Intelligent automatic deviation compensation for wireless modems

• US 5,408,695 A
• Filed: 12/31/1992
• Issued: 04/18/1995
• Est. Priority Date: 12/31/1992
• Status: Expired due to Fees

• Current Assignee: Radio Mobile Solutions, Inc.
• Original Assignee: Coded Communication Corporation
• Inventors: Dorr, Barry L.
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): LE, THANH C
• Application Number:
  US07/999,308
• Time in Patent Office: 838 Days
• Field of Search: 455/226.1, 455/226.4, 455/67.1, 455/70, 455/71, 455/234.1, 455/234.2, 455/245.1, 455/250.1, 455/254, 455/240.1, 375/8, 375/9, 375/98
• US Class Current: 455/226.1
• CPC Class Codes: H04B 17/104 of other parameters, e.g. DC offset, delay or propagation times
• CPC Class Codes: H04B 17/18 Monitoring during normal operation
• CPC Class Codes: H04L 27/152 using controlled oscillators, e.g. PLL arrangements

Abstract

An apparatus automatically determines the frequency deviation of a transmitter by analyzing a signal received from the transmitter. The apparatus automatically corrects the incoming signal for the improper deviation and maintains a record of the frequency deviation errors. Transmitters which consistently exhibit errors may then be recalled for servicing. This device increases data transmission reliability, improves compliance with FCC licensing regulations and reduces maintenance costs.

Claims

1. An automatic frequency deviation monitoring system comprising:

a remote station communications system, said remote station communications system further comprising;

a remote data processor which generates a data signal having a preamble portion and a data portion;

a remote modem which is connected to said remote data processor, wherein said remote modem receives said data signal from said remote data processor and generates a corresponding modulated data signal; and

a frequency modulated (FM) transmitter which is connected to said remote modem, wherein said FM transmitter receives said modulated data signal from said remote modem and generates a corresponding FM data signal having a frequency deviation value which is characteristic of said FM transmitter;

and

a base station communications system, said base station communications system further comprising;

an FM receiver which receives said FM data signal from said remote station communications system and generates a corresponding audio data signal;

a frequency deviation monitoring feedback loop connected to receive said audio data signal from said FM receiver, said frequency deviation monitoring feedback loop further comprising;

an amplitude detector for measuring the amplitude of said preamble portion of said audio data signal;

a comparator for comparing said measured amplitude of said preamble portion of said audio data signal to a reference value and generating a frequency deviation correction factor which reflects differences between said measured amplitude and said reference value; and

a variable gain element which uses said deviation correction factor to adjust the amplitude of said audio data signal until said measured amplitude of said preamble portion of said audio data signal is substantially equal to said reference value;

and

a base modem, wherein said base modem comprises a controller which is configured to;

(a) sense the presence of said preamble portion of said audio data signal and enable said frequency deviation monitoring feedback loop when the presence of said preamble portion of said audio data signal is initially sensed;

(b) control the digitization and storage of said frequency deviation correction factor which causes said adjusted amplitude of said preamble portion of said audio data signal to be substantially equal to said reference value; and

(c) append said digitized frequency deviation correction factor to said audio data signal.

2. A frequency deviation monitor comprising:

an automatic gain control circuit, said automatic gain control circuit further comprising;

a variable gain element means having a signal input, a signal output and a gain control input, wherein;

(a) said signal input receives an input signal having a first portion and a second portion, said input signal further characterized by an input frequency deviation value, said first portion characterized by a first peak to peak amplitude which is a function of said input frequency deviation value;

(b) said gain control input receives a gain factor; and

(c) said signal output delivers a frequency deviation corrected output signal characterized by a second peak to peak amplitude of said first portion of said signal which is a function of said first peak to peak amplitude and said gain factor;

an amplitude detector means connected to said variable gain element means signal output for measuring said second peak to peak amplitude;

a comparison element means connected to said amplitude detector means for receiving said second peak to peak amplitude and for comparing said second peak to peak amplitude to a reference value, said comparison element means having an output indicative of the real time difference between said second peak to peak amplitude and said reference value;

a signal averager means connected to said comparison element means output for receiving said real time difference between said second peak to peak amplitude and reference value and for producing a frequency deviation correction factor which is proportional to the time average of said real time difference; and

a storage element means connected to said signal averager means for receiving and holding said frequency deviation correction factor, wherein said storage element means makes said frequency deviation correction factor available to said variable gain element means gain control input and to an analog to digital converter;

and

a modem means connected to said automatic gain control circuit for receiving said frequency deviation corrected output signal from said variable gain element means signal output and for decoding said frequency deviation corrected output signal and controlling the operation of said automatic gain control circuit wherein said modem means;

