An electronic oscillator

[Bondguy]

From a definition that an electronic oscillator converts the direct current signal into an alternating current signal,
I understand that  I must supply DIRECT current FIRST and then I receive ALTERNATING current signal.

But  if I measure voltages on that  oscillator , why do I measure alternating voltage ONLY?Where is the DIRECT signal that was supplied to the crystal first?

[Audioguru]

An oscillator circuit is powered by a DC voltage and current. You can measure the DC.
Oscillation might begin by the sudden input voltage change when the power supply is turned on or by noise in the circuit.

[Bondguy]

Yes, but when the oscillation starts, I can measure the oscillation ONLY( AC voltage). Where is DC voltage - the crystal is still supplied with DC voltage.

[barry14]

Where are you making your measurements?  The output of most oscillators is an AC signal with no DC by design as that is the most useful signal to apply to other equipment.  However, internally, the oscillator is supplied with DC to power the circuits that generate the AC signal. Some function generators do have an offset feature so that a DC level (either positive or negative) can be added to the output signal. Note that the DC power to the oscillator circuits must be continuously applied or the AC signal will disappear.

[Zero999]

Yes, but when the oscillation starts, I can measure the oscillation ONLY( AC voltage). Where is DC voltage - the crystal is still supplied with DC voltage.

That's the whole point of an oscillator: DC in, AC out.

What you most likely have is both AC and DC on top of one another. Please pose a schematic.

[danadak]

In general terms an oscillator takes in energy, AC or DC, and
produces an ac signal that has some specific characteristic, not
necessarily related to the energy source used. Typically in our
terms a waveform with a specific frequency(s).

In short wavelength work, optical spectrum and above, the oscillator
is pumped by an ac signal and produces, typically, a fixed frequency
of light or wave.


Regards, Dana.

[Bondguy]

My  problem is that oscillator has a lower frequency than it should have.I replaced crystal and frequency is still lower. So I expect that DC voltage is lower, so that frequency is also lower. Is it possible? Or what can be a reason for a  lower frequency?

[IanMacdonald]

More likely there is some other feedback mechanism than the crystal causing a lower frequency oscillation. If the feedback is way too strong, then one of the capacitors in the feedback loop may be acting as the timing component instead of the crystal. A smaller capacitor may solve this. 

The DC is usually blocked form reaching the output by a capacitor. Though at higher frequencies a transformer may be used.