# Sinusoidal wave to a discete wave

Discussion in 'General Electronics Chat' started by dumindu89, Aug 24, 2012.

1. ### dumindu89 Thread Starter Member

Oct 28, 2010
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Hi!
What are the possible ways to convert a sinusoidal wave to a discrete wave using transistors?
I know about the schmitt trigger. But I would like to know any other easier transistor configuration for this and also how to do the calculations too.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Do you mean by discrete a square wave?

3. ### Ron H AAC Fanatic!

Apr 14, 2005
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We need to know frequency and amplitude ranges.

4. ### dumindu89 Thread Starter Member

Oct 28, 2010
113
0
Yeah.. It should be sinusoidal to square wave.

Frequency is 100MHz and I need to convert 4Vpp sinusoidal wave to a square wave which has characteristics of LVTTL. (high level threshold is 2V and low level threshold is 0.8V).
Simply I want to send the converted square wave to a Altera MAX II EPM240 CPLD as an input.

5. ### crutschow Expert

Mar 14, 2008
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You could use a high speed comparator such as one of these with TTL output.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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What is the source of your 100MHz sine wave? What is its output impedance?

7. ### dumindu89 Thread Starter Member

Oct 28, 2010
113
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Thanks.
Are there any other method to convert to a square wave using a transistor configuration?

Sinusoidal wave coming from an amplified wave of a VCO and its output impedance is around 1 kΩ.

8. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Just capacitor couple the 4v sinewave with a small series resistor into the CPLD digital input. That input will have TTL or ST input thresholds and clamp diodes to Vdd and Vss.

No special hardware is necessary.

9. ### dumindu89 Thread Starter Member

Oct 28, 2010
113
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didn't get it. Can you explain it in bit more detail?

10. ### Ron H AAC Fanatic!

Apr 14, 2005
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That's what I was going to suggest, perhaps with the addition of a couple of small Schottky diodes to the rails, in case the 4V p-p happened to be 5V or whatever.
Also - the CPLD has Schmitt trigger input options, which will square up the sine wave, if needed.

11. ### dumindu89 Thread Starter Member

Oct 28, 2010
113
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I didn't get that option well. Can you show me it as a schematic?

Furthermore, If I design a schmitt trigger for this, how I calculate the resistor values?

12. ### Ron H AAC Fanatic!

Apr 14, 2005
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Attached is a schematic. The Schottky diodes are probably not needed if you are sure the input signal will never exceed 4.0V peak-to-peak.
As I said, the Max II input buffers have optional Schmitt trigger settings. You don't have to design and build one.

From the datasheet in the above link:

• ###### sine input to CPLD.png
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13. ### dumindu89 Thread Starter Member

Oct 28, 2010
113
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Thanks.
We need the D2 diode to do clamping and You added the D1 diode with 3.3V Vcc to clip the voltage if it goes beyond 4V. Isn't it?

Anyway why do you used C3 capacitor?

Are there any other shottcky diodes which have inverse voltage around 4 V? Becuase items in Avago brand is hard to find in the local market in my country.

14. ### Ron H AAC Fanatic!

Apr 14, 2005
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I failed to notice that the Avago diodes have 4V max reverse voltage rating. That is marginal.
You need small-signal (RF, mixer, etc.) Schottky diodes, if you elect to use them. With 4V p-p, both diodes act as clamps and clippers. If you can get BAT54 (single diode) or BAT54S (both diodes in one package), they should work, although the capacitance is higher than I would like.
You could try eliminating both diodes, and adding 1MegΩ from the input pin to ground. This will allow the internal diode to act as the clamp.