The AM Forum

I just finished a little circuit for converting a low level sine wave into a TTL or CMOS logic level square wave. I'm using it to drive the input of an IXDD414 in my 160 Meter single FET Class E transmitter.

I purchased an NE521 dual comparator (manufactured by ON Semiconductor) in a 14 pin DIP package.

It requires +/- 5 volts, regulated ... which I am supplying with a pair of voltage regulators; mounted, along with the comparator, on a 2" x 3" Radio Shack prototyping board.

With a 250 milliVolt, 1.885 MHz or 3.885 MHz, sine wave input: it provides a very nice TTL logic level square wave out. It will work with a much smaller input (25 milliVolts), and it will work at higher frequencies (but I didn't try it beyond 3.885 MHz).

It works as expected with my Class E transmitter: 250 milliVolt rf sine wave in => NE521 => IXDD414 => 11N90

Some application information:

a. Note that the maximum common mode and difference signals specified by the manufacturer are +/- 5 volts

b. To make my unit work, I had to place a 100 ohm resistor to ground on each of the inputs of the unused comparator (this is a dual comparator), and I ran a 5600 ohm resistor from +5V to one of those inputs... so that the unused comparator would be fixed in the low output state

c. There are 3 "stobe" inputs... all of which should be connected to +5V supply with a 5600 ohm resistor.

b. Bypass the +/- 5V supplies with a 10uF capacitor and a .001uF capacitor in parallel

Stu

A 74LS13 schmitt trigger with a 50K or so pot on the input to +5 works well and only needs a single supply.  You can also adjust the duty cycle of the output with the pot.

I think the comparator is the best part for the application if the other factors are not an annoyance. Is it necessary to adjust the duty?

I built up one using an LTS(?) comparators.
(Frank GFZ's design.)
Very simple, works well up through 30mhz.
Duty cycle adjust and all.

But most guys here wouldn't like it...
It's all SMDs!
 ;D


For Class-D and Class-E , duty cycle adjustment is not required until you get to 40 meters..
But is very handy to get peak efficiency.

Bruce,
I think my circuit generated both phases for driving a push pull AMP.
There is no reason that circuit can't be duplicated using different comparators.
The parts I used were very fast to get to 30MHz.