Mike H wrote:IIRC the resistors and caps values for the bridge must be perfectly matched, and track together, for the lowest distortion. ANY mismatch adds distortion...
This was one of the main issues I was having with the Jim Williams design - I had a high quality dual gang pot, but the tracking was still a couple of % out, enough to throw stability off kilter.
Layout is also critical in order to get the really low noise figures - grounding, physical configuration, screening etc. Reading Williams' articles you realise just how much time he spent during the prototype phase just getting things right...
things are moving on pretty quickly at sig gen central....I'm looking at dds at the moment.....havn't seen any thd figs quoted yet but I'm sure all will be revealed..........
the wien bridge I have is stable enough it's just the thd that lets it down. For some reason I never measured my Farnell so I don't even know if I'm improving over it.....I have the thermistor from it and the dual variable condensor so If I'm bored one day......
onward
reading a bit more of Rod Elliot's findings...he says a 10 bit dac on a dds(which is what I was looking at) will weigh in at about 0.125% distortion.......but he reckons he gets about 0.05% from his wien bridge....well I'm pretty sure I have a lamp equal or better to the one in the example so I might juggle with my resistor network before I go any further....unless he just got lucky with his.
There's nowhere you can be that isn't where you're meant to be
I've just redone the measurement for my wien bridge and as far as I can see I'm getting 0.025% distortion. This is twice as good as Rod Eliiot's version and 5 times better than he says I should get with dds and 10bit DAC. This is measured on the netbook so it can't be taken as exact, but it does hint that I'm chasing rainbows in trying to improve on my wien bridge.
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Last edited by ed on Sun Jan 05, 2014 9:34 am, edited 1 time in total.
There's nowhere you can be that isn't where you're meant to be
I've just redone the measurement for my wien bridge and as far as I can see I'm getting 0.025% distortion. This is twice as good as Rod Eliiot's version and 5 times better than he says I should get with dds and 10bit DAC. This is measured on the netbook so it can't be taken as exact, but it does hint that I'm chasing rainbows in trying to improve on my wien bridge.
If you don't mind me asking, how are you arriving at the 0.025% figure?
Nick wrote:Only thing I can see is that the 1k signal is at -10dB, so everything else in the list should have 10 added to it, so 2nd at 70dB and so on.
Indeed - the reference is the fundamental, which is at around -10dB
yes, I wondered long and hard about that..although the scope says 775mv going in, the span thing seldom reads 0(it does sometimes), which is why I question the accuracy on the laptop with the behringer usb i/f.
But bottom left hand side of span shows RMS i/p at -6.5db and peak RMS at -1.8db....
not really sure how to adjust for this, and if it's even worth it on this machine.
when I get the generator back in it's box I will take it to the studio machine and measure it using the layla 3g i/f......and then I will check it on the clio machine...it all gets a bit anal.
I checked out a few expensive sub £300 units on the web and some quote 0.5%(tg210) and most of the others quote -55db at 1vp-p......I must a least be in the right ball park with those figures.
edit: taking the -6.5db as shown, the thd comes out at 0.053%, which is the figure Rod had.....mmmmm
as an example an AFG2012 at about £250 gives a figure of -55db at 1vp-p although I'm not sure what that represents or whether it's a total distortion measure at all.....
There's nowhere you can be that isn't where you're meant to be
Don't forgert to allow for the THD of the measuring equipment.
When I was messing with the Wien notch THD meter filter, I think I established a THD from my sig-gen of 0.1% once. Extremely difficult to see mind you, on a 'scope screen
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
dBu to voltage is 0.775 × 10^(dBu/20).
but I used one of the online conversion routines, which seem to ever so slightly disagree with the equation at about the 9th decimal place..c'est la vie
