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Old 25th November 2012, 05:20 AM   #871
RNMarsh is offline RNMarsh  United States
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Default FFT

Quote:
Originally Posted by 1audio View Post
If the analyzer section is well behaved (doesn't add a lot of its own distortion) its a great front end for a PC sound card. Almost infinite protected dynamic range and the notch extends the distortion floor 20-60 dB depending. A 16 bit sound card, or an 8 bit digital scope adapter even, on the monitor output will show virtually all there is to see.
I have several FFT options to use... my TEK scope with FFT option to 300MHz, an old, slow analog FFT -HP 3580A (used with my old Sound-Tech but gave that to local college years ago) or a new 24/192kb ADC/DAC and PC/FFT software. Maybe even an iPAD with an FFT application. All of which will get me way below the sine wave source.

My main concern was first a very low thd ocillator source and found Victor's ultra low thd but single freq. on eBAY. But, then I wanted a more flexible oscillator with much less distortion than .001% rather than a collection of many single freq ones. Enter HP339A. -Thx RNM
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Old 25th November 2012, 05:57 AM   #872
richiem is offline richiem  United States
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My HP 339A has significant 2nