Harmonics on oscillators. Do all of them have these?

[neazoi]

Hello,
I am wondering if harmonics are generated in the next types of oscillators:

1. Typical (Colpitts, Pierce, Armstrong, Hartley, Clapp) transistor oscillators
2. DDS oscillators
3. Microcontroller based (eg. PIC16Fxxx) oscillators.
4. Astable multivibrators

 

[LvW]

Neazoi, the answer simply is: YES.
Nothing in the (electronic) world is ideal - that means in this case: Each oscillator produces harmonics (no absolutely clean sinus).
And it is a very challenging task for an engineer to find for a particular application (a) the suitable scillator topology and (b) an "optimum" dimensioning which gives an acceptable THD (which satisfies his requirements).

 

[neazoi]

Does it apply also on astable multivibrators and microcontroller based oscillators? There are just voltage levels that switched ON/OFF in these cases (square wave). Can they have harmonics?

 

[FvM]

There are just voltage levels that switched ON/OFF in these cases (square wave). Can they have harmonics?

Yes, a square wave oscillator is a reliable method to generate a lot of harmonics. Seriously speaking, i'm surprized by the question. https://en.wikipedia.org/wiki/Fourier_series

An asymmetric square wave with low duty cycle contains mainly harmonics and can be used as a comb generator.

 

[betwixt]

Absolutely YES. The only harmonic free signal is a pure sine shaped wave. Any deviation from this implies that other frequencies are present and distorting the wave shape. Rectangular shaped waves are the ultimate in distortion, if you feed one in to a spectrum analyser it will show it is a wide "comb" of power spikes across a wide frequency range. In fact the cleaner the square wave, the more harmonics it has.

Pure sine is almost impossible to achieve because of non-linearity in the oscillator and amplifier stages, that's why so much attention is paid to filtering to prevent anything but the fundamental frequency reaching later stages in most circuits.

In some RF receivers a distorted local oscillator is deliberately used and then followed by a tunable filter so that one, and only one, harmonic can be picked from the spectrum to save using different oscillators for each band.

Brian.

 

[neazoi]

Thank you all for the replies, this is clear to me now.
Do oscillators produce only higher harmonics or lower as well? (I.e. does a LPF needed afrer them or BPF?)

 

[LvW]

Thank you all for the replies, this is clear to me now.
Do oscillators produce only higher harmonics or lower as well? (I.e. does a LPF needed afrer them or BPF?)

No, there are no "sub-harmonics". Thus, an LPF is sufficient - if needed. There are some good oscillators either with inherent filtering or with such a good amplitude stabilizing that additional filtering is not necessary (in most cases)

 

[neazoi]

No, there are no "sub-harmonics". Thus, an LPF is sufficient - if needed. There are some good oscillators either with inherent filtering or with such a good amplitude stabilizing that additional filtering is not necessary (in most cases)

To state two examples:
https://www.qsl.net/va3iul/Very_Low_Phase_Noise_VFO/Very_Low_Phase_Noise_VFO.htm (VFO)
**broken link removed** (XO)
These are two of the best circuits I have found in terms of noise, but I am not aware of their harmonics.

 

[sky_123]

I'm not familiar with Vackar, but it looks like the main reason that the harmonics are (relatively) low is because
there is a filter at the end of it, so it's not completely a result of the oscillator topology, but rather the filter. The
general rule is to always have a filter if you're using a VCO for a mixer, regardless of the osc topology.

 

[edpalmer42]

Thank you all for the replies, this is clear to me now.
Do oscillators produce only higher harmonics or lower as well? (I.e. does a LPF needed afrer them or BPF?)

Although oscillators typically do not produce subharmonics, depending on your situation there could be lower frequency issues that require filtering. In particular, power supply harmonics can leak into the output. This can be a real problem for systems that use switching power supplies.

Ed

 

[sky_123]

depending on your situation there could be lower frequency issues that require filtering

Agree, also, some people use DDS as the LO, but this could have spurii all over the place, so again you'd probably want
to filter as much as possible (or try not to use just a DDS as LO).

 

[biff44]

There are oscillators that do have subharmonics. They are the Push-Push type of devices with two transistors. For example, each transistor is oscillating at 1 GHz, but because of the phasing of the Push-Push output structure, it looks like they are oscillating at 2 GHz. There would be a lot of 1 Ghz leakage though.

DDS's have harmonics as well as totally unrelated spurious tones, that can become huge as you approach an output frequency of around 1/2 the clock rate.

To get a pure sine wave out with the minimum of harmonic energy, you need to have a non-linear element in the oscillator circuit to reduce the active device gain to be "just enough" to sustain oscillation. Under this condition, the active device is not clipping, and produces low harmonics. Then you add a lowpass filter on the output for further clean up, and that is about as good as you are going to get.

But don't go nuts trying to eliminate all harmonics, because the very next thing you apply this signal to (amplifier, mixer, etc) will regenerate its own harmonics.