Micro-controller based frequency calibration

Abstract: A radio modem is disclosed that includes a radio receiver for receiving radio signals comprising data frequency multiplexed onto a carrier signal having a carrier frequency. An oscillator generates a reference frequency for an FM discriminator, which is connected to the antenna and to the for demodulating said data from said radio signals using said reference frequency. A microprocessor has an input connected to the output of the FM discriminator. The microprocessor also has an output. The microprocessor is programmed to sample the signal voltage output by the FM discriminator, average the signal voltage samples, and compare the average to a preset voltage. The microprocessor is programmed so that if the average differs from the preset voltage, the microprocessor generates an error signal at the microprocessor output. The oscillator is coupled to the microprocessor output so that the error signal adjusts the frequency of the oscillator.

Description: Background of the Invention The present invention relates to modems, and particularly to packet data radio modems, and the radio communication systems incorporating such modems. In further particularity, the present invention pertains to ensuring that the frequency at the receiving modem matches the carrier frequency of the transmitting modem. Modems modulate digital data onto analog carrier signals for transmission over analog communication channels, and demodulate received signals to recover the digital data. The analog communication channel may be a conventional wired telephone system, or it may be a wireless radio link. The modes of modulating the data onto the carrier signal in radio communication links may be different than the modes in a wire system. A radio communication link provides a number of complications for a modem using that link to transmit data. One technique used by modems for use in radio communication systems is to package the data to be transmitted into discrete packets of data. The data packets are frequency modulated onto the carrier signal. The receiving modem recovers the data from the frequency modulated signal. In a radio communication link such as is used with a packet data radio modem, the carrier signal may not always be present. The periodic lack of a carrier signal may cause the receiving modem to conclude that the communication link has been lost, or to erroneously alter its expectations of the received carrier frequency. As is well known, the quality of a radio transmission link can vary considerably during a single communication session. The radio transmission link may be substantially less than ideal. When the radio link is poor, the receiving modem may misinterpret the data information contained in the received signal. The rate at which errors are made is called the bit error rate. The bit error rate is affected by such factors as thermal noise, oscillator phase noise, group delay distortion, interfering sources, and frequency errors. The present invention reduces errors due to frequency errors. The frequency at which the receiving modem is operating must match the frequency of the transmitting modem. In the case of a radio modem, the mobile modem must match that of the fixed base station with which it is communicating. There are events that occur in radio data transmission that may require that the mobile unit recalibrate its operating frequency. For example, as it moves, the mobile unit may switch to a different operating cell, which has a different fixed base unit. The mobile unit should also retain it knowledge of the carrier signal frequency when it loses the radio signal for short periods of time. Various automatic frequency control techniques for locking the local oscillator of the mobile modem to the same frequency as that of the base station have been proposed. Such techniques include a local oscillator in the mobile unit that is to produce a frequency that matches the frequency of the transmitting station. Circuitry is used to analyze the output of the demodulator and determine if it appears that the carrier signal has been correctly removed from the received signal. However, such techniques generally require extremely accurate (and expensive) components, and careful calibration. Thus, a low cost, simple arrangement for an automatic frequency control technique for locking the local oscillator of the mobile modem to the same frequency as that of the base station is desired. In addition, a low cost, simple mechanism is desired for maintaining the last known frequency during brief periods when the carrier signal is not present. U.S. Patent No. 5,109,544 issued to Mittel et al. is directed to a paging receiver with automatic frequency control. More specifically, this reference discloses tracking the transition of the received signal by counting and averaging high and low signals in order to calculate a frequency error. The frequency error is then used to determine a correction factor which adjusts the frequency of an oscillator. U.S. Patent No. 5,606,731 issued to Pace et al. is directed to a Barber type zero-intermediate frequency (zero-IF) receiver circuit which is configured to receive a radio frequency signal detected by an antenna. Such receiver circuit comprises oscillator circuits which are identical and track each other for providing an accurate reference to process a demodulated signal. This allows the receiver circuit to have an improved battery life and to eliminate the need for a high stability local oscillator and expensive crystal. GB Patent App. No. 2,287,144 assigned to Motorola is directed to a radio device such as a radio receiver or a radio transceiver. Such radio device comprises an adjustable reference oscillator which is adapted to provide a reference frequency signal. Automatic frequency control is implemented to adjust the reference oscillator so that it may provide the desired reference frequency signal. Summary of the Invention Aspects of the invention are set out in claims 1, 7, 10 and 16 appended hereto. Embodiments include an