Nokia Mobile Phones Limited

Introtek International, L.p.

Juha H Tenhunen

Raimo K Kivari

Esko K Strommer

Antti Rasanen

Brian T Rivers

A frequency detection system is based on a digital phase locked loop, the detection system being especially suitable for use in noisy environments like supervisory audio tone (SAT) detection in AMPS and TACS mobile telephone systems. In addition to the digital phase locked loop (4), the frequency detection system according to the invention incorporates a detector circuit (5), which comprises a detection timer (6) and two phase detectors VI1 (7) and VI2 (8). The timer (6) forms a detection sequence of desired length, at the end of which the output signal (SATVAL) of the detector circuit is updated. The first phase detector VII (7) has a phase window in which it counts those falling edges of the synchronized input signal (SSAT) that coincide with the window. The second phase detector VI2 (8) also has a phase window of its own in which it counts those falling edges of the synchronized input signal (SSAT) that coincide with its window. The phase windows are made up of the basic output signal of the digital phase locked loop (SO1) and its 2nd, 4th, and 8th harmonics (SO2, SO4 and SO8). If at the end of a detection sequence the counter of the first phase detector VII (7) exceeds, and the counter of the second phase detector VI2 (8) falls short of their predetermined threshold values, the detection circuitry (5) will indicate that the frequency of the received signal is correct.

Frequency detector system on a digital phase locked loop

A data valid detector circuit for Manchester encoded data monitors two parameters to determine single bit validity, namely, the location and number of edges in one bit period, and the output count of an integrator type Manchester decoder circuit. Circuits analyze each received bit waveform to see that the data falls within reasonable limits of what can be said to be a valid Manchester bit. By retaining bit-by-bit information on validity for a sample of N bits, a determination is made of the validity of this string of N bits. A data valid signal is then generated indicating reception of a usable signal, even when disturbed by noise, if a sufficient number of bits in the string are valid.

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Oulu, Finland

Juha H Tenhunen