Digital signal processor exciter

• US 5,418,818 A
• Filed: 09/22/1992
• Issued: 05/23/1995
• Est. Priority Date: 09/22/1992
• Status: Expired due to Fees

• Current Assignee: Glenayre Electronics Incorporated (EDCI Holdings Incorporated)
• Original Assignee: Glenayre Electronics Incorporated (EDCI Holdings Incorporated)
• Inventors: Cox, Charles B., Hinkle, Richard J., Shade, Robert J., Linderer, Timothy J., Goud, Paul A., Kroeger, David W., Marchetto, Robert F., Stewart, Todd A.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Le, Amanda T.
• Application Number:
  US07/950,241
• Time in Patent Office: 973 Days
• Field of Search: 375/5, 375/39, 375/59, 375/69, 375/106, 375/62, 375/107, 455/51.2, 455/92, 455/102, 455/112, 455/118, 455/151.1, 455/154.1, 455/22, 332/103, 332/117
• US Class Current: 375/264
• CPC Class Codes: H03C 1/00 Amplitude modulation H03C5/00, H03C7/00 take precedence

Abstract

A digital exciter (30) for selectively modulating digital or analog input data. The digital exciter includes a digital signal modulator (32) and a digital quadrature modulator (DQM)(36) comprising two digital signal processors (DSPs). The signal modulator is controlled by a control (40). An operator can selectively determine whether the digital exciter is used for modulating either two level or four level NRZ digital data and whether the device is to provide linear modulation or frequency modulation (FM) of the input signal. An interpolator (38) interpolates a 662/3 kHz timer interrupt rate used in the signal modulator to a 400 kHz rate, thereby reducing the processing load on the DSP comprising the DQM by simplifying the sine and cosine values used in the quadrature modulation. An operator can select from among a plurality of operating parameters on a menu for controlling the signal modulator, either from a local or a remote video display terminal (VDT).

Claims

1. An exciter for a radio transmitter, comprising:

(a) means for supplying an input signal to the exciter for processing and transmission as a radio signal;

(b) input signal modulating means, coupled to the means for supplying the input signal, for modulating the input signal and producing a modulated signal that is complex, having both in-phase and quadrature components, said input signal modulating means including a digital voltage controlled oscillator that comprises a first digital signal processor, said first digital signal processor producing the in-phase and quadrature components by processing the input signal with quadrature related periodic functions;

(c) interpolating means, coupled to the input signal modulating means to receive the modulated signal, for sampling the modulated signal to produce sampled values and interpolating between the sampled values of the modulated signal to determine interpolated values intermediate the sampled values, thereby increasing an effective sample rate at which the modulated signal is sampled, the sampled and interpolated values together comprising an interpolated signal;

(d) quadrature modulating means, coupled to the interpolating means to receive the interpolated signal, for converting the interpolated signal from a complex signal to a pass band signal that is not complex; and

(e) up converting means, coupled to the quadrature modulating means to receive the pass band signal, for converting the pass band signal from a lower frequency signal to a higher frequency signal for output and transmission as the radio signal.

2. The exciter of claim 1, wherein the up converting means comprise:

(a) a digital-to-analog converter that receives the pass band signal and converts it from a digital signal to an analog signal;

(b) a first local oscillator that produces a first periodically varying signal having a frequency higher that of the pass band signal;

(c) a first multiplier that multiplies the first periodically varying signal by the analog signal to produce a first intermediate frequency signal;

(d) a band pass filter that filters the first intermediate frequency signal to produce a filtered signal;

(e) a second local oscillator that produces a second periodically varying signal having a frequency higher than that of the filtered signal; and

(f) a second multiplier that multiplies the second periodically varying signal by the filtered signal to produce a second intermediate frequency signal, which comprises the output signal.

3. The exciter of claim 1, wherein the interpolating means and the quadrature modulating means comprise a second digital processor.

