At 06:45 PM 4/15/97 -0500, you wrote:
>|>By the way I did run across a programmable sine wave generator chip
>|>good from DC to 500 kHz from MicroLinear -- I suppose this could be
>|>squared up and used as a clock.
>
>|Why don't you try a programmable divider (from 20MHz)? It probably gives
>|you enough resolution.
>
>[nb: Does anyone know a good way to make MS Exchange format
>replies properly without having to hand-insert all the "|" or
>">" characters?]
>
>A "conventional" clock divider running from 20MHz won't have much
>resolution around 500KHz.  Your steps would be:
>
>  487,805Hz
>  500,000Hz
>  512,821Hz
>
>etc.  [i.e. about 12KHz resolution].  You may get much finer
>resolution if you don't mind a little bit of "jitter" in the
>output.  You may get any frequency you want, to arbitrary
>accuracy, with 1/2 clock-period of jitter by building a suit-
>able-length accumulator (i.e. adders and latches).  If you
>build, say, a 16-bit accumulator and put 2193 on the input,
>then the MSB of the output will toggle at a speed (2193/65536)
>times the cycle rate.
>
>Alternatively, if you don't mind having a bit more jitter, you
>may use a simpler technique as found in a 4089 [a cute but not
>terribly useful part which has been discontinued; if you can
>find the data sheet for it somewhere, though, you'll see what
>I'm talking about].  A 22v10 fed with a 20MHz clock should be
>able to produce any frequency from 39,139Hz to 10,000,000Hz
>within 0.4% [if you were to use this route, you'd need 11 out-
>puts from the PIC to set the desired output speed but the PIC
>itself wouldn't have to do anything].  If more accuracy is need-
>ed, you could change the circuit a little bit and have the PIC
>"fine-tune" the output (the circuit itself works as a divide-down
>counter that selectively "drops" counts; the PIC could "drop" a
>few more if needed).
>
>This technique, by the way, works best when the output frequency
>is much below the input frequency, or when it will be divided down
>so jitter doesn't matter too much (the device works by producing
>a frequency between 1/2 and 1/4 of the input frequency and then
>dividing that by a power of two; the more stages you divide it the
>less jitter will remain).  If you use it for audio, you need to
>divide it by at least 100 in order to eliminate the jitter comp-
>onents.  For your applications, though, it sounds as though that
>shouldn't be too much of a problem.
>
>
Hi John,
        If you need a fine resolution high speed clock source, why don't
you try
the new technology Direct Digital Synthesizer, Like AD9850, it can generate
up to 42MHz with a 125MHz clock input , output tuning resolution of 0.0291 Hz.
The AD9850 also contains a high speed comparator which can be configured
to accept the (externally) filtered output of the DAC to generate a low
jitter
square wave output. But the cost is much higher.



Jerry Meng, BA1FB

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