(a) enables said automatic gain control circuit only when it detects the presence of said first portion of said input signal;

(b) sends a begin conversion command to said analog to digital convertor causing it to digitize said frequency deviation correction factor;

(c) decodes said frequency deviation corrected output signal to create a decoded message; and

(d) appends said frequency deviation correction factor held by said storage element means to said decoded message.

3. A frequency deviation monitor comprising:

an input connection for receiving a frequency modulated (FM) signal, wherein a portion of said frequency modulated (FM) signal is deterministic in character;

a discriminator which receives said FM signal and converts said FM signal into a corresponding audio waveform signal;

an automatic gain control feedback loop which receives said audio waveform signal and generates a frequency deviation correction factor for said FM signal by comparing the amplitude of said deterministic portion of said corresponding audio waveform signal to a reference standard, said automatic gain control feedback loop further having means to generate a frequency deviation corrected output signal by applying said frequency deviation correction factor to said audio waveform signal which corresponds to said FM signal; and

a modem for receiving said frequency deviation corrected output signal from said automatic gain control feedback loop and decoding said frequency deviation corrected output signal and controlling said automatic gain control feedback loop wherein said modem;

(a) enables said automatic gain control feedback loop when it detects the presence of said deterministic portion of said audio waveform signal;

(b) sends a begin conversion command to an analog to digital convertor causing it to digitize said frequency deviation correction factor;

(c) decodes said frequency deviation corrected output signal to create a decoded message; and

(d) appends said frequency deviation correction factor to said decoded message.

4. A frequency deviation monitor as defined in claim 3 wherein said automatic gain control feedback loop further comprises a variable gain element having a signal input connected to said discriminator for receiving said audio waveform signal, a signal output and a gain control input, wherein:

(a) said signal input receives said audio waveform signal and said deterministic portion of said signal is characterized by a first peak to peak amplitude;

(b) said gain control input receives said frequency deviation correction factor; and

(c) said signal output delivers said frequency deviation corrected output signal characterized by a second peak to peak amplitude which is a function of said first peak to peak amplitude and said gain factor to said modem.

5. A frequency deviation monitor as defined in claim 4 further comprising an amplitude detector for measuring said second peak to peak amplitude of said frequency deviation corrected output signal.

6. A frequency deviation monitor as defined in claim 5 further comprising a comparison element for comparing said second peak to peak amplitude of said frequency deviation corrected output signal to a reference value, said comparison element having an output indicative of the real time difference between said second peak to peak amplitude and said reference value.

7. A frequency deviation monitor as defined in claim 6 further comprising a signal averager for producing said frequency deviation correction factor which is the time average of said real time difference.

8. A frequency deviation monitor as defined in claim 7 further comprising a storage element for holding said frequency deviation correction factor, wherein said storage element makes said frequency deviation correction factor available to said variable gain element gain control input and to said analog to digital converter.

9. A method for monitoring frequency deviation comprising the steps of:

receiving a frequency modulated (FM) signal having a preamble of known characteristics followed by a data message;

converting said FM signal to a corresponding audio waveform signal;

measuring the peak to peak amplitude of said preamble of said audio waveform signal;

processing said audio waveform signal in a feedback loop by adjusting the gain of a variable gain element in said loop until the peak to peak amplitude of said preamble is substantially equal to a reference value;

storing the value of the gain which causes said peak to peak amplitude of said preamble to be substantially equal to said reference value as a frequency deviation correction factor;

sending said processed audio waveform signal wherein the peak to peak amplitude of said preamble is substantially equal to said reference value to a modem; and

appending said frequency deviation correction factor to said data message.

10. A method for monitoring frequency deviation as defined in claim 9 further comprising the step of transmitting said data message to a data processor.

11. A method for monitoring frequency deviation as defined in claim 10 further comprising the step of maintaining a historical log of said frequency deviation correction factors for data messages received by said data processor.