4. The exciter of claim 1, wherein the input signal modulating means further include a low pass filter coupled to receive and filter the input signal before the input signal is supplied to the digital voltage controller oscillator.

5. The exciter of claim 1, wherein the first digital signal processor includes memory in which a lookup table of a limited number of values for the periodic functions are stored, said first digital signal processor interpolating values for the periodic functions intermediate those that are stored in the lookup table to improve the resolution with which the values of the periodic functions used to determine the quadrature and in-phase components are defined.

6. The exciter of claim 5, wherein the first digital signal processor interpolates values of the periodic functions a second order interpolation.

7. The exciter of claim 1, wherein the quadrature modulating means include complex multiplying means that operate at a frequency selected so that the quadrature modulation can be implemented by multiplying the interpolated signal by sine and cosine functions having integer values.

8. The exciter of claim 1, wherein the means for supplying the input signal comprise an edge detector that produces an edge detect signal when a data input signal changes logical state, and wherein the input signal modulating means include a timer responsive to the edge detect signal, said timer determining a time interval between when the input signal was last sampled at a predefined sample rate and the time at which the edge detect signal is produced, said input signal modulating means including filtering means to scale samples of the input signal based on the time interval to determine data transitions that are synchronous with changes in the logical state of the data input signal.

9. The exciter of claim 8, wherein the filtering means comprise a finite impulse response low pass filter.

10. The exciter of claim 1, wherein the input signal modulating means comprise frequency shifting means for adjusting a center frequency of the complex signal by a variable increment.

11. The exciter of claim 1, wherein the input signal modulating means include modulation selecting means that enable operator selection of one of a linear modulation and a frequency modulation of the input signal.

12. A method for using an input signal to modulate a radio frequency output signal for transmission, comprising the steps of:

(a) processing the input signal with quadrature related periodic functions using a digital signal processor, to produce a digital complex signal having both in-phase and quadrature components;

(b) sampling the digital complex signal to produce sampled values;

(c) interpolating between the sampled values of the digital complex signal to determine interpolated values intermediate the sampled values, thereby increasing an effective sample rate at which the digital complex signal is sampled, sampled and interpolated values together comprising an interpolated signal;

(d) convening the interpolated signal from a complex signal to a pass band signal that is not complex; and

(e) converting the pass band signal from a lower frequency signal to a higher frequency signal for output and transmission as a modulated radio signal.

13. The method of claim 12, wherein the step of converting the pass band signal comprises the steps of:

(a) converting the pass band signal from a digital signal to an analog signal;

(b) producing a first periodically varying signal having a frequency higher than that of the pass band signal;

(c) multiplying the first periodically varying signal by the analog signal to produce a first intermediate frequency signal;

(d) band pass filtering the first intermediate frequency signal to produce a filtered signal;

(e) producing a second periodically varying signal having a frequency higher than that of the filtered signal; and

(f) multiplying the second periodically varying signal by the filtered signal to produce a second intermediate frequency signal, which comprises the output signal.

14. The method of claim 12, wherein the step of processing the input signal further comprises the step of low pass filtering the input signal to produce a filtered input signal that is then processed with the quadrature related periodic functions.

15. The method of claim 14, wherein the step of processing the input signal further comprises the steps of:

(a) storing a limited number of values for the periodic functions in a lookup table;

(b) interpolating values for the periodic functions intermediate those that are stored in the lookup table to improve the resolution with which the values of the periodic functions used to define the in-phase and quadrature components are determined.

16. The method of claim 12, further comprising the step of converting the in-phase and quadrature components from digital signals to analog signals, after step (d) and before step (e).

17. The method of claim 12, wherein the input signal is a digital signal, further comprising the steps of:

(a) producing an edge detect signal when the input signal changes logical state;

(b) sampling the input signal at a predefined sample rate;

(c) determining a time interval between when the input signal was last sampled at the predefined sample rate and the time at which the edge detect signal was produced; and

(d) scaling samples of the input signal based on the time interval to determine data transitions that are synchronous with changes in the logical state of the data input signal.

