GB2023954A - A signal mixer - Google Patents

Abstract

In an image rejection mixer including an input balun 4A Figure 4, feeding a received signal at 1A along balanced conductors 2A, 3A to diode ring circuits 5'A, 6'A, each of which is connected to the balanced ports of a balun 5''A, 6''A, two local oscillator signals 17, of equal amplitude and in phase quadrature are fed to unbalanced ports of the baluns 5''A, 6''A, a quadrature 3dB coupler, like that shown at 13 Figure 2, has two inputs connected to receive respective intermediate frequency signals from respective shunt connections 18 each connected across the balanced port of an associated balun 5''A, 6''A, and means absorbs image signals at one output of the quadrature 3dB coupler. The mixer is of microstrip construction and includes coaxial baluns 4A, 5''A and 6''A. This construction enables the mixer not only to recover the image signal generated in the mixer, but also reject source noise entering the mixer at image frequencies. The broad bandwidth baluns also allow recovery of the sum frequency and prevent unwanted resonances which would upset noise cancellation. <IMAGE>

Description

SPECIFICATION A mixer This invention relates to a mixer and particularly, though not exclusively, to a mixer for mixing an RF signal of frequency wo received by a radar receiver with a signal of frequency w generated by a local oscillator to give an intermediate frequency (w - (o5).

In practice the signal frequency mixing with the local oscillator frequency and its harmonics (e.g. 2w) generates a whole spectrum of frequencies. Some of these are shown on Figure 1 and include (2w - Ws) which is called the "image frequency and (w + ws) which is called the "sum frequency". Thus a high percentage of the energy entering the mixer is not transferred to IF. There exists a technique known as image recovery" which enables the unwanted image frequency to be terminated reactively in such a way that most of the signal energy entering the mixer is converted to the intermediate frequency. A known image recovery mixer arrangement is described in the paper "A Quiet Mixer" by Don Neuf published in the May 1973 edition of The Microwave Journal.A mixer like this is illustrated schematically in Figure 2.

Referring to Figure 2, the received signal enters the mixer at a terminal 1 which is connected to balanced lines 2 and 3 by a microstrip balun indicated schematically at 4. The signal is split in equal phase and amplitude between two identical circuits 5 and 6. A local oscillator signal enters the mixer at 7 and is fed into a quadrature 3dB coupler 8, the effect of which is to produce, at outputs 9 and 10, two local oscillator signals having a 900 phase difference. These local oscillator signals are fed to respective circuits 5 and 6.

Each of the circuits 5 and 6 includes a diode ring 5', 6' and a microstrip balun 5", 6" indicated schematically by the symbol for a centre-tapped transformer.

Signal input from the conductors 2, 3 is applied across opposite corners of each diode ring whilst local oscillator input is applied to, and intermediate frequency output is taken from, the remaining corners of each diode ring. In operation, each circuit 5 or 6, in conjunction with the conductors 2 and 3 and the balun 4 mixes the received signal with the local oscillator signal to produce an intermediate frequency signal on line 11 or 12. The intermediate frequency signals from circuits 5 and 6 are combined in another quadrature 3dB coupler 13 where they appear on output port 14.

It can be shown mathematically that the image generated in circuit 5 appears on lines 2 and 3 and that the image generated in circuit 6 appears in opposite polarity on lines 2 and 3. Thus no resultant image is generated which means that image recovery is achieved.

Such image recovery results in low conversion loss and therefore a better noise figure. This circuit arrangement also achieves rejection of incoming signals at image frequency, the intermediate frequency derived from such incoming signals at image frequency being directed to port 15 of coupler 14 where it is absorbed by a matched load.

The relationship between conversion loss and mixer noise figure is given by a formula derived by W.L. Pritchard (see "Notes on Crystal Mixer Performance" IEEE PUG MITT pages 37 to 39 January1955).

This relationship is shown by the centre curve on Figure 3 from which it is apparent that, anywhere on the curve, a significant reduction of conversion loss results in a con espondingiy significant reduction of the mixer noise figure. A second stage noise figure of 1.5 dB is included in the noise figure values given on Figure 3. Hitherto, efforts to achieve a good noise figure have concentrated on obtaining low conversion loss.

