WO1985003824A1

WO1985003824A1 - Fm subcarrier transmission system

Info

Publication number: WO1985003824A1
Application number: PCT/US1985/000242
Authority: WO
Inventors: John Yoh-Han Ma
Original assignee: Ma John Yoh Han
Priority date: 1984-02-16
Filing date: 1985-02-14
Publication date: 1985-08-29

Abstract

An RF communications system (10) comprising at least two RF transmitters for broadcasting RF signals, each transmitter (12) broadcasting on a different frequency but each signal including an identical pilot signal (16). The RF communications system also comprises an RF receiver (11) which includes a tunable circuit (31) for receiving one of a plurality of RF signals, including the RF signals containing the pilot signal, a circuit (33) for detecting the pilot signal and generating a control signal if the received RF signal does not include the pilot signal, and a circuit (34) for generating in response to the first control signal a first tuning signal for tuning the tunable circuit (31) to a second RF signal.

Description

Ϊ SUB ARRIER TRANSMISSION SYSTEM

Field of"the Invention The present invention relates to radio frequency (RF) communications systems. In particular, it relates to receivers for RF communications systems.

Background of the Invention In an RF communications system, information (e.g., data, audio or video information) is broadcast from a transmitter to a receiver. An RF paging system is one example of an RF communications system. A message is broadcast from a transmitter to a paging device (i.e., a pager) carried by a person who subscribes to the paging system (i.e., a subscriber). Such a system is extremely useful for people who need to be quickly informed of a message (for example, a doctor who needs to know of a change in a patient's condition or an executive who needs to know of a business matter that requires his immediate attention) . Paging systems frequently use the transmitter of a standard FM station to broadcast messages to their subscribers. The messages are transmitted by a participating FM station as a special portion of its standard FM broadcast which only pagers can receive. The pager includes a receiver which is fixedly tuned to the frequency of the participating FM station for receiving the broadcast message and an identification unit for determining if the message is directed to the subscriber.

This type of RF paging system has proven highly effective and economical. Its use, however, has been limited to the broadcast range of the FM station to which the receiver is fixedly tuned. It would be more effective and most convenient if a paging system could be used nationwide. This would enable, for example, a subscriber on a business trip out of town to still be quickly informed of a message. A central proble with establishing a nationwide paging system using standard FM stations scattered throughout the country is that most of these FM stations broadcast at different frequencies. Summary of the Invention

A general object of the present invention is to provide an improved RF paging system. A more specific object of the present invention is to provide a nationwide RF paging system which utilizes an existing nationwide network of transmitters which may- broadcast at different frequencies and a pager which, if within range of any of these transmitters, will receive a broadcast message.

Another general object of the present invention is to provide an improved receiver for an RF communications system. In this regard, a more specific object of the present invention is.to provide an improved receiver which in every location having a participating RF transmitter will respond to the RF signal broadcast by the transmitter regardless of the frequency of the signal. It is a further object of the present invention to provide an improved receiver which will respond to the RF signal broadcast by every participating transmitter and yet is economical to manufacture and reliable and convenient to use.

The present invention realizes these objects by featuring an RF paging system including at least two participating RF transmitters for broadcasting RF signals, each broadcasting on a different frequency but each broadcast signal including an identical pilot signal. The RF paging system also includes a tunable receiver which may receive any of a plurality of RF signals, including the two RF signals broadcast by the participating RF transmitters, and a circuit for detecting a pilot signal in the received RF signal. The RF paging system further includes a circuit for tuning the tunable receiver to another RF signal if the pilot signal is not detected in the received RF signal and a circuit for maintaining the tunable receiver tuned to the received RF signal if the pilot signal is detected.

Another aspect of the invention features a receiver which when transported from one location to another will automatically seek an RF signal broadcast by a participating transmitter in the destination location. The receiver includes an electronically tunable circuit for receiving a first RF signal. Since the RF signal broadcast by a participating transmitter includes a pilot signal identifying it as a participating transmitter, the receiver also includes a pilot signal detecting circuit for generating a control signal if the first RF signal does not include the pilot signal. The receiver further includes a circuit for generating in response to the control signal a tuning signal for tuning the electronically tunable circuit to a second RF signal.