18. The method of claim 12, further comprising the step of adjusting a center frequency of the digital complex signal by a variable increment.

19. The method of claim 12, wherein the step of processing comprises the step of selecting one of a linear modulation and a frequency modulation of the input signal.

20. In an exciter for a radio transmitter, apparatus for selectively setting and adjusting a characteristic parameter used in modulating a radio frequency signal produced by the exciter, without the need for monitoring the radio frequency signal to determine the effect of such adjustment, said apparatus comprising:

(a) digital signal processor means, coupled to receive an input signal, for producing a complex modulated signal in which the input signal determines a modulated condition according to a selected modulation scheme;

(b) processor control means, coupled to the digital signal processor means, for controlling the digital signal processor means according to a set of predefined program instructions, said processor control means using an operator selected digital value for said characteristic parameter to control the digital signal processor means; and

(c) operator interface means, coupled to the processor control means, for accepting an operator selection of the digital value for said characteristic parameter, said processor control means thereby supplying the digital signal processor means with said digital value for use in producing the complex modulated signal and thus eliminating any drift or variation in said characteristic parameter with time and as a function of changes in ambient conditions that might otherwise result from an analog adjustment of said characteristic parameter.

21. The apparatus of claim 20, wherein the characteristic parameter comprises one or more parameters selected from a group that includes:

a frequency deviation limit for a frequency shift keying modulation, an exciter center frequency, an analog frequency deviation limit, an analog frequency deviation level, a frequency shift keying frequency deviation level, an analog signal modulation delay, and a digital signal modulation delay.

22. The apparatus of claim 20, wherein the operator interface means comprise a remote terminal that is coupled to the processor control means from a different geographical location than that where the processor control means are disposed, said remote terminal enabling an operator at the different geographical location to select the characteristic parameter.

23. The apparatus of claim 20, wherein the operator interface means comprise a key panel and a display, the key panel including a plurality of discrete switches selectively actuated by the operator to select the digital value for said characteristic parameter.

24. The apparatus of claim 20, wherein the processor control means monitor the modulation condition, producing a signal indicative of said modulation condition, said operator interface means displaying a value corresponding to the signal to an operator.

25. The apparatus of claim 24, wherein the signal indicative of the modulation condition is selected by the operator from among a group of signals that include:

an input data detection signal, a modulation mode signal, a frequency deviation limit signal for frequency shift keying modulation, an analog peak frequency deviation signal, an analog frequency deviation level signal, a frequency shift keying frequency deviation level signal, an analog signal delay signal indicative of a delay applied to the modulation of an analog input signal, a digital delay signal indicative of a delay applied to the modulation of a digital signal, and a center frequency offset signal.

26. The apparatus of claim 24, wherein the operator interface means comprise a remote terminal that is coupled to the processor control means from a different geographical location than that where the processor control means are disposed, said remote terminal enabling an operator at the different geographical location to access the signal indicative of the modulation condition and to select from among a group of such signals, said group including:

an input data detection signal, a modulation mode signal, a frequency deviation limit signal for frequency shift keying modulation, an analog peak frequency deviation signal, an analog frequency deviation level signal, a frequency shift keying frequency deviation level signal, an analog signal delay signal indicative of a delay applied to the modulation of an analog input signal, a digital delay signal indicative of a delay applied to the modulation of a digital signal, and a center frequency offset signal.

27. The apparatus of claim 20, wherein the digital signal processor means includes a selectable pre-emphasis filter, said processor control means responding to an operator input to cause the digital signal processor means to selectively use the pre-emphasis filter when producing the complex modulated signal.