automatic frequency control system for radio modem that uses a microprocessor to generate a feedback signal for adjusting the receiver's frequency so that the receiver's frequency coincides with the carrier frequency of the received signal. A preferred embodiment is a radio modem that includes a radio receiver for receiving radio signals comprising data frequency multiplexed onto a carrier signal having a carrier frequency. An oscillator generates a reference frequency for an FM discriminator, which is connected to the antenna and to the for demodulating said data from said radio signals using said reference frequency. A microprocessor has an input connected to the output of the FM discriminator. The microprocessor also has an output. The microprocessor is programmed to sample the signal voltage output by the FM discriminator, average the signal voltage samples, and compare the average to a preset voltage. The microprocessor is programmed so that if the average differs from the preset voltage, the microprocessor generates an error signal at the microprocessor output. The oscillator is coupled to the microprocessor output so that the error signal adjusts the frequency of the oscillator. Description of the Drawing Figure 1 is a block diagram of a radio transmitter and receiver system incorporating the present invention. Figure 2 is a block diagram of a radio transceiver incorporating an embodiment of the present invention for use in a packet data radio modem. Figure 3 illustrates a typical packet data radio signal. Detailed Description The invention will be described in the context of the mobile device in receive mode, receiving data from a transmitting base station. The described embodiment of present invention is a radio transceiver that includes a single voltage controlled, temperature compensated crystal oscillator (VCTCXO) in a phase locked loop (PLL) frequency synthesizer in both transmit and receive modes. A system including a radio communication link appropriate for use with a packet data radio modem is shown conceptually in Figure 1. A radio transmitter 11 frequency modulates digital data onto a carrier signal. The carrier signal has a carrier frequency. The transmitter uses conventional pseudo-random Gaussian Minimum Shift Keyed (GMSK) modulation. A feature of such pseudo-random GMSK modulation is that, over an extended time, the modulation averages out to the DC voltage of the carrier signal. The transmitter sends the modulated signal from a transmit antenna 13. The modulated signal is received at a receive antenna 21, and processed by a receiver 23. The receiver 23 is shown in Figure 2. The radio frequency (RF) signal is received at the antenna 21. A mixer 31 mixes the received signal with a first local oscillation signal 41 for a first downconversion to a first intermediate frequency. A second mixer 33 then mixes the signal with a second local oscillation signal 43 for a second downconversion to a second intermediate frequency. An FM demodulator or discriminator 51 then demodulates the signal. A limiting amplifier 53 ensures a constant amplitude for the signal input to the FM discriminator. The FM demodulator 51 demodulates the data from the carrier signal. If the local oscillator is operating at the same frequency as the oscillator in the transmitter that generates the carrier signal, the output of the FM discriminator will be an analog representation of the data that was modulated onto the carrier signal. The received frequency modulated RF signal is dual downconverted to an intermediate frequency in a conventional manner. In accordance with the invention, a single reference frequency is used for both downconversions. A crystal oscillator 61 generates the frequency that is the reference for the synthesized local oscillator (the first local oscillation signal 41) for downconversion to the first intermediate frequency. In addition, a harmonic of the crystal oscillator frequency is used as the second local oscillator (the second oscillation signal 43) for downconversion to the second intermediate frequency. Thus, the receiver may be locked to the received signal by adjusting a single frequency source, the crystal oscillator 61. An automatic frequency control loop ensures that the local oscillation signals are consistent with the frequency of the carrier frequency of the received radio signal. The automatic frequency control loop includes a single voltage controlled, temperature compensated crystal oscillator (VCTCXO) 61. The VCTCXO is the crystal reference oscillator. The output of the VCTCXO is connected to a phase lock loop (PLL) 63. The output of the PLL is applied to a voltage controlled oscillator 65 to generate the first local oscillation signal 41. The output of the VCTCXO 61 is also applied to a tuned collector amplifier 71 that creates the harmonic of the VCTCXO frequency as the second local oscillation signal 43. The tuned collector amplifier 71 in the illustrated embodiment multiplies the frequency of the VCTCXO 61 by three. The FM discriminator 51 is a conventional limiter-discriminator type FM demodulator. The FM discriminator includes a Gilbert cell mixer or multiplier. In accordance with the invention, a digital microprocessor 121 provides the feedback to the automatic frequency control loop 101 and generates the adjustments necessary. The microprocessor 121 is connected to the output of the FM demodulator 51 to receive the received data output. The microprocessor examines the output of the FM demodulator to determine if the local oscillation frequencies are consistent with the carrier frequency of the received signal. If the microprocessor determines that the local oscillation frequencies are not appropriate, the microprocessor produces an error signal to the automatic frequency control loop. That error signal is used to adjust the frequency of the crystal oscillator VCTCXO 61. The microprocessor 121 may be any conventional microprocessor. Ideally, the microprocessor is the controller microprocessor of the modem. The controller microprocessor controls other radio functions, such as