Another aspect of the invention features a receiver which having located an RF signal broadcast by a participating transmitter will automatically lock onto the RF signal and will remain tuned to that signal even during a brief loss of the signal. Consequently, neither signal interference nor brief transmitter power outages will result in a loss of the RF signal.

Another aspect of the invention features a receiver which includes a memory element which allows the receiver to remain tuned to an RF signal broadcast by a participating transmitter even after brief loss of power by the receiver. Consequently, power to the receiver may be periodically switched off (for example, to conserve batteries) and the receiver will remain tuned to the RF signal.

Further objects, aspects, and advantages of the present invention will become apparent upon studying the following detailed description and the accompanying drawings of an embodiment of the invention. Description-of the Drawings of the Invention

In the accompanying drawings:

FIG. 1 is a block diagram of an RF paging system of the present invention;

FIG. 2 is a detailed block diagram of one embodiment of the receiving unit of FIG. 1;

FIG. 3 is a partial circuit diagram of a portion of the receiving unit of FIG. 2;

FIG. 4 is a flow chart illustrating the programming for the receiving unit of FIG. 2; FIG. 5 is a detailed block diagram of an alternative embodiment of a portion of the receiving unit of FIG. 2; and

FIG. 6 is a partial circuit diagram of a portion of another embodiment of the receiving unit of FIG. 1.

Description of -a Preferred Embodiment of the Invention

Although the present invention is illustrated and described with reference to a specific exemplary embodiment, i.e., an RF paging system, there is no intention to limit the invention to this particular embodiment. Modifications and alternative embodiments may be made by those skilled in the art, particularly in light of the following teachings, and yet still be encompassed by the invention. Accordingly, the present invention is intended to cover all modifications, alternatives, and equivalents which may fall within the spirit and scope of the invention as defined by the appended claims.

As shown in FIG. 1, the paging system 10 of the exemplary embodiment comprises a pager 11 and the transmitting unit 12 of an FM station transmitting on any of the standard FM channels, i.e., any of the 100 channels comprising the 88 to 108 MHz RF spectrum. The transmitting unit 12 includes a subsidiary communications authorization (SCA) generator 13 for frequency modulating an SCA subcarrier, e.g., a 42 or 67 KHz signal, by several message-related signals. These message-related signals include signals comprising the message for a particular subscriber and signals identifying the subscriber's pager as the pager to which the message is directed. These signals are . supplied by a message input 14 and a page identification input 15, respectively. Another message-related signal is supplied by a pilot signal input 16. According to one aspect of the invention, the pilot signal identifies the FM station as one participating in the RF paging system and all participating FM stations use an identical pilot signal, e.g., a 130 Hz tone.

The transmitting unit 12 further includes a combining unit 20 for combining the SCA modulated signal supplied by the SCA generator 13 and the remainder of the signals to be broadcast, e.g., monophonic audio signals supplied by a monophonic audio input 21. An FM transmitter 22 frequency modulates the FM channel carrier by the combined signal supplied by the combining unit 20. The FM signal output by the FM transmitter 22 is then broadcast from the antenna 23.

According to a central aspect of the invention, the pager carried by a subscriber to the paging system may be transported between distant locations, and a receiving unit of the present invention, which is incorporated into the pager 11, seeks the pilot signal and locks onto a participating FM station in the destination location, allowing the subscriber to be readily and conveniently tuned to a participating FM station regardless of where he travels. As shown in FIG. 1, the pager 11 generally comprises the receiving unit 24 an identification unit 25, and a message unit 26. The identification unit 25 decodes the identification signals and identifies the pager 11 as the pager intended to receive a broadcast message. The message unit 26 decodes the signals comprising the message and presents the message to the subscriber, e.g., by "beeping", by providing voice or printed messages, or by storing messages for later display. Neither the identification unit 25 nor the . message unit 26, both of which are well known in the art, form a part of the present invention.

