Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-05-12
2002-07-30
Chang, Vivian (Department: 2682)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S328000
Reexamination Certificate
active
06427069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to high-frequency mixers and more specifically, to an odd-subharmonic balanced mixer for sub-millimeter-waves. This new mixer yields improved conversion efficiency, lower output noise, superior cross-port isolation, improved impedance matching, and ease of fabrication at receiver frequencies on the order of several hundred gigahertz.
2. Prior Art
A mixer is a critical component of modern radio-frequency (RF) systems. It is usually the first or second device after the RF antenna input. Various mixer parameters, such as bandwidth and inter-port isolation, must be optimized to produce devices capable of performing in modern RF systems.
In recent years there has been an increase in sub-millimeter-wave receiver applications requiring easily producible and improved performance mixer technology at these extremely high frequencies.
One reason frequency conversion is done is to make signal processing easier and less expensive. Changing the frequency of a signal without altering its information content is necessary because signal processing components, such as amplifiers, are much less expensive and work better when they are designed to operate at lower frequencies.
Another reason for frequency conversion is to allow for the practical transmission of low-frequency information (such as audio information) through space. Transmitting audio frequency (up to 20 KHz) signals directly would require antennas of impractically large size because of the relatively long wavelengths of audio signals. However, if the audio signals are first converted up in frequency to a much higher carrier frequency, antennas of practical size can be utilized. At the receiving end, it is necessary to capture part of the electromagnetic energy of the transmitted signal and reconvert it back down to the audio frequency range in order to extract the original low-frequency information. Thus, both transmission and receiving require an input signal to be converted. Mixers perform this frequency conversion.
Mixing an input RF signal with an LO signal yields frequency products below and above the RF and LO frequencies. Each frequency product corresponds to the sum of the input RF and LO frequencies, while the lower frequency product corresponds to the difference between the input RF and LO frequencies. Usually, it is the lower frequency product (the “down-converted” RF signal) which is used in receiving systems, whereas the high frequency product (the “up-converted” RF signal) which is used in transmission systems. A non-linear element, e.g., diode, is essential for frequency conversion of any sort.
There are basically four types of mixers: each with a different number of diodes: single-ended, single-balanced, double-balanced, and double double-balanced (also called triple-balanced). However, all types are three-port devices and comprise an input port (the RF port), a local oscillator input port (the LO port), and an output port (the IF port). Single-ended mixers are the simplest type and are realized using only a single diode. The LO, RF and IF ports are separated only by filters to provide some degree of inter-port isolation. Single-ended mixers, however, have a narrow bandwidth, limited dynamic range and poor inter-port isolation.
Broader bandwidths and better isolation can be obtained with a single-balanced mixer. A single-balanced mixer consists of two single ended mixers. The mixer diodes are fed by the LO and RF signals through a BALUN which yields inter-port isolation between the LO and RF ports. Double-balanced mixers are so called because they have two BALUNs instead of just one and comprise two single-balanced mixers. This enables inter-port isolation both between the LO and RF ports and between the LO and IF ports. Double-balanced mixers use twice the number of diodes (four) as a singly-balanced mixer and the diodes are usually, although not always, arranged in a diode “quad” ring configuration.
Triple-balanced mixers are so called because, in addition to BALUNs on the RF and LO ports, the IF port is also balanced. A triple-balanced mixer requires twice the number of diodes (eight) as a doubly-balanced mixer and a triple-balanced mixer is usually realized by combining two diode quad ring mixers.
The mechanism by which a diode mixer converts energy from one frequency to another is generally well known. A diode which is pumped by a signal from a local oscillator at a frequency fLO responds as a switch that is closed during the conducting portion of the LO voltage cycle and open during the non-conducting part of the cycle. This switching action modulates an incident RF signal at a frequency fRF and thereby makes available output signals at numerous frequencies different from and in addition to the incident RF and LO signals. The relationship between the various additional output signals is fIF=½kfLO±fRF½, where fIF is the output frequency and k and I are integers (at low level external inputs I=1). If, for example, k=1, the mixer is considered a fundamental mixer.
Harmonic mixers have been utilized in which k>1, hence the principal output is fIF=½mfLO±fRF½ that is, at a particular “m” th harmonic of the LO frequency. These mixers have used filters to block responses corresponding to k
1
m. The main disadvantage to this type of mixer is that the conversion-loss is relatively high because of the limited success of filters to prevent RF signal energy to convert into unintended outputs.
A more recent type of even subharmonic (2n) pumped mixer uses two diodes connected in parallel and opposing polarity, that is, antiparallel (n is any integer). These form a semiconductor switch which is toggled at 2nf
LO
and, consequently, output frequencies fIF are available only as combination of even harmonics of the local oscillator frequency.
As background, existing forms of the two-diode, subharmonic pumped mixers are described in literatures such as:
E. R. Carlson et al, “Subharmonically Pumped Millimeter-Wave Mixers,” Vol. MTT-26, pp. 706-715, October 1978;
M. V. Schneider and W. W. Snell, Jr., “Harmonically Pumped Stripline Down-Converter,” Vol. MTT-23, pp. 271-275, March 1975; and
M. Cohn et al, “Harmonic Mixing With An Antiparallel Diode Pair,” Vol. MTT-23, pp. 667-673, August 1975.
Said “MTT” volumes being the IEEE Transactions on Microwave Theory and Techniques.
An example of a balanced mixer utilizing a subharmonic pumped antiparallel diode pair is disclosed in U.S. Pat. No. 3,983,489, issued Sept. 28, 1976 to Gittinger. This type of mixer, in which transformer coupling is used to isolate the RF and LO ports, can achieve wide bandwidth in the UHF frequency range but is not usable at microwave frequencies due to limitations of the transformer devices used therein.
A search of the prior art has revealed the following U.S. patents which appear pertinent in varying degrees to the present invention:
U.S. Pat. No. 3,311,811 Rupp
U.S. Pat. No. 4,145,692 Armstrong et al
U.S. Pat. No. 4,320,536 Dietrich
U.S. Pat. No. 4,392,255 Del Giudice
U.S. Pat. No. 4,394,632 Hu
U.S. Pat. No. 4,485,488 Houdart
U.S. Pat. No. 4,491,977 Paul
U.S. Pat. No. 4,730,169 Li
U.S. Pat. No. 4,803,419 Roos
U.S. Pat. No. 4,817,200 Tanbakuchi
U.S. Pat. No. 4,994,755 Titus et al
U.S. Pat. No. 5,060,298 Waugh et al
U.S. Pat. No. 5,077,546 Carfi et al
U.S. Pat. No. 5,266,963 Carter
U.S. Pat. No. 5,446,923 Martinson et al
U.S. Pat. No. 5,517,687 Niehenke et al
U.S. Pat. No. 5,553,319 Tanbakuchi
U.S. Pat. No. 5,721,514 Crockett et al
U.S. Pat. No. 5,740,528 Drennen
U.S. Pat. No. 5,819,169 Fudem
U.S. Pat. No. 5,844,939 Scherer et al
Of the aforementioned patents, the following appear to be the most relevant:
U.S. Pat. No. 5,553,319 to Tanbakuchi is directed to a routing YIG tuned mixer. A routing YIG tuned resonating filter integrated with an image enhanced double-balanced mixer is provided. FIG. 10 is a schematic diagram of an image enhanced double-balanced YIG tuned mixer 305 which mixes a fundame
Chang Vivian
Nguyen Duc
Tachner Leonard
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