# Square wave oscillator based on comparator AD8611

#### 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.

Could someone please advise the working circuit with output amplitude 3+VDC?

Thank you in advance.

#### ericgibbs

Joined Jan 29, 2010
18,850
hi llaa,
Welcome to AAC.
Is this a Homework or College assignment.?

Mod

#### Papabravo

Joined Feb 24, 2006
21,225
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
14,317
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?

#### BobTPH

Joined Jun 5, 2013
8,975
I texted Doc Brown on this and he calculated that you need a bandwidth of 1.21 GHz (pronounced Jiggahertz.)

#### Papabravo

Joined Feb 24, 2006
21,225
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.

#### llaabbss

Joined Oct 11, 2022
6
hi llaa,
Welcome to AAC.
Is this a Homework or College assignment.?

Mod
This is entirely a homework. Need to make a generator to test some ideas.

#### 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

#### 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.

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#### Papabravo

Joined Feb 24, 2006
21,225
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".

#### Jerry-Hat-Trick

Joined Aug 31, 2022
552
I need to create a square wave oscillator with variable frequency up to 200MHz
May I ask why?

#### llaabbss

Joined Oct 11, 2022
6
May I ask why?
Need to treat materials with a pulsed current. 200 MHz is a self-set goal.

#### Papabravo

Joined Feb 24, 2006
21,225
Need to treat materials with a pulsed current. 200 MHz is a self-set goal.
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!

#### 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
21,225
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.