The receiving unit 24 of the exemplary embodiment generally comprises (1) an antenna 30 for collecting broadcast RF signals; (2) a tunable FM receiver 31 for selecting an FM signal broadcast by an FM station and demodulating it to obtain the FM carrier modulating signals; (3) an SCA detector 32 for obtaining the message-related signals from the SCA modulated signal; (4) a pilot signal detector 23 for determining if the message-related signals include the pilot signal identifying the received FM signal as originating from a participating FM station; and (5) a frequency controller 34 for generating a tuning signal to electronically tune the tunable FM receiver 31 to another FM signal if the received FM signal does not include the pilot signal.

As shown in FIG. 2, the tunable FM receiver 31 includes an RF amplifier 35 which amplifies the RF signals received by the antenna 30 and provides an amplified, wideband RF signal to a mixer 36. The mixer 36 also receives a single-frequency mixing signal from a tunable local oscillator 40, typically a voltage controlled oscillator (VCO) . The output of the mixer 36 is a narrowband, intermediate frequency (IF) signal which comprises a frequency-shifted FM signal corresponding to the FM signal broadcast by an FM station, but which has a fixed center frequency, typically 10.7 MHz. This frequency-shifted FM signal is then amplified by an IF amplifier 41 and supplied to an FM detector 42.

The FM detector 42 demodulates the frequency-shifted FM signal and supplies the FM carrier modulating signals to the SCA detector 32. It also generates an automatic frequency control (AFC) signal used to lock the output of the mixer 23 onto the desired 10.7 MHz IF signal. The FM detector 42 may include a limiter and may comprise any standard design, e.g., a balanced-diode discriminator, a ratio detector, or a quadrature detector. The FM detector 42, as well as the RF amplifier 35, the mixer 36, the tunable oscillator 40, and the IF amplifier 41, are all well known and commonly available in the art.

The tunable FM receiver 31 also includes a tuning coordination unit 43 which receives the AFC signal generated by the FM detector 42 and the tuning signal generated by the frequency controller 34 and supplies a composite tuning signal to the VCO 40. As shown in FIG. 3, the tuning coordination unit 43 comprises a charge pump 44, a capacitor 45, and an IGFET analog switch or transmission gate 46.

The charge pump 44, which includes first and second bipolar transistors 50, 51 and a resistance network, supplies a tuning signal at its output 52 in accordance with the AFC signal at its input 53. Specifically, the charge pump 44 acts as a current source or sink for the capacitor 45 and supplies or drains charge from the capacitor 45 in accordance with the voltage of the AFC signal but independent of the voltage of the capacitor 45. The resistance network is balanced so that no net current is supplied from the charge pump 44 to the capacitor 45 in response to the AFC signal generated when the center frequency of the IF signal is at the desired 10.7 MHz center frequency of the IF amplifier 41. Consequently, if no tuning signal is generated by the frequency controller 34, the voltage across the capacitor 45 remains constant.

Since this voltage is applied to a varactor 54 in the VCO 40 to determine the frequency of the mixing signal, the constant voltage maintains a constant mixing signal frequency. However, if the center frequency of the I.F.. signal drifts either direction from the desired 10.7 MHz center frequency, the AFC signal will vary accordingly, causing one or the other of the bipolar transistors 50, 51 to conduct and, thereby, supply or drain charge from the capacitor 45 in accordance with the variation in the AFC signal. The voltage across the capacitor 45 ramps up or down in response to this charge supply or drain, and since the voltage across the capacitor 45 is applied to the varactor 54, the frequency of the mixing signal varies up or down to pull the center frequency of the IF signal in on the desired 10.7 MHz center frequency.

In addition to providing a ramp voltage output to the varactor 54, the capacitor 45, in conjunction with a resistor 55, provides a low pass filter which suppresses noise in the AFC loop. The resistor 55 improves the dynamic response of the filter. In accordance with one aspect of the invention, the capacitor 45 also serves as a memory element for the VCO 40. Pagers, which are battery powered, frequently incorporate battery-saving procedures in which power to certain components, including the tunable FM receiver 31, is switched off for a given length of time. The capacitor 45 "remembers" the frequency of the mixing signal generated by the VCO 40 prior to switching the power off since the capacitor voltage, which determined the frequency of the mixing signal prior to switching the power off, is maintained while the power is off. Once the power is switched on again, this voltage is again supplied to the VCO 40, causing it to generate a mixing signal of the same frequency as before the power was switched off. The IGFET switch 33, which is switched on and off with the power, serves to isolate the capacitor 45 when the power is off and prevent the capacitor voltage from changing due to leakage current paths.

