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square wave generator

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Janelda

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Hi all,

I need to design a square wave generator that can give off pulses with a period that can be selected between 2Mhz-20Mhz in steps of 2ns. A computer must set the period. Can anyone guide me on how to start?
 

It would seem that you need to start with a 500MHz clock and an 8 bit counter. Then you should be able to achieve what you need by adding some logic.

Keith
 

Dividing a 500 MHz clock is a straightforward design, but unfortunately involving unpleasant high frequency logic (e.g. ECL).
A PLL based design would scale down the reference frequency to values, that can be handled by usual HC logic.
 

I am wondering how to build such a high frequency clock? Most of the multivibrators and circuits on the net are for lower speeds. I need the pulse to be very stable as well to 10ps. how can i start to design such a clock?
 

I think what FvM is suggesting is a PLL/synthesiser based design so you can do it all at lower frequencies simply by changing the synthesiser values.

Keith.
 

In all cases you can build it with 500MHz Clk_nowadays its not the biggest problem...
You have ECLiPs Family (usable as LV PECL system) from On Semi, Secondsourced from Micrel, most functions over 2 GHz, Counter over 1.4GHz(10years befor), & tiny version in SO8 :)...
A good XTAL osci on 500MHz is 7x5mm, has rms Jitter of max 1psec(!) & price is max 20$!...
https://octopart.com/eg-2121ca+phpa,+500.00mhz-epson-10546449]
K.
 

Another possibility is to select a DDS solution i.e. from AnalogDevices & programm it :)...
I think, this is the most simple way, enough to have a smaller PIC to control it, or communicate it from a PC. The first ADI DDS chips are clocked at 50MHz, today in region of x100MHz too...
K.
 

Hali Keith,
Its a nice IC(tnx for sharing!), but he has to have steps in 2 nsec between 2...20MHz:
for me is a more likable idea to design with shift registers (clocked from 500MHz), they outputs are gated to some counters to have the specs...
These solution would has the least values of jitter_if its a issue_in relation to PLLs. :)
K.
 

karesz said:
Hali Keith,
Its a nice IC(tnx for sharing!), but he has to have steps in 2 nsec between 2...20MHz:
for me is a more likable idea to design with shift registers (clocked from 500MHz), they outputs are gated to some counters to have the specs...
These solution would has the least values of jitter_if its a issue_in relation to PLLs. :)
K.
Click to expand...

I thought that at first, but I think by describing it as 2ns steps makes it sound harder than it is. 20MHz has a 50ns period. 52ns is 19.23MHz. That is a huge step. If you put those numbers into the ICS525 it is no problem (within a certain tolerance). There may be better chips than that around - it is quite a long time since I used it. There is a calculator for it here:

https://www.idt.com/?app=calculators&device=525_04

Maybe I have missed something?

Keith.
 

its very simple ...we all have the common magic IC-555 .it generates square pulse of time period=1.1*r*c ....for 50% duty cycle u can use diode across r1 n r2 ...if not clear here u can c its datasheet ...its simple boy
 

I guess it depends on the accuracy and stability you want. I don't think the 555 will oscillate at 20MHz anyway, and even if it could, it wouldn't maintain a period within 2ns with temperature etc. Nor is it capable of being digitally programmed. Other than that, it is probably ideal ;)

Keith.
 

Hi all,
Im not a "digital & uP soul", but I must repeat it; its a really good job for DDS circuits, programmed with PIC or PC_or other "arbitrary generator" solutions :)...
K.
 

I had a look at the PLL synthesizers, i see that they use crystal oscillators as inputs, but what i don't get is, that the crystal has a sine wave output, i need square waves?
 

I haven't used the PLL chips much, but the ICS525 has a square wave output.

Keith
 

I guess, you have seen synthesizers with a crystal stabilized VCO (VXCO), these designs are quite different from your specification.
They have only a small frequency pull range, e.g. a few 100 ppm. A VCO with 1:10 frequency variation range can be realized as
a current steered relaxation or ring oscillator.

A 74HC4046A at 6V VCC could e.g. achieve the intended frequency range. Generally a sine output wouldn't be an issue,
because it can be always converted to a square wave. This is e.g. done with the ADI DDS generators.
 

I am now a bit lost as to what i should do. Is it possible to directly design a 500 Mhz square wave oscillator stable to 10ps or do i have to design a sine wave first and then put it into a PLL synthesizer to get the stablility from the crystal oscillator? Does this mean that i must design a Clapp,wein bridge etc. oscillator(bec i cant find equations and circuits that help me design this from scratch, the bias circuits and the colpitts are always seperate, how do i combine them)? using s-parameters and such? I have tried some square wave circuits i found on the web, but they don't work to well at certain frequencies-something tells me that each circuits have limitations to the frequency they can produce.

How can i understand the fundamentals of electronics/electronic components in order to build my own square wave generator? I am in urgent need of help, this must be done, and fast - PLS HELP!!
 

Unfortunately, you come out with the 10 ps at the end. Of course 10 ps it's still just a number without telling about peak-to-peak,
rms phase or period jitter. Seriously, you'll have difficulties to find a ready made 500 MHz crystal oscillator with a suficient jitter specification,
you also must to decide about an I/O standard, ECL or LVDS are promising. Or build a sine oscillator. I see, that karesz as mentioned
Epson products, this can be a starting point.
 

Hi,
pls refer to my comment from 06.Apr. 16:45 over EG2121CA Seiko-Epson XTAL-Osci Family (up to 700MHz).
I think their are the good commercial possibility for so a problem and price of ca. 18-20 $ is not so bad...
Othersiede; I can not understand the specifications, then so a project with 10psec or less jitter, 2nsec stepping of periodes controling between 2-20MHz = in my opinion are not for a "beginner project"...
The first problem is for mee a good oscillator to design-for me its nothing for elementary questions...
If its possibly to use a DDS for the project_must be mostly simple, but I dont know about jitter specs...
K.
 

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