Variable Frequency Waveform Synthesizer

This application relates to a portion of a patent application, DTMF Communication System, by Hayes et al, Ser. No. 728,725, filed simultaneously herewith. A more complete understanding of the system to which the portion described herein relates may be had by reference to the relation application.

1. Field of the Invention

This invention relates to a digital waveform synthesizer and more particularly to a synthesizer generating a waveform at a plurality of different frequencies from a single clock signal.

2. Description of the Prior Art

When a periodic waveform is synthesized as a plurality of discrete steps derived by repetitively counting a high frequency clock signal, the number of counts must be an integral number and the percentage error in the frequency of the synthesized waveform is reduced by increasing the count number. Thus, rounding a required count number of 50.5 to 50 produces a 1% error while rounding a required count number of 100.5 to 100 produces only a 0.5% error. The count number can be increased by increasing the clock signal frequency or decreasing the number of steps in one cycle of a synthesized waveform. However, practical restraints limit these parameters. Therefore, better frequency accuracy has heretofore required higher frequency counting circuits or lower quality synthesized waveforms.

A waveform synthesizer in accordance with the invention includes a clock signal generator providing a reference signal at a known frequency, a counter circuit connected to count cycles of the reference signal and to recycle after a number of counts at an intermediate frequency less than a desired stepping frequency, a decoder coupled to sense count states of the counter circuit and generate a plurality of step increment signals for each cycle of the counter, and a waveform generating circuit coupled to generate a periodic waveform as a sequence of discrete steps occurring at the stepping frequency, the waveform generating circuit being incremented from step to step in response to the step increment signals.

The total variation in time duration between the successive step increment signals is preferably less than 15% to avoid substantial distortion of the synthesized waveform. As few as four steps for each cycle of the intermediate frequency signal may substantially reduce the frequency error where waveforms at several different frequencies must be synthesized from a single reference clock signal. A waveform such as a sinewave may thus be synthesized at a plurality of frequencies from a single practical frequency base with both good frequency accuracy and low harmonic distortion.

A better understanding of the invention may be had from a consideration of the following detailed description, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram representation of a waveform synthesizer in accordance with the invention; and

FIG. 2 is an illustration of a waveform synthesized by the synthesizer shown in FIG. 1.

Referring now to FIG. 1, a frequency synthesizer 330 includes a low frequency tone generator circuit 332 and a high frequency tone generator circuit 334. The low frequency tone generator circuit 332 includes a programmable counter 336 which is implemented in this example as a 6 bit shift register 338 and a feedback matrix and decoder 340 which controls the least significant bit or D1 input to shift register 338 to cause it to repeatedly sequence through a selected number of steps or counts as determined by low tone frequency select signals TCL1-TCL4. It should be appreciated, however, that while the programmable counter 336 has been implemented in a manner that is particularly advantageous for fabrication as a P MOS integrated circuit, in general programmable counter 336 may be any recycling counter having a selectable number of different sequential count states.

A waveform generator 341 includes a 4 bit binary counter 342 and a code translator 344. The 4 bit binary counter 342 has a clock input coupled to receive a signal STEP INCREMENT L from feedback matrix and decoder 340. Binary counter 342 divides each sine wave period into 16 discrete steps or intervals and is clocked by signal STEP INCREMENT L at a rate of approximately 16 times the desired DTMF (dual tone multiple frequency) at output LOGP tone frequency to cause counter 342 to sequence through one count cycle for each sine wave output cycle. Counter 342 has a complementing reset input connected to signal TONE and therefore is free to count repetitions of clock signal STEP INCREMENT L in the presence of a TONE signal but is constrained to a reset condition in the absence of a TONE signal. Signal TONE is a control signal which enables generator of sine wave outputs at LO GP and HI GP. A code translator 344 receives the 4 bit binary output from binary counter 342 and in response thereto generates a digital binary code output signal that is indicative of a desired sine wave magnitude for each of the 16 portions of a sine wave period defined by binary counter 342. A digital-to-analog converter 346 receives the digitally coded 4 bit binary sine wave magnitude signal from code translator 344 and converts this signal to an analog signal LO GP having an analog magnitude proportional to a binary count defined by the output of code translator 344. Signal LO GP becomes the low tone group frequency DTMF output signal for frequency synthesizer 330.

Except that different frequencies are involved, the high frequency tone generator circuit 334 is generally similar to low frequency tone generator circuit 332. High frequency tone generator circuit includes a programmable counter 350 including a 6 bit shift register 352 and a feedback matrix and decoder 354 responsive to the 6 outputs of shift register 352 and the high frequency group tone command signals TCH1-TCH4.

