Abstract:
A monolithic microwave integrated circuit (MMIC) downconverter for a direct broadcast satellite low noise block downconverter is comprised of a mixer that produces sum and difference frequency signals from an oscillator signal and a radio frequency (RF) signal. An intermediate frequency (IF) signal is obtained by selecting the difference signal of the sum and difference frequency signals using a low pass filter. An active load and amplifier circuit coupled to the low pass filter both provides a DC bias voltage to the mixer that is substantially insensitive to DC current and amplifies the IF signal from the low pass filter. The combined active load and amplifier circuit conserves DC power and reduces the MMiC circuit size, thus reducing the Gallium Arsenide area and therefore, the component cost.

Description:
FIELD OF THE INVENTION 
     This invention relates to Monolithic Microwave Integrated Circuit (MMIC) downconverters used in Low Noise Block Downconverters for Direct Broadcast Satellite (DBS) television transmissions. 
     BACKGROUND OF THE INVENTION 
     Direct Broadcast Satellite television transmission receivers comprise a Low Noise Block Downconverter (LNB) mounted on an antenna or satellite dish connected, via coaxial cable, to an indoor tuner attached to a television set or VCR. The LNB is connected to the antenna or dish and converts the satellite signal received by the antenna to a frequency and signal level suitable for processing by the tuner and television or VCR electronics. 
     The Low Noise Block Downconverter assembly typically includes a monolithic microwave integrated circuit (MMIC) downconverter mounted on a printed circuit board along with support circuitry, additional amplifier stages and filters to provide increased amplification and reduced front end noise. Typically, the LNB Downconverter receives microwave frequencies of approximately 11 GHz to 12 GHz and first amplifies the signals through several high electron mobility transistors (HEMTs) and two amplifier stages in the MMIC. The MMIC also downconverts the RF signals to an intermediate frequency (IF) of approximately 1000 MHz in a mixer, then amplifies the IF signal. 
     The Direct Broadcast Satellite television market is a large consumer market with sales of many millions of units each year. The market is price sensitive with competitive devices offered by several companies. Reduction of the LNB Downconverter product price is important for a manufacturer to remain competitive in the market. Product price reduction can be achieved through reduction of the component costs, especially the MMIC. 
     One design approach, as shown in FIG. 3, is to bias the mixer through a voltage dropping resistor that is separate from the amplification circuitry as discussed in the paper &#34;A High-Performance, Miniaturized X-Band Active Mixer for DBS Receiver Application with On-Chip IF Noise Filter&#34; published in the IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 9, dated September 1990. This approach yields a mixer bias voltage that is sensitive to DC bias current and additional power dissipation in the bias resistor. 
     Another design, as shown in FIG. 4, utilizes an active load to bias the mixer which provides a stable mixer bias voltage. The active load circuit does not act as an intermediate frequency (IF) amplifier, requiring additional circuitry and increased power dissipation. 
     Another configuration, as illustrated in FIG. 5, connects the local oscillator amplifier in parallel with the mixer so that the sources and drains are common. There is no known teaching of how to bias the mixer. Assuming the bias is provided through the IF filter, there is no suggestion that the bias voltage is provided by the IF amplifier circuit. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention allows a reduction in the cost of the MMIC by skillfully combining the active load and amplifier circuits. The resulting improvement is a MMIC downconverter that simultaneously provides a stable mixer bias voltage and intermediate frequency amplification. By integrating the implementation of the active load and amplifier circuits, the device is smaller and more power efficient than competitive devices. 
