Abstract:
A built in test circuit within a frequency synthesizer for use in selective call radio receivers provides a selectable one of a plurality of internal signals as a test signal for diagnosing the frequency synthesizer&#39;s output in real time.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to a frequency synthesizer, and more particularly to a frequency synthesizer having a built in test circuit for sampling in real time a number of signals within the frequency synthesizer for diagnosing the output of the frequency synthesizer. 
     BACKGROUND OF THE INVENTION 
     Typically, a frequency synthesizer comprises a first divide-by counter providing a reference signal to a phase detector in response to an oscillator signal from a crystal controlled reference oscillator. The phase detector provide a filtered d.c. voltage to a voltage controlled oscillator. A second divide-by counter provides a feedback signal from the output of the voltage controlled oscillator to the phase detector. This circuit arrangement is commonly referred to as a phase locked loop. Frequency synthesizers provide virtually an unlimited number of discrete frequencies directly related to the frequency of the reference oscillator for mixing with the carrier frequency within a selective call radio receiver. However, a disadvantage of known frequency synthesizers is that there is no known method of testing signals within the synthesizer in real time for diagnosing a faulty output signal. 
     Thus, what is needed is a frequency synthesizer having a built in test circuit for sampling in real time a number of signals within the frequency synthesizer for diagnosing the output of the frequency synthesizer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved frequency synthesizer. 
     Another object of the present invention is to provide a built in test system for a frequency synthesizer within a selective call radio receiver. 
     In carrying out the above and other objects of the invention in one form, there is provided a phase locked loop comprising a first counter providing a first signal having a first frequency in response to a reference signal having a reference frequency and a phase detector providing second and third signals in response to the first signal and a fourth signal. A sink source float provides a fifth signal in response to the second and third signals and an oscillator provides an output signal in response to the fifth signal. A second counter provides the fourth signal in response to the output signal and a test controller selectively provides selected ones of the reference, first, second, third, fourth, and fifth signals as a test output. 
     The above and other objects, features, and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a plurality of phase locked loops coupled to a serial peripheral interface. 
     FIG. 2 is a block diagram of a phase locked loop in accordance with the present invention. 
     FIG. 3 is a block diagram of a test controller within each of the phase-locked loops. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, phase locked loops 11, 12, and 13 each serve as a frequency synthesizer for a selective call radio receiver such as a pager. Although three phase locked loops are shown, the invention would work equally as well with any number of phase locked loops. A serial peripheral interface 14 has three inputs comprising SELECT, CLOCK, and DATA IN, and one output DATA OUT and is coupled by the bus 10 to the phase locked loops 11, 12, and 13 for transmitting these signals therebetween. A crystal controlled reference oscillator 15 provides an oscillator frequency to a reference divider 16 for providing a reference frequency F REF  to each of the phase-locked loops 11, 12, and 13. 
     In the normal data operation, the select signal SELECT enables the system and the clock signal CLOCK clocks in the input signal DATA IN. The input signal DATA IN comprises an eight bit control word for selecting one of the phase locked loops 11, 12, and 13 and programming the selected loop for a desired frequency. The control word is followed by four eight bit data words transmitting data to the selected loop. Only one reference oscillator 15 is required for a plurality of phase locked loops 11, 12, and 13. 
     Referring to FIG. 2, each of the phase-locked loops 11, 12, and 13 comprises a divide-by-K counter 21 providing an output F.sub.φ, where K is a number programmed by the data control word that selects the desired loop reference frequency. A phase frequency detector 22 provides outputs pump up PU and pump down PD signals to the sink source float 23 indicative of the phase/frequency relation between inputs from the divide-by-K counter 21, signal F REF , and the sink source float 23 by the feedback loop, signal F V , from the divide-by-N counter 24. The pump up PU and pump down PD signals are digital signals triggered by the loop reference frequency F.sub.φ  and the signal F V , respectively, that dictate pulses representing the output voltage from the sink source float 23. The divided-by-N counter 24 provides the signal F V  for matching the signal F.sub.φ in frequency and phase. The filter 25 provides a filtered DC control voltage to a voltage controlled oscillator VCO 26. The memory 27 comprises a plurality of registers for clocking data in from the serial peripheral interface 14 to both the divide-by-K and divide-by-N counters 21 and 24. 
     In accordance with the present invention, the test controller 28 allows for the monitoring of critical signals within each phase locked loop through a single output line TEST OUT, which is routed to the serial peripheral interface 14 and provided as the output DATA OUT during the test mode. Once the test mode is selected for a particular loop, one of six internal signals may be monitored in real time through three programming bits. During the diagnostic mode, a control word and a single data word are clocked in as the input signal DATA IN. The control word enables the test mode, selects one of the phase locked loops 11, 12, or 13, and selects the desired signal F REF , F.sub.φ, F V , PU, PD, PCOUT, SQ, and the output DATA from the plurality of flip-flops 31 as output TEST OUT. 
     Referring to FIG. 3, the test controller 28 comprises a plurality of flip-flops 31 (registers) coupled in series for series for clocking the eight bit words through the test controller 28. A plurality of flip-flops 32 (latches) are coupled in series for latching the eight bit word and providing signals TSTK and TSTN to the divided-by-K and divided-by-N counters, respectively, for selecting additional signals within the respective counter. The multiplexer 33 provides one of the signals F REF , F.sub.φ, F V , PU, PD, PCOUT, SQ, and the output DATA from the plurality of flip-flops 31 in response to the eight bit test word. Signal PCOUT is, for example, a signal internal to the divide-by-N counter 24 that provides an indication of the functionality of the counter 24. Signal SQ is an output signal from the sink source float 23 that comprises the input signal DATA IN as it clocks through the divide-by-K counter 21, the divide-by-N counter 24, and the sink source float 23. 
     One skilled in the art would be able to set up any number of methods to control the test selection. 
     By now it should be appreciated that there has been provided a frequency synthesizer having a built in test circuit for sampling in real time a number of signals within the frequency synthesizer for diagnosing the output of the frequency synthesizer.