A more recent noise formula derived by W.W.

Mumford and E.H. Scheibe (seethe book "Noise Performance Factors in Communications Systems" published by Horizon House-Microwave Inc. page 65 1968) gives the top curve shown in Figure 3 and we have found that this agrees more closely with experimental results. An important feature which we have noticed about this curve is that it flattens out at low conversion loss figures. In other words the achievement of a very low conversion loss does not significantly improve the noise figure. Even if it were possible to reduce conversion loss to zero, the noise figure would still be around 4dB.

These facts have led us to question the usefulness of the conventional approach of simply reducing conversion loss to obtain a low noise figure. There is another factor which affects the noise figure of the mixer; namely the amount of noise appearing at the intermediate frequency output. A simple conventional mixer takes incoming RF at signal frequency and converts it to intermediate frequency. It also takes incoming RF at image frequency and converts it to intermediate frequency. In the same way the noise generated by the input source resistance at image frequency is transferred to intermediate frequency as well as that generated at signal frequency.

Thus, although we now believe that the generation of image frequency in the mixing process is not of crucial importance for the achievement of a low noise figure, the image as a window through which noise may enter, is important.

Following from this belief we have realised that, if the noise entering through the image window can be suppressed, using the technique of image rejection, the output noise can be greatly reduced and the noise figure improved.

Experiments we have conducted further indicate that the conversion loss and therefore the noise figure of a mixer is dependant, to an extent not hitherto realised, on the generation of the sum frequency. It follows therefore that it is necessary to recover this sum frequency (w + w,) if a substantial improvement in the noise figure is to be achieved.

With these factors in mind we have carried out detailed experimental and theoretical investigation into the behaviour of image rejection mixers and have designed a mixer having a very good noise figure over a wide bandwidth.

The invention provides an image rejection mixer comprising: two diode ring circuits; an input balun arranged to feed a received signal, along balanced conductors, to opposite corners of each diode ring circuit, the other corners of each diode ring circuit being connected to the balanced ports of respective second baluns; means for generating two local oscillator signals of equal amplitude in phase quadrature with each other and for feeding these local oscillator signals to unbalanced ports of the respective second baluns; two shunt connections, each connected across the balanced port of an associated second balun; a quadrature 3dB coupler having two inputs connected to receive respective intermediate frequency signals from respective shunt connections; and means for absorbing image signals at one output of the quadrature 3dB coupler; characterised in that the three baluns are all co-axial baluns of decade bandwidth.

The term "bandwidth" as used in this Specification is to be construed as referring to a 1 dB bandwidth.

The invention also provides an image rejection mixer comprising; two diode ring circuits; an input balun arranged to feed a received signal along balanced conductors, to opposite corners of each diode ring circuit, the other corners of each diode ring circuit being connected to the balanced ports of a respective second balun; means for generating two local oscillator signals of equal amplitude in phase quadrature with each other and for feeding these local oscillator signals to unbalanced ports of the respective second baluns; two shunt connections, each connected across the balanced port of an associated second balun; a quadrature 3dB coupler having two inputs connected to receive respective intermediate frequency signals from respective shunt connection; and means for absorbing image signals at one output of the quadrature 3dB coupler; characterised in that the length of each line extending from a diode ring to the end of the input balun, and the length of each line extending from a diode ring to the end of its associated second balun, are f/4 at approximately sum frequency.

The value of the sum frequency is of course of the order of twice the local oscillator frequency since the local oscillator is tuned so that its frequency is similar to that of the received signal.

The significance of the use of co-axial, wide bandwidth baluns and of the aforementioned limitations in the lengths of the lines will become apparent from the following description of one particular embodiment, given with reference to figures 4, 5 and 6 of the accompanying drawings in which Figure 4 is a schematic perspective view from above (not to scale) of an image rejection mixer constructed in accordance with the invention, the mixer being mounted on a dielectric sheet shown partly broken away to reveal components on the underside thereof; Figure 5 is a vertical cross section through one of the baluns shown in Figure 4; and Figure 6 is a table showing what frequencies appear at different ports of the circuit shown in Figure 4.