tuning, power management, and timing. By using the controller microprocessor that is already present in the modem, the invention simplifies the modem construction. The microprocessor uses an analog to digital converter to sample the data produced by the FM discriminator. The input to the analog to digital converter of the microprocessor is connected to the output of the FM discriminator. The analog to digital converter may be internal to the microprocessor. As mentioned above, over a substantial period of time, the modulation averages out to the carrier DC voltage. The DC voltage, or offset, at which the receiver is locked is preset and known. If the carrier frequency of the received signal differs from the receiver's lock frequency, the DC offset produced by the output of the FM discriminator will be proportionately higher or lower than the set value. The microprocessor is programmed with a software equivalent of a low-pass filter. This software low pass filter averages the data signal from the FM discriminator. By averaging a large number of samples of the output of the FM discriminator, the microprocessor determines the DC voltage or offset of the FM discriminator output. The difference between the known DC offset and the average of the samples of the output of the FM discriminator is used to generate an error signal. The error signal is used to adjust the frequency of the crystal oscillator. By comparing the DC voltage of the FM discriminator output with the set (and expected) DC voltage, the microprocessor determines the amount that the frequency of the local oscillator should be changed so that the local oscillator frequency matches the carrier frequency of the received signal. The microprocessor produces at an output pin 123 an error signal corresponding to the needed frequency change. That error signal is a digital representation of the voltage that should be applied to the voltage control port of the voltage controlled crystal oscillator 61 to adjust the frequency of the oscillator 61 to correct the frequencies of the first and second oscillation signals 41, 43. That digital signal may be a three bit value. The digital signal is converted to an analog error correction voltage using a digital to analog converter (DAC) 125. The analog voltage from the DAC 125 is applied to the voltage control port of the crystal oscillator 61. The calculated error signal adjusts the frequency of the crystal oscillator in small increments. The microprocessor may generate an incorrect error signal (correction voltage) if it has averaged a statistically inadequate number of samples of the output of the FM discriminator. For example, when a calibration is initially attempted, the system may attempt to set the oscillator voltage before sufficient samples of the output of the FM discriminator have been made. Therefore, an additional check on the calculated correction voltage is made to determine if the calibration calculations have settled on a stable calibration value. The microprocessor compares the calculated correction voltage to the current correction voltage. If the newly calculated correction voltage and the current correction voltages are equal (within a tolerance), the system concludes that the microprocessor has converged to a stable correction value for the oscillator. The microprocessor then stops calculating a value for the error correction voltage. If, however, the newly calculated voltage and the current voltage differ significantly, the digital voltage signal is incremented or decremented (as appropriate). The calculation process is then repeated. A maximum number of iterations is set to ensure that the system is not attempting to calibrate to an erroneous or inappropriate signal frequency. In addition, the range of the voltage output is limited to prevent calibration to an off-frequency base station. The above calibration process is ideally repeated periodically. Periodic repetition assures that the mobile station will track frequency variations that arise due to temperature. In addition, periodic repetition allows the mobile unit to correct itself if it has converged on an incorrect frequency. The calibration process is also repeated whenever the radio modem is switched to a new base station. In addition, the microprocessor samples the Received Signal Strength Indication (RSSI) in the downconverted signal. The microprocessor may have a second analog to digital converter input for sampling the RSSI. If the RSSI is high enough to indicate that the received signal is strong enough to be used as a reference, the above calibration process is undertaken. A low RSSI indicates that the signal detected is not truly a modulated data signal. By examining for sufficient RSSI strength before undertaking the calibration process, the system avoids unnecessary attempts to calibrate to spurious signals, such as may occur when the transmitting modem is not actually transmitting. The output of the digital error correction voltage from the microprocessor is maintained during periods when the carrier signal drops out so that the lock on the carrier frequency is not lost during carrier off times. The memory capabilities of a digital microprocessor can be used as a sample and hold arrangement to store error parameters when the carrier is off. Figure 3 illustrates a packet radio RF link, showing that during periods 131 the carrier signal is on and contains data. During periods 133, the carrier signal is off. The DC voltage that indicates that the modem is frequency locked to the frequency of the carrier signal produced by the base station is stored in the modem's non-volatile random access memory (RAM). After the calibration process, the voltage signal produced by the microprocessor to the crystal oscillator (calibration value) is also stored in the modem's non-volatile RAM, prior to removing power from the modem. That value is used as the starting value for the first calibration after power is restored to the modem. When the transceiver is operating in transmit mode, direct modulation of the crystal oscillator is employed.

