On Fri, 20 Feb 2004 09:56:22 GMT, (Mike W) wrote:



Mice one Jim.
1. buy your PAL programmer
2. buy your PAL
3. learn how to obtain the required function
4. burn the PAL and discover it does'nt work
5. while patience lasts go to 2. wend
6. revert to PLL


Well, true... Happens I have access to that sort of thing, but I do see
your point.

Another approach might be to divide by 5 and then you can double twice
with a couple of chips, and then divide by 2 to get the 50% duty cycle.

A PLL system works, but for something this simple a couple of generic glue
logic IC's will serve the purpose.

Let me see, it'd require a 7490, a hex inverter for some delay in the
multiplier, and a 7400 nand. If memory serves, that ought to do it.


Jim Pennell
N6BIU

Anyone care to nominate an RF mixer chip for 100Mhz that's readily
available in the UK?
--

The BBC: Licensed at public expense to spread lies.

On Fri, 20 Feb 2004 09:56:22 GMT, (Mike W) wrote:



Mice one Jim.
1. buy your PAL programmer
2. buy your PAL
3. learn how to obtain the required function
4. burn the PAL and discover it does'nt work
5. while patience lasts go to 2. wend
6. revert to PLL


Well, true... Happens I have access to that sort of thing, but I do see
your point.

Another approach might be to divide by 5 and then you can double twice
with a couple of chips, and then divide by 2 to get the 50% duty cycle.

A PLL system works, but for something this simple a couple of generic glue
logic IC's will serve the purpose.

Let me see, it'd require a 7490, a hex inverter for some delay in the
multiplier, and a 7400 nand. If memory serves, that ought to do it.


Jim Pennell
N6BIU

Anyone care to nominate an RF mixer chip for 100Mhz that's readily
available in the UK?
--

The BBC: Licensed at public expense to spread lies.

On Fri, 20 Feb 2004 09:56:22 GMT, (Mike W) wrote:

On Fri, 20 Feb 2004 03:08:14 GMT, "Jim Pennell"
wrote:



I'd be tempted to use a PAL chip. It is possible to make a multiply by 4
and then divide by 10 in the same chip, which would make the total parts
required to get the 4 MHz 50% Sq Wave your original 10 MHz input and a
single chip.


Mice one Jim.
1. buy your PAL programmer
2. buy your PAL
3. learn how to obtain the required function
4. burn the PAL and discover it does'nt work
5. while patience lasts go to 2. wend
6. revert to PLL

I could do the same analysis for a PLL - it isn't a two minute job.

Do you have a problem with an XOR chip and a 74(xx)90?

On Fri, 20 Feb 2004 03:08:14 GMT, "Jim Pennell"
wrote:



I'd be tempted to use a PAL chip. It is possible to make a multiply by 4
and then divide by 10 in the same chip, which would make the total parts
required to get the 4 MHz 50% Sq Wave your original 10 MHz input and a
single chip.


Mice one Jim.
1. buy your PAL programmer
2. buy your PAL
3. learn how to obtain the required function
4. burn the PAL and discover it does'nt work
5. while patience lasts go to 2. wend
6. revert to PLL

"Mike W" wrote in message
...
I have a need to produce an accurate 4Mhz 50% dutycycle
TTL squarewave to use as a timebase.

I have a 10Mhz IQD frequency standard of suitable accuracy.
How can I divide this to produce the 50% duty cycle 4Mhz
signal?. Is it even possible with logic alone?. I can
see how to mix with either 6Mhz or 14Mhz, but this then
detracts from the required accuracy.


I'd be tempted to use a PAL chip. It is possible to make a multiply by 4
and then divide by 10 in the same chip, which would make the total parts
required to get the 4 MHz 50% Sq Wave your original 10 MHz input and a
single chip.


Jim Pennell
N6BIU

In article ,
(Mike W) writes:

I have a need to produce an accurate 4Mhz 50% dutycycle TTL squarewave
to use as a timebase.

I have a 10Mhz IQD frequency standard of suitable accuracy. How can I
divide this to produce the 50% duty cycle 4Mhz signal?. Is it even
possible with logic alone?. I can see how to mix with either 6Mhz or
14Mhz, but this then detracts from the required accuracy.

A more direct way, with very little jitter, is to double the 10 MHz
reference with a dual diode arrangment, output tuned to 20 MHz,
then into amplification (if needed), through a Schmitt inverter,
another (!) dual diode doubler with output tuned to 40 MHz. Again
amplification (if needed) and another Schmitt inverter to square
up the 40 MHz. Divide the 40 MHz by 10 in a Johnson counter to
achieve the 50% duty cycle. Broad tuning, no real problems there.
Very direct. Minimal jitter.

Johnson counters aren't common but they aren't made from
unobtainium either. The CD4017 is an example and still available
although it isn't fast enough for this application. Dividing by 10 via
a Johnson counter needs a 5-stage shift register arrangement
which can be done from 74LS or 74F or 74AC components (a
4-bit SR IC plus a flip-flop to complete the 5 stages).

If operating at slower rates, the 50% duty cycle would appear at
the "Carry Out" pin of a CD4017.

A good simple explanation of Johnson counters is at:

http://www.play-hookey.com/digital/johnson_counter.html

Some other suggestions might suggest themselves if you explain
the "timebase" in more detail.

Len Anderson
retired (from regular hours) electronic engineer person

Do you need the accuracy of the frequency standard? Why not just buy a
little 4MHz CMOS crystal oscillator?

If jitter and accuracy were important I'd either (hobby use) build a VXCO
with an 8MHz crystal referenced to 2MHz from the standard or (for a customer
design) buy an integrated 4MHz VXCO and reference it to 2MHz from the
standard. In either case I'd use the phase comparator from a 74HC4046 or an
exclusive OR gate. You'll be using a divide-by 5 which can be had from a
74HC390 if they're still available, and a divide-by 2 or 4, which can be had
from just about anything -- including the same 74HC390 if your hookup is a
little perverse. All the logic _could_ be done on a PAL, of course.

Come to think of it if absolute accuracy is important but jitter isn't you
can just use the oscillator from the '4046 as well, with the same division
scheme.

--------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com


"Mike W" wrote in message
...
I have a need to produce an accurate 4Mhz 50% dutycycle TTL squarewave
to use as a timebase.

I have a 10Mhz IQD frequency standard of suitable accuracy. How can I
divide this to produce the 50% duty cycle 4Mhz signal?. Is it even
possible with logic alone?. I can see how to mix with either 6Mhz or
14Mhz, but this then detracts from the required accuracy.

atb Mikw