28. A method for selectively setting and adjusting a characteristic parameter used by an exciter in a radio transmitter for modulating a radio frequency signal, without the need for monitoring the radio frequency signal to determine the effect of such adjustment, said method comprising the steps of:

(a) producing a complex modulated signal having both in-phase and quadrature components, the input signal being used in modulating the complex modulated signal to produce the radio frequency signal, according to a selected modulation scheme;

(b) accepting an operator selection of a digital value for said characteristic parameter; and

(c) using said digital value selected by the operator in producing the complex modulated signal, so that drift and variations in said characteristic parameter with time and as a function of changes in ambient conditions, as might otherwise have occurred when an analog adjustment and setting for said characteristic parameter had otherwise been used, are eliminated.

29. The method of claim 28, wherein the step of accepting an operator selection comprises the step of the operator selecting the characteristic parameter from among a group that includes:

a frequency deviation limit for a frequency shift keying modulation, an exciter center frequency, an analog frequency deviation limit, an analog frequency deviation level, a frequency shift keying frequency deviation level, an analog signal modulation delay, and a digital signal modulation delay.

30. The method of claim 28, wherein the step of accepting the operator selection of the digital value includes the step of transmitting the digital value selected by the operator from a different geographic location than that where the radio frequency signal is being modulated.

31. The method of claim 28, wherein the step of accepting the operator selection of the digital value includes the step of selecting the digital value by entering the digital value on a key pad.

32. The method of claim 28, further comprising the steps of:

(a) producing a signal indicative of the modulation condition of the RF signal; and

(b) displaying a value corresponding to said signal to the operator.

33. The method of claim 32, further comprising the step of selecting the signal indicative of the modulation condition from a group of signals that include:

an input data detection signal, a modulation mode signal, a frequency deviation limit signal for frequency shift keying modulation, an analog peak frequency deviation signal, an analog frequency deviation level signal, a frequency shift keying frequency deviation level signal, an analog signal delay signal indicative of a delay applied to the modulation of an analog input signal, a digital delay signal indicative of a delay applied to the modulation of a digital signal, and a center frequency offset signal.

34. The method of claim 32, wherein said steps of producing the signal and displaying the value are responsive to an operator disposed a geographically remote location, enabling that operator to access the signal as it is displayed at the remote location.

35. The method of claim 28, further comprising the step of selectively using a pre-emphasis filter in modulating the RF signal.

36. The method of claim 28, further comprising the step of selecting a mode for modulating the radio frequency signal, said mode thus selected including one of a data mode in which modulation of the radio frequency signal with a digital input data signal occurs, and an analog mode in which modulation of the radio frequency signal with an analog input data signal occurs.

37. An exciter for a radio transmitter in a simulcast radio system comprising a plurality of transmitters that transmit a common signal at about the same time comprising:

(a) digital signal processor means, coupled to receive an input signal, for producing a complex modulated signal in which a carrier signal is modulated by the input signal, thereby producing a modulated signal;

(b) delay means for delaying a signal that is being modulated by the digital signal processor means, for a selected interval of time; and

(c) means for operator entry of the selected interval of time used by the delay means, said selected interval of time being selected by an operator so that the delay means delay the signal being modulated by the digital signal processor means sufficiently to synchronize the modulated signal with corresponding modulated signals produced by other transmitters in the simulcast radio system.

38. In an exciter for a radio transmitter used in a simulcast radio system that include a plurality of transmitters, a method for synchronizing a signal transmitted from the radio transmitter with corresponding signals transmitted from other transmitters in the simulcast radio system, comprising the steps of:

(a) producing a complex modulated signal in a digital exciter in which a carrier signal is modulated by an input signal;

(b) entering a delay time into the digital exciter, a duration of said delay time being selected so as to synchronize the complex modulated signal with corresponding modulated signals transmitted from other transmitters in the simulcast radio system; and

(c) at a point integral with the digital exciter, delaying a signal that is being modulated, for the delay time.

