Abstract:
The inventive circuit and corresponding method are used to form a universal referenece cycle interface for any synchronous source, whereby compensation processes and level compensations adaptively occur at decision thresholds.

Description:
CLAIM FOR PRIORITY 
   This application claims priority to PCT/DE00/03839, published in the German language on Jul. 12, 2001, which claims the benefit of priority to German Application No. 199 63 804.7, filed in the German language on Dec. 30, 1999. 
   TECHNICAL FIELD OF THE INVENTION 
   The invention relates to a circuit for generating at least one reference clock. 
   BACKGROUND OF THE INVENTION 
   Clock generators, in particular for switching systems, have been built using submodules. In this case, the submodules are each designed for specific reference clock types. With this design, each submodule had only one reference clock interface. Adaptation to a transmission frequency or a cable variant was otherwise achieved by manual setting; a specification of alarm thresholds for level monitoring was achieved by a one-off setting, preferably during production of the submodules. 
   SUMMARY OF THE INVENTION 
   The invention discloses a circuit and a corresponding method to form a reference clock interface. 
   The invention is advantageous since automatic adaptation to the respective reference clock type is performed and since settings for the cable variant are not required. 
   The invention is also advantageous since different input signals are recognized, and for each input signal an adaptive threshold compensation for sampling the input signal is formed. 
   A multiplicity of input signals can be used as an input signal, such as an atomic frequency, a PCM24 recovered signal, a PCM30 recovered signal or DS 1  in BITS applications, without any further outlay for circuitry and without special settings during production. In one embodiment, it is possible to derive from the input signal at least one first signal for deriving level information and threshold setting for a second signal, which serves at the same time as a trigger signal for the clock generator. 
   In one aspect, a continuous frequency and continuity monitoring can be performed for both binary and ternary input signal types. 
   In another aspect, the frequency of the input signal is determined autonomously and parasitic interference effects such as glitches and dropouts are selected. 
   The invention is also advantageous since with the quantization it is simultaneously possible to determine the jitter tolerance at which the selection of the parasitic interference effects is still ensured. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Further features of the invention emerge from the detailed description of the exemplary embodiments below with reference to drawings, in which: 
       FIG. 1  shows a structure of a reference clock interface. 
       FIG. 2  shows a mask. 
       FIG. 3  shows a mask. 
       FIG. 4  shows the incorporation of a reference clock interface in a clock generation unit. 
       FIG. 5  shows a block diagram. 
       FIG. 6  shows a circuit diagram. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to one embodiment of the invention, there is a universal, compensation-free reference clock interface, in particular for a clock generator in switching systems, which is described below along with a corresponding method. 
     FIG. 1  shows a structure of a universal reference clock interface having an input IN+/IN−. The block diagram is divided into a first module B 1  and a second module B 2 . 
   Arranged in the first module B 1 , a “physical interface”, is a transformer U at which the input signal present on the primary side of the transformer U is divided on the secondary side into a first signal S 1  and a second signal S 2 . The signal ES present on the input side of the transformer U may be a reference clock signal for example. A first channel K 1 , a so-called “level” channel, is fed the first signal S 1 , and a second channel K 2 , a so-called “trigger” channel, is fed the second signal S 2 . The level information (alarm criterion) is captured with the first signal S 1 ; a triggering or sampling of the second signal S 2  is performed with the second signal S 2  depending on the level information obtained from the first signal S 1 . On the secondary side of the transformer U is an overvoltage protector EMV and an impedance matching element (Za) in every channel, which together form a terminating impedance for the line connected to the input IN of the circuit. 
   Provided in the second module B 2  are units for determining the required information with respect to the reference clock signal to be sampled and individually provided. The second module B 2  can also be designated an adaptive digital system. The analog signals supplied by the first module B 1  on the first and second channel K 1 , K 2  are digitized separately in each case in the modules D 1  and D 2  in the second block B 2 . Arranged in the modules D 1  and D 2  is a type of 1-bit analog/digital converter with an adaptively controlled decision threshold. The decision threshold is supplied with control signals, incl. clock signal, from a programmable logic unit L, e.g. a programmable gate array. The programmable logic unit L enables the reference clock signal ES present at the input of the circuit S to be acquired optimally with respect to sampling and monitoring. The criteria obtained from the level information are determined with the aid of a processor subassembly arranged in the second module B 2 . The processor subassembly is formed by a microprocessor μP and a module FW. A local clock generator SYS CLK makes clock and timing signals available for the reference clock interface. The second module B 2  has a logic module L in which are integrated a first processing unit V 1  for adaptive level monitoring, a second processing unit V 2  for adaptive threshold setting for the first and second signal, and a third processing unit V 3  for clock signal adaptation of the input signal ES. A clock signal conditioning corresponding to the type of input signal is performed in the third processing unit V 3 . 