Referring to Figure 4, the mixer is constructed, mainly in strip line form, on a dielectric sheet 16.

Parts corresponding to those shown in Figure 2 are denoted by the same reference numerals as used in Figure 2 but with the addition of the suffix A. The signal input is connected to terminal 1A and two local oscillator signals, from a quadrature 3dB coupler like that shown at 8 on Figure 2, are fed respectively to the input 17 of balun 5"A and to an identical input of balun 6"A. (the balun 5"A and its connections to the diode ring 5'A, the local oscillator input 16, and the intermediate frequency output 11A are identical to the balun 6"A and its corresponding connections. The latter are not fully shown on Figure 4, being obscured by the sheet 16.) Intermediate frequency signals are taken from two shunt stubs, one of which is shown at 18, and are combined in a quadrature 3dB coupler, like that shown at 12 in Figure 2, so as to reject image signals.

The lines taking the intermediate frequency signals to the associated quadrature 3dB coupler have co-axial shunt capacitors 24 to earth.

The description of a known mixer, given with reference to Figure 2, is also applicable to the mixer of Figures 4 and 5 and will therefore not be repeated.

The following description concentrates therefore only on the novel features. Each balun 4A, 5"A and 6"A is a co-axial balun of decade bandwidth operating from 1 -10 GHz and mounted on the dielectric sheet 16. Referring to Figure Sit comprises two tubular balanced arms 19, of length B/4 where is the wavelength corresponding to the centre frequency which is about 5.5 GHz. The arms 19 are mounted on opposite sides of the sheet 16 and a central unbalanced conductor 20 passes through them, the conductor 20 being held in position by dielectric material 21.

The features of the baluns which make them of wide bandwidth are: 1. that arms 19 form a high impedance balanced line stub because of their separation from an earthed casing (not shown) of the mixer; and 2. that a low impedance exists between the upper arm 19 and the upper part of the conductor 20, this upper part being of increased diameter as shown on Figure 5.

A mixer as shown in Figures 4 and 5 has been built and found to exhibit a low noise figure over almost an octave bandwidth, this being due, it is believed, partly to rejection of image source noise and partly to recovery of sum frequency signals generated in the mixer. One necessary requirement to achieve this is that the length of the lines 2A and 3A is short compared with the wavelength of the signal frequency cy. In the illustrated embodiment it is about 1/50th of the wavelength but it could be say 1/20th. Thus, the diode rings have, in effect, common input connections 2B, 3B.

Another requirement for the rejection of image source noise is that the baluns 5"A, 6"A be of wide bandwidth, preferably of the order of at least a decade. In order to understand why this is so it is necessary to consider which frequencies appear at the local oscillator input 17, at the intermediate frequency output 11A, and at the signal input 1 a. For a diode ring arranged as in Figure 4 it can be shown, by a Fourier analysis of the non-linear time varying impedance of the four diodes, that some of the frequencies indicated on Figure 1 are suppressed and that others appear at different ports. Figure 6 shows which of the frequencies of Figure 1 appear at which ports.

From Figure 6 it is apparent that frequencies wand 3w appear at the local oscillator port 17 (and the corresponding port of balun 6" A). In fact all the odd harmonics of the local oscillator frequency appear at these ports. It has been shown that reactively terminating the third harmonic has a significant effect on the signal input impedance and the conversion phase and amplitude. This would not be important if the reactive termination were identical for both local oscillator ports since the correct phase and amplitude relationship between the two mixers would still be maintained. However, the tolerances in manufacturing the circuitry mean that, in practice, the reactive terminations are far from identical if they are resonant. By using broadband, co-axial baluns 5"A, 6"A, such resonances can be avoided.If the mixer is designed to operate from say 1 to 2 GHz the value of 3w can be as high as about 6GHz. The baluns therefore preferably have a bandwidth which covers w and 6w. By choosing (as is preferred) a bandwidth which covers the range w to 1 or, resonances up to the fifth harmonic are also prevented.

