| 摘要 |
A tone receiver, for receiving audio frequency tone signals typically used in telephone signalling, includes a reference frequency generator and a modulator for up-converting audio frequencies. Telephone network tone signalling is up-converted and coupled to a filter including a piezoelectric element. Alternating current signals passing through the filter indicate the presence of a tone signal of a predetermined frequency. Multiple frequency tone signals are detectable using a plurality of modulators and filters, each modulator being supplied with one of a plurality of reference frequencies. In one example, balanced modulation is used with reference frequencies being supplied in the form of square waves.
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