Several medium to high speed ADC’s which I would like to test required a low jitter square wave input for optimum performance. In addition, I wanted a >20MHz square wave signal to test my active oscilloscope probe and simulate high speed digital signals in a simple manner.
Since my function generator is only able to produce square waves up to 5MHz I decided to build a simple comparator based sine to square converter. For a first try I selected the LT1016 high speed comparator from Linear:
- Single 5V supply Rail produces ~3.3V Output swing
- Complimentary TTL / CMOS output
- 3000V/s small signal gain
- Very high bandwidth product (50GHz)
- 5$ / psc
- Maximum ~10mA output current – Not able to drive 50 Ohm loads directly to 3.3V CMOS / TTL level
The working principle is very simple. The sine wave input gets AC coupled and biased to half the supply voltage. Which gets compared against half the supply voltage on the second comparator input. The LT1016 produces from this a complimentary output which can be very handy as well.
The first test board was wired up with a big continuous ground plane underneath and plenty of decoupling capacity on the supplies. Series output resistance is 470Ohms resulting in a 330mVpp Square wave into a 50Ohms load (measured 420mVpp).
Measrment of the prototype with a 20MHz sine wave input.
This worked pretty good but I really wanted a more permanent solution. Schematic of the new design is as simple as the prototype:
My prototype PCB was ordered. The schematic stayed the same but the PCB board offers better routing options with power and signal layers directly over the ground plane, via stitching and 50 Ohm impedance matching for the signals. Sine input is via a W.FL2 micro coaxial, output via two SMA edge mount connectors. I choose the W.FL2 connector since the maximum frequency as well as the input power for the sine wave will stay low and since it allows for a more flexible mounting into a (eg) Hammond aluminum case.
The finished PCB is shown below.
The results were slightly better than the prototype. Raise/fall time were faster compared to the first prototype (3.5/3.5nS vs 4.9/3.7nS) – Probably because the lower series resistance. I was, however, only able to test to 20MHz sine input today and will certainly try with another signal generator. Note that series output resistance was chosen to be 200Ohms resulting in a 628mVpp square wave into a 50 Ohm load (500mV measured).
One improvement I might make is to add a potentiometer to the second comparator input to adjust the duty cycle to exactly 50% if desired. Also, a 5V input regulator would make the device a bit more flexible.
Schematic and Gerber files as well as the measurement data in form of cvs data can be downloaded from my Github account.