I read the article Matt did on Notch Filters in Feb. 2020 and decide to try a simple one for use with my new QA402 while I wait for QA480’s to be in stock again.
I built a simple twin-t passive notch.
The depth showed 52db centered at 1008 when I ran the chirp. I exported and then loaded and retested with chirp. I ran test at 1khz with the oscillator of the QA402.
Somehow I got the following though you see I had to raise the output of the generator to 4.6dbv to get the 1khz signal to close to 0db.
Then entire noise level was below 150db and thd+n was -116db. Thd was -138db. So I was feeling pretty good. Then as I started messing around with parameters, the noise floor rose, and it all went to hell.
I can’t seem to paste another pic in this post so will try another.
Trying another paste.
So I have no idea how I got rid of the noise in the first place then am not able to get back to those good results. Any thoughts much appreciated!
Ok. Got back to good results again.
But this is what was on front panel of QA402. No relays clicking. No message on screen.
Seems like is getting into some odd state unless I’m just screwing something up.
Decided to check my unit against specs. Noise floor was ok at -115.31 but way off in self thd and thd+n. Not sure what I could be doing wrong.
Also checked Gen2. Same. Also checked Gen1 and Gen2 into right channel. About 2 db better thd but nothing near spec.
Sorry I’m answering my own question but perhaps someone else will find it useful. I assumed device soecs on thd etc were with input range 0 even though Matt has said elsewhere to keep input range 15-20db above signal of in this case -2dbv in specs. As I said I wringly assumed 0 input range since its not stated in specs. Moved it to 12db and got thd -108.5 and thd+n -97.42.
Tried 18 and got thd -118.8 and thd+n -93.9.
So much closer to spec at 12 but still not quite there after trying laptop on battery only and moving devices around.
On another note I happened to turn both gen1 and gen2 on at -10dbv. Left and right rms dnv boxes showed -4dbv while the gen1 and gen2 boxes showed -10dbv. This also turned on the atten light on the box but full scale input range showed 0 and there was no message on screen. Even after I turned off gen2 and waited, I had to manually press 42 input range on screen and then it reset. Maybe that’s normal operation but some sort of on screen annunciation would have been helpful.
Still trying to duplicate some of the results from the spec document from May 2021. I am not dure of all settings required to generate graphs like self THD vs output level.
My results are clearly not as good.