   In order to operate the reference clock interface with the different standardized cable variants, an optimized input impedance (cable connection) is determined. When determining the input impedance, it is ensured in accordance with international norm ITU-T G.703 that a return loss of greater than 15 dB is maintained for both existing impedance types 75 and 120 ohm. From that an optimal input resistance of approximately 96 ohm for both impedance types can be derived, thus the return loss for both impedance types is approximately 18 dB. The input resistance of 96 ohm also correlates with the terminating test impedance defined in the American norm (Bellcore GR-499-CORE) of 100 ohm±5% for a DS 1  pulse mask in the BITS applications. 
   It is particularly advantageous to use a broadband transformer U here. When a broadband transformer U is used, the input resistance can be maintained in a wide frequency range, e.g. of 1.5 (10) MHz. It is possible to connect to said circuit application all, for example, input signals of 1 MHz (atomic freq.), 1544 kHz (PCM24 recovered), 2048 kHz (PCM30 recovered), 5/10 MHz (atomic freq.) up to the 1.5 MB/s data signals (DS 1  with BITS applications) without any additional configuration outlay. 
   The cable variants are defined in the cable connector by appropriate wiring of the inputs. Given a symmetrical 120 ohm cable, the two wires are connected to the inputs IN+ and IN−. The cable shielding is contacted to the GND terminal. Given coax cable applications, the signal wire of the coaxial cable is connected to IN+ and the cable shield of the coaxial cable is connected to GND. The inputs IN− and GND are short-circuited in the cable connector. 
   By virtue of the compatibility achieved, both cable variants can be connected to any reference clock inputs. As a result of a fixed realization of the input resistance, when monitoring the level it is ensured that, owing to the input impedance Rin=95 . . . 96 ohm, the level measured is increased by approximately 11% for 75 ohm applications (with coax cable) and reduced for 120 ohm applications (with symm. cable). 
   The requirements for digitization and the control thereof will be explained with reference to the signal mask shown in  FIG. 2  in accordance with ITU-T G.703 for 2048 kHz, and the Bellcore GR-499-CORE signal mask for 1.5 MB/s synchronization signals shown in  FIG. 3 . To enable comparison of the two masks shown in  FIGS. 2 and 3  with one another, only the upper halves of the pulse are represented in each case. 
   The two systems according to ITU-TG.703 and Bellcore GR-499-Core exhibit fundamental differences. A mask (T 3  mask) as shown in  FIG. 2  are observed directly at the output of a synchronization source. The mask was designed for a sinusoidal signal or a digital clock signal (two-value signal). A first decision threshold P and a second decision threshold Q for a hysteresis are defined for the level monitoring (alarm detection). The second decision threshold represents an alarm switch-on threshold for a level failure. If the level at the reference clock input falls below the second decision threshold, a loss of signal alarm is triggered and at the same time the second decision threshold is increased to the potential of the first decision threshold P. When the reference clock signal returns, the level alarm is then withdrawn here, and the decision threshold is again lowered to the Q level. There is a sufficient gap (hysteresis) between the levels P and Q in order to ensure the unambiguity of the alarm triggering at all times even in the case where the level is fluctuating and subject to interference. Too great a hysteresis sets the Q level very low. As a result, reference clock signals having too low a level are still accepted, which is disadvantageous since too low a level is prone to interference and in addition causes increased jitter during digitization. 
   In a building integrated time source BITS system, in contrast to the T 3  mask, the pulse mask is controlled at the system input DDF (digital distribution frame), which means that the connection cable is included in the check. Existing systems are designed for cable attenuation up to 6 dB. The criterion for a level failure is defined on the digital side by the evaluation of the spectral density following sampling. This method requires an adaptive comparator compensation (approx. 50% of the present amplitude). As shown in  FIG. 3 , the mask was designed for a B 8 ZS coded data signal (ternary signal). 
   Table 1 shows a summary of the alarm criteria in comparison between the two abovementioned signal types (system). Supplementary thereto, Table 2 shows the norm requirements according to ITU-T G.775 and an actual realization approach with discrete thresholds for both signal types. 
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Alarm criteria 
             
