GB2237684A

GB2237684A - Mixer circuit for antenna

Info

Publication number: GB2237684A
Application number: GB8801319A
Authority: GB
Inventors: Colin Dougal Mcewen
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-01-21
Filing date: 1988-01-21
Publication date: 1991-05-08
Anticipated expiration: 2008-01-21
Also published as: GB2237684B; GB8801319D0

Abstract

A diode ring mixer has two input ports coupled to respective folded dipole antennas, 40, 41 the output port being provided by centre-taps of the folded dipoles. In one embodiment the dipoles are disposed in orthogonal relationship on a semiconductor substrate, the mixer being implemented by diode ring integrated in the substrate. <IMAGE>

Description

MIXER CIRCUIT FOR PLANAR ANTENNA This invention relates to a mixer circuit for a planar antenna wherein direct coupling of radio frequency (RF) and local oscillator (LO) signals is effected.

It is known to provide a construction of a double balanced mixer circuit for use with conventional dipole antennas. However, implementation of such a circuit without the need for centre-tapped transformers in a monolithic construction is not a practical proposition and has a high conversion loss.

The present invention provides a circuit which is implementable in a monolithic planar construction and has a low conversion loss.

According to the invention there is provided a mixer circuit arrangement for a planar antenna structure including a balanced mixer circuit havingfirst and second input ports and one output port, and a folded dipole the feed of which is coupled to one input port of the mixer, the folded dipole having a centre tap connection to provide an output coupling of the mixer.

In a preferred embodiment of the invention the mixer circuit arrangement includes first and second folded dipoles the feeds of which are coupled to the first and second input ports respectively of the mixer circuit, both folded dipoles having centre tap connections which together form the output port of the arrangement, the folded dipoles being tuned to receive RF and LO frequencies respectively.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a conventional dipole with centre feed, Figure 2 illustrates a folded dipole with a centre tap connection, Figure 3 illustrates an equivalent circuit for the folded dipole of Figure 2, Figure 4 illustrates a conventional balanced mixer circuit, Figure 5 illustrates a mixer circuit arrangement according to the invention, and Figure 6 illustrates a mixer circuit arrangement utilising a planar array of orthogonally disposed folded centre-tapped dipoles.

A conventional dipole antenna, Figure 1, usually consists of two collinear conductors 10, 11 with a total length of substantially 1 /2, where X is the operating wavelength. The antenna is centre fed. An arrangement which increases the value of impedance at the feed point is the folded dipole, Figure .2, in 'which another /2 element 12 is connected directly. between the ends of the simple dipole antenna. If now the folded antenna is provided with a third connection 13 to its effective centre point, a so-called centre tap", the equivalent circuit will be as shown in Figure 3.

A well known method of demodulation is the superheterodyne receiver in which the received radio frequency (RF) signal is mixed with a local oscillator (LO) signal at a different frequency to produce an intermediate frequency (IF) output signal. A well known mixer circuit is that shown in Figure 4 where two centre-tapped transformers 30, 31 are coupled by a diode ring configuration having four diodes 32, 33, 34, 35.

Typically the RF is coupled into one transformer and the LO into the other transformer, the IF being coupled out via the centre taps.

In the arrangement according to the invention one or both transformers is replaced by a respective folded centre-tapped dipole antenna, as shown in Figure 5. Each dipole antenna 40, 41 is tuned to receive a specific frequency. If the RF is a radiated signal then it can be received by one folded centre-tapped dipole antenna while the LO is fed in via a conventional centre-tapped transformer. Conversely the LO can be a radiated signal while the RF is transformer coupled. If both the RF and the LO are radiated signals then both inputs to the mixer circuit are via folded centre-tapped antennas 40, 41. The arrangement can be implemented for either a single or a doubled balanced mixer circuit.

Figure 6 illustrates a preferred embodiment of the invention utilising two orthogonally disposed folded centre-tapped dipoles implemented in a planar structure on a semiconductor substrate. Advantageously the mixer diodes are implemented as integrated semiconductor devices in the substrate material. The four diodes 32-35 are first fabricated in the semiconductor substrate, preferably gallium-arsenide (GaAs), using conventional monolithic integrated circuit technology. A passivation layer (not shown) is then formed on the substrate surface. This is followed by fabrication of one folded dipole 40 deposited as a metallization layer, an insulation layer (not shown) and the second folded dipole 41 as a second metallization layer. The centre taps to the dipoles are provided by conductors 42, 43.

This structure finds application in implementing the so-called 1MARTS1 concept as outlined in RSRE Memorandum No. 3482, UPlanar Antenna - Coupled Mixer Circuits for the Microwave and Millimetre Wavebands - The MARS Principle by D.J. Gunton and H.D. Rees, March 1983.

Claims

CLAIMS.

1. A mixer circuit arrangement for a planar antenna structure including a balanced mixer circuit having first and second input ports and one output port, and a folded dipole the feed of which is coupled to one input port of the mixer, the folded dipole having a centre tap connection to provide an output coupling of the mixer.

2. An arrangement according to claim 1 including first and second folded dipoles the feeds of which are coupled to the first and second input ports respectively of the mixer circuit, both folded dipoles having centre tap connections which together form the output port of the arrangement, the folded dipoles being tuned to receive RF and LO frequencies respectively.

3. An arrangement according to claim 2 wherein the mixer circuit is a double balanced mixer implemented by a diode ring.

4. An arrangement according to claim 2 or 3 wherein the folded dipoles are in orthogonal relationship.

5. An arrangement according to claim 4 wherein the mixer is implemented by diodes integrated in a semiconductor substrate and the folded dipoles are implemented as metallisation patterns on the substrate surface.

6. An antenna circuit arrangement substantially as described with reference to Figure 5 or Figure 6 of the drawings.

6. A mixer circuit arrangement substantially as described with reference to Figure 5 or Figure 6 of the drawings.

Amendments to the claims have been filed as follows CLAIMS.

1. An antenna arrangement including a balanced mixer having a first two-terminal input port, a second two-terminal input port and a first terminal of an output port, and a folded dipole the ends of which are connected to the two terminals of the first input port, the folded dipole having a centre tap connection to provide a second terminal of the output port of the mixer.

2. An antenna arrangement according to claim 1 including a second folded dipole the ends of which are connected to the two terminals of the second input port of the balanced mixer, the second folded dipole having a centre tap connection which forms the first output terminal of mixer.