     An embodiment of the invention is a monolithic microwave integrated circuit (MMIC) downconverter for a direct broadcast satellite low noise block downconverter. The MMIC comprises a mixer circuit that combines a local oscillator signal with a radio frequency input to produce sum and difference frequency signals. An intermediate frequency signal (the difference signal) is selected by a low pass filter coupled to the mixer circuit. An active load and amplifier circuit provides the mixer circuit with a DC bias voltage that is substantially insensitive to DC current. The active load and amplifier circuit simultaneously amplifies the intermediate frequency signal from the low pass filter for an output signal of the MMiC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a Low Noise Block Downconverter system and embodies the present invention; 
     FIG. 2 depicts the portion of the monolithic microwave integrated circuit at 23 in FIG. 1 and embodies the present invention; and 
     FIGS. 3, 4 and 5 are schematic drawings of prior art circuits. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a block diagram of a Low Noise Block (LNB) Downconverter 1 for use in a Direct Broadcast Satellite (DBS) receiver system. The radio frequency (RF) input is received by an antenna or dish (not shown) from a satellite transmission. The RF input may be horizontally polarized, RF IN (Horizontal) 2, or vertically polarized, RF IN (Vertical) 4, depending on the broadcast channel to which the television is tuned. The LNB Downconverter 1 is mounted on the antenna. It converts the received RF signal frequency from approximately 11 GHz to 12 GHz to an intermediate frequency of approximately 1000 MHz and amplifies the signal for processing by receiver electronics (not shown) inside, for example, the home. 
     The RF IN (Horizontal) 2 or RF IN (Vertical) 4 are received by high electron mobility transistors (HEMTs), HEMT 10 and HEMT 12 respectively, for amplification of the signal. The HEMTs provide high frequency, low noise amplification of the RF signals. The amplified RF IN (Horizontal) 2 or RF IN (Vertical) 4 signal is further amplified by HEMT 14. The amplified RF signal is filtered by the BPF 16, an image-reject band pass filter, to remove unwanted signals from other sources or noise produced by the HEMTs outside of the RF signal frequency band. The output, RF IN 24, is input to the MMIC 22 for downconversion to an intermediate frequency (IF) signal, IF OUT 8, for processing by the receiver electronics. 
     The MMIC 22 receives the RF IN 24 and amplifies the signal in the RF AMP 26. The amplified RF signal is filtered in the BPF 28, an image-reject band pass filter, to reduce noise produced by the RF AMP 26 in the image band. The filtered RF signal, RF 30, is input to the mixer 32 where it is combined with local oscillator signal, LO 38, to produce sum and difference frequency signals of the RF 30 and LO 38 signals. The LPF 42, a low pass filter, selects an intermediate frequency signal, which is the difference signal, and provides it to IF amplifier, IF AMP 44. Amplified intermediate frequency signal, IF 40, is further amplified in four additional amplifier stages with the MMIC 22 comprising IF AMP 76, IF AMP 78, IF AMP 80, and IF AMP 82. The amplified signal, IF OUT 8, is then output to a television or VCR receiver electronics for processing. 
     FIG. 2 illustrates portion 23 of the MMIC 22 and is referenced in the following discussion. 
     The mixer 32 is a single gate field effect transistor (FET). The RF signal 30, having a frequency range by way of example of 10.7 GHz to 11.8 GHz, is input to the gate, g, of mixer FET 32. The local oscillator (LO) signal 36, having a frequency by way of example of 9.75 GHz, is connected to the gate, g, of FET 34 which produces an amplified local oscillator (LO) signal, 38, at the drain, d, of the mixer FET 32 at 64. The sources of FETs 32 and 34 are connected to ground. In this configuration, the nonlinear drain resistance of FET 32 is used as the primary mixing element. A spectrum of frequencies from the sum and difference of the RF signal 30 and LO signal 38 and harmonics are generated at the output of the mixer 32 at 64. 
     The low pass filter, LPF 42, is implemented using a series inductor 46 and shunt capacitor 48. The sum and difference signals from the mixer 32 are received by the inductor 46 of the low pass filter 42. The filter 42 attenuates all but the difference frequencies to provide an intermediate frequency signal by way of example of 950 MHz to 2050 MHz. 
