# Square wave generator 370kHz to 1MHz

Discussion in 'General Electronics Chat' started by Tilen Suhadolnik, Jul 16, 2015.

Jul 16, 2015
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So, I would like to make a square wave generator (any amplitude, about 50% duty cycle), where you could set a frequency from 370kHz to 1MHz.
I have found a solution to do that with 555 timer. Datasheet says it goes up to 3MHz.
For the 555 astable circuit I would use fixed R1 1kOhm and C 1pF. The R2 will vary from 720kOhm to 2MOhm.
Now the question is, is that C too small?
What could be a minimum R1 value?
R1, R2 and C are defined as here: http://www.ohmslawcalculator.com/555-astable-calculator

Also, is there any other simple way to do that?

2. ### dl324 Distinguished Member

Mar 30, 2015
3,242
619
Post a schematic so we can see what you're talking about.

1pF is probably too small. Touching something with your finger would add a few pF of stray capacitance...

Adjusting a 555 for a square wave with varying frequency will be a challenge. It would be easier to double the frequency and use a flip flop to give you a square wave.

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3. ### Roderick Young Member

Feb 22, 2015
408
168
I agree with @dl324 - try using 1000 pF (1 nF) as your timing capacitor, and resistors in the 100's of ohms. Generate a frequency that varies from 740 kHz to 2 MHz, which is right on the edge of what the 555 is capable of. Then input that into a flip-flop to get an absolutely square wave as the output. You could use a 74HC74 - put the input frequency into the clock input of a flip-flop, and tie the notQ output of that flip-flop back to the D input. Your square wave output will be on the Q output.

4. ### AnalogKid Distinguished Member

Aug 1, 2013
4,523
1,247
The resistors are so large that error currents within the 555 will affect the results. And, as noted, 1 pf is the capacitance of waving your finger at the circuit and less than the input capacitance of the chip input. Increase the cap 100x and decrease both Rs 100x. That should be the same frequency, but the lower network impedance will be more immune to external influences.

A better way to get closer to a symmetrical output is not to use the Discharge input, as shown here. R1 in this drawing corresponds to R2 in your description. A 150 pF capacitor and 4.7K fixed resistor in series with a 10 K pot should do it.

f= 0.722/R1C

ak

5. ### Bordodynov Active Member

May 20, 2015
637
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Timer MIC1557.

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