Apparatus for determining the frequency of an electrical signal has long been known. A conventional apparatus for determining the frequency is usually referred to as a frequency counter and frequently includes a visual display showing in digital form the frequency measured. The apparatus typically counts the number of cycles of an incoming signal during a sampling period. Some of the prior art frequency counters inhibit display of a count unless the signal detection method used by the counter indicates that a dominant signal is present in signals input to the frequency counter.
Frequency agile radio receivers are available that allow a listener to monitor conversations on each of numerous fixed frequencies. The receiver may monitor fixed frequencies or channels, tuning one at a time, typically in response to programmed instructions. These receivers typically include a memory in which frequency information for tuning the receiver is stored. The receivers are usually programmable for altering or establishing the stored frequency information and monitoring process. The programming feature permits the stored frequency information to be changed, for example, in response to location changes or changes in frequency allocation information. However, when a frequency agile radio receiver is in operation in an area where there are many transient transmitters, for example, mobile transceivers. The user of the frequency agile radio receiver may not know the frequencies of transmission of the nearby transmitters. The typical receiver is incapable of determining the frequencies of these transmissions so the receiver cannot tune to monitor them. Since listeners desire to monitor these nearby transmissions, it is desirable to include, within the radio, apparatus for determining the channel including the frequency of these nearby transmissions and, preferably, to provide for manual or automatic tuning of the radio receiver to that channel for monitoring them.
Some prior art frequency counters for tuning radio receivers have been available in housings separate from the receivers and connected to the receivers by cables, making use awkward. Counting of the frequency and tuning of the associated receiver are disadvantageously slow because the special purpose counters require at least two complete frequency determinations before producing an output.
Accordingly, there is a need for a frequency determining and radio tuning apparatus that can rapidly and accurately determine the frequency of a received radio frequency signal, that can ensure that only the frequency of a coherent signal is determined, and that, in application to a radio receiver, can tune the receiver to the channel including the frequency determined before loss of the signal.