Low-distortion Audio-range Oscillator

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#1,441
Samuel Groner said:
Have you tried it? Even with several percent amplitude mismatch output ripple is likely still much lower than for an average or RMS detector (with the same filtering). And you can easily trim the amplitude match for each range.

The S & H peak detector is cleary better, but also more difficult to implement right.

Samuel
Click to expand...

I have a pair of AD RMS detectors coming from Scott. The output ripple from these IC's is the square of the input. If I use one on the output and one on a quadrature output I can sum the two together and null the ripple. This can be done with the currents before filtering so no
summing amplifier is required, then follow it with the internal buffer(s). This should be interesting to try.
 
#1,442
This is the output of the RMS detector for the auto set level in the 339A.
1KHz input at 1Vrms and 1Vdc output.

I can live with this but it not as good at lower frequencies.
This can be corrected with more filtering.
 

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#1,446
OK just confirmed that the cap is necessary with the photovoltaic/JFET method and was able to trim the seconds into the noise with it (.033uF). I am using a bridge technique comparing a perfect resistor attenuator to one with the FET. I'm nulling a J310 against 100 Ohms with about .5V p-p across the FET (I assume this is too much). What are good numbers to take a measurement at?
 
#1,447
scott wurcer said:
OK just confirmed that the cap is necessary with the photovoltaic/JFET method and was able to trim the seconds into the noise with it (.033uF). I am using a bridge technique comparing a perfect resistor attenuator to one with the FET. I'm nulling a J310 against 100 Ohms with about .5V p-p across the FET (I assume this is too much). What are good numbers to take a measurement at?
Click to expand...

Hi Scott,

The 339A Jfet has about 50mVpp Vds.
 
#1,451
dimitri said:
Why not two of them in control loop: (A*sin(wt))^2+(A*cos(wt))^2)=A^2, no ripple (in theory)
Click to expand...

Hi Dimitry,

I've been giving this some thought.
If the summed levels are out by 1% this causes roughly 165mV ripple.
If we use one output signal as a reference and use an AGC on the other output signal,
we might be able to make this work. But it will come with extra complexity.

If we do this with a Gilbert multiplier core, we have access to every part of the multiplier.
Once we get it right then look at some of the 'simpler to use' multiplier ic's.

Cheers,
 
#1,452
davada said:
This is the circuit used and spectrum of the output.

For 6Vrms input to the lamp there is a 5.56Vdc output.
At 3Vrms the output drops to 0.823Vdc.

That's not very linear.

I'm concerned about the level of distortion as well.
This is with a 0.1uF feedback cap on the output amplifier.
I's much worse with out the cap.
R is 10M.
Click to expand...

An ignorant question
dont you need at least 3 points to determine if it is linear or nonlinear?
 
#1,457
RNMarsh said:
My take is this --- if you cut the input from 5.56v by exactly 1/2, the output should do same (2.78). It was way off at .823v and thus not deemed a worthy candidate. Have I got that right? -RNM
Click to expand...

That's about it.

Now this could just be scaling and not necessarily non linear.
But I was here watching the points as they went by.
It was a poor communication on my part just to say it's non linear and I think this is
what was being said.
 
#1,459
There is one approach to peak detection that I haven't ventured into and that is FFT.

Here is a complete project I found on the web which can be scaled down and modified for this purpose.

This is really quite cool. Coded for a PIC. I'm using the smaller pin version of this PIC to control the oscillator but I can upgrade to the larger pin count PIC. It looks like I need more pins anyway.

Since the peak value is not required to stabilize the loop, the lamps do this, this may be workable. The sample speed has to increase to cover a greater band width or we can just let it wrap around since it's a single frequency we know what that is.
We don't need the LCD driver code so that's a big saving there.


Check this out.

Real-Time Audio Spectrum Analyser - WFFwiki

Cheers,
 
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