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Commutating Mixer with Output Voltage Multiplication for Enhanced Gain

IP.com Disclosure Number: IPCOM000246444D
Original Publication Date: 2016-Jun-07
Included in the Prior Art Database: 2016-Jun-07
Document File: 5 page(s) / 408K

Publishing Venue

Motorola

Related People

Heck, Joe: INVENTOR

Abstract

Improved Commutating Mixer configurations are shown which achieve greater voltage gain than the conventional configuration. The increased gain allows for improved receiver system noise figure owing to the increased gain ahead of the Post Mixer Amplifiers, whose noise typically dominates the noise floor of the receiver. The increased gain of the mixers can allow for less gain in the Low Noise Amplifier (LNA) ahead of the mixers, or in some cases, the LNA may be completely eliminated.

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Commutating Mixer with Output Voltage Multiplication for Enhanced Gain

By Joseph Heck

Motorola Solutions, Inc.

 

ABSTRACT

Improved Commutating Mixer configurations are shown which achieve greater voltage gain than the conventional configuration.  The increased gain allows for improved receiver system noise figure owing to the increased gain ahead of the Post Mixer Amplifiers, whose noise typically dominates the noise floor of the receiver.  The increased gain of the mixers can allow for less gain in the Low Noise Amplifier (LNA) ahead of the mixers, or in some cases, the LNA may be completely eliminated.

PROBLEM

The Commutating Mixer configuration, also known as the Tayloe Mixer or as the Quadrature Sampling Detector or Mixer, has advantages in several performance criteria in Direct Conversion Receiver applications. In particular, the voltage gain of the Commutating Mixer is greater than other passive mixer types. A conventional double balanced Switching Mixer implemented in a CMOS IC process may have a voltage gain (loss) of about -4 dB, whereas a double balanced Commutating Mixer based on the same switching devices may have a voltage gain close to 0 dB, while maintaining good 2nd and 3rd order intermodulation performance. The higher gain of the Commutating Mixer provides greater system gain ahead of the Post Mixer Amplifier (PMA). Since the noise of the PMA limits the receiver system noise figure, the additional gain can improve the overall receiver system noise figure significantly. Even so, the PMA typically still remains the dominant stage in setting the system noise figure.

SOLUTION

The improved mixer configurations described in this paper achieve significantly greater voltage gain through a modified Commutating Mixer configuration, allowing better system noise figure, and/or reduction in the gain of the Low Noise Amplifier (LNA). In some cases, it may be possible to completely eliminate the LNA and still achieve an acceptable noise figure with the concepts described.

OPERATION

Figure 1 shows a conventional Commutating Mixer in a single-ended or single-balanced configuration.  It employs four phases of Local Oscillator (LO) signal to convert the input signal, Vs to baseband, or Zero IF.  For Direct Conversion or Zero IF applications, the frequency of the LO signals is essentially the same as the carrier or center frequency of the input signal, Vs.

Figure 2 shows an improved single-balanced Commutating Mixer with increased gain.  In the improved mixer, each pair of Cs capacitors to the right of each of the 4 mixer switches is connected in parallel and charged during its respective mixer LO time slot. During the opposite time slots, 180 degrees later, each pair of Cs capacitors is connected in series, doubling the output voltage. During this opposite phase, the output voltage of each of the 4 outputs is also transferred to the output hold capacitors, labeled Ch.

Figure 3 shows a variation on the implementation of Figure 2 in which the...