Thus the wide bandwidths of baluns 5"A and 6"A prevent resonances which would occur with narrower bandwidth baluns; and which would interfere with the rejection of image source noise.

Another requirementforthe rejection of image source noise is that the balun 4A be physically short and therefore of broad bandwidth. In order to understand the reason for this, reference is again made to Figure 6 which shows that two frequencies of importance appear at the intermediate frequency port 1 ZA: the required intermediate frequency and the sum frequency. A capacitor 24 is used to match the intermediate frequency output impedance of the mixer to 50 Ohms which is the input impedance of the quadrature 3dB coupler. To achieve low conver sion loss the sum frequency at the intermediate frequency port 11A must be reactively terminated.It can be shown that to obtain a reactive termination of the sum frequency at the intermediate frequency port 11A it is necessary for the lengths x andy as shown on Figure 4 to be approximately W/4 at the sum frequency. In practice, when the received signal is around 1 to 2 GHz, the sum frequency is 2 to 4 GHz which means that k/4 is somewhere in the region of 3cms. In order that the baluns be separated by a sufficient distance to prevent coupling this means that the balun stubs must be more than about 1 cms. It follows that the baluns have a centre frequency of about 5.5 GHz. They must therefore be of approximately decade bandwidth or more in order that the lowest signal frequency of 1 GHz may pass through them.

Finally, it is also apparent from Figure 6 that the only frequency of importance appearing at the signal port 1A is the signal reflection, this being caused by mismatchingofthe mixer impedance to the impe dance Gt the signal source. A shunt connection 22 and high impedance parts 23 of the lines 2A, 3A are used to give correct matching.

Claims (7)

1. An image rejection mixer comprising: two diode ring circuits; an input balun arranged to feed a received signal along balanced conductors, to opposite corners of each diode ring circuit, the other corners of each diode ring circuit being connected to the balanced ports of a respective second balun; means for generating two local oscillator signals of equal amplitude in phase quadrature with each other and for feeding these local oscillator signals to unbalanced ports of the respective second baluns; two shunt connections, each connected across the balanced port of an associated second balun; a quadrature 3dB coupler having two inputs connected to receive respective intermediate frequency signals from respective shunt connections; and means for absorbing image signals at one output of the quadrature 3dB coupler; characterised in that the three baluns are all co-axial baluns of approximately decade bandwidth or more.

2. An image rejection mixer according to claim 1 in which the length of each line extending from a diode ring to the end of the input balun, and the length of each line extending from a diode ring to the end of the associated second balun, are approximately B/4 at sum frequency.

3. An image rejection mixer comprising: two diode ring circuits; an input balun arranged to feed a received signal along balanced conductors, to apposite corners of each diode ring circuit, the other corners of each diode ring circuit being connected to the balanced ports of a respective second balun; means for generating two local oscillator signals of equal amplitude in phase quadrature with each other and for feeding these local oscillator signals to unbalanced ports of the respective second baluns; two shunt connections, each connected across the balanced port of an associated second balun; a quadrature 3dB coupler having two inputs connected to receive respective intermediate frequency signals from respective shunt connections: and means for absorbing image signals at one output of the quadrature 3dB coupler; characterised in that the length of each line extending from a diode ring to the end of the input balun, and the length of each line extending from a diode ring to the end of-its associated second balun, are B/4 at approximately sum frequency.

4. An image rejection mixer according to claim 3 characterised in that the baluns are all co-axial baluns of decade bandwidth.

5. An image rejection mixer according to claim 3 or 4 in which the length of each balun arm is approximately 1i18 or less at sum frequency.

6. An image rejection mixer substantially as described with reference to Figure 3 of the accom panying drawings and substantially as illustrated therein.

7. A radar receiver including an image rejection mixer as claimed in any preceding claim, said mixer serving as afrequencychangerto produce, from a received radio frequency signal, an intermediate frequency signal.