MONOLITHIC DISTRIBUTED MIXER

Abstract: A monolithic distributed mixer 10 includes a plurality of dual gate field effect transistors (FETs) 32, 38, 44 and 50 and first 18 and second 18′, 26 and 20′, 22′, 24 and 26′, correspond­ ing pluralities of T shaped constant K-filters 20, 22, 24 con­ nected respectively to first and second gates of the dual gate FETs 32, 38, 44 and 50 for displaying in time the LO and RF voltages applied thereto whereby the phase shift is the same at each filter and the phase difference between the LO and RF at each FET 32, 38, 44 and 50 is equal. Thus, the IF energy developed at each drain is in phase. The plurality of dual gate FETs 32, 38, 44 and 50 have their drains commonly connected for summing the IF outputs thereof to provide a flat response throughout the bandwidth which is limited only by the cutoff voltages of the FETs or first and second pluralities of K-filters which act as low pass filters.

Description: The present invention relates to a monolithic distributed mixer and more particularly to a microwave mixer employing a traveling wave structure and processing a local oscillator signal and a radio frequency signal to produce an intermediate frequency signal and output it via an intermediate frequency output and comprising a plurality of field effect transistors and lumped element impedance means coupled to said plurality of field effect transistors for holding the input impedance constant over a bandwith limited only by the cutoff frequency of the structure. In the past mixer techniques have included single or dual gate field effect transistors (FETs) used in numerous types of configurations ranging from a single ended to dual balanced rings. Nevertheless, none of the techniques have achieved broadband performance, and this includes circuits employing external balance and multiple devices. The use of balance provides the best performance of all, but they cannot be implemented using monolithic circuit fabrication methods. There are also other circuits constraints limiting the performance of single or dual gate FET mixers. A single gate FET mixer is, for example, described in "A Microwave Distributed MESFET Mixer" (14th European Microwave Conference, September 1984, A. Tang et al). The microwave mixer disclosed therein comprises a lumped impedance means and a plurality of single gate FETs connected to the lumped impedance means for holding the input impedance constant over a bandwidth limited only by the cutter frequency of the structure. With single gate devices, however, it is difficult, even with balance, to inject the required local oscillator (LO) energy needed for operation. Broad band performance is also difficult to achieve owing to the wide impedance variations encountered at the gate and drain of any FET. Accordingly, it is an object of the invention to provide a monolithic mixer having improved impedance matching characteristics. This object is achieved in that said plurality of FETs are dual gate FETs having first and second gates, a source and drain, said lumped element impedance means includes a first lumped impedance means having its input coupled to said LO signal and its outputs coupled to the respective first gate of said FETs, and a second lumped impedance means having its input coupled to said RF signal and its outputs coupled to the respective second gate of said FETs. Briefly stated the invention comprises a microwave mixer which employs a traveling wave structure to alleviate any bandwidth and impedance matching problems. The traveling wave structure includes lumped element transmission lines connected to a dual gate type active elements formed in a common substrate. Although distributed amplifiers require transmission lines structures on the drain sides of the FETs, this requirement is eliminated in the traveling wave mixer by correctly designing the input transmission lines. If the phase shift, as a function of frequency, is the same at each corresponding section of transmission line, the phase difference between f1 and f2 at each active device is equal. Thus, the IF energy developed at each drain will be in phase and hence can be summed with a simple impedance matching network. With the microwave mixer, operation over a very large bandwidth is possible. Thus a variety of receiver functions for EW systems are made possible on a single chip, such as, for example, phase tracking RF converters and broadband upconverters. Other features of the invention will become more readily apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: Figure 1 is a schematic view of the microwave mixer constituting the subject matter of the invention; Figure 2 is a chart showing the conversion gain of the microwave mixer of Figure 1; and Figures 3a and 3b are charts showing, respectively, the VSWRs for the RF and LO ports of the microwave mixer of Figure 1. Referring now to Figure 1, the microwave mixer 10 includes a LO circuit and a RF circuit operatively connected to a mixer for combining the incoming RF signal with the LO signal to produce an IF signal. The LO circuit includes a coaxial cable connector 12 for connection to a LO (not shown) for receiving LO signals at a preselected frequency f1. A coupling capacitor 14 has a first plate connected to LO connector 12 and a second plate connected to the junction of a dc power supply potentiometer 16 and a lumped element transmission line 18. The lumped element transmission line 18 includes a plurality of series connected T shaped constant K filter circuits 20, 22, 24 and 26 for delaying in time the application of the LO voltages at each dual gate FET. The K filter circuit 20 includes an inductor 28 having first terminal connected to the junction of the second plate of the coupling capacitor and dc power supply potentiometer 16 and a second terminal connected to the junction of a first terminal of an inductor 30 and the junction of a first gate of a dual gate FET 32 and ground capacitor 34. Inductor 30 has its second terminal connected to the junction of the first terminal of inductor 36 and junction of a first gate of a dual gate FET 38 and ground capacitor 40. Similarly, the second terminal of inductor 36 is connected to the junction of the first terminal of inductor 42 while the second terminal of inductor 42 is connected to the junction of the first terminal of inductor 48 and junction of a first gate of FET 50 and ground capacitor 52. Finally, the second terminal of inductor 48 is connected to a capacitor 54 and the capacitor 54 is connected to ground through resistor 56. The RF circuit is identical in construction to that of the LO circuit except that the T shaped constant K-filters 20', 22', 24' and 26' are connected to the second gates of dual gate FETs 32, 38, 44 and 50, and the coaxial input terminal is connected to an RF source. The sources of dual gate FETs 32, 38, 44 and 50 are connected to ground through RC circuits 58, 60, 62 and 64; while their drains are commonly connected to the junction of the first plate of a capacitor 66 and first terminal of inductor 68. The second terminal of inductor 68 is connected to the junction of a grounded capacitor and first terminal of a resistor 72. Resistor 72 has its second terminal connected to the dc power supply. The second plate of capacitor 66 is connected to the junction of a grounded variable capacitor 74 and IF coaxial cable output terminal 76. By employing the above described distributed structure having a lumped equivalent transmission line coupled to several active devices the input impedance can be held constant over very large bandwidths until the cutoff frequency of the structure is reached. The transmission line network is in part realized by employing the input capacitance of the active devices as the shunt reactance. When the active devices are dual gate FETs a similar structure is used to couple energy to a second gate thereof to establish a second broadband port. Either port can be used for LO (f1) injection with the remaining port used for RF (f2). In operation, the phase shift, as a function of frequency, is the same at each section 20, 22, 24 and 26 of transmission lines 18 and 18' and the phase difference between the LO frequency (f1) and the RF (f2) at each active device (dual gate FETs 32, 38, 44 and 50) are equal. Thus, the IF energy developed at each drain is in phase and can be summed with a simple impedance matching network such as, for example, a quarter wavelength matching transformer. A lumped element frequency scaled design of the microwave mixer (Fig. 1) has been constructed and tested. The circuit (Fig. 1) exhibited excellent bandwidth, conversion gain, and low input VSWR. As calculated, the frequency response was limited by the cutoff frequency of each dual gate FET and the cut off (corner) frequency of the lumped element transmission lines (equivalent to a low pass filter). The conversion gain and VSWR are shown in Figures 2 and 3.