Once the FM detector 42 demodulates the frequency-shifted FM signal and supplies the FM carrier modulating signal to the SCA detector 32, the SCA detector 32 filters the signal to obtain the SCA modulated signal and demodulates the SCA modulated signal to obtain the message-related signals, including the message, identification, and pilot signals. Since the SCA modulated signal typically occupies the upper portion of the frequency band of the FM carrier modulating signal, the SCA detector 32 includes a highpass or bandpass filter 55 for obtaining the SCA modulated signal as shown in FIG. 2. Since the SCA modulated signal is frequency modulated by the message-related signals, the SCA detector 32 also includes an FM detector 56, which may be similar to the FM detector 42 in the tunable FM receiver 31. Both the highpass or bandpass filter 55 and the FM detector 56 are well known and commonly available in the art.

Since any message-related signals obtained by the SCA detector 32 might include both signals comprising a message and signals identifying whether or not the message is directed to the subscriber's pager, the output of the SCA detector 32 is supplied to both the message unit and the identification unit. To determine if the message-related signals originated from a participating FM station, the output of the SCA detector 32 is also supplied to the pilot signal detector 33. Since the pilot signal is typically a low frequency signal (e.g., a 130 Hz tone), the pilot signal detector 33 comprises a lowpass or bandpass filter 60 followed by an amplitude detector 61 for sensing the amplitude of the pilot signal. For example, as shown in FIG. 3, the amplitude detector 61 may comprise a rectifier circuit comprising first and second diodes 62, 63 and a parallel combination of a resistor 64 and a capacitor 65 for generating a DC signal only when the pilot signal is present. Alternatively, for a more precise indication of when the 130 Hz tone is present, a phase-lock loop tune decoder may be substituted for the rectifier circuit. The DC signal is supplied to the frequency controller 34. According to one aspect of the invention, the frequency controller 34 responds to the absence of a pilot signal, as indicated, for example, when the DC signal goes low, by generating a tuning signal for tuning the FM receiver 31 to an FM signal broadcast by a different FM station. As shown in FIGS. 2 and 3, in one embodiment of the receiving unit 24, the frequency controller 34 comprises a microprocessor unit (MPϋ) 70 which includes a resettable timer 71, a sweep generator 72, and a tristate driver 73. The tristate driver 53 is typically provided for allowing a single input/output pin or lead of the MPO 70 to function as an output during the two binary states and as an input during the third or high-impedance state. Since transient interruptions in the received

FM signal may occur due to interference or momentary transmitter failure, the resettable timer 71 serves to delay the generation of the tuning signal for an appropriate interval of time (e.g., 20 seconds) after the pilot signal disappears. A flow chart illustrating the MPϋ programming for implementing this procedure is shown in FIG. 4. The MPϋ 70 first samples the output of the pilot signal detector 33 (STEP 1) and then determines if the pilot signal is present (STEP 2) . If the pilot signal is present, i.e., if the DC signal is high, the MPϋ 70 resets the timer (STEP 3) and issues a disable signal to the tristate driver 73 (STEP 4) , which prevents a tuning signal from being supplied to the FM receiver 31. The MPϋ 70 then again samples the output of the pilot signal detector 33 (STEP 1) and continues to reset the timer 71 and issue a disable signal until the pilot signal disappears, i.e., until the DC signal goes low.