Claims: A radio modem, comprising: an antenna (21) for receiving radio signals comprising data frequency multiplexed onto a carrier signal having a carrier frequency; an oscillator (61) for generating a reference frequency; an FM discriminator (51) connected to said antenna and to said oscillator for demodulating said data from said radio signals using said reference frequency; and a microprocessor (121) having an input connected to the output of said FM discriminator, and an output, wherein: said microprocessor is programmed to sample the signal voltage output by said FM discriminator; said microprocessor is programmed to average said signal voltage samples; said microprocessor is programmed to compare said average to a preset voltage; and said microprocessor is programmed so that if said average differs from said preset voltage, said microprocessor generates an error signal (123) at said microprocessor output only when said microprocessor detects that the received signal has a strength above a preset threshold. wherein said oscillator (61) is coupled to said microprocessor output so that said error signal adjusts the frequency of said oscillator. The radio modem of Claim 1, wherein said preset voltage corresponds to a particular signal frequency, such that if said carrier frequency differs from said particular frequency, the difference between the average of said voltage samples and said preset voltage corresponds to the difference between said carrier frequency and said particular frequency. The radio modem of Claim 2, wherein said oscillator (61) comprises a voltage controlled oscillator, and said modem additionally comprises a digital to analog converter (125) coupled between said microprocessor output and said oscillator, wherein said digital to analog converter converts said error signal (123) into a voltage to adjust said voltage controlled oscillator. The radio modem of Claim 3, additionally comprising a tuned collector amplifier (71) coupled to said oscillator for generating from the frequency produced by said oscillator a harmonic of said frequency, wherein said FM discriminator (51) uses said reference frequency and said harmonic of said frequency. The radio modem of Claim 1, additionally comprising nonvolatile RAM, wherein: said preset voltage is stored in said nonvolatile RAM; and said microprocessor stores said error signal in said nonvolatile RAM. The radio modem of Claim 1, wherein said microprocessor is additionally programmed to stop generating said error signal if a newly computed error signal is substantially equal to a previously computed error signal. A receiver for a packet data radio modem, said receiver comprising: a radio antenna (21) for receiving a radio signal comprising data frequency modulated onto a carrier signal; a voltage controlled oscillator (61) for producing a reference frequency; a downconverter (31) coupled to said antenna and to said oscillator for downconverting said radio signal to an intermediate frequency using said reference frequency; an FM discriminator (51) coupled to said downconverter for demodulating said data from said downconverted signal; a microprocessor (121) coupled to the output of said FM discriminator, wherein said microprocessor: includes an analog to digital converter for sampling the signal at the output of said FM discriminator; computes the average of said samples of said FM discriminator signal; compares said average with a preset value that corresponds to a particular frequency; and produces an output error signal (123) corresponding to the difference between said average and said preset value only if said microprocessor detects that the signal received at said antenna (21) has strength above a preset threshold.; a digital to analog converter (125) coupled to the output of said microprocessor for converting said microprocessor output error signal into an analog voltage, wherein the output of said digital to analog converter is coupled to said voltage controlled oscillator (61) so that said voltage from said digital to analog converter adjusts the reference frequency produced by said oscillator. The radio modem of Claim 7, wherein said microprocessor (121) is additionally programmed to stop generating said error signal (123) if a newly computed error signai is substantially equal to a previously computed error signal. A method of adjusting the frequency of a radio modem receiver (23) to match a carrier frequency of a received signal, comprising: receiving a radio signal comprising data frequency multiplexed onto a carrier signal (131 ) having a carrier frequency; producing a reference frequency using a voltage controlled oscillator (61); using said reference frequency to demodulate (31, 33) said radio signal to produce a discriminated signal; using a microprocessor (121) to sample said discriminated signal; using said microprocessor to average said samples of said discriminated signal; using said microprocessor to compare said average to a preset voltage; producing from said microprocessor a digital error signal (123) corresponding to the difference between said average of said voltage signal and said preset voltage only if the microprocessor detects that the received signal has a strength above a preset threshold; converting (125) said digital signal from said microprocessor into an analog voltage signal; and applying said analog voltage signal to said voltage controlled oscillator (61) to adjust said reference voltage. The method of Claim 9, wherein said step of using a demodulating said radio signal comprises: generating (71) from said reference signal a harmonic of said reference signal; and using said reference signal and said harmonic signal to downconvert (33) said received signal. The method of Claim 10, wherein said step of using said reference signal and said harmonic signal to downconvert said received signal comprises: combining (31) said reference signal (41) with said received radio signal to produce an intermediate frequency signal; and combining said harmonic signal (43) with said intermediate frequency signal. The method of Claim 9, additionally comprising: storing said digital signal produced by said microprocessor (123) in nonvolatile RAM before turning off said modem; and using said stored digital signal as the initial value for said oscillator (61) to use to generate a reference signal when said modem is turned on. The method of Claim 9, additionally comprising: comparing the new digital signal produced by said microprocessor with a previous digital signal; and if said digital signals are not substantially equal, a voltage corresponding to said new digital signal is not applied to said oscillator (61). The method of Claim 13, wherein said step of comparing said average voltage from said FM discriminator (51) with said preset voltage is not performed if said microprocessor determines that the strength (RSSI) of said received signal is below a predetermined threshold. A radio communication system, comprising: a radio transmitter (11) configured to frequency modulate data onto a carrier signal (131) having a carrier frequency, and to transmit said modulated signal; a radio receiver comprising: an antenna (21 ) for receiving said modulated signal; an oscillator (61) for generating a reference frequency; an FM discriminator (51) connected to said antenna and to said oscillator for demodulating said data using said reference frequency; a microprocessor (121) having an input connected to the output of said FM discriminator, and an output, wherein: said microprocessor is programmed to sample the signal voltage output by said FM discriminator; said microprocessor is programmed to average said signal voltage samples; said microprocessor is programmed to compare said average to a preset voltage; and said microprocessor is programmed so that if said average differs from said preset voltage, said microprocessor generates an error signal at said microprocessor output only if said microprocessor detects that the signal received at said antenna (21) has strength above a preset threshold.; and wherein said oscillator is coupled to said microprocessor output so that said error signal adjusts the frequency of said oscillator; said error signal adjusts the frequency of said oscillator. The radio communication system of Claim 15, wherein said preset voltage corresponds to a particular signal frequency, such that if said carrier frequency differs from said particular frequency, the difference between the average of said voltage samples and said preset voltage corresponds to the difference between said carrier frequency and said particular frequency. The radio communication system of Claim 16, wherein said oscillator (61) comprises a voltage controlled oscillator, and said modem additionally comprises a digital to analog converter (125) coupled between said microprocessor output and said oscillator, wherein said digital to analog converter converts said error signal into a voltage to adjust said voltage controlled oscillator. The radio communication system of Claim 17, additionally comprising a tuned collector amplifier (71) coupled to said oscillator for generating from the frequency produced by said oscillator a harmonic of said frequency, wherein said FM discriminator uses said reference frequency and said harmonic of said frequency.

Inventors: DIFRANCISCO LOUIS R, RAUSCH KRIS, TRAINOR MICHAEL J, DIFRANCISCO, LOUIS R., RAUSCH, KRIS, TRAINOR, MICHAEL J.
Applicants: NORTHROP GRUMMAN CORP, NORTHROP GRUMMAN CORPORATION
Application year: 1997
Main classification: H04H1
IPC codes: H04L27, H03J1