           
        
         
             
                 
                 
                 
               Level of 
             
             
               System 
               Alarm on 
               Alarm off 
               evaluation 
             
             
                 
             
             
               T3 
               Q level not 
               P level 
               Analog 
             
             
               2048 kHz 
               reached 
               exceeded 
             
             
                 
               For 5 . . . 124 μs 
               For 5 . . . 124 μs 
             
             
               BITS 
               Signal density &lt; 
               Signal density = 
               Digital 
             
             
               1544 kB/s 
               12.5% 
               12.5% 
             
             
                 
               for 65 . . . 162 μs 
               for 65 . . . 162 μs 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
           
             
             
             
             
             
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Alarm thresholds 
             
           
        
         
             
                 
               Norm 
                 
             
             
               System 
               requirement 
               Realization approach 
             
             
                 
             
           
        
         
             
               T3 
               P: 
               = −9 dB 1   
               0.47 V 
               −12.6 dB/Zo = 75Ω 
               −14.8 dB/Zo = 120Ω 
             
             
               Zo = 75/120Ω 
               Q: 
               = −35 dB 1   
               0.23 V 
               −20.8 dB/Zo = 75Ω 
               −23.0 dB/Zo = 120Ω 
             
           
        
         
             
               BITS 
               Adaptive: 50% 
               0 dB threshold: 1.70 V (4.4 V 3 ) 
             
             
               Zo = 100Ω 
               of amplitude 
               −3 dB threshold: 1.15 V (3.4 V 3 ) 
             
             
                 
               value 2   
               −6 dB threshold: 0.85 V 
             
             
                 
             
             
                 1 Relative to nominal level 
             
             
                 2 The norm does not require any analog-side monitoring 
             
             
                 3 Transition to next lower level 
             
           
        
       
     