Once the pilot signal disappears, the MPϋ 70 increments the resettable timer 71 (STEP 5) and determines if the timer 71 has been incremented through the desired delay interval (STEP 6) . If not, the MPϋ 70 continues to issue a disable signal to the tristate driver 73. The MPϋ 70 then again samples the output of the pilot signal detector 33 (STEP 1) and continues to increment the timer 71 and issue the disable signal as long as the pilot signal is not present and the delay interval has not been exceeded. Once the pilot signal has not appeared for the entire delay interval (e.g., because the subscriber has left the broadcast region of the participating FM station) , the MPϋ 70 sets the timer the delay interval (STEP 7) and issues an enable signal to the tristate driver 73 (STEP 8) . The sweep generator 72 then provides a tuning signal, e.g., a square wave signal, to the tunable FM receiver 31 sufficient to sweep the receiver 31 across the entire FM spectrum. The MPϋ 70 then again samples the output of the pilot signal detector (STEP 1) and continues issuing the enable signal until the pilot signal reappears. At any time after the pilot signal disappears, both during and after the delay interval, the MPϋ 70 will reset the timer 71 (STEP 3) and issue a disable signal if the pilot signal reappears, preventing any tuning of the FM receiver 31 and allowing the AFC signal to lock onto the received FM signal containing the pilot signal. If the pager 11 includes a battery-saving feature which recurrently switches off the power to any of the components of the receiving unit 24, the procedure of FIG. 4 should not be implemented until for some time after the power is switched back on. This delay allows the receiving unit 24 time to route the pilot signal, if any, through the tunable FM receiver 31, the SCA detector 32, and the pilot signal detector 33 and, thereby, avoid false indications that the pilot signal has been lost. Also, the delay interval to which the timer 71 is incremented may be lessened since the timer 71 is only recording time while power to the receiving unit 24 is switched on.

The tuning signal generated by the frequency controller 34 for sweeping the FM spectrum is supplied to the tuning coordination unit 43 and serves as a charge source or sink for the capacitor 45. In an optimal configuration, the frequency controller 34 generates a tuning signal which is independent of the voltage across the capacitor 45. As charge is supplied to or drained from the capacitor 45 by the tuning signal, the voltage across the capacitor 45 ramps up or down. Since this voltage is applied to the varactor of the VCO 40, the frequency of the mixing signal varies accordingly, altering the frequency of the received FM signal.

The magnitude of the tuning signal generated by the frequency controller 34 is established according to several constraints. This tuning signal is combined with the tuning signal generated by the charge pump 44, a d it is the composite signal which is applied to the capacitor 45. As the tuning signal from the frequency controller 34 sweeps the tunable FM receiver 31 to a new FM signal, the AFC signal will vary in an attempt to maintain the FM receiver 31 tuned to the old FM signal. Consequently, the magnitude of the tuning signal from the frequency controller 34 must be sufficient both to override the tuning signal generated by the charge pump 44 in response to the AFC signal and to cause the tunable FM receiver 31 to sweep the entire FM spectrum. Yet, the magnitude of the tuning signal from the frequency controller 34 must not be so large . as to cause too steep of a ramp voltage output by the capacitor 45. Otherwise, the tunable FM receiver 31 sweeps across the FM spectrum so rapidly that the pilot signal detector 33 and the frequency controller 34 have insufficient time to detect the presence of a pilot signal and disable the tristate driver 73. In this regard,- it is an advantage to simultaneously tune the VCO 40 by both the tuning signal from the frequency controller 34 and the tuning signal from the charge pump 44 because the tuning signal from the charge pump 44 acts to slow the rate of sweep in the vicinity of an FM signal and thereby provide the pilot signal detector 33 and the frequency controller 34 more time to detect the pilot signal and react accordingly. An alternative embodiment of the receiving unit 24 which allows the use of a smaller capacitor 45 in the tuning coordination unit 43 is shown in FIG. 5. This embodiment includes a digital-to-analog (D/A) converter 74 for generating an additional tuning signal. In the illustrated alternative embodiment, the D/A converter 74 comprises a network of four resistors 75, 76, 80, 81 whose values are in the ratio of 8:4:2:1. Thus, the D/A converter 74 can generate 16 different tuning signals based on the various combinations of MPϋ outputs. The tuning signal generated by the D/A converter 74 is delayed in a fashion similar to the delay for the tuning signal issued by the sweep generator 72 and is then supplied directly to the VCO 40, providing 16 discreet voltage levels to a second varactor 82. These tuning signals serve as coarse adjustments in the received FM signal while the tuning signal generated by the sweep generator 72 in the MPϋ 70 and supplied to the tuning coordination unit 43 serves as a fine adjustment between the coarse adjustments. Consequently, a smaller capacitor 45 may be used since the sweep needn't be across the entire FM spectrum. Alternatively, the sweep generator 72 may be eliminated if the D/A converter 74 is provided with sufficient discreet levels to tune directly to each of the 100 FM channels in the FM spectrum.

