Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-08-22
2003-11-25
Tran, Pablo N (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S324000, C455S326000, C455S338000, C327S113000, C327S116000, C327S119000
Reexamination Certificate
active
06654595
ABSTRACT:
INTRODUCTION
1. Field of the Invention
The invention relates generally to frequency conversion systems, devices, and methods, and more specifically to radio frequency communication devices and systems including, mixers, radio tuners, transmitter, and receivers incorporating FET mixer type frequency conversion devices for up- and down-frequency conversion.
2. Background of the Invention
Conventional heterodyne receivers down convert a radio-frequency (RF) signal to a baseband signal using one or more intermediate stages in which the RF signal is converted to one or more intermediate-frequency signals, lower than the RF signal, until the base-band frequency is reached. A heterodyne transmitter generates a higher frequency RF signal from a baseband signal using one or more intermediate stages to up-convert the frequency. A transmitter provides both transmit and receive components and function.
In simplified terms, a homodyne receiver directly down-converts radio-frequency (RF) signals to baseband frequency without intermediate stages. Analogously a homodyne transmitter up-converts from base-band to RF without intermediate stages. A radio system (frequency conversion stage, tuner, receiver, transmitter, or transceiver) may include homodyne and heterodyne components. In this disclosure the term system may be used when referring to any or a combination of such stage, tuner, receiver, transmitter, or transceiver, so as to simplify the description.
Conventional homodyne systems may typically have a poor dynamic range, unacceptably high distortions for some applications, and other undesirable characteristics as compared to non-homodyne systems. The poor dynamic range is typically the result of at least two significant factors. First, distortions, including input second order intercept point (IP
2
), input third order intercept point (IP
3
), and so-called “N×N” distortions, cause unwanted spurious responses to fall within the frequency band of interest. Second, amplitude and/or phase imbalances contributed by an imperfect quadrature local oscillator, may cause errors in the in-phase channel (I) or quadrature-phase channel (Q) signals before they are digitized by the analog-to-digital converter (ADC) in the digitizer, resulting in non-linearities in the conversion process. (These quadrature or I/Q channels are sometime referred to as sine and cosine channels or signals as a result of the out-of-phase relationship between the channels and the manner in which they are conventionally generated.) These non-linearities directly or indirectly result in distortion and loss of useful dynamic range.
These conventional homodyne systems may beneficially employ software algorithms, residing in the Digital Signal Processing (DSP) section of the tuner, transmitter and/or receiver down the signal path from the homodyne frequency conversion stage, to compensate for some of the distortions, errors, and other anomalies in the such conventional homodyne systems (especially receivers) with minimal success, but this additional DSP task undesirably requires a higher clock rate than would otherwise be required for a given bandwidth. Wider signal bandwidth may typically need a processor clock rate that is from about 10 times to about 20 times or more the clock rate required without compensation, in order to compensate phase and amplitude errors over the entire receiver bandwidth of interest. The higher clock rate presents additional problems in itself. Digital compensation after digitization reduce the wanted spectrum bandwidth. Without compensation, homodyne receivers or direct conversion receivers employing mixers are limited to around 40 dB of dynamic range and bandwidth in the audio frequency range.
The trend in new radio systems technology receiver/tuner development is predicted to concentrate on moving the RF spectrum down to baseband frequencies where it will be digitized and processed under software control. This will impose even more stringent demands for dynamic range, increased sensitivity, and lower distortion. Reducing size, weight, and power consumption to provide longer operating times under battery power, are also concerns for commercial and non-commercial applications. A key system performance challenge involves keeping the spectrum dynamic range (sensitivity vs. distortion) as high as possible before digitization in the ADC while maintaining high sensitivity and controlling distortion.
An additional problem with conventional wireless (radio) communication systems pertains to frequent requirements for skilled radio operators to initiate and maintain contact between multiple radio stations or transceivers. This problem is particularly acute because of the need to monitor or provide surveillance over a large HF/VHF/UHF frequency spectrum. Both commercial and non-commercial communicators have been working to achieve automatic, reliable and robust communications using the HF/VHF/UHF spectrum, particularly the HF spectrum. One goal of this work has been an attempt to eliminate or reduce the need for highly skilled radio operators while simultaneously increasing the reliability of the HF spectrum as a communication medium.
Automatic Link Establishment (ALE), also known as Adaptive HF, is an integral part of this effort. ALE is defined as the capability of an HF radio station to make contact between itself and another station or stations under automatic processor control. ALE techniques include automatic signaling, selective calling, and automatic handshaking at different bands in the HF spectrum. Monitoring and following all these activities requires a near simultaneous full band HF receiver. Digitizing the entire HF frequency band, and handling ALE protocol with Digital Signal Processing (DSP) presents many challenges. For example, if the monitoring sites are not ideal in location, dynamic range, resulting from near by transmitters masking far away ALE signals, presents a problem. It has been estimated that an adaptive HF monitoring solution requires full simultaneous HF coverage with 100 dB of Spur Free Dynamic Range (SFDR). The cost for implementing and deploying such ALE systems also remains problematic.
This and other performance challenges have been addressed in part by the development of analog-to-digital converters (ADCs) which have increased resolution (sensitivity), increased Spur Free Dynamic Range (SFDR), and greater baseband spectral bandwidth. ADCs having 14-bit resolution and 30 MHz baseband bandwidth, and which can be clocked out at 65 mega samples per second (MSPS), with a projected SFDR of 85-90 dB or more are available and narrower bandwidth ADCs (for example, bandwidths less than about 10 Mhz) and providing 16-bit resolution at an even greater 95-100 dB SFDR are under development. These devices provide the needed ADC performance improvement over earlier 12-bit ADCs. Even though higher-performance ADCs have been developed, other problems remain.
Frequency conversion or mixer stages in conventional RF systems have heretofore been unable to attain the approximately 85-100 dB Spur-Free Dynamic Range required in certain tuner/receiver systems, particularly where the output of that mixer stage was intended as the input to high performance Analog-to-Digital Converters (ADCs) where the 100 dB SPRD, is required at the input. In fact such systems have been limited to substantially lower performance. The last or final mixer stage just prior to output to the ADC (baseband frequency converter stage) typically has the highest signal amplitude level in the tuner. A state-of-the-art ADC requires about a 2 volt peak-to-peak signal for full ADC conversion scale, and should have all spurious signal products down by about 100 dB in order to utilize the capabilities of the ADC without introducing other undesirable artifacts. These ADC performance specifications correspond to a baseband spectrum mixer stage coupled to the ADC input terminals having an input third order intercept point (IP
3
) of about +50 dBm and an input second order intercept point (IP
2
) of about +100 dBm.
Another
Ananian R. Michael
Dorsey & Whitney LLP
Signia-IDT, Inc.
Tran Pablo N
LandOfFree
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