Welcome to the first release of the SDR Cube software containing full support for AM Mode transceive operation.  A major feature “AM Mode” has been added to the SDR Cube Transceiver software, enabling the operator to select and use the AM mode for voice communications. Mode selections of either AM▲ (upper) or AM▼ (lower) provide a unique and powerful approach for AM reception, and an efficient method is implemented in software to generate AM transmissions. This latest software release gives the SDR Cube owner a distinctly capable way to communicate in AM mode, and when coupled with some RF power amplification he will be able to participate in AM discussion nets on the air with other hams using this classic mode.

Another new and useful feature is included in this release: “CW Spotting Indicator”. When in either CW or CW-R mode, a tiny triangular indicator is placed on the bandscope at the “CW Offset” frequency to assist in zero beating an incoming CW signal for ideal reception. By presenting this indicator, an operator who may not have “good pitch” would be able to tune the received CW signal to be positioned at the CW Offset frequency (which is a setting in the User menu), thus ensuring that the Cube’s transmitted signal will be heard on the receiving side as the same tone he is hearing. That is, the transmit and receive stations will be “zero beat” with each other.

Operational Characteristics

Conventional modulation techniques for transmitting AM usually generate an RF carrier signal and then directly modulate its amplitude. However in the SDR Cube architecture there is no way to bypass the RF I-Q mixer present in the Softrock RF deck, so we first generate a CW tone signal and then amplitude modulate it with the baseband voice signal. This resultant signal is then treated as any other SSB signal in the rest of the signal chain.

   
AM modulation envelope in the frequency domain as seen on the
spectrum analyzer display (left) and in the time domain with
the oscilloscope display (right). A 1 kHz modulation
signal is used in these measurements.

The typical SSB signal chain imperfections (carrier leakage and opposite sideband rejection) are present and tend to affect AM quality. If you have no modulation (no audio input signal) you can see the 1/2 power AM carrier and you would notice any SSB signal imperfections. When modulating you may also notice that the AM "two teeth" spectrum envelope may not be symmetrical and "clean". However for our purposes in the SDR Cube this is an acceptable and even good result.

AM Reception

AM reception is accomplished in SSB mode, by selecting and listening to either the upper sideband of the transmitted AM signal, or to its lower sideband. The spectral information is the same and the operator can decide which sideband to listen to. This is a unique and useful feature in that one can avoid interference and noise signals by selecting the quieter sideband. No ordinary AM receiver can do this. Hence we use AM▲ (upper) and AM▼ (lower) symbols for the Mode indicators on the Cube display. In receive, the Cube’s bandscope shows the AM signal at its original place around zero (carrier) frequency.

AM Transmission

AM transmission always occurs at the dial frequency. With I-Q modulator, the AM carrier generation is somewhat unconventional. First we generate an audio signal (a CW tone) that is amplitude modulated with the baseband audio signal, and then the AM carrier offset is added . This signal is then fed to the SSB modulator where the carrier frequency is set “CW offset” amount off so that the AM carrier appears at the dial frequency. The easiest way to verify this is to transmit AM without modulation so just the carrier frequency peak would be visible in a spectrum analyzer.

Adjustment

The default settings present in the software are likely sufficient for use of AM more "out of the box", but they may also be tweaked as indicated below:

1) Adjust Cube SSB TX to "perfect" opposite sideband attenuation.

2) Adjust the AM envelope so that the signal envelope remains sine shape. With Cube hardware it is not possible to get clean modulation with 100% modulation index. The most negative audio signal is not exactly zero carrier.

An oscilloscope is helpful when doing the AM gain adjustment. Default recommendations are given for different bands (TXPA modules). The EEPROM default value is 128 which is about 50% of the adjustment range. This value is quite good for the 40-meter TXPA module. For the 80-meter TXPA module, full gain (256) is needed for good signal.

 
Optimal gain (163) shown in two different time scales

Too much gain                                                                                                    Too little gain

AM Modulation Symmetry

An HP8902A receiver is used to measure Cube AM modulation symmetry.

 
Positive envelope (50.2%)                                                                                     Negative envelope (56.7%)

Symmetry is good when these two measurements are as close as shown above.  It is quite instructive to see about 700mW of carrier power compared with the 1.5W total power (sidebands and harmonics included). This illustrates how "inefficient" the old AM modulation is.