Feedback matrix and decoder 354 provides a STEP INCREMENT H signal at a frequency approximately 16 times a desired high tone group DTMF output frequency which increments a 4 bit binary counter 356 within a waveform generator 357 which also includes a code translator 358. Counter 356 is reset in the absence of signal TONE and is permitted to count only in the presence of signal TONE. The 4 binary outputs of counter 356 are coupled as inputs to a code translator 358 which is identical to code translator 344. Code translator 358 has 4 binary outputs which are coupled as inputs to a digital-to-analog converter 360 that may be identical to A-D converter 346. Digital-to-analog converter 360 provides an output signal HI GP that is the high group DTMF tone frequency output for frequency synthesizer 330.

Frequency synthesizer 330 divides each output sine wave period into a plurality of steps or intervals. Because a large harmonic noise signal is generated at the stepping frequency, the chosen number of step, N, must be sufficiently large that the lowest frequency times N must be greater than twice the highest frequency that is to be generated. This relationship permits the harmonic noise at the stepping frequency to be attenuated by a low pass filter. The attenuation capability increases with N and it is therefore desirable that N actually be substantially greater than the minimum.

In the present example, low group tones and high group tones are independently generated and can therefore be separately filtered, although two separate low pass filters would be required where only one would be required if the signals are combined before filtering. For the low group standard DTMF tones, the highest frequency is 946 Hz and the lowest frequency is 698 Hz. It is therefore necessary that N be greater than 2 .times. 946/698 = 2.6. Similarly, for separate filtering of the high group, N must be greater than 2 .times. 1638/1217 = 2.7. If the low group and high group DTMF signals are to be combined for filtering, then N must be greater than 2 .times. 1638 divided by 698 = 4.7. In the present example, the number of steps has been selected to be 16. Actual tests showed that with N selected to be 16, the greatest harmonic noise occurred at the 14th and 16th harmonics. All lower harmonics were at least 45 DB below the magnitude of the fundamental sine wave frequency. The 14th harmonic was down 26 DB and the 16th harmonic was down approximately 28 DB. With a simple single pole RC filter, the 14th harmonic was reduced to approximately 45 DB below the fundamental and the 16th harmonic was reduced to approximately 48 DB below the fundamental. In each case, the lower frequency harmonics were below the 14th harmonic in magnitude.

Having selected the stepping frequency to be 16 times the fundamental output tone frequency, the problem arises of selecting a master clock frequency from which all of the output tone frequencies can be derived. Because a master clock frequency can practically be divided only by an integral number to provide the tone stepping frequencies, it is unlikely that a master clock frequency can be found that is an integral multiple of all of the tone stepping frequencies, except at an extremely high frequency that would be impractical to implement as an integrated circuit. However, the higher the master clock frequency, the smaller will be the percentage error in the fundamental output tone frequencies.

In the present example, a master clock frequency of 170.3 KHz has been selected for generation on differently phased signals .phi.1 and .phi.2 by a clock signal generator 362. To generate the highest DTMF frequency-stepping signal from this master clock frequency, it is necessary to divide the master clock frequency by a number M = 170,300/1638 .times. 16 = 6.53. However, since the master clock frequency can be divided only by an integral number, the master clock frequency must be divided by either 6 or 7. If divided by 7, the actual fundamental frequency turns out to be 1520.5 for an error of 7.23%. On the other hand, if the diviser is selected to be 6, the fundamental frequency turns out to be 1773.96 Hz for an error of 8.3%. While this error can be reduced by increasing the master clock signal, such an increase will place constraints on the method of manufacture and yield of the frequency synthesizer 330.

However, this error may be reduced in accordance with the invention by utilizing an intermediate frequency which is an integral submultiple of the stepping frequency and also an integral submultiple of the master clock frequency. For example, an intermediate frequency of 1/4 the stepping frequency or four times the fundamental output tone frequency has been selected for frequency synthesizer 330. For the highest tone of the high group, the intermediate frequency is therefore 4 .times. 1638 = 6552 Hz. Dividing the master clock frequency by the intermediate frequency produces a required diviser of M = 25.99 for zero error. Rounding this off to M = 26 produces negligible error in the actual DTMF output tone fundamental frequency.

