GB2266206A - A pager usable with more than one network - Google Patents

Abstract

A pager responsive to broadcasts from two networks consists of; a radio frequency receiver stage consisting of two local oscillators 8, 9, one for each pager network, an antenna 5, amplifier 6, band pass filter 7 and a mixer 11 for mixing signals from one or other local oscillator with signals from the antenna; an intermediate frequency stage; and a control system which electronically switches on and off the local oscillators and which electronically tunes the antenna, amplifier and band pass filter. <IMAGE>

Description

PAGER FIELD OF THE INVENTION The present invention relates to pagers.

PRIOR ART Traditional paging systems comprise a radio broadcasting frequency network for broadcasting to a specific geographical area, typically a city, and a large number of subscribers each carrying a pager, that is to say a portable battery operated radio frequency receiver. The network broadcasts on a particular radio frequency channel chosen from the very limited number of channels which in accordance with international air wave conventions are reserved for pager broadcasts, and the pagers carried by the subscribers are tuned to receive on that particular channel. Alignment is carried out in the same way as for a conventional portable radio, that is to say manual alignment by means of a variable capacitor.

A disadvantage with existing pagers is that they are generally useable in one pager broadcasting network only. A businessman who commutes between two geographical areas each having a separate pager network and requires a pager service in both geographical areas is obliged to purchase and to carry two pagers and to pay subscriptions to the two networks.

OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a pager which is responsive to two or more pager networks to that a user commuting between the two geographical areas need carry only pager and is not obliged to adjust the pager in moving from one network to the other.

It will be appreciated that some networks may be very similar to one another, while others are quite different. Two networks may overlap geographically or may be thousands of miles apart.

Two networks may handle off peak transmissions in the same manner or in different manners (networks transmit and pagers listen less frequently during off peak hours) Two networks may by agreement give the subscriber the same identification code, or in the absence of agreement the subscriber has two different identification codes. It is an object of the present invention to provide a pager which is "universal", that is to say a pager which can be used in a variety of different "two-network" situations.

The invention provides a pager responsive to broadcasts from two or more networks comprising a radio receiver of the type comprising a radio frequency stage followed by an intermediate frequency stage including; an antenna, an amplifier, and a band pass filter, and two or more local oscillators in the radio frequency stage in each of which oscillators a signal may be generated for mixing with a signal received by the antenna, and; a programmable control system which may be programmed to suit the characteristics of the two or more networks in which the pager is intended to be used so as to switch on and off the local oscillators at the appropriate times so as to receive signals from the two or more networks and identify and act on these signals, and which electronically tuned the antenna, amplifier, and band pass filter to optimize reception from the networks.

The pager is pre-programmed by the manufacturer to suit the particular two or more networks.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing the architecture of the pager; Figure 2 is a circuit diagram of the radio frequency (RF) stage of the receiver; Figure 3 is a table illustrating the factory pre-programmed control options that permit different versions of the pager to suit different paging networks; Figure 4 is a timing chart showing how the pager handles a paging signal that switches back and forth between two data transmission rates; Figure 5 is a diagram illustrating how the user's decision and the pager's decision on channel switching are handled in the "intelligent mode", which is described hereinafter.

DETAILED DESCRIPTION Referring to Figure 1, the illustrated pager is for use in two networks and is a double conversion stage digital numeric display pager. It receives on one pager broadcast network frequency at a time and is capable of receiving paging signals on two different pager broadcast network frequencies less than 25 MHz apart in the VHF band. When suitable programmed it can receive paging signals at either 512 bps or 1200 bps.

The receiver is made up of a first conversion radio frequency (RF) section contained iI the box at the top left hand side of Figure 1 and a second conversion intermediate frequency (I.F.) section contained in the box at the top right hand side of Figure 1. The digital portion includes a Post Office Code Standardization Advisory Group (POCSAG) code decoder 1 and a central processing unit (CPU) 2.

The receiver of the pager is designed to be able to switch receiving frequencies with a high sensitivity and a low electromagnetic susceptability. The first conversion RF section at the front end of the receiver allows the capability of switching receiving frequencies. The RF section is directly controlled by the CPU 2 with two control lines 3, 4 at logic levels, one logic level being the inversion of the other Most of the amplification is done at the second conversion I.F.

section and therefore sensitivity is more or less the same for both channels as the frequency of the first I.F. remains unchanged. To achieve low electromagnetic susceptability, the antenna 5, the RF amplifier 6, and the bandpass filter 7 are all made electronically tunable. The two oscillators 8, 9 the power of which the CPU 2 directly controls provides a simple way of changing local oscillator frequencies electronically.

Demodulated signals from the second conversion I.F. stage are fed into the decoder 1 for address codewords comparison. If the stored address codeword matches that of the incoming one, the decoder 1 generates an interrupt to notify the CPU that a call has been received. The CPU controls all the user interfaces and generates an alert when appropriate.

The frequency of the system clock 10 connected to the decoder governs the data rate of the pager and the system clock can be changed to produce pagers for 512 bps and for 1200 bps.

