Telecommunications – Transmitter and receiver at separate stations
Reexamination Certificate
1993-08-18
2001-05-15
Banks-Harold, Marsha (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
C455S343200, C455S039000
Reexamination Certificate
active
06233431
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to radio receivers, and particularly to systems and circuits which can enhance the operation of paging receivers.
BACKGROUND TO THE INVENTION
A conventional superheterodyne receiver (superhet) is based upon the mixing of a received signal with a local oscillator (“L.O.”) signal to yield a new (intermediate frequency or “I.F.”) signal which is the difference or sum of the L.O. and off-air signal frequencies. Tuning of the receiver is effected by varying the local oscillator frequency, thereby changing the frequency of the off air signal which when mixed with the L.O. signal, will yield a signal at a fixed I.F.
A disadvantage of the superheterodyne receiver is that received signals having a frequency higher or lower than the L.O. frequency and differing from the L.O. frequency by the I.F. either positively or negatively will mix to yield signals at the I.F. This is known as the reception of the “image” and may be partially overcome by the use of a higher intermediate frequency. This is often followed by a second conversion to a lower frequency where adjacent channel filtering is performed. Double superheterodyne receivers commonly have the first I.F. at 10.7 MHz so that the image frequency is 21.4 MHz away from the desired signal.
In order to save battery power, pager receivers “hibernate” or “sleep” for certain periods of time, turning themselves on regularly at predetermined intervals in order to check for addressing from a transmitter. One such form of time division multiplexed addressing is that specified in CCITT 584-1 paging format, also known as POCSAG. A disadvantage of this form of time division addressing is that power consumption is constant regardless of channel traffic. Another disadvantage is that if the receiver is required to receive large amounts of data to be transmitted during off peak hours it is necessary in current state of the art pagers to continuously operate, according to the paging code format.
Current state of the art radio pagers generally have their features and functions defined by firmware designed into the product, and most have a level of programmability, using special attachments, to provide for selection of alternate response characteristics from a small range of options offered. A disadvantage of this is that all of the receiver response characteristics have to be defined at the time of design and the receiver cannot be modified to meet the requirements of a specific user or service provider.
Current state of the art radio pagers generally include vibrators within the body of the pager so that the user may be silently alerted to the receipt of a page. For maximum effectiveness, it is necessary for such device to be firmly coupled to the wearer, as is the case when worn on a belt. The disadvantage of this is that when the pager is not so carried, the vibrator has limited effectiveness and an alternative form of alert is desirable.
Radio paging receivers commonly use replaceable Alkaline batteries or rechargable batteries of the same form interchangeably. The detection of low battery reserve is generally determined by evaluation of the terminal battery voltage under load. A disadvantage of this is that disposable battery discharge characteristics differ from those of rechargeable batteries and accurate detection of low battery reserve for both types using a common detection method is generally only accurate for one type.
AFC (Automatic Frequency Control) is very desirable on UHF (Ultra High Frequency) receivers operating above 500 MHz. At these frequencies the frequency errors in the crystal oscillators used to generate the local oscillator signals in the receiver amount to a few parts per million, and are comparable with, and can exceed, the frequency tolerance of the receiver (for maximum sensitivity). These errors result from sensitivity to temperature and from long term frequency drift caused by aging of the crystal.
AFC is widely used on analog systems. For such receivers on channel center, the integral of the deviation over a period of a few hundred milliseconds or less is zero, and because transmission (and reception) is continuous, capacitors can be used to integrate and store the AFC feed back signal.
Simple analog systems cannot be used in receivers which are strobed (for channel sampling) or which are designed to be used with digital, time dependent addressing techniques (such as CCITT 584-1). There are three principal difficulties:
a) In digital systems, the FSK (Frequency Shift Keyed) modulated carrier may contain very low frequency components due to long streams of 1's or 0's.
b) The periods between samples can be as long as one second so that analog techniques to store feedback signals (such as sample and hold) are marginal and errorrprone.
c) To minimize power consumption, the receiver should be prealigned to minimize the period time required by the receiver to sample the channel.
SUMMARY OF THE PRESENT INVENTION
One embodiment of the present invention is a novel form of superheterodyne receiver which does not have an image frequency.
In this embodiment, near and adjacent signal rejection filtering is largely achieved and the requirements for filtering are much reduced over that of the prior art. Further, low cost highly miniaturize IF filter elements, widely used in conventional superheterodyne receivers, can be used.
This receiver, unlike conventional single stage zero I.F. receivers, will reproduce analog frequency modulated signals, essential requirements for voice or fast digital signals.
The receiver of the present invention can be used in a digital radio pager which can have the following important novel features, structures and methods of operation:
1. Operation according to CCITT-584-1 paging format but with fully compliant extensions whereby battery saving can be substantially improved, including a means for the receiver power up infrequently but at defined times to provide for reception of data even when switched off.
2. Automatic Frequency Control of the pager which is able to operate on FSK data without regard to the length of consecutive equal data states transmitted, and which can operate on a time division multiplex address scheme such as CCITT 584-1.
3. Operation according to CCITT 584-1 Paging format but with fully compliant extensions whereby unlimited instructions can be transmitted to one or more than one pager, so as to tailor its response or operating characteristics to suit those of individual users or service providers.
4. Meanings of buttons or keys which constitute the users means of operating the pager can have their function altered, and the new meaning illustrated or indicated by the use of images presented on a dot matrix or other form or programmable display, in close proximity to the control button.
5. A holster designed to carry the pager has an integral clip for attachment to the wearer's apparel, in which is installed a vibrator. A detection means is included so that the alerting means, being audible, optical or vibrational can be automatically changed according to the users programmed preference when in or out of the holster.
6. A means for detecting the rate of change of the battery terminal voltage or internal resistance to provide for accurate low battery detection can be provided for either alkaline or rechargeable batteries.
An “intelligent” automatic frequency control (AFC) system can be provided in which the discriminator output level is integrated for a specific period, starting a fixed period after a logical transition. If data remains good (i.e. the state is retained for a minimum data period) the integrated voltage is stored and compared with a similar sample taken for the other logical state. The AFC signal is generated, proportional to the difference between these two samples, and the correction signal applied to a varactor in the local oscillator circuit, to control the frequency.
A “Script” method for controlling the pager response, comprised of a special escape sequence (e.g. a non-printable character) followed by a
Panther Gyles
Williams J. Peter
Banks-Harold Marsha
Pascal & Associates
Silcom Research Limited
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