Claims: A microwave mixer employing a traveling wave structure and processing a local oscillator signal (LO signal) and a radio frequency signal (RF signal) to produce an intermediate frequency signal (IF signal) and output it via an IF output and comprising a plurality of field effect transistors (FETs) and lumped element impedance means coupled to said plurality of FETs for holding the input impedance constant over a bandwith limited only by the cutoff frequency of the structure, characterized in that said plurality of FETs are dual gate FETS having first and second gates, a source and drain, said lumped element impedance means includes a first lumped impedance means having its input coupled to said LO signal and its outputs coupled to the the respective first gate of said FETs, and a second lumped impedance means having its input coupled to said RF signal and its outputs coupled to the respective second gate of said FETs. The mixer of claim 1 further characterized in that the drains are connected in common to the IF output via a simple impedance matching network. The mixer of claim 1 further characterized in that the the sources are connected through RC networks to ground. The mixer of claims 1, 2 and 3 further characterized in that the first and second lumped impedance means each comprise a plurality of T shaped constant K-filters. The mixer according to claims 1-4 further characterized in that the circuit elements of said mixer are integrated in a monolithic circuit. The mixer according to claims 1-5 further characterized in that the monolithic circuit is of GaAs.

Inventors: PAVIO, ANTHONY M.
Applicants: TEXAS INSTRUMENTS INCORPORATED
Application year: 1986
Main classification: H03D9
IPC codes: H03D9, H03D7