Factory preset control options are stored in the non-volatile memory of the decoder 1 to make different versions of the pager to suit different characteristics or paging networks.

Figure 2 is a circuit of the first conversion RF section of the receiver. The antenna assembly 5 is made of the antenna ANT, a variable capacitor VC1, a diode D1, a decoupling capacitor C4, an isolation resistor R1 and a variable resistor VR1. A bypass capacitor C1 couples the intercepted signal into the RF amplifier 6 comprising two decoupling capacitors C5 and C6, two biasing resistors R14 and R2, a transistor Q1, an inductor L1, a variable capacitor VC2, a diode D2, an isolation resistor R3, and a variable resistor VR2. Bypass capacitors C2 and C3 couple the amplified signal in and out of a band pass filter 7 comprising an inductor L2, a variable capacitor VC3, a diode D3, an isolation resistor R4 and a variable resistor VR3.

The antenna assembly 5, the RF amplifier 6, and the bandpass filter 7 are all made electronically tunable by using step grade P-N junction diodes D1, D2 and D3. A step grade P-N junction diode has doping levels changed like a unit step function and consequently possesses properties of decreasing depletion capacitance with respect to increasing reverse bias voltage owing to the thickening of the depletion layer developed across the P-N junction. Variable resistors VR1, VR2 and VR3 are employed to vary the biasing voltage of the diodes.

The output of the bandpass filter 7 is fed into the mixer 11 through an impedance matching network made up of an inductor L3 and a variable capacitor VC4. The mixer 11 is composed of a transistor Q5, decoupling capacitors C7 and C17, biasing resistors R5 and R12, an intermediate frequency transformer IFT which couples the down coverted signal from the radio frequency stage to the intermediate frequency stage, and a series resonant circuit made up of an inductor L6 and a capacitor C16 resonating at the frequency of the first I.F. which is the difference between the carrier frequency and the local oscillator frequency.

The two local oscillators 8, 9 are identical with the exception of crystals XTAL1 and XTAL2. The first local oscillator 8 is made up of a transistor Q4, a crystal XTAL1, a variable capacitor VC6 to adjust the oscillation frequency, a parallel resonant circuit resonating at twice the crystal frequency formed by an inductor L4 and a variable capacitor VC5 to adjust the amplitude of the oscillation, decoupling capacitors C8 and C10, biasing resistors R7 and R13, and a feedback network with resistor R8 connected in parallel with capacitor C11, the time constant of which is approximately the reciprocal of the crystal's frequency, and a variable capacitor VC6 already mentioned. Likewise, components Q6, XTAL2, VC8, L5, VC7, C12, C14, R9, R10, R11, and C15 make up the second local oscillator 9.

The power into the collectors of these two local oscillators 8, 9 is controlled by the CPU 2 through signal 3/0 V and signal 0/3 V and through level shifters and buffers formed by the transistor Q2 and Q3 and resistors R6 and R15, and through inductors L4 and L5.

The output of the two local oscillators 8, 9 is coupled into the mixer 11 through bypass capacitors C9 and C13. As only one oscillator 8, 9 is turned on at a time, the oscillator 8 or 9 that is off can be regarded as an impedance connected in parallel with the input of the mixer 11 at the base of transistor Q5. The amplified signal and the local oscillator signal are added together at the base of transistor Q5.

Owing to the inherent logarithmic characteristics of transistors, the output at the collector of Q5 is a multiplication of the two signals as log (A + B) = log A x log B ---------(1) where A represents the amplified RF signal, B represents the local oscillator (8, 9) signal, log A represents the amplified RF signal after it has been put through a logarithmic device, and log B represents the local oscillator (8, 9) signal after it has been put through a logarithmic device, If cos (W1t) is the RF signal at the collector of Q5, and cos (W2t, is the local oscillator (8, 9) signal at the collector of Q5, where W1 represents the radian frequency of the RF signal, W2 represents the radian frequency of the local oscillator (8, 9) signal, and t represents time, then the R.H.S. of equation (1) becomes cos[W1t] x cos [W2tj - 1/2 cos [(W1 + W2)t] + 1/2 cos [(W1 W2)t] -------- (2) The second term in equation (2) which represents the difference between the frequencies of the RF and the local oscillator 8, 9 signals is filtered out by the intermediate frequency transformer IFT to be the output of the first conversion RF section.

The signal BIAS is connected to the base of transistors Q1, Q4, Q5 and Q6 through biasing resistors R14, R13, R12 and R10 in order that the whole RF section can be shut down to reduce power consumption.

Figure 3 is a table showing the control options stored in the non-volatile memory of the decoder 1. On power up, the CPU reads the non volatile memory in the decoder and configures the pager accordingly.

