| 摘要 |
A transmitter-receiver is disclosed for multichannel systems wherein a search for an idle channel is carried out with the help of a second intermediate frequency which is modulated with the station's own call number or with a call number not otherwise used in the system and which is injected into the receiver portion. If, during the channel search, a selective call receiver, adjusted to this call number, responds, the channel is determined to be idle because the additional IF voltage is higher than the IF noise voltage.
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