Phase-shift oscillators with HPF vs LPF.

Thread Starter

StuckStudent123

Joined Sep 23, 2016
20
What is the difference between a phase-shift oscillator with a high-pass frequency network and one with a low-pass frequency network? I've read that the HPF one has lower distortion but is that all?

Also, if you have three low-pass filters in series in the network I know that you should have an amplifier with a phase of -180o. Is this the same with high-pass filters in series? My assumption is yes because you will still have the phase offset from the filters which needs to be corrected to satisfy the Barkhausen criteria.
 

LvW

Joined Jun 13, 2013
909
It is correct that you can use a chain of RC lowpass or highpass elements to achieve a 180deg phase shift at one single - the desired - frequency fo. The remaining phase shift (another 180deg) are provided by an inverting amplifier.
The amount of distortion is solely caused by the amount of excess gain which must be provided by the amplifier because we need an overall loop gain (at f=fo) which is slightly larger than unity (to ensure safe start of oscillations). This is because the oscillation amplitude will rise until limiting occurs.
Hence, the question if a highpass or a lowpass should be used has no influence on the distortion.

However, in many cases it is beneficial to use the lowpass alternative because, in this case, we can use the oscillation signal at the lowpass output (instead of the opamp output), which is a filtered version of the distorted opamp signal. However, in this case, we need an additional amplifier for buffering.
As an alternative, the opamp output can be used if a suitable amplitude regulation is incorporated which will avoid hard limitation (amplitude clipping at the power supply rails)
 

crutschow

Joined Mar 14, 2008
24,391
I would think using LPFs would, in general, give lower distortion than HPFs, since the distortion consists of higher frequency harmonics, and the LPF would tend to suppress them.
For that to work, you need to take the signal from the end of the filter chain.
 

danadak

Joined Mar 10, 2018
3,906
Use of HPF enables input referred noise to cause more jitter in the oscillator,
which LPF helps to ameliorate.

Regards, Dana.
 
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