Radio-frequency transmitter

A method comprising:

• converting, by a digital-to-analog converter (DAC) based on a clock rate (fs) a digital signal to an analog signal comprising frequency content from signal images in more than one Nyquist region;
• filtering out, by a bandpass filter configured to remove, frequencies within a lowest Nyquist region to generate an intermediate-frequency (IF), wherein the lowest Nyquist region comprises frequencies between zero and one-half of fs;
• up-converting, by a mixer based on a local oscillator (LO) signal, the IF signal from an output of the DAC to a radio-frequency (RF) signal;
• filtering the RF signal by at least removing, from the RF signal, frequency components greater than a difference between a frequency of the LO signal and one-half of fs and less than a sum of a frequency of the LO signal and one-half of fs, wherein the filtered signal comprises frequency components greater than the sum of the frequency of the LO signal and one-half of fs;
• and
• transmitting the filtered RF signal.

A system comprising:

• a field-programmable gate array (FPGA) configured to encode a surveillance message in a digital signal;
• a digital-to-analog converter (DAC) configured to convert the digital signal to an analog signal, comprising frequency content from signal images in more than one Nyquist region, based on a clock rate (fs);
• a bandpass filter configured to remove, frequencies in a lowest Nyquist region from the analog signal to generate an intermediate-frequency (IF) signal, wherein the lowest Nyquist region comprises frequencies between zero and one-half of fs;
• a fixed-frequency local oscillator (LO) circuit configured to generate an LO signal;
• a mixer configured to up-convert the IF signal to a radio-frequency (RF) signal based on the LO signal;
• and
• a RF filter configured to generate a filtered signal by at least removing, from the RF signal, frequency components greater than a difference between a frequency of the LO signal and one-half of fs and less than a sum of a frequency of the LO signal and one-half of fs, wherein the filtered signal comprises frequency components greater than the sum of the frequency of the LO signal and one-half of fs.