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A Method to Suppress Local Oscillator Harmonics in a Switch Type Frequency Mixer Using Both Time and Amplitude Quantization

IP.com Disclosure Number: IPCOM000200503D
Original Publication Date: 2010-Oct-17
Included in the Prior Art Database: 2010-Oct-17
Document File: 4 page(s) / 227K

Publishing Venue

Motorola

Related People

Caruthers, Rodger: INVENTOR

Abstract

In RF receivers frequency mixers may be used to convert RF signals to some other desired frequency. One type of mixer that is popular for this application is comprised of high speed switches who’s advantage is high dynamic range. These switches are driven by a Local Oscillator (L.O.) to enable the needed frequency conversion. Given the nature of these switches, harmonics of the L.O. are created. These harmonics produce undesirable spurious responses. Herein is described a method to suppress these harmonics which in turn reduces the undesired spurious responses. This method uses series connected switch-controlled attenuators to approximate a sine wave waveform for the L.O.. This approach may be extended to suppress more harmonics by adding more switched-controlled attenuator stages. This allows a higher number of harmonics to be suppressed as the L.O. frequency is lowered, supporting the use of less aggressive pre-mixer filtering. The method described is envisioned to be used in mixed signal ASICs (Application Specific Integrated Circuits) and MMICs (Microwave Monolithic Integrated Circuits). It is especially suited for SDR (Software Defined Radio) receiver architectures where frequency assignment flexibility is highly desirable.

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A Method to Suppress Local Oscillator Harmonics in a Switch Type Frequency Mixer Using Both Time and Amplitude Quantization

By Rodger W. Caruthers

Motorola, Inc.

Applied Technology Department

 

ABSTRACT

In RF receivers frequency mixers may be used to convert RF signals to some other desired frequency.   One type of mixer that is popular for this application is comprised of high speed switches who’s advantage is high dynamic range.  These switches are driven by a Local Oscillator (L.O.) to enable the needed frequency conversion. Given the nature of these switches, harmonics of the L.O. are created. These harmonics produce undesirable spurious responses.

Herein is described a method to suppress these harmonics which in turn reduces the undesired spurious responses.  This method uses series connected switch-controlled attenuators to approximate a sine wave waveform for the L.O..   

This approach may be extended to suppress more harmonics by adding more switched-controlled attenuator stages.   This allows a higher number of harmonics to be suppressed as the L.O. frequency is lowered, supporting the use of less aggressive pre-mixer filtering.  

The method described is envisioned to be used in mixed signal ASICs (Application Specific Integrated Circuits) and MMICs  (Microwave Monolithic Integrated Circuits). It is especially suited for SDR (Software Defined Radio) receiver architectures where frequency assignment flexibility is highly desirable.

PROBLEM

Spurious responses in an RF receiver are undesirable in that they provide opportunities for the receiver to be desensitized by signals that are outside of the desired reception frequency band for the receiver.  Typically these spurious responses force the use of additional pre-mixer filtering which increases the receiver’s size and cost, and reduces its frequency range assignment flexibility.

The frequency mixer in a receiver can be a significant source of these spurious responses. The type of mixer chosen for the receiver can have a significant impact on the level and number of these spurious responses.  For ASICs and MMICs  a mixer based on high speed switches is frequently used because of its high dynamic range and low current drain.   Since switching techniques are used, in lieu of analog multiplying techniques, L.O. harmonics are created that produce significant spurious responses.

A method is herein described that is envisioned to suppress the creation of these L.O. harmonics.

SOLUTION

The use of a switch based mixer forces the L.O. to act as a square wave which has a frequency spectrum that consists of odd harmonics that role off slowly in amplitude relative to the L.O.’s fundamental frequency.

A method that may be used to supp...