Telecommunications – Transmitter and receiver at same station – With frequency stabilization
Reexamination Certificate
2001-12-10
2004-02-17
Trinh, Sonny (Department: 2685)
Telecommunications
Transmitter and receiver at same station
With frequency stabilization
C455S078000, C455S088000, C455S208000
Reexamination Certificate
active
06694129
ABSTRACT:
BACKGROUND
1. Field
This invention relates in general to wireless communications. Specifically, this invention relates to systems and methods for direct conversion transceivers.
2. General Background and Related Art
The field of communications has experienced a tremendous growth due in large part to the improved capabilities of wireless devices. Wireless devices employ radio waves to enable distant communications without the physical constraints of wire-based systems. Information, such as voice, data, or paging information, is conveyed by radio waves transmitted over predetermined frequency bands. Allocation of available frequency spectra is regulated to ensure that numerous users may communicate without undue interference.
Information to be transmitted from a source to a destination is seldom acquired in a format that is ready for radio transmission. Typically, a transmitter takes an input signal and formats it for transmission in a predetermined frequency band. The input signal, also referred to as a baseband signal, modulates a carrier in the desired frequency band. For example, a radio transmitter that receives an audio input signal modulates a carrier frequency with the input signal.
A corresponding remote receiver tuned to the same carrier frequency as the transmitter must receive and demodulate the transmitted signal. That is, the remote receiver must recover the baseband signal from the modulated carrier. The baseband signal may be directly presented to a user or may be further processed prior to being presented to the user. Many consumer wireless devices, such as radios, televisions, and pagers, are solely receivers.
Transceivers are wireless devices that integrate a transmitter and receiver in a single package. Transceivers enable nearly instantaneous two-way communications. Examples of transceivers include two-way radios, walkie-talkies, two-way pagers, and wireless phones.
Several figures-of-merit are important in assessing the effectiveness of a receiver design. Sensitivity determines the ability of a receiver to detect a weak signal. Receiver sensitivity must be such that the receiver can detect the minimal discernible signal (MDS) from background noise. Noise represents random fluctuations in voltage and current. The MDS is a receiver-specific measure of sensitivity that incorporates the bandwidth of a given system. Receiver selectivity, on the other hand, indicates the protection afforded a receiver from off-channel interference. The greater the selectivity, the better the receiver can reject unwanted signals.
Desense is a reduction in a receiver's overall sensitivity due to man-made or natural radio frequency interference (RFI). Desense occurs when a very strong interfering signal overloads the receiver and makes the detection of weaker signals more difficult. The desensitization characteristic of the receiver determines its ability to operate successfully under strong interferors, such as jammers.
The noise figure is another key measure of a receiver's performance. The noise figure degrades, that is, increases, at each successive stage in the receive path. Amplification or attenuation techniques may be applied within a receiver to achieve an acceptable noise figure. Noise, along with distortion, determines signal to noise and distortion (SINAD), a ratio in decibels which describes a receiver's performance in the presence of noise.
Distortion is the presence of unwanted signals at the output of devices in the RF path of a receiver. Distortion may include harmonic distortion, intermodulation distortion, and cross-modulation distortion. Harmonic distortion occurs when the desired input signal is large enough to compress the receiver and is typically measured at the baseband output as a function of frequency offset from the desired signal and as a function of the desired signal power. Crossover distortion occurs when the amplitude-modulated component from the transmitter (e.g., a CDMA wireless phone) is transferred to another carrier (jammer) at the output of the device (LNA output). The most common form of distortion is intermodulation distortion (IMD).
Intermodulation distortion is the result of two or more signals mixing together to produce additional unwanted distortion within the signal bandwidth. For two inputs, the intermodulation products occur at the sum and difference of integer multiples of the original frequencies. That is, for two input signals having frequencies f
1
and f
2
, the output frequency components can be expressed as mf
1
±nf
2
, where m and n are integers ≧1. The order of the intermodulation product is the sum of m and n. “Two tone” third order components (2f
1
−f
2
and 2f
2
−f
1
) can occur at frequencies near the desired or interfering signals and thus cannot be easily filtered. Higher order intermodulation products have lower amplitude; as such, they are less problematic. Second order intermodulation jamming products may be generated at baseband frequencies if the tone spacing is within half of the signal bandwidth.
FIG. 1
is a graph plotting the levels of fundamental, second order, and third order IMD components against input level. Theoretical points where the second order and third order levels intercept the fundamental are known as the second order intercept point (IP2 or SOI) and third order intercept point (IP3 or TOI). The IIP2 of a receiver is the input level second order intercept point. The IIP3 is the input level third order intercept point.
The third order intercept point and noise figure of a receiver are directly related to the receiver's dynamic range. The dynamic range defines the range of signals that the receiver can handle within the specified performance of the receiver, that is, the range over which the receiver can produce an accurate output with acceptable SINAD. Specifically, for a baseband receiver, such as an analog-to-digital converter, the dynamic range may be represented as spurious free dynamic range (SFDR), which ranges from the noise floor of the device to the maximum signal before clipping occurs.
Local oscillator (LO) leakage occurs when an LO signal leaks to the receiver input. Such leakage may be transmitted by the transceiver antenna as spurious emissions, which may interfere with other devices. In addition, LO leakage may be reflected back into the receiver itself and may desense the receiver if not removed prior to demodulation.
Jammer leakage occurs when a jammer signal leaks to an LO input or output of a device within a receiver. Such leakage may mix with the jammer signal to produce undesired signals, such as DC signal levels that are proportional to the amplitude modulation (AM) component of the jammer signal. AM jammer signals may be located at any frequency within a receive frequency band.
Low-frequency flicker (l/f) noise is caused by defects in the emitter-base junction of bipolar junction transistors. Although typically small, flicker noise and other such noise may need to be removed in a receiver in order to maintain signal integrity at baseband.
Isolation is the ratio (in dB) of the power level applied at one port of a device to the resulting power level at the same frequency appearing at another port. Reverse isolation, which is the inverse (reciprocal) of isolation, is a figure-of-merit for receiver components. Reverse isolation is a measure of how much energy injected into an output port makes it back into the input source. To achieve low LO and jammer leakage, high reverse isolation is desired.
The 1 dB compression point of an amplifier is a measure of the output power level when the amplifier gain is 1 dB lower than the small signal gain. The saturation point of an amplifier is a measure of the maximum output power capability of the amplifier. These figures-of-merit are illustrated in FIG.
1
.
The above figures-of-merit and signal phenomena should be considered when designing wireless communication devices. More generally, the wireless communications landscape has been dominated by Code Division Multiple Access (CDMA), a form of spread
Peterzell Paul E.
Sahota Gurkanwal
Brown Charles D.
Cheatham Kevin T.
Qualcomm Incorporated
Trinh Sonny
Wadsworth Philip R.
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