IF exciter for radio transmitter

Details IF exciter for radio transmitter IF exciter for radio transmitter

1998-06-17

2001-11-13

Pham, Chi (Department: 2631)

Pulse or digital communications

Systems using alternating or pulsating current

Amplitude modulation

C375S271000

Reexamination Certificate

active

06317468

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention relates to radio frequency transmitters and more particularly to modulating an intermediate frequency of a radio frequency transmitter with an information signal.
BACKGROUND OF THE INVENTION
The use of intermediate frequencies (IF) in radio frequency (RF) transmitters and receivers is known. Typically an IF frequency is of a relatively constant frequency that varies only by a modulation level of an impressed information signal. In the case of a transmitter, the modulated IF frequency is shifted upwards to a final channel frequency by mixing the IF frequency with a variable frequency from a frequency synthesizer.
Philosophically, the greater the difference between the IF frequency and the final channel frequency, the easier it becomes to filter an information signal from other, unwanted artifacts. When an IF frequency is mixed with the final channel frequency, the result is a summation frequency and a difference frequency and multiples thereof (aliases). Most of the information energy lies in the summation and difference signals and typically one (or both) are amplified for transmission to a distant receiver. The aliases represent interference, both to the information signal and to information signals on adjacent channels and, consequently, must be filtered from the information signal before transmission.
When the difference between the IF frequency and final channel frequency is great, the difficulty of filtering the aliases from the information signal becomes much easier. As a consequence, an IF frequency is typically chosen to maximize the frequency difference between the IF frequency and any frequency the synthesizer may assume in transmitting on an assigned channel.
The information signal translated to an IF frequency may be analog or digital. Historically, the information signal has been analog, originating from such sources as microphones. Where the information signal is analog, the technology used for translation of the information signal to the IF frequency has included the use of mixers and filters.
More recent systems have used digital processing. Analog to digital converter (A/D) converters and digital signal processors (DSP) may be used to transform analog signals (e.g., from a microphone) into a digital format. The conversion to a digital format has also allowed computers to be used as sources of an information signal transferred over a radio frequency link.
The availability of digital signal processing has resulted in some simplification of signal processing hardware. For example, techniques have been developed which allow the generation of transmitted waveforms from a digital information source using waveform lookup tables. Multiplexers may be used to combine waveforms from the waveform tables to generate combinations of waveforms to simulate a variety of modulation levels.
Other techniques include interpolating digital signals and delta-sigma modulating an input driving digital to analog (D/A) converters to generate an IF waveform. Bandpass filters may be used to control aliasing of the digitally converted products.
In general, the generation of the IF signal inevitably involves modulation of a source information signal onto an IF reference frequency. IF exciters have been developed which rely on various mixing philosophies and phase change algorithms. While such methods have generally been effective, their effectiveness is dependent upon extensive filtering and the use of analog components having relatively precise tolerances. The introduction of digital techniques have generally not changed these requirements. Because of the importance of radio frequency communications, a need exists for a method of IF excitation that is not dependent upon the precise tolerances of analog signal generation and extensive filtering.
SUMMARY
A method and apparatus are provided for modulating an intermediate frequency of a radio transmitter with a digital sample stream. The method includes the steps of interpolating the digital sample stream by an over sampling factor and modulating the interpolated digital sample stream to a single bit sample stream, while spreading quantization noise created by the modulation outside a bandwidth of the intermediate frequency. The method further includes the steps of converting the single bit sample stream to an analog signal and bandpass filtering the analog signal around the intermediate frequency.
Use of the apparatus and method of the invention allows for the use of analog components with less precise tolerances. For instance, D/A converters with very low bit differentiation (e.g., one bit) becomes practical. Reducing the requirements for bit differentiation enhances integration of such signal processing devices (having N actual bits) into very large scale integrated (VLSI) circuits (having M effective bits) and eliminates the need for high precision “stairstep” M-bit D/A transfer functions (where N<<M).
The invention also allows for the use of analog devices with less precise tolerances. The filter cutoff frequencies of such devices may be reduced. The “brickwall” characteristics of filtering arrays may also be eliminated.
Further, the balance of the in-phase (I) and quadrature (Q) components under the invention may be regarded as substantially perfect. Prior state-of-the-art devices have required calibration to achieve a “good” balance, which is still not comparable to that offered under the invention. Further, calibration is not required under the invention.
Conventional digital modulation techniques require tight control of gain/phase processing to achieve good I, Q balance. This invention facilitates good overall system performance because of the inherent I, Q gain/phase balance. (Note that I, Q gain/phase imbalance acts to increase bit error rate (BER) in digital systems.)


REFERENCES:
patent: 5418818 (1995-05-01), Marchetto et al.
patent: 5627499 (1997-05-01), Gardner
patent: 5701106 (1997-12-01), Pikkarainen et al.
patent: 6111531 (2000-08-01), Farag
A CMOS IF Transceiver with Reduced Analog Copmplexity by Tod Paulus et al.

Affiliated with

Also associated with

Rating

IF exciter for radio transmitter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with IF exciter for radio transmitter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and IF exciter for radio transmitter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2579666