An embodiment of the receiving unit 24 which does not incorporate an MPϋ 70 and therefore may be more economical is shown in FIG. 6. This embodiment of the receiving unit 24 comprises a pilot signal detector 33, including a lowpass or bandpass filter 60 for the 130 Hz tone and an amplitude detector 83, and a frequency controller 34, including an oscillator 84 and a tristate driver 85. The amplitude detector .83 both rectifies the received pilot signal, through the interaction of the diode 86 and the bipolar transistor 90, and provides the delay interval between the loss of the pilot signal and the enabling of the tristate. driver 85. The resistor 91 in the amplitude detector 83 takes a certain length of time to charge the capacitor 92 sufficiently to enable the tristate driver 85.

The oscillator 84 generates the tuning signal which is supplied to the tuning coordination unit 43 once the tristate driver 85 is enabled. The oscillator 84 employs CMOS NAND gates (part No. 4011) 93, 94 having an extremely high input impedance so that the frequency of oscillation is sufficiently low, e.g., about .02 Hz, when reasonable component values are used, e.g., a capacitance 95 of 33 microfarads and a resistance of 3.3M ohms for each resistor 96, 97. The tristate driver also includes two CMOS

NA D gates 100, 101 and illustrates the use of directional diodes 102, 103 for converting two-state or binary logic outputs to a tristate signal. The binary logic outputs are normally complementary and are driven to reverse bias by the diodes 102, 103 during the third or high impedance state. Alternatively, an FET analog switch or transmission gate may be used and switched off to attain a third or high impedance state.

Claims

HAT IS CLAIMED IS:

1. In an RF communications system, a device for receiving a specially modulated FM signal transmitted within an FM channel and including a pilot signal identifying the specially modulated FM signal, said device comprising: tunable means for receiving one of a plurality of FM signals, each transmitted within a different FM . channel; means coupled to the tunable receiving means for detecting the pilot signal; means coupled to the pilot signal detecting means for generating a first control signal if the pilot signal is not detected; and means coupled to the tunable receiving means and the first control signal generating means for generating in response to the first control signal a tuning signal for tuning the receiving means to another of the plurality of FM signals.

2. The device of Claim 1 wherein the tuning signal generating means comprises a microprocessor unit including a sweep generator means for generating the tuning signal and timer means for delaying the output of the tuning signal for a certain interval of time after the first control signal is generated. 3. The device of Claim 2 further comprising means for generating a second control signal if the pilot signal is detected and wherein the microprocessor unit further includes means coupled to the second control signal generating means for resetting the timer means to the beginning of the interval in response to the second control signal.

4. The device of Claim 1 wherein the tuning signal generating means comprises a microprocessor unit and a digital-to-analog converter means coupled to the microprocessor unit for generating the tuning signal, said tuning signal generating means further including timer means for delaying the output of the tuning signal for a certain interval of time after the first control signal is generated.

5. The device of Claim 1 wherein the tuning signal generating means includes a delay means coupled to the first control signal generating means and an oscillator coupled to the delay means and the receiving means, said delay means generating a control signal a certain length of time after the first control signal is generated and said oscillator providing the tuning signal to the receiving means upon receiving the second control signal.

6. In an RF communications system, a device for receiving an FM signal transmitted within an FM channel and including a specially modulated subcarrier having a pilot signal for identifying the FM signal, said device comprising: tunable means for receiving one of a plurality of FM signals, each transmitted within a different FM channel, said tunable receiving means including means for detecting the specially modulated subcarrier; means coupled to the subcarrier detecting means for detecting the pilot signal, said pilot signal detecting means including means for generating a first control signal if the pilot signal is not detected; and means coupled to the tunable receiving means and the first control signal generating means for generating in response to the first control signal a first tuning signal for tuning the receiving means to another of the plurality of FM signals.

7. The device of Claim 6 wherein the subcarrier is in the upper portion of the band of frequencies modulating the FM channel carrier signal and is frequency modulated by the pilot signal and wherein the subcarrier detecting means includes a filter passing the subcarrier and an FM detector coupled to the filter for demodulating the subcarrier. 8. The device of Claim 7 wherein the pilot signal comprises a low frequency tone and the pilot signal detecting means includes a filter for passing the low frequency pilot signal and means coupled to the filter for generating a DC signal in response to the pilot signal.

