[sdiy] clock frequency shifter?

[Magnus Danielson]

From: tom wisdom <twisdom at students.miami.edu>
Subject: [sdiy] clock frequency shifter?
Date: Mon, 09 Apr 2001 00:51:13 -0400

> hi all,
> 
> i'm building a lo-fi sampler box for an audio electronics workshop at 
> school, using the ISD voice recorder chips that have been mentioned 
> previously on this list in connection with a Radio Shack kit. i want to 
> make them behave a bit more interestingly. :)
> 
> this chip has an internal clock that sets the sample rate at 8kHz. 
> there's also an XCLK input on the chip, that, according to the datasheet 
> (which is at 
> http://www.winbond-usa.com/products/isd_products/chipcorder/datasheets/1400/), 
> can be connected to an external clock signal. for the chip's default 8 
> kHz sampling rate (yep, it is going to sound like pants), the clock rate 
> is 1.024 MHz.
> 
> there's an anti-aliasing filter on the audio input, so i don't think 
> it's possible to increase the frequency range of the thing by 
> overclocking it. but i would like to be able to vary the speed of sample 
> record and playback by varying the clock rate, say with a pot.
> 
> so: is there a way to take the output of a 5MHz crystal and vary it 
> semi-smoothly down to somewhere near 1 MHz? or alternately, a 
> very-simple VCO that does this range? could i just use an LFO circuit 
> with component values adjusted accordingly, or are there weird effects 
> that come into play? linearity and stability are obviously not very 
> large concerns. any suggestions are welcome (it's due in a couple weeks).

If you want precission and flexibility, then I would say a phase
accumulator. If you want a stable and traceable (to a crystal)
frequency, then I would recommend a pair of frequency dividers and
4046 (or variant thereof).

Do something like this:

+----+   +---+   +----+                +----+
|5MHz|---|% N|---|4046|  +----------+  |4046|   Fout
+----+   +---+   | PD |--|RC-lowpass|--|VCO |-*---
               +-|    |  +----------+  |    | |
               | +----+                +----+ |
               |             +---+            |
               +-------------|% M|------------+
                             +---+

Set up the VCO for oscillation around 1 MHz (follow datasheet).
Select a suitable RC filter (follow datasheet).
Select the N and M for suitable frequency and frequency steps.

       M
f    = - 5 MHz
 out   N

If you let N be a static divider and M be a setable divider, then N
will set the steps of M.

Either way, this is not rocket-science.

I strongly recommend you to look at the Philips datasheets for
74HC4046, 74HC7046 and 74HC9046 both to see improved versions as well
as bunches of usefull dimensioning formulas.

Neither frequency dividers or the PLL stuff should be very hard.
This way you can do almost arbitrary frequency synthesis well above 10
MHz with not too strange components and not too much of hair-pulling.

If you ever want to go higher, there are loads of components wich
allows you to go several decades further up before getting very expensive.

Cheers,
Magnus