     The IF amplifier 44 amplifies the intermediate frequency signal and operates as an active load for the mixer. The FET 62 is configured as a common gate amplifier with its gate, g, grounded through capacitor 50. The IF signal from the low pass filter 42 is received on the source, s, of the FET 62. FET 60 operates as an active load for the amplifier FET 62. Capacitor 58 and resistor 56 connected between power source VDD 6 and the gate, g, of FET 62 serve to compensate the frequency response of the active load circuit as described in the article WIDEBAND HIGH GAIN SMALL SIZE MONOLITHIC GaAs FET AMPLIFIERS by Vlad Pauker and Michel Binet (CH1875-4/83/0000-0081, 1983 IEEE). The amplified IF signal, IF 40, is output from the drain of the FET 62 and applied to the four cascaded amplifier stages IF AMP 76, IF AMP 78, IF AMP 80, and IF AMP 82 for additional amplification. The resulting signal, IF OUT 8, is applied to the output terminal of the MMIC 22. 
     The IF amplifier 44 also establishes a bias voltage on the drain of the mixer FET 32. The voltage by way of example of 1.0 volt was experimentally determined to be an optimum for mixer operation. A voltage divider comprised of resistor 52, resistor 54, and resistor 56 serially connected between supply voltage VDD 6 and ground receives the bias supply voltage, VDD 6, from the input of the MMIC 22 at 74 and establishes a bias voltage, VG, relative to ground on the gate, g, of FET 62 at the junction of resistors 52 and 54 determined by the equation R52/(R52+R54+R56)*VDD 6, which is by way of example 0.7 volt. The gate, g, to source, s, voltage, VGS, of the FET 62 varies with the drain to source current, IDS, through the FET 62, however, the variation is relatively small. The voltage, VS, on the source, s, of the FET 62 is equal to the sum of the gate voltage, VG, and the magnitude of the gate to source voltage, VGS, or VG+.linevert split.VGS.linevert split.. Since VGS is by way of example -0.3 volt, the source voltage, VS, of the FET 62 is approximately 1.0 volt. The source, s, of the FET 62 is connected to the drain, d, of the mixer FET 32. The source voltage, VS, is therefore the bias voltage of the mixer FET 32 and is substantially insensitive to DC current. Since the DC current of the mixer 32 varies when the local oscillator signal, LO 38, power is applied and also due to variations in the implementation process, it is necessary for the active load and amplifier 44 to supply the proper bias voltage with low sensitivity to the DC current. 
     As previously described, the amplified local oscillator signal, LO 38, having a frequency of preferably 9.75 GHz is applied to the drain of the mixer FET 32 to be combined with the RF signal 30. The LO signal 38 is generated by a local oscillator 36 and amplified by the LO amplifier FET, LO AMP 34, in the MMIC 22. A dielectric resonator 18, external to the MMIC 22, is coupled to the local oscillator 36 via a transmission line 35 as shown in FIG. 1. The dielectric resonator 18 stabilizes the local oscillator 36 to a fixed frequency of preferably 9.75 GHz. The 50 ohm termination resistor 20 prevents unwanted oscillations at other frequencies. 
     The MMIC 22 of FIG. 1 is fabricated from Gallium Arsenide, GaAs, and assembled into a single package with input terminals 22a for the radio frequency signal RF IN 24, 22b for the bias supply VDD 6, 22c for the input to the local oscillator, 22d for the intermediate frequency output signal IF OUT 8, and four ground terminals (not shown, but depicted by ground symbols). The bias supply input terminal is connected to the active load and amplifier circuit IF AMP 44, the RF AMP 26, the LO 36, and the IF signal amplifiers IF AMP 76, IF AMP 78, IF AMP 80, and IF AMP 82 for supplying the DC bias voltage. 
     While the invention has been described and an embodiment disclosed, it is anticipated that other modifications and adaptations will occur to those skilled in the art without departing from the scope of the invention. It is intended therefore that the invention be limited only by the claims appended hereto. The frequency, voltage and other values given in the disclosure are by way of example and may be varied within the scope of the invention. Although the word connected is used to refer to the connection of two components, when a connection is recited there may be one or more components in between those connected.