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Author Topic: Changing Sinusoidal Wave from 0..+1V to -0.5..+0.5V  (Read 764 times)
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London
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I have this nice little waveform generator, which I bought on eBay, that goes all the way up to 50MHz. That's good stuff. Bad stuff is that it outputs sinusoidal waves from the minimum of 0V to the maximum of +5V. Now my table top power supply or batteries, supply only the voltage from zero upwards.

I would much prefer for this waveform generator to outputs sinusoidal wave from -0.5V to +0.5V. Is there a some kind of mysterious circuit that can drop the output 0.5V, so I can get a negative voltage of 0.5V?
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You just need to couple its output via a capacitor - the value depends on the frequency and the load impedance.
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The issue of the coupling capacitor C has some subtleties. What if it drives an input to an amplifier with R=100 Gig ohms? The voltage on the capacitor is almost floating. For a one microfarad ac coupling capacitor, the RC decay time is:

10^11 * 10^-6 = 1 day = RC

The initial conditions of the capacitor will determine its static voltage. It could have more than 12 volts on it for hours. It is possible that the connector tip touched a 12 volt node and charged the capacitor.

Conclusion

There are some systems in which the coupling capacitor should have a resistor of about 1 Meg ohm to ground or any appropriate bias node. That leaker will discharge any initial conditions from the capacitor so that high frequencies are sent to the load without a DC bias being there for days.
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Lacey, Washington, USA
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I have this nice little waveform generator, which I bought on eBay, that goes all the way up to 50MHz. That's good stuff. Bad stuff is that it outputs sinusoidal waves from the minimum of 0V to the maximum of +5V. Now my table top power supply or batteries, supply only the voltage from zero upwards.

I would much prefer for this waveform generator to outputs sinusoidal wave from -0.5V to +0.5V. Is there a some kind of mysterious circuit that can drop the output 0.5V, so I can get a negative voltage of 0.5V?

Are you saying that the negative peak is at 0V? Have you looked at this with a 'scope?

What is the brand and model?
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Steve Greenfield AE7HD
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London
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Yes, I checked it with oscilloscope many times. Just have two questions:

- how is capacitor going to reverse polarity if one of cpacitor's terminals is always ground?

- is there a formula for matching C and frequency?
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Lacey, Washington, USA
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Brand and model?


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Steve Greenfield AE7HD
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Lacey, Washington, USA
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Oh, did you have a load on the signal generator, other than just the 'scope probe? Try connecting a 1k resistor from the output to ground and check it again.
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Steve Greenfield AE7HD
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I just bought it on eBay. It is bare board with LCD and few buttons. DDS signal generator. Works well, just need to pull the output down 0.5V.
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Lacey, Washington, USA
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Can you tell me what IC is on the board?

This is making more sense, now.
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The capacitor doesn't shift the DC bias to 0V, it (almost) completely decouples the circuits at DC.  No (that is, very very little) DC current passes, so it's up to the input to establish the DC reference point.  You can do this with a 10K to 1M resistor to ground (which will center the 1VAC around gnd for +0.5 and -0.5VAC), or some arbitrary bias voltage like 1/2Vcc (e.g. 2.5V for single-ended circuits running at 5V, giving you +2V to +3VAC), whichever is appropriate for your circuit.
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Quote
- how is capacitor going to reverse polarity if one of cpacitor's terminals is always ground?
The capacitor is in series with the load so neither end is directly connected to ground.   A series capacitor (along with the load resistance) acts as a high-pass filter.  DC is zero Hz and it gets blocked.   Almost all audio equipment that runs from a battery (including most car stereos) have an output capacitor going to the speakers.   

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- is there a formula for matching C and frequency?
Capacitive Reactance (in Ohms) is:  Xc = 1/(2 x PI x C).  Note that capacitance is in Farads, whereas your actual capacitor will be marked in microfarads or picofarads.

A 1uF capacitor is probably fine for whatever you are doing.
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