   
   With 75 Ω and 120 Ω cable, the limit values P and Q for T 3  produce different thresholds because a uniform signal power is assumed for the definition. For this reason the relative norm specifications are given. The fixed thresholds (the same for both impedances) used in the realization approach are well within the permitted limits (see comparison in dB). The mismatch was already taken into account in the conversion with ±1 dB. The hysteresis is approx. 240 mV. 
   In order to achieve sufficient safety clearance to the interference level, the reference signal is transformed up by 1:2 on the “level” channel. Since the norm specifies the amplitude in Vos, the alarm threshold in V can be simply compared with the associated level in Vss 1:1. The reference signal is transmitted 1:1 and forwarded to the digitization stage on the “trigger” channel. For T 3  reference signals the threshold is 0 V (signal center). 
   In conjunction with a downstream pulse width control, according to the invention the discrete BITS limit values, in this case: 0 dB, −3 dB and −6 dB (relative to the nominal level), satisfy the prerequisites for an optimal bit error-free sampling of the 1.5 MB/s synchronization signals. 
   The control for the threshold definition for BITS applications is performed in an adaptive manner, such that the amplitude is compared with the threshold 4.4 V on the “level” channel. This measured value corresponds to a level 2.7 dB below the nominal value of 3.0 Vos according to ITU-T G.703. The associated 0 dB trigger threshold is at 1.70 V (approximately 50% of the maximum amplitude of 3.6 Vos. If the level is insufficient, a switch is made to the next lower level value of 3.4 V (corresponds to −5 dB). This includes the −3 dB trigger threshold with 1.15 V (corresponds to approx. 50% of the 0 dB limit with 4.4 V). If the level is still not sufficient, the trigger threshold is lowered to the −6 dB limit with 0.85 V (corresponds to exactly 50% of the −3 dB limit with 3.4 V). The lowest trigger threshold corresponds to an analog side input sensitivity of −11 dB (−9 dB) relative to the nominal level (minimum permitted level). In particular the negative overshoot of the reference signal was taken into account when optimizing the discrete thresholds, so that no sampling of this signal segment can occur at any level value. 
   A downstream pulse width check is intended to detect and filter out configured signal segments (pre-equalization) and increased cable reflections. 
   With this method, for example in the case of a ternary BITS signal, the positive pulse halves are detected and the synchronization offered. 
     FIG. 4  shows an incorporation of a reference clock module ER in a central clock generator CCGES. This representation shows the modules connected to a processor P of the clock generator. In this case, the processor is connected for example via a bus connection to the logic module L and to a storage module SM. A time base unit ZB supplies both the clock pulse for the processor P, the logic module L and the reference clock module ER. The logic module L is connected to a multiplicity of interface units  2  to n. The first interface unit  1  will be explained in greater detail with reference to the figures and the associated descriptions described. With this circuit, the functionality of the circuit of the logic module L, a programmable module, e.g. a field programmable gate array (FPGA), is provided in connection with the processor P. 
   An interaction of a first or second unit D 1 , D 2  with a programmable logic unit downstream thereof is explained in  FIG. 5 . The interface between the analog and digital part of the circuit is formed in each case by a comparator K which can be integrated for example in a module with the designation MAX916. In a refinement of the invention, the comparator K, a dual 1-bit analog/digital converter, has a high input sensitivity (approx. 2 mV) as well as a high processing speed of up to 50 MS/s for better processing of the analog signals applied. An extended input voltage range of ±5 V permits use for standardized reference clock signals. A 40 MHz quantization clock already ensures a reliable sampling of reference frequencies between 1 and 10 MHz. 
   The reference voltages Uref, 1,2  for the first and second signal S 1 , S 2  are supplied by the programmable logic unit L via a low-pass filter LF. The low-pass filter smooths here the voltage jumps coming from the digital/analog converter (threshold switchover) and also the suppression of a.c. voltage components such as noise levels, ripple, etc. The dynamic properties of the low-pass filter LF are determined by the time parameters of the alarm criteria. The time parameters for, for example, a level alarm were set uniformly for both systems (T 3  and BITS) to 100 μs. To reach the steady-state condition quickly and at the same time obtain the greatest possible low-pass effect, a short low-pass time constant of approximately 5 μs is selected. 
   The digital/analog converters DAC are part of the programmable logic unit L. Two methods for analog voltage generation for continuous value control by means of the DC content implemented in the pulse width can be considered here by way of example. With a first method, the value scaling is determined by the height of the quantization frequency. The advantages of this method lie in the increased flexibility of the adaptive threshold control and in the fact that the low-pass filter is controlled via one line. This saves on external components, pins and space. 
   The second method realized in the exemplary embodiment permits a discrete value control limited to a few values by means of a plurality of static control symbols from the programmable logic unit. According to Table 2, three discrete voltage values per channel (level and trigger) are preferably used to ensure the level detection function for reference clock signals. Two control signals with three possible logic states (L, H and high Z) are required for this. The advantage of this method is that no great demands are placed on the programmable logic unit with respect to the dynamic, and no residual ripple overlays the reference voltage. 
   The first and second signals supplied by the comparator stage K are subjected to a digital filtering dF to enable a correct evaluation of the digitized analog signals. The circuit according to the invention autonomously determines the frequency of the connected reference clock signals and selects parasitic interference effects such as glitches and dropouts for example. As a result of the quantization, the jitter tolerance at which the selection of the parasitic interference effects still functions reliably is simultaneously determined. 
   The input frequency is checked and selected in accordance with Table 3 on the basis of the number of 40 MHz quantization clock periods per reference clock period. 
   
     
       
             
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Reference 
                 
                 
                 
                 
                 
                 
             
             
               freq. 
               10 MHz 
               5 MHz 
               2048 kHz 
               1544 kHz 
               1 MHz 
               BITS 
             
             
                 
             
           
           
             
               Number 
               3 
               7 
               18 
               24 
               38 
               48 
             
             
               min. 
             
             
               Number 
               4 
               8 
               19.5 
               25.9 
               40 
               51.8/ 
             
             
               nom. 
                 
                 
                 
                 
                 
               77.7 1   
             
             
               Number 
               5 
               9 
               21 
               28 
               42 
               84 
             
             
               max. 
             
             
               Frequency 
               8.00 . . . 
               4.444 . . . 
               1905 . . . 
               1429 . . . 
               0.952 . . . 
               1429 . . . 
             
