Adaptive Impedance Matching (AIM) for Electrically Small Radio Receiver Antennas

Overview

A method of tuning a high impedance, high Q antenna circuit using the Receive Signal Strength Indicator (RSSI) of the corresponding radio receiver. In this specific application, earbud wires were used as part of the FM receiving antenna in portable Hybrid Digital, HD-Radio Receivers.  

Abstract

The earbud wire is approximately the correct length or 1/4 Lambda at commercial FM Radio frequencies.  The antenna counterpoise, however is the actual portable unit itself and is considered electrically small or simply put, not big or long enough and thus appears to be a parasitic capacitance to the radio receiver.  The concept behind AIM is to resonate this "capacitive" antenna with a fixed inductor and an additional variable capacitor that is dynamically tuned by the AIM Algorithm running in the HD-Radio receiver.  This produced a high Q dynamically tuned structure that in theory could provide significant gain over typical portable radio receivers that used earbud wire antennas and traditional 50 ohm systems.  Further technical details can be found on the downloads page in the attached AIM Patent including explanation of the advantages  of High Q, High Impedance antenna matching verses 50 matching in the commercial FM band.

Contributions

Dean's role in the overall project included developing a very high input impedance test Low Noise Amplifier in the FM Band with an input capacitance of 2pf maximum.  This enabled the high impedance antenna circuit to be built in the real world and allowed the team to characterize how high of a Q could really be obtained.    Once this was completed, Dean identified the need for a "nested loop" in the AIM Algorithm as the AIM tuning was based on RSSI and the receiver's Automatic Gain Control (AGC) was also driven by the RSSI.  With two equations, AIM value and AGC value, and only one unknown, RSSI, the answer was to deign a nested loop and only tune one parameter at a time.  Dean observed the AIM algorithm's time constant was significantly faster than the AGC time constant and proposed the solution of nested loops with proper timing to simultaneously solve the two equations based on only one unknown.   Once prototype systems of this existed, with hardware built and carefully characterized by Dean, it was decided to integrate this into the HD Radio chipset under development.  

Results

Dean personally briefed this new high impedance design change to the IC design team at Samsung Electronics, Korea, that lead to a HD-Radio chipset optimized for mobile performance and products.  This resulted in Portable HD Radio consumer products with exceptional radio receiving capabilities when using earbud wires as the primary antenna with a measured advantage of 6-8 dB over traditional technology.  

HD Radio Consumer Product with AIM Technology

This review is from: Insignia Portable HD Radio (Best Buy Electronics Website)

“I use this as a passenger on a multi-hour daily commute in the Colorado Front Range, where there are about 12 commercial and 3 non-commercial stations broadcasting HD. Sensitivity is amazing with the headphone cord antenna. On travel in Rock Hill, SC, I received distant stations from North Wilkesboro and Raleigh”

This review is from: Insignia Portable HD Radio (Best Buy Electronics Website)

“I just received this HD Radio today. I live about 50 to 60 miles north of NYC and the reception from the terrestrial FM stations is greatly improved over my bedside radio. The HD Stations that come in are crystal clear and the upgrade in audio that you hear is really improved over the terrestrial FM stations”