However, the intermediate frequency is now derived by sequencing the programmable counter 350 through 26 different sequential count states. Because the intermediate frequency is only 1/4 of the stepping frequency, these 26 count states must be further subdivided to provide 4 pulses on signal STEP INCREMENT H for each cycle of the 26 count states. Since the 26 count states are not integrally divisible by the number 4, the stepping pulses on signal STEP INCREMENT H cannot occur at exactly uniform periods. However, 4 stepping pulses can be derived for each 26 step sequence by alternately generating a stepping pulse after 6 count states and then 7 count states. These unequal periods will result in some harmonic distortion of the fundamental sine wave signal, but each 1/4 sine wave period or 90.degree. of phase repeatedly corresponds to exactly 1/4 of the period of the actual output sine wave tone signal. Therefore, the noise from harmonic distortion is of relatively small magnitude and was found to be down more than 26 DB from the fundamental even without filtering. Furthermore, the worst case occurred with the 14th harmonic so that a single pole filter permitted the distortion to be reduced to more than 45 DB below the magnitude of the fundamental frequency signal.

In the absence of a tone command latch input signal, the feedback matrix and decoder 340 is connected to provide an output signal to register 338 input D1 of logic 1 when shift register 338 contains all 0's or when Q5 .times. Q6 = 1. This arrangement causes the programmable counter 336 to sequence repeatedly through 63 different count states in the absence of a low tone command input signal TCL1-TCL4. However, in the event that one of the signals TCL1-TCL4 goes true, feedback matrix 340 provides an output signal which causes some of the sequencing states to be skipped to provide an actual integral number of sequence states M to provide a desired intermediate frequency for a selected tone frequency. In the present example, M = 45 for tone L4 = 946 Hz; M = 50 for tone L3 = 851 Hz; M = 55 for tone L2 = 774 Hz; and M = 61 for tone L1 = 698 Hz. The actual tone signals L1-L4 have been given as the standard tone signal frequencies and it will be appreciated that the frequencies of the signals actually generated will vary by an acceptable amount from standard frequencies. The required sequences are obtained by generating input signal D1 to shift register 338 with the logic function.

similarly, signal STEP INCREMENT L is derived by detecting different count states depending upon the commanded low group tone frequency and has the logic function

STEP INCREMENT L =

four bit binary counter 342 is incremented for each pulse on signal STEP INCREMENT L and repeatedly cycles through 16 count states, 0-15, as indicated on the horizontal axis for synthesized sine wave signal waveform 370 as shown in FIG. 2. Code translator 344 responds to 4 bit count signals from binary counter 342 to provide a 4 bit binary coded output signal that indicates a desired instantaneous magnitude for the synthesized sine wave signal waveform 370 for each count state. For example, code translator 344 provides a digital output of 9 for count states 0 and 7, an output of 12 for count states 1 and 6, an output of 14 for count states 3 and 5, and an output of 15 for count states 3 and 4. The remaining portion of the sine wave output signal waveform 370 is a mirror image of the first portion with code translator 344 providing a digital output of 6 for count states 8 and 15, 3 for count states 9 and 14, 1 for count states 10 and 13, and 0 for count states 11 and 12. It will be appreciated that by changing the correlation between count state inputs and magnitude outputs for code translator 344, any desired periodic waveform may be generated and synthesizer 330 need not be limited to the generation of sine waves. The 16 varying magnitude steps of the waveform 370 thus occur in sequence in response to successive cycles of signal STEP INCREMENT L. However, because 4 cycles of SIGNAL STEP INCREMENT L occur for eacy cycle of recycling counter 336, signal STEP INCREMENT L is truly periodic only at an intermediate frequency having a period defined by 4 successive cycles of signal STEP INCREMENT L.

Programmable counter 350 operates in a manner similar to programmble counter 336 except that different intermediate frequencies are utilized to provide the different high tone group of DTMF output signals and clock signal step increment H is generated in response to different count states. The D1 input to shift register 352 has the logical function.

this provides count sequence states of M = 26 for 1638 Hz tone H4, 29 for 1468 Hz tone H3, 32 for 1330 Hz tone H2 and 35 for 1217 Hz tone H1.

Similarly, the clock signal, STEP INCREMENT H, is periodic at an intermediate frequency requiring 4 cycles thereof and has the function

STEP INCREMENT H =

while there has been shown and described above a waveform synthesizer in accordance with the invention for the purpose of enabling a person of ordinary skill in the art to make and use the invention, it will be appreciated that the invention is not limited thereto. Accordingly, any modifications, variations, or equivalent arrangements within the scope of the attached claims should be considered to be within the scope of the invention.