Yes, in fact the C7 lamp oscillator might be very low THD for a lamp based one but at 4kHz it takes 10's of thousands of cycles to stabilize.
I agree with all of this... and yes, the circuit was seen this spring but forgotten about as we were try to just get the THD down on the 339A's circuitry which is cheap and plentyful for DIY'ers. Not to reinvent the wheel. But now all the easy stuff has been done. The result is well below -100db over a wide range of freq.
In that context, and the direction we have been going, it is time to re-evaluate the direction by using the example given.
As for the time constant --- me, personally, I dont care how long it takes to settle down. Ten minutes is OK. 100 seconds is no problem.... We dont need it for (and the 339A wasnt made for) fast assembly line testing. Nice if it wasnt that long, though. I'm going to do that freq thd test once in a blue moon. However, the sine wave thd and individual harmonics is a more important priority to be as low as possible - is what is needed to be able to even measure what we can design as amplifiers. For a budget price, no less.
-130 ... -145 ... -160 and more even are seen on FFT plots from SIM's. That's why test equipment is needed to verify. Reality checks. If the distortion is below what can be measured with accuracy and precision and repeatabliity, then thats just about the right amount..... Thx-RNMarsh
Last edited:
Is this -130dB THD or just -130dB for any harmonic?
Something that raises my eyebrow is the possibility of harmonic cancellation with in the signal chain most notably from inconsistent measurement. The 339a does this for the same input level seen by the input amplifier but with different attenuation between the analyzer input and oscillator output. I can understand the noise changing but the distortion??? The only real change is the input amplifier is seeing a different source resistance. The analyzer input resistance is constant and the output resistance of the oscillator is constant as well.
Just a rough order of magnitude--usually only 2nd and 3rd are significant, so THD is anyway not much higher than the individual harmonics.
That's exactly the point--the distortion contribution of the notch filter does (with a 50:50 chance) either add or subtract to/from the oscillator, depending on the phase relation of the harmonics (if the phase relation is exactly +90 or -90, the magnitude will remain unaltered--but this case is extremely unlikely). So we don't even know if the notch filter increases or decreases the observed distortion!
Samuel
Hi David,
As I understand the distortion originated in the notch filter and(or) in the load resistor is the main concern. Assume that the filter is fully passive and we are using the same best components (R and C) as in oscillator feedback network(s). With 6-10dB attenuation distortion which may be generated in the notch filter will be reduced considerably. If there will be small difference between 6 and 10dB measurements one can say that this is oscillator residual.
One can ask - what about distortion is attenuator resistors? I would replace one resistor with parallel resistors in series
As I understand the distortion originated in the notch filter and(or) in the load resistor is the main concern. Assume that the filter is fully passive and we are using the same best components (R and C) as in oscillator feedback network(s). With 6-10dB attenuation distortion which may be generated in the notch filter will be reduced considerably. If there will be small difference between 6 and 10dB measurements one can say that this is oscillator residual.
One can ask - what about distortion is attenuator resistors? I would replace one resistor with parallel resistors in series
Last edited:
Well yes we can see if the notch filter is poking it nose in with distortion by testing this way
but there is also the spectrum analyzer or FFT. There is usually a buffer before an ADC and then there is the ADC residual as well. It is difficult to know exactly which is the contributing factor.
You are correct. AD797 stages before notch filter don't allow me to measure reliable <-125dB with AP2. After rejection of fundamental amplifier before an ADC and ADC itself aren't very important, at least there is always the option to use different amplifier/ADC and compare results.
Last edited:
This true for the buffer but we are assuming the ADC behaves the same way. Different animal.
Are you familiar with the concept of missing codes in ADC's?
It's a funny way of putting it. The aren't really missing they are simply not there.
It a type of inherent non linearity in ADC's. Even ADC's that say no missing codes have missing codes. If the missing codes just happen to be in the dynamic range of our measurement being as low as they are then we have distortion. This mechanism of distortion is far more significant at low levels than it is at the ADC's FS.
Well it depends on the ADC, what type of conversion it uses and more than anything how much you paid for it. If I put that into the QA400 I would have so much distortion generated
I wouldn't be able to tell what's DUT and what QA400.
My point is there are other non linearity that need to be considered which add to the uncertainty. For a sanity check we should measure our distortion by more than one means.
I find inconsistency with the method which I can't explain. For example I have a selective voltmeter which I use to confirm the measurements I get from something like a QA400 or EMU0204. But I'm still using a notch filter to get the measurement and so this aspect of the measurement still has some uncertainty. All measurement has uncertainty even the AP's. Look at the differences of what's been posted with Victor's oscillator. It's all over the board. And then there is the issue of calibration.
What you pointing out is proportionality but any error is proportional as well.
The purpose of notching and then amplifying is to make some sources of error less significant.
Last edited:
You don't need to add a trim.
If your notch filter is variable You will change the phase by tuning the frequency of the notch filter away from the oscillator frequency. Your notch will rise and you can see if the distortion rises, falls or stays the same. If you have a fixed frequency notch filter you can tune the oscillator frequency away from notch center this will change the phase. your notch will rise. Observe you distortion. The phase is at it's highest velocity at the deepest notch. The phase reverses rapidly at the deepest notch, notch center.
You seem to have missed the discussion; essentially a lamp is a very linear positive tempco thermistor. David showed a really elegant solution for making a multiplier with a lamp and some dc voltage. The remaining task is a level detector to "steer" the level with higher precision that can be had from the lamps. SG suggested using a simple rectifier/integrator, which should be fine since the response time is slow anyway.
This could be used with either a Wein bridge or a state variable oscillator. I'm not sure how practical this would be for an HP339.
Yes, that's what I'm using to gain some measurement confidence. As you say, this again rises the question if the attenuator introduces some distortion, or the possibly changed impedance conditions (different loading of the oscillator) have some effect. Or perhaps the connectors or switches..?
What is of relevance is the phase at the harmonics. Detuning the notch filter won't change much here.
Samuel
"or the possibly changed impedance conditions (different loading of the oscillator) have some effect."
Or different loading of the filter.
OKay. The phase of the harmonics should be predictable at the output of a Twin T if it is the Twin T that has introduced the phase shift. We can see this in simulation.
Where is this phase shift of the harmonics coming from that cause cancellation?
There is a distinct possibility that what I'm seeing is an effect of the FFT windowing ripple.
This can be tested buy using flat top.
Last edited: