GB2362522A - Frequency mixer - Google Patents

Abstract

A broadband GaAs FET mixer for use in an adaptive mobile telecommunications terminal has source followers Q5, Q6 used to drive FET gates Q9, Q10 to reduce source impedance, thereby improving linearity.

Description

2362522 HIGH LEVEL MIXER

The present invention relates to the field of high level mixers.

More specifically, the present invention. relates to a high level Gallium Arsenic (GaAs) field effect transistor (FET) mixer for use in a wireless telecommunications system.

Presently, mixers are used in the receiver of a wireless telecommunications system to provide a desired intermediate frequency (IF). By adjusting the frequency of a local oscillator (LO) the IF can be tuned to match the maximum transmission frequency of the receiver's bandpass filters.

The operation and use of high level mixers in wireless telecommunications systems is well known and will now be briefly described with reference to figure 1, in which a known high level GaAs FET mixer 10 is shown in circuit diagram form. The GaAs FET mixer is a resistive single switch device in which the radio frequency (RF) power is commuted by the ON/OFF resistance of the channel in a GaAs FET 11. The channel resistance is controlled via the GaAs FET's gate 18 and therefore large local oscillator power is not necessary.

The basic components and operation of the mixer 10 are as follows. An RF signal 12 is input to the mixer 10 and transmitted via a narrow bandpass filter 13 to a drain 19 of the GaAs FET 11. A local oscillator 14 regenerates a LO signal 15 which is transmitted via a 2 resistor 16 to the gate 18 of the GaAs FET. The LO signal 15 switches the GaAs FET channel ON/OFF, and hence commutes the RF signal 12 ON/OFF. The resulting intermediate frequency (IF) signal 22 then passes via a conductor 20 through a second narrow bandpass filter 21 and is output from the mixer. The mixer 10 does not require bias across the drain 19 and the source 17, thus no excess noise is generated resulting in the mixer having extremely linear characteristics.

An example of the device shown in figure I is the Siemens CMY2 10 MMIC. This device typically achieves a +20 dBm intercept point and has low LO drive power requirement.

The rapid growth in wireless telecommunications has created an interest in multi-band and multi-mode mobile phones. To date mobile phones work in predefined frequency ranges using predefined modes. For example, GSM terminals transmit in 880-915 MHz frequency range and receive in 925-960 MHz frequency range and operate in time division duplex (TDD) mode. Other types of mobile phones operate in different frequency ranges and different modes.

Currently, each type of mobile phone requires a mixer designed specifically for the predefined frequency range in which that phone operates. In order for a mobile phone to operate in a broadband ftequency range, frequency specific RF filtering would be required. Alternatively, a broadband high level mixer capable of processing a 3 number of broadband signals would, amongst other things, need to be installed in the phone's handset.

It is an object of the present invention to provide a high level mixer which can operate over a wide frequency bandwidth.

It is a further object of the present invention to provide a high level mixer which has low power consumption, moderate local oscillator drive levels, and a linear response curve.

According to the present invention there is provided a high level mixer for use in a wireless telecommunications system comprising: a local oscillator signal input means including a differential amplifier and arranged to receive a local oscillator signal; said differential amplifier coupled to a pair of source followers of an impedance adjustment means; said impedance adjustment means arranged to adjust the impedance of said local oscillator signal; said pair of source follower coupled to gates of a first and a second FET; a radio frequency input means including a first and a second RF amplifier and arranged to receive and amplify a radio frequency signal; said first RF amplifier coupled to a drain of said first FET and said second RIF amplifier coupled to a drain of said second FET; said drains of said first and second FETs being further coupled to a differential amplifier of an intermediate frequency output means; said intermediate frequency output means arranged to output a signal at a frequency which is between the frequencies of said local oscillator signal and said radio frequency signal.

4 Preferably said differential amplifier of said local oscillator signal input means. said pair of source followers, said first and second RF amplifiers, and said differential amplifier of said intermediate frequency output means are FETs.

According to a further aspect of the present invention, the mixer is formed as an integrated circuit.

Preferably, the integrated circuit is a monolithic microwave integrated circuit.

While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawing and detailed description of a preferred embodiment, presented by way of example only in which; Figure 2 is a circuit diagram of a high level mixer according to the present invention.

In figure 2 a high level mixer 30 is shown generally comprising a local oscillation (L0) input means 32, an impedance adjustment means 34, a radio frequency (RF) input means 36, an intermediate frequency (IF) output means 3 8, a first FET Q9 and a second FET Q 10.

The LO input means 32 comprises a LO differential amplifier, Q I and Q2. The LO differential amplifier is preferably formed by FETs.

The impedance adjustment means 34 comprises a first and second source follower, Q5 and Q6. The source followers are preferably FETs. The drains, Q I d and Q2d, of the LO differential amplifier are coupled to the gates of the second and first source follower, Q6g and Q5g, respectively. The gates of the LO differential amplifier, Q I g and Q2g, are coupled to a local oscillator (not shown). The sources of the LO differential amplifier, QIs and Q2s, are coupled to a current source 33. The drains of the source followers, Q5d and Q6d, are coupled to a positive power supply (not shown). The source of the first source follower Q5s is coupled to the gate of the first FET Q9g and the source of the second source follower Q6s is coupled to the gate of the second FET Q 1 Og.

The RF input means 36 comprises a first and second RF amplifier, Q3 and Q4 respectively. The RF amplifiers are preferably FETs. The gates of the first and second RF amplifiers, Q3g and Q4g, are arranged to receive an RF signal. The sources of the first and second RF amplifiers, Q3s and Q4s, are coupled to ground 37. The drain of the first RF amplifier Q3d is coupled via a capacitor 42 to the drain of the first FET Q9d. The drain of the second RF amplifier Q4d is coupled via a capacitor 40 to the drain of the second FET Q 10d.

The sources of the first and second FETs, Q9s and Q 1 Os, are coupled to ground 35. The drains of the first and second FETs, Q9d and Q 1 Od, are coupled to the gates of an IF differential amplifier, Q8g and Q7g respectively, of the IF output means 38. The sources of the IF differential amplifier, Q7s and Q8s, are coupled to a current source 39. The IF differential amplifier is preferably formed by FETs.

6 The drains of the IF differential amplifier, Q8d and Q7d, output a positive IF signal (+IF) and a negative IF signal (-IF) respectively.

The operation of the high level mixer shown in Figure 2 is as follows. The local oscillator (not shown) generates a positive and negative LO signal, +LO and -LO respectively. The +LO signal is input to the gate of Q I and the -LO signal is input to the gate of Q2. Q I and Q2 operate to amplify +LO and -LO respectively. The amplified +LO signal is then input to the gate of source follower Q6 and the amplified -LO signal is input to the gate of source follower Q5. The source follower Q5 and Q6 operate in differential mode and reduce the impedance levels of +LO and -LO. Advantageously, the low impedance levels of +LO and -LO improves the linearity of the FETs Q9 and Q 10.

The low impedance -LO signal is then input to the gate of FET Q9 and the low impedance +LO signal is input to the gate of FET Q10. The FETs Q9 and Q 10 operate as resistive mixers. Q9 and Q 10 are preferably GaAs FET devices.

An RF signal (+RF) is input at the gates of Q3 and Q4 of the RF input means 36. Q3 and Q4 amplify the incoming RF signal. The amplified RF signal is output via drains Q3d and Q4d via capacitors 42 and 40 to the drains of Q9 and Q 10 respectively. In addition, the low impedance +LO signal is coupled from the source of Q6 to the gate of Q 10 and the low impedance -LO signal is coupled via the source of Q5 to the gate of Q9. The LO signals alternatively commute Q9 and Q 10 to 7 ground 3 5. As a result, the RF signal at the drains of Q9 and Q 10 is alternatively shunted to ground 35. The commuted U signal is applied to the gates of the differential amplifier Q7 and Q8. The resulting signals are then output from the IF output means 38. The output IF signals are at a frequency which is between the LO and RF signal frequencies but have the original modulation of the RF signal. The resulting +IF and -IF signals are output from the mixer to a mobile terminal IF stage (not shown) for use in a wireless telecommunications system.

Preferably, the mixer described in figure 2 is an integrated circuit and the FETs are GaAs FETs. More preferably, the integrated circuit is a monolithic microwave integrated circuit (MMIC).

Advantageously, the mixer according to the present invention can operate over a large bandwidth and reduces the need for bandpass filters within the mobile terminal. Furthermore, the mixer has high linearity, low noise levels, low power requirements, and low LO signal leakage.

As will be appreciated by those skilled in the art, various modifications may be made to the embodiment hereiribefore described with out departing from the scope of the present invention. For example, level shifts may be disposed between the various amplifier stages to ensure correct voltage bias levels.

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Claims (6)

1. A high level mixer for use in a wireless telecommunications system comprising: a local oscillator signal input means including a differential amplifier and arranged to receive a local oscillator signal; said differential amplifier coupled to a pair of source followers of an impedance adjustment means; said impedance means arranged to adjust the impedance of said local oscillator signal; said pair of source followers coupled to gates of a first and second FET; a radio frequency input means including a first and a second RF amplifier and arranged to receive and amplify a radio frequency signal; said first RF amplifier coupled to a drain of said first FET and said second RF amplifier coupled to a drain of said second FET; said drains of said first and second FETs being further coupled to a differential amplifier of an intermediate frequency output means; said intermediate frequency output means arranged to output a signal at a frequency which is between the frequencies of said local oscillator signal and said radio frequency signal.

2. A mixer as claimed in Claim 1, wherein said differential amplifier of said local oscillator signal input means, said pair of source followers, said first and second RF amplifiers, and said differential amplifier of said intermediate frequency output means are FETs.

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3. A mixer as claimed in Claim 2, wherein said FETs are GaAs FETs.

4. A mixer as claimed in any preceding claim, wherein said mixer is formed as an integrated circuit.

5. A mixer as claimed in Claim 4, wherein said integrated circuit is a monolithic microwave integrated circuit.

6. A mixer as hereinbefore described with reference to accompanying 10 figure 2.