Square wave oscillator based on comparator AD8611

Thread Starter

llaabbss

Joined Oct 11, 2022
6
Hi,
I need to create a square wave oscillator with variable frequency upto 200 MHz and (optionally) variable duty cycle.
I've got several components for this: a comparator AD8611, an operational amplifiers AD8009 and AD8055.
I tried mutiple circuits, but it didn't help.
Circuit.pngЗнімок екрана з 2022-10-12 19-08-46.png
Could someone please advise the working circuit with output amplitude 3+VDC?

Thank you in advance.
 

Papabravo

Joined Feb 24, 2006
19,578
From the datasheet your goal is quite plainly impossible using the AD8611. The maximum input frequency is 100 MHz. and the prop delay is maximum 5.5 ns. You need to look for a part with a higher open loop bandwidth. Since you are doing square waves, the bandwidth needs to be much greater than the fundamental to allow for the rich harmonic content.
 

Alec_t

Joined Sep 17, 2013
13,229
Welcome to AAC!
To get anything like a square-wave at 200MHz is going to need an op-amp/comparator with a bandwidth greater than ten times that.
At those frequencies circuit layout will be critical, to minimise parasitic capacitance and inductance. What are your pcb design skills?
You realise this is not something you could knock up on a breadboard?
 

Papabravo

Joined Feb 24, 2006
19,578
Welcome to AAC!
To get anything like a square-wave at 200MHz is going to need an op-amp/comparator with a bandwidth greater than ten times that.
At those frequencies circuit layout will be critical, to minimise parasitic capacitance and inductance. What are your pcb design skills?
You realise this is not something you could knock up on a breadboard?
Oh yeah. I completely neglected to think about the implementation, since no implementation would work with the chosen part. Even an OP847 won't get you to 2 GHz.
 

Thread Starter

llaabbss

Joined Oct 11, 2022
6
You realise this is not something you could knock up on a breadboard?
Yes, I realize this thing wouldn't work on a breadboard. But I try to test these elements on a breadboard indeed, but on lesser frequencies. Seems it didn't help either :)
 

Thread Starter

llaabbss

Joined Oct 11, 2022
6
From the datasheet your goal is quite plainly impossible using the AD8611
Considering my tries to assemble the circuit on a breadboard, I didn't get results even on 1 MHz. It greatly differs from the circuit using a GHz transistor RD15HVF1-101, but transistor outputs a perfect sine wave 120 MHz even on a breadboard (but I didn't manage to get a readable signal from its gate). I'm using a 100 MHz oscilloscope.
 
Last edited:

Papabravo

Joined Feb 24, 2006
19,578
I am not at all surprised. If you do a Fourier analysis of a square wave with quantified rise and fall times you will see that it looks like a sum of sin waves that are harmonically related. The effect of using a limited bandwidth amplifier on square waves is equivalent to using a low pass filter. What is left is a sinewave at the fundamental frequency with the 3rd harmonic down 40 dB or more.

As we say in the South: "That dog won't hunt".
 

Thread Starter

llaabbss

Joined Oct 11, 2022
6
We wish you good luck in your attempts. Especially the rise time of 25 picoseconds which is 0.5% of the 5-nanosecond period.
That will require some prodigious bandwidth!
I got your sarcasm :)
That's why I tend to use a RD15HVF1-101 transistor. But definitely need to replace an oscilloscope to get a wider ability of seeing.
 

Papabravo

Joined Feb 24, 2006
19,578
I got your sarcasm :)
That's why I tend to use a RD15HVF1-101 transistor. But definitely need to replace an oscilloscope to get a wider ability of seeing.
More of a challenge than sarcasm. I don't think you appreciate how difficult this will be. I'll be interested to hear if you succeed.
 
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