio levels of I & Q for minimum energy being transmitted on the opposite sideband.


First adjust to the desired power level ...


Place a 50-ohm dummy load on the Cube

Lightly couple another receiver (in CW mode with the narrowest filter) and search for the Cube's signal while pressing "Tune" in LSB mode on the Cube.

Switch the Cube to CW mode and go into the User Menu to select TX Gain I, TX Gain Q, or TX Gain X.

Adjust each of these three settings for the lowest signal heard on the other receiver.



A) Set TX Power Level -- The Tx Gain settings in the user Menu will first be adjusted to set the desired transmit power level. Place a wattmeter in series with a dummy load to view the RF power being generated during this procedure. When using a Softrock as the RF front end, conventional wisdom advises placing the power level between 0.7 and 1.0 watts for optimal signal linearity.


B) Opposite Sideband Suppression Adjustment --  For this step the Cube will transmit into a dummy load, which is lightly-coupled to another receiver set to listen on the opposite sideband from what is being transmitted.  Then the Tx Gain I, TX Gain Q and TX Gain X settings will be adjusted to produce the lowest-heard signal on the receiver.

NOTE:   This adjustment is accomplished using an internally-generated "soft DDS" tone. This tone is a little rough, however, and the opposite sideband suppression will be better achieved if a clean audio signal is used in this procedure. If you have an audio frequency signal generator do following test setup ...


Step 5:    CALIBRATING THE Si570


There is a small variability in the default XTAL setting for the Si570.  This calibration step corrects for that variability and allows the Cube's VFO to accurately report the exact frequency on the display.

Instead of using a signal generator you can also use the known frequency of an AM station in the Shortwave band. Put the VFO dial reading to that frequency and adjust the Si570 XTAL setting to move the the AM station's carrier to the 0Hz position on the display.  Then exit the User menu to return to calibrated normal operation.

Typical chip default is 114.271011 and (in one case) the calibrated value is 114.2704


Step 6:    Observing the spectrum output of the RF front end

This is a quick view of the RF spectrum of a 1.1 watt transmitted signal on 20 meters.  (The Cube is in Tune mode.)  Note that the 2nd and third harmonics are down almost 50 dB from the fundamental 14 MHz transmitted signal.







Copyright 2010 Midnight Design Solutions, LLC.  All Rights Reserved.

Page last updated:  Jan 9, 2011