**Exponential RF amplifier for reducing harmonics without filtering
**by sv3ora

Here
is a forum discussion, showing an idea I had for reducing harmonics in
an oscillator without filtering. It turns out to be impossible. I have
also learned that an amplifier won't process the carrier and the
harmonics as independent signals (like displayed in the frequency
domain of a spectrum analyzer), but it will treat them as a single
distorted waveform (as shown in the time domain of an oscilloscope).

ME

Considering a wideband RF oscillator followed by an amplifier.

This system usually produces harmonics can be filtered with a LPF.

In multi-band operation more than one LPFs are switched accordingly.

My idea is to use an ANTI-logarithmic amplifier (if such thing exists)
to reduce these harmonics without any LPF on multi-band operation.

It works like this:

The anti-logarithmic amplifier will amplify the larger signals (which
is the carrier) more, but it will amplify the smaller signals (which
are the harmonics) less.

Using such an amplifier and combined with an output attenuator
possibly, the harmonics levels compared to the carrier, will be reduced.

1. Does such an anti-logarithmic amplifier exist?

2. Will this idea work?

OTHERS

1. yes, an exponential circuit. Maybe extended to symmetrical form, like y = sign(x)*exp(abs(x))

2. not at all

The "anti-log" circuit distorts the sine wave, creating new harmonics.

I think, the problem is that you are confusing the intended signal
manipulation in frequency domain with manipulation in time domain.

ME

Yes I understand, but even the LOG amplifier is also non-linear and yet used in measurement equipment.

Moreover the exponential amplifier may create harmonics, but these
won't be amplified as much as the carrier. So more harmonics possibly
but at lower levels compared to the carrier.

OTHERS

you confuse frequency domain amplitude and time domain. The amplifier
would process a voltage that is the sum of sine wave + harmonics. It
doesn't process the harmonics level independently from the sine carrier.

And even an ideal sine wave processed with an exponential amplifier
will generate harmonics. Nonlinear amplifier -> distortion from the
sine-> harmonics

ME

I am sorry but I struggle to understand this.

Won't an amplifier amplify whatever signals come into the input?

A distorted input signal isn't the sum of the carrier and the harmonics?

I am talking about a "hypothetical" exponential amplifier not an ordinary amplifier.

Please let me know.

OTHERS

>>>Won't an amplifier amplify whatever signals come into the input?

Yes, but your idea is based on frequency domain with harmonics signal =
small level = less amplification. You treat signal separately, as if
the amplifier would see carrier only (large amplification), then
harmonics only (small amplification).

>>>A distorted input signal isn't the sum of the carrier and the harmonics?

Exactly, it is the sum! So with your exponential amplifier, the
amplifier gain will then depend on the time varying instantaneous
voltage of carrier + harmonics. Example for a heavily distorted sine
wave: make it look more like a square wave, by adding a lot of odd
harmics. How can any theoretical amplifier make that a sine again?

>>>I am talking about a "hypothetical" exponential amplifier not an ordinary amplifier.

Your amplifier would need to split signals into spectral components
first, and amplify all of them separately. FvM and I understand that
your idea is inspire by the spectrum analyzer display, where that
frequency separation happens. But an RF amplifier sees all the spectral
components added, and amplifies at any time the input voltage which is
the sum of all these spectral components. Then there is no "small
harmonics voltage", there is only one sum voltage that is carrier +
distortion.

ME

All right, I think I understand now,

So a signal waveform in the time domain, can be analyzed, or is the
RESULT of, the carrier and the harmonics. And if one adds the same
harmonics as separate signals to a clean carrier then he will end up
with the result distorted waveform.

However, the opposite is not true, it does not mean that the amplifier
seed this distorted waveform as separate signals (like displayed on the
spectrum analyzer), although the signals are there because you can
detect them in a near by receiver.

Am I correct on this?

OTHERS

Yes, it is really just two views (time domain: distortion, frequency domain: multiple frequency components) of the same signal.