Assembling the SDR Cube Transceiver

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 ...

With the Softrock front end installed, the typical current draw from a 12V supply will be about 280 ma (In transmit this will increase to about 350 ma.)

Make sure that you still have the full use of controls and display, as seen in the Digital Subassembly Test section

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.

Input about 1-2 mV rf signal to Cube's BNC connector. The level must be high enough to get meaningful voltage readings from the audio we are going to measure.

Select USB.

Turn the Filter control on the front panel to the widest filter position (fully CCW).

Tune the VFO so that you have audio between 500 and 1000Hz. (Anywhere within the Cube's filter pass band will do.)

Change to LSB using the Mode pushbutton on the front panel. You should now be hearing a much lower volume tone corresponding to the test signal being received on the USB.

Go into the User Menu and select RX Gain I. (Both I and Q gains should be at the default setting of 30000 when you start.)

Start adjusting the RX Gain I setting downward. If the level of the tone being heard on the LSB also decreases you are adjusting the correct setting. If the audio level increases rises, return the RX Gain I setting to 30000 and instead adjust the RX Gain Q setting ... . Now, decreasing this setting should decrease the audio amplitude of the LSB.

Adjust the setting to hear the minimum audio volume. (The other branch should be 30000 and the one you adjust should be lower.)

Now select RX Gain X gain. Adjust it up or down to produce a minimum audio level.

Exit the User menu to save settings and return to normal operation.

Now switching between LSB and USB should yield a clear difference in the audio levels, with the opposite sideband being nearly inaudible

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.

Select CW

Go to Menu and adjust TX Gain I and Q down to level where you have 700mW to 1W RF output. At this point you should dial same number for both I and Q gain settings. Leave TX Gain X to zero.

Tune button can also be used for the adjustment if you do not have a key or switch instead of the key or paddle. If tune button is used you have to release it while tuning the I or Q value.

CW or tune signal give you full power reference level. You can not use any signal in the mic input to do this easily.

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.

Place a 50-ohm dummy load on the Cube

Lightly couple the signal to another receiver set in CW mode (or USB) with the narrowest filter

With the Cube in CW mode, press "Tune" and search for the CW/USB signal on the external receiver until a clear 1 kHz tone is heard.

Switch the Cube to CW-R mode (LSB) 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.

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 ...

Connect a toggle switch to the PTT pin on the rear panel Mic jack

Connect a 4.7uF cap from the Mic audio pin, a 100-ohm resistor to ground from the other end, and a 10k resistor from this point to AF generator. (This is just a DC block and attenuator for the Mic input, as it is easier to set the generator output level.)

Set the generator frequency between 600 Hz and 1 kHz

Select USB or LSB mode for the Cube and initiate transmit by switching the toggle switch.

With external watt meter or scope adjust the AF generator level so that you have about 0.5W RF output into a 50-ohm dummy load. Something around half output power (~400 mW) is better than full power for this adjustment.

Continue following the sub-steps identified in "B above.

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.

Connect a known-accurate single tone reference signal (about 1-2 mV) to the antenna connector. For example: 14.200.000 Hz.

Tune the Cube to 14.200.00, LSB or USB (does not matter). A spectrum spike should be seen near the zero Hertz zero center position of the display.

Go to the User Menu and select Si570 XTAL. Adjust the setting to move the spectrum spike closer and closer toward the center of the display, until it completely disappears at 0 Hz.. (The 0 Hz bin is not shown, and thus is a good indicator of when the frequency is exactly calibrated).

Write down the calibration figure. Exit the menu to save the values and return to normal operation.

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.