Digital transmitter system and method
View Patent ↗A digital transmitter converts a digital signal into analog form with a digital to analog converter (DAC) and uses an analog signal image produced from the DAC to provide an analog signal at a transmission frequency and/or uses a projected analog signal image to produce analog signals for transmission. Rather than removing analog signal images with a low pass filter at the output of the DAC and/or using analog signal images and analog mixers for frequency conversion, the digital transmitter uses the analog signal images from the DAC to produce the analog signals at the desired frequency/frequencies. By setting and/or adjusting the conversion rate for the DAC and/or the digital signal frequency/frequencies, the analog signal images produced from the DAC can be positioned in the desired frequency band(s). For example, the digital transmitter can position the digital signals within non-overlapping portions of a conversion bandwidth defined as one-half the conversion rate for the DAC. When the digital signals are converted into analog form, the DAC produces analog signal images periodically repeated at multiples of one-half the conversion rate such that analog signal images are produced at the appropriate frequency band(s) for amplification and transmission.
1. A method of processing digital signals to be transmitted in analog form, said method comprising:
in a digital-to-analog converter (DAC) having a conversion frequency, converting a digital signal having an input frequency to produce an analog signal image output at a radio frequency greater than the input frequency, CHARACTERISED IN THAT:
a) said digital signal is one of a plurality of digital input signals each of said digital input signals having an input frequency, and the input frequencies are positioned within non-overlapping portions of a frequency band whose width is one-half the conversion frequency;
b) the analog signal image output of the DAC includes a plurality of images of the frequency band, and said plurality of images are grouped in successive non-overlapping frequency bands referred to as zones such that each zone contains one image of each of the plurality of digital input signals; and
c) the method further comprises selecting digital input signal images for transmission, wherein at least two digital input signal images are selected from different zones and correspond to different ones of the plurality of digital input signals.
2. The method of claim 1 , further comprising:
directing each selected digital input signal image onto a respective path;
amplifying each said selected digital input signal image on said respective path; and
transmitting each amplified selected digital input image using at least one respective antenna.
3. The method of claim 1 further comprising:
adjusting the conversion frequency so as to produce at leas one digital input signal image at a transmission frequency.
4. The method of claim 1 further comprising:
adjusting at least one of the plurality of digital input frequencies so as to produce at least one digital input signal image at a transmission frequency.
5. A transmitter comprising:
a digital to analog converter (DAC) having a conversion frequency and configured to receive a plurality of digital input signals, each having a different input frequency, and to convert each digital input signal of said plurality of digital input signals into analog form, thereby to produce corresponding digital input signal images as analog signals at different radio frequencies, each of which is greater than its corresponding input frequency;
CHARACTERISED IN THAT the transmitter further comprises:
signal processing circuitry configured to receive said plurality of digital input signals and to condition said digital input signals for input to the DAC by positioning their respective input frequencies in non-overlapping portions of a frequency band whose width is one-half the conversion frequency; and
transmitter circuitry configured to select certain of said digital input signal images for transmission, wherein said digital input signal images are grouped in a plurality of non-overlapping successive frequency bands referred to as zones, such that each zone contains one digital input signal image of each digital input signal, and the transmitter circuitry is configured to select from different zones at least two digital input signal images that correspond to different ones of the plurality of digital input signals.
6. The transmitter of claim 5 wherein said transmitter circuitry comprises:
signal distribution circuitry configured to direct a first selected digital input signal image of a first frequency band on a first path and a second selected digital input signal image of a second frequency band on a second path;
a first amplifier on said first path for amplifying said first selected digital input signal image on said first path;
a second amplifier on said second path for amplifying said second selected digital input signal image on said second path;
a first antenna connected to said first path for transmitting said first selected digital input signal image after amplification; and
a second antenna connected to said second path for transmitting said second selected digital input signal image after amplification.