39. An exciter for a radio transmitter, comprising:

(a) means for supplying an input signal to the exciter for processing and transmission as a radio signal, said means for supplying the input signal including an edge detector that produces an edge detect signal when a data input signal changes logical state;

(b) input signal modulating means, coupled to the means for supplying the input signal, for modulating the input signal and producing a modulated signal that is complex, having both in-phase and quadrature components and wherein the input signal modulating means include a timer responsive to the edge detect signal, said timer determining a time interval between when the input signal was last sampled at a predefined sample rate and the time at which the edge detect signal is produced;

(c) interpolating means, coupled to the input signal modulating means to receive the modulated signal, for sampling the modulated signal to produce sampled values and interpolating between the sampled values of the modulated signal to determine interpolated values intermediate the sampled values, thereby increasing an effective sample rate at which the modulated signal is sampled, the sampled and interpolated values together comprising an interpolated signal;

(d) quadrature modulating means, coupled to the interpolating means to receive the interpolated signal, for converting the interpolated signal from a complex signal to a pass band signal that is not complex; and

(e) up converting means, coupled to the quadrature modulating means to receive the pass band signal, for converting the pass band signal from a lower frequency signal to a higher frequency signal for output and transmission as the radio signal.

40. The exciter of claim 39, wherein said input signal modulating means further include filtering means to scale samples of the input signal based on the time interval to determine data transitions that are synchronous with changes in the logical state of the data input signal.

41. A method for using a digital input signal to modulate a radio frequency output signal for transmission, comprising the steps of:

(a) producing an edge detect signal when the digital input signal changes logical state,

(b) sampling the digital input signal at a predefined sample rate;

(c) determining a time interval between when the digital input signal was last sampled at the predefined sample rate and the time at which the edge detect signal was produced;

(d) scaling samples of the digital input signal based on the time interval to determine data transitions that are synchronous with changes in the logical state of the digital input signal,

(e) processing the data transitions to produce a digital complex signal having both in-phase and quadrature components;

(f) sampling the digital complex signal to produce sampled values;

(g) interpolating between the sampled values of the digital complex signal to determine interpolated values intermediate the sampled values, thereby increasing an effective sample rate at which the digital complex signal is sampled, sampled and interpolated values together comprising an interpolated signal;

(h) converting the interpolated signal from a complex signal to a pass band signal that is not complex; and

(i) converting the pass band signal from a lower frequency signal to a higher frequency signal for output and transmission as a modulated radio signal.

42. An exciter for a radio transmitter comprising:

(a) digital signal processor means, coupled to receive an input signal, for modulating the input signal to produce a complex modulated signal according to a selected modulation scheme;

(b) delay means for delaying a signal that is being modulated by the digital signal processor means for a selected interval of time;

(c) means for selectively determining a mode in which the digital signal processor means operate, including an analog mode in which the input signal is an analog signal, and a data mode in which the input signal is a data signal; and

(d) mode transition means for switching between the analog mode and the data mode, said mode transition means setting the input signal to zero during a switching between the analog mode and the data mode, said delay means delaying one of the data signal and the analog signal so as to equalize times required by the digital signal processor means to process the analog and the data signals, thereby ensuring that at most, an insignificant time shift in a modulated signal output from the exciter occurs when a change between the data and analog modes occurs.

43. In an exciter for a radio transmitter, a method for equalizing delays between modulation of an analog signal and modulation of a data signal, comprising the steps of:

(a) modulating an input signal to produce a complex modulated signal according to a selected modulation scheme;

(b) delaying a signal that is being modulated for a selected interval of time;

(c) selectively determining a mode in which the exciter operates, including an analog mode in which the input signal is an analog signal, and a data mode in which the input signal is a data signal;

(d) setting the input signal to zero during a switching between the analog mode and the data mode; and

(e) delaying one of the data signal and the analog signals so as to equalize times required to modulate the analog and the data signals, thereby ensuing that at most, an insignificant time shift in a modulated signal output from the exciter occurs when the exciter changes between the data and analog modes.