9. The device of Claim 8 wherein the subcarrier comprises one of a plurality of signals within a frequency range from 20 to 75KHz and wherein the pilot signal comprises a 130 Hz tone. io. In an RF communications system, a device for receiving a specially modulated FM signal transmitted within an FM channel and including a pilot signal identifying the specially modulated FM signal, said device comprising: tunable means for receiving one of a plurality of FM signals, each transmitted within a different FM channel, said tunable receiving means including means for memorizing the frequency at which the tunable receiving means is tuned to receive said one FM signal and means for generating a first tuning signal to maintain said tunable receiving means tuned to said one FM signal; means coupled to the tunable receiving means for detecting the pilot signal, said pilot signal detecting means including means for generating a first control signal if the pilot signal is not detected; and means coupled to the tunable receiving means and the first control signal generating means for generating in response to the first control signal a second tuning signal for tuning the receiving means to another of the plurality of FM signals. 11. The device of Claim 10 wherein the tunable receiving means includes a tunable oscillator comprising a voltage-controlled oscillator responsive to a voltage signal and wherein the memorizing means memorizes the voltage signal.

12. The device of Claim 10 wherein the memorizing means retains said frequency through brief power interruptions to the tunable receiving means.

13. The device of Claim 10 wherein the tunable receiving means further comprises means for simultaneously tuning the tunable receiving means by both first and second tuning signals.

14. The device of Claim 13 wherein the first tuning signal generating means includes means for generating an AFC signal in response to the received FM signal and a charge pump coupled to the AFC signal generating means for generating the first tuning signal in response to the AFC signal.

15. In an RF communications system, a device for receiving one of a plurality of modulated signals wherein each modulated signal comprises signals within a frequency band, said one modulated signal including a pilot signal identifying said one modulated signal, said device comprising: tunable means for receiving a first modulated signal, means coupled to the tunable receiving means for detecting the pilot signal, said pilot signal detecting means including means for generating a first control signal if the pilot signal is not detected and means for generating a second control signal if the pilot signal is detected; and means coupled to the tunable receiving means and pilot signal detecting means for generating in response to the first control signal a first tuning signal for electronically tuning the receiving means to a second modulated signal.

16. The device of Claim 15 wherein the tunable receiving means includes means for generating a second tuning signal for maintaining the tunable receiving means tuned to the first modulated signal and wherein the device further comprises means for simultaneously tuning the tunable receiving means by the first and second tuning signals.

17. The device of Claim 15 wherein the first tuning signal generating means includes means for inhibiting said first tuning signal if the second control signal is generated within a certain interval of time after the first control signal is generated.

18. An RF paging system comprising: first means for transmitting a first modulated signal within a first band of frequencies, said first modulated signal including first message-related signals including a pilot signal second means for transmitting a second modulated signal within a second band of frequencies different from the first band of frequencies, said second modulated signal including second message-related signals also including the pilot signal; and means for receiving one of the first and second modulated signals, said receiving means including tunable means for receiving one of a plurality of modulated signals, said plurality of modulated signals including the first and second modulated signals, means coupled to the tunable receiving means for detecting the pilot signal, means coupled to the pilot signal detecting means and the tunable receiving means for tuning the tunable receiving means to another of said plurality of modulated signals if the pilot signal is not detected; and means coupled to the pilot signal detecting means and the tunable receiving means for maintaining the tunable receiving means tuned to said one of a plurality of modulated signals if the pilot signal is detected. 19. The RF paging system of Claim 18 wherein the plurality of modulated signals comprises a plurality of FM signals, each transmitted within a different FM channel and wherein the first and second modulated signals comprise first and second FM channel carriers modulated by a first and second SCA subcarriers, respectively, said first and second SCA subcarriers being modulated by said first and second message-related signals, respectively.

20. The RF paging system of Claim 18 wherein the receiving means includes means for maintaining the tunable receiving means tuned to said one of a first and second modulated signals during brief loss of the signal and during brief loss of power to the tunable receiving means.