True Root-Mean-Square Detection with a Sub-Threshold Transistor Bridge Circuit

• US 20100041356A1
• Filed: 08/18/2008
• Published: 02/18/2010
• Est. Priority Date: 08/18/2008
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Arnborg, Torkel
• Granted Patent: US 8,060,040 B2
• Time in Patent Office: Days
• US Class Current: 455/130
• CPC Class Codes: G01R 17/16 with discharge tubes or semiconductor devices in one or more arms of the bridge, e.g. voltmeter using a difference amplifier
• CPC Class Codes: G01R 21/10 by using square-law characteristics of circuit elements, e.g. diodes, to measure power absorbed by loads of known impedance G01R21/02 takes precedence
• CPC Class Codes: H04B 17/327 Received signal code power [RSCP]

Abstract

A sub-threshold transistor bridge circuit for use in detecting the signal level of an input radio-frequency signal is disclosed. In an exemplary embodiment each branch of the sub-threshold transistor bridge circuit comprises a transistor configured to operate in the sub-threshold region over a predetermined range of input signal levels. An input radio-frequency signal applied to a first pair of opposing corners of the bridge circuit yields a bridge output signal at the remaining pair of opposing corners that has a low-frequency component substantially proportional to the squared-amplitude of the envelope of the input radio-frequency signal. Also disclosed are various detector circuits including a sub-threshold transistor bridge circuit, as well as methods for detecting a signal level for an radio-frequency signal using a sub-threshold transistor bridge.

Claims

1. A detector circuit comprising a sub-threshold transistor bridge circuit, each of the four branches of the bridge circuit comprising a transistor configured to operate in the sub-threshold region over a predetermined range of input signal levels so that an input radio-frequency signal applied to a first pair of opposing corners of the bridge circuit yields a bridge output signal at the remaining pair of opposing corners, the bridge output signal having a low-frequency component substantially proportional to the squared-amplitude of the envelope of the input radio-frequency signal.

2. The detector circuit of claim 1, wherein each of the transistors comprises a field-effect transistor having a low or negligible threshold voltage, and wherein the gate of each field-effect transistor is DC-coupled to the gate of the field-effect transistor on the opposite branch.

3. The detector circuit of claim 1, wherein each of the transistors comprises a field-effect transistor DC-biased for sub-threshold operation, and wherein the gate of each field-effect transistor is AC-coupled to the gate of the field-effect transistor on the opposite branch.

4. The detector circuit of claim 1, further comprising:

a DC-to-radio-frequency converter circuit configured to convert the bridge output signal to a second radio-frequency signal having an amplitude proportional to the bridge output signal level;

a radio-frequency amplifier configured to amplify the second radio-frequency signal with a pre-determined gain; and

a rectifier circuit configured to convert the amplified second radio-frequency signal to a low-frequency detected signal proportional to the envelope of the amplified second radio-frequency signal.

5. The detector circuit of claim 4, wherein the DC-to-radio-frequency converter circuit comprises a frequency mixer circuit configured to mix the bridge output signal with a local oscillator signal to obtain the second radio-frequency signal, the local oscillator signal having a center frequency that differs from the center frequency of the input radio-frequency signal.

6. The detector circuit of claim 4, further comprising a limiting amplifier configured to amplify the input radio-frequency signal to obtain an amplitude-limited radio-frequency signal, wherein the DC-to-radio-frequency converter circuit comprises a frequency mixer circuit configured to mix the bridge output signal with the amplitude-limited radio-frequency to obtain the second radio-frequency signal.

7. The detector circuit of claim 4, further comprising:

an analog-to-digital converter configured to convert the low-frequency detected signal to a sampled power level signal; and

a digital processing circuit configured to calculate the square root of the sampled power level signal to obtain a sampled true root-mean-square signal.

8. A method of detecting a signal level of a radio-frequency signal, the method comprising:

applying an input radio-frequency signal to a first pair of opposing corners of a sub-threshold transistor bridge circuit, each of the four branches of the bridge comprising a transistor configured to operate in the sub-threshold region over a predetermined range of input signal levels so that a bridge output signal at the remaining pair of opposing corners of the bridge has a low-frequency component substantially proportional to the squared-amplitude of the envelope of the input radio-frequency signal; and

detecting a signal level for the input radio-frequency signal based on the bridge output signal.

9. The method of claim 8, wherein detecting a signal level based on the bridge output signal comprises:

converting the bridge output signal to a second radio-frequency signal having an amplitude proportional to the bridge output signal level;

amplifying the second radio-frequency signal according to a pre-determined gain; and

converting the amplified second radio-frequency signal to a low-frequency detected signal proportional to the envelope of the amplified second radio-frequency signal.

10. The method of claim 9, wherein converting the bridge output signal to a second radio-frequency signal comprises mixing the bridge output signal with a local oscillator signal to obtain the second radio-frequency signal, the local oscillator signal having a center frequency that differs from the center frequency of the input radio-frequency signal.

11. The method of claim 9, further comprising amplifying the input radio-frequency signal, using a limiting amplifier, to obtain an amplitude-limited radio-frequency signal, wherein converting the bridge output signal to a second radio-frequency signal comprises mixing the bridge output signal with the amplitude-limited radio-frequency to obtain the second radio-frequency signal.

12. The method of claim 9, further comprising:

converting the low-frequency detected signal to a sampled power level signal, using an analog-to-digital converter; and

calculating the square root of the sampled power level signal to obtain a sampled true root-mean-square signal.