             
               band 
               13.333 
               5.714 
               2222 
               1667 
               1.053 
               1667 kB/s 
             
             
               Jitter 
               25 ns 
               25 ns 
               37 ns 
               48 ns 
               50 ns 
               144 ns 
             
             
               tolerance 
             
             
                 
             
             
                 1 Frame gap in 1.5 MB/s “all ones” BITS signal 
             
           
        
       
     
   
   Another function of the digital filtering dF is the checking of the pulse width of the reference clock signals coming from the comparator stage. A spike suppression is performed on the trigger channel and the correct phasing on upper pulse halves of the BITS signals where reflections are present on the cable connection to the reference clock interface. 
   The control logic SL of the logic module L performs the adaptive threshold control of the comparator stage K via the μP interface by making control signals for generating the reference voltages available to the comparator stage. Furthermore the quantization clock is offered phase-adjusted by the clock block CLK taking account of the propagation delay conditions of the I/O cells. 
   The forwarding of the digitally filtered reference clock signals (“trigger” channel) and the level measurement signals on the first “level” channel to the μP interface is likewise performed by means of the control logic SL. 
   The μP interface establishes the connection between the programmable logic unit L and the processor subassembly μP and FW, as shown in  FIG. 1 . Non time-critical mathematical operations and time measurements are performed by the μP itself. These include the determination of the alarm criteria as well as the coding for adaptive threshold control. 
   In this development, the universal reference clock interface including four identically structured channels in accordance with the circuit diagram shown in  FIG. 6 . Shared for four channel&#39;s the programmable gate array, e.g. FPGA:XC4044XLA which, apart from the interface functions, combines in itself all time-critical HW components of the central clock generator CCGES. 
   Connected to the control outputs L 1 , 2  and T 1 , 2  is a simple DAC (digital/analog converter) including low-pass filter (LF) comprised of RC components. The low-pass effect at the outputs Uref 1  and Uref 2  is variable due to the control with 3-value code. Matched to the norm requirements, the code-dependent time constants lie in the range of 0.5 . . . 10 μs. The coding with the assignment to the various thresholds is shown in Table 4. 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Coding of the threshold control realized in FIG. 6 
             
           
        
         
             
                 
               Level comparator 
               Trigger comparator 
             
           
        
         
             
               System 
               Designation 
               L1 
               L2 
               U ref1   
               T1 
               T2 
               U ref2   
             
             
                 
             
             
               T3 
               P 
               L 
               Z 
               0.47 V 
               L 
               L 
               0 V 
             
             
                 
               Q 
               L 
               L 
               0.23 V 
               L 
               L 
               0 V 
             
             
               BITS 
                0 dB 
               Z 
               Z 
               4.4 V  
               H 
               H 
               1.70 V 
             
             
                 
               −3 dB 
               H 
               H 
               3.4 V  
               L 
               H 
               1.15 V 
             
             
                 
               −6 dB 
               — 
               — 
               — 
               H 
               Z 
               0.85 V 
             
             
                 
             
           
        
       
     
   
   The comparator K is operated with 30 5 V supply voltage. The digital inputs and outputs (CLK A,B  and Q A,B ) work with TTL level so that a direct connection to the first module in LV-CMOS with 3.3 V is possible without level adaption. The connection lines are HF-matched serially by means of resistors, for example 33 ohm. The high-resistance analog inputs of the MAX916 (level and trigger) are protected against overvoltage peaks by the resistors R 4 , R 5  with 330 ohm resistance each, since the internal clamp diodes allow limited current peaks. 
   The two resistors R 2 , R 3 , for example 215 ohm, together with a resistor R 1 , for example 1 kohm, (directly at their input) realize the optimal impedance matching (Za,  FIG. 1 ) of the reference clock inputs for 95.96 ohm. The two LCDA05 modules from the firm of Semtech ensure sufficient overvoltage protection (ESD, EFT) with the aid of integrated suppressor diodes (TVS array). Owing to the integration of serial Schottky diodes in this chip, a very low load capacitance is achieved, which significantly improves the HF properties of the reference clock interfaces. 
   The last module is the interface transformer T1068 from the firm of Pulse. This octal toroidal core transformer in SMD assembly technology has been specifically developed for the 1.5 MB/s and 2 MB/s data rate. It includes four transformers with ü=1:1 (CT) and four with ü=1:2 (CT). Owing to the excellent HF properties up to about 5 MHz, the four up-transforming transformers are ideally suited for use in the reference clock inputs. The function can be extended to 10 MHz, in which case a worsening of the input reflection by up to 6 dB is accepted.