Referring to the first column from left to right of Figure 3, there are two ways the address codes of the pager can be arranged. The four address codes of the pager can be split into two groups each containing two address codes or they can all belong to the same group. If they are split into two groups, each group can have a unique frame number assigned to it. The two groups of address codes can therefore be used separately by two paging networks to identify the same pager. In such circulustances, an alert will only be generated by the pager if the address code received belongs to the group associated with the appropriate paging network. Generally, the four by one arrangement is selected if the same set of address codes is used by both paging networks to identify the same pager.The two by two arrangement is selected if different address codes are used by each paging network to identify the same pager. This saves the need for a mutual agreement that an address code which is assigned to a pager by a paging network will not be used by the other paging network.

The second column from left to right of Figure 1 shows how long a pager can be selected to wait on the channel when the pager fails to recognize a paging signal. To determine if a paging signal exists, the pager looks for the preamble and synchronization codeword simultaneously. The detection of either one of them will yield a positive conclusion. The purpose of allowing the pager to wait in the absence of a paging signal is to make the pager applicable in situations where a paging signal from a particular network switches back and forth between two data transmission rates. Further elaborations will follow the description of Figure 4.

The third column from left to right of Figure 3 shows the two modes that the pager can be pre-programmed to operate in.

When pre-programmed to operate in the "basic" mode, the user makes all the decisions on selecting the channel and the pager makes no decision.

When pre-programmed in the "intelligent" mode, both the user and the pager make decisions on selecting the channel. The user's decision can overide the pager's decision and likewise the pager's decision can overide the user's decision. The final decision depends on the physical location of the pager.

Referring to Figure 5, if the pager lies in the area where coverage of paging network A and paging network B overlaps, the user's decision is final. If the pager lies in an area outside the overlapped area, the pager's decision is final.

In the "intelligent" mode of operation, the user can leave the channel switching operation to the pager but any persistence by the user in changing the channel of reception will be respected by the pager. A prequisite for the "intelligent" mode of operation is the continuous transmission of dummy batches made up of idle codewords during the off peak hours of each of the paging networks so as to keep the pager logged into a channel.

Networks have other ways of off peak handling such as stop modulation at mark, at key on position, at key off position, and turning the transmitters off; in these situations the pager is pre-programmed in the "basic" mode.

The last column from left to right of Figure 3 shows how the pager can be adjusted to suit variations in coverage and the way two paging networks are linked. There are two ways the decoder 1 can achieve synchronization with the incoming pager signal.

One way is through intermittent reception and the other way is through continuous reception. The intermittent reception method saves battery but is slow as it relies heavily but not solely on the detection of the preamble which is not transmitted very often. The continuous method is fast as it mainly detects the synchronization codeword which is transmitted at the beginning of every batch and approximates to about once a second for 512 bps signals. The continuous method achieves synchronization rapidly at the expense of battery consumption. Some tradeoffs have to be made based upon the coverage and modulation of the two paging networks in which the pager is intended to be used.

If the two paging networks that provide services to the pater are linked in such a way that they broadcast the same paging signal on their own channels, the pager does not expect the need for resynchronization after channel switching is made. In such a case, a low value for the number of times the decoder 1 goes through fast synchronization process on channel switching helps cut down power consumption.

If the coverage of one paging network is thousands of milts away from another, keeping the receiver turned on while switching channels is a waste of battery when the pager is in an uncovered area.

In an area where radio coverage overlaps and when the paging signal on one channel differs from another, fast synchronization after switching channel will help ensure a higher call successful rate as the blind spot of one paging network might not be a blind spot of the other paging network. Under such circumstances, a high value for the number of times the decoder goes through fast synchronization process on channel switching is preferred.

Referring to Figure 4, (a) is a condition where the air time of a particular network is evenly distributed between two data rates of 512 bits per second and 1200 bits per second. (b) is where most of the air time is used to service the 512 bps pagers and (c) vice versa. (d) shows how the paging signal of (b) is modified by adding 576 bits of preamble at the other data rate prior to the preamble of the signal that is about to be transmitted. By limiting the maximum number of batches a preamble carries to 29 at 512 bps and 69 at 1200 bps for a channel hold time of 32 seconds selected (see Figure 3), the pager will be able to operate in the "intelligent" mode previously described in an environment where data rate switches back and forth. The paging signal of (c) can be similarly modified. The maximum number of batches a preamble carries does not have to be 29 batches and 512 bps and 69 batches at 1200 bps and values on the domain of Figure 4 are just for illustration as there are other values of channel hold time to choose from in Figure 3. Should modifications to the signalling format of a paging network be impossible for some reason, the only way that the pager can adapt itself to data rate switching is to operate in the "basic" mode.

Claims (1)

1A pager responsive to broadcasts from two or more networks comprising a radio receiver of the type comprising a radio frequency stage followed by an intermediate frequency stage including; an antenna, an amplifier, and a band pass filter, and two or more local oscillators in the radio frequency stage in each of which oscillators a signal may be generated for mixing with a signal received by the antenna, and; a programmable control system which may be programmed to suit the characteristics of the two or more networks in which the pager is intended to be used so as to switch on and off the local oscillators at the appropriate times so as to receive signals from the two or more networks and identify and act on these signals, and which electronically tunes the antenna, amplifier, and band pass filter to optimize reception from the networks.