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SELF-ACTUATING LOW-POWER RECEIVER FRONT-END PROTECTION CIRCUITYF

IP.com Disclosure Number: IPCOM000007865D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-Apr-30
Document File: 3 page(s) / 150K

Publishing Venue

Motorola

Related People

Scott Kunesh: AUTHOR

Abstract

A system requires a small, low current, high per- formance VHF receiver to operate along side one or more high-power VHF or UHF transmitters. Because the receiver's antenna is physically close to the trans- mit antenna(s), the RF power that the receiver sees can be as high as 10 watts for as long as one minute. This amount of energy is enough to permanently damage the receiver's front end. The physical sepa- ration of the receiver's antenna and the transmit antenna(s) can't be increased. The operating fre- quency of the transmitter(s) can be the same as the receiver's operating frequency In many instances, the receiver will be powered "off' when a transmit- ter keys. The receiver and transmitter(s) are sepa- rate units with no interconnections. The transmit- ter(s) can't be modified to provide a "transmitter keyed" signal line. In summary, some means must be provided to protect the receiver's front end, even when the receiver is off, that is transmit frequency insensitive and has a minimal impact on the receiv- er'sperformance and size.

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Technical Developments

SELF-ACTUATING LOW-POWER RECEIVER

FRONT-END PROTECTION CIRCUITYF

by Scott Kunesh

  A system requires a small, low current, high per- formance VHF receiver to operate along side one or more high-power VHF or UHF transmitters. Because the receiver's antenna is physically close to the trans- mit antenna(s), the RF power that the receiver sees can be as high as 10 watts for as long as one minute. This amount of energy is enough to permanently damage the receiver's front end. The physical sepa- ration of the receiver's antenna and the transmit antenna(s) can't be increased. The operating fre- quency of the transmitter(s) can be the same as the receiver's operating frequency In many instances, the receiver will be powered "off' when a transmit- ter keys. The receiver and transmitter(s) are sepa- rate units with no interconnections. The transmit- ter(s) can't be modified to provide a "transmitter keyed" signal line. In summary, some means must be provided to protect the receiver's front end, even when the receiver is off, that is transmit frequency insensitive and has a minimal impact on the receiv- er'sperformance and size.

RF Output. Protection (i.e., attenuation) is enhanced by lowering the switch's "on" impedance. The tie- quency where maximum attenuation is attained occurs when the transmission line's electrical length measures v4 wavelength. The power detector block does not affect circuit operation in this mode of operation.

  Receiver powered on without high-level RF pres- ent: The Power Detector block is inactive in this mode of operation. With Receiver B+ present, the switch is normally open. This results in a low-loss path from RFZnput to RF Output. Keeping both the transmission line's Q and the switch's "off imped- ance high ensures low insertion loss. Minimum inser- tion loss occurs at the same frequency as maximum attenuation mentioned above.

  Receiver powered on with high-level RF pres- ent: As in the previous mode of operation, Receiver B+ is present which has the switch normally open. When high-level RF is applied to RF Input, it is assumed that the time required for the level to go horn zero to full power is not instantaneous and is compatible with this circuit's response time. As the input power ramps up, the DC voltage out of the Power Detector also ramps up proportionately. At a predetermined threshold of input power, the detec- tor will output a voltage that is high enough to change the state of the switch to closed. At this point, the impedance seen at RFInput changes horn nominal (such as 50 ohms) to very high (greater than 500 ohms) due to the action ofthe !&wavelength transmission line. This, in turn, causes the DC out- put of the power detector to increase even further, thus providing switching hysteresis. The switch will stay closed, thereby protecting the receiver's front end, as long as the high-level RF is present at RF Input. The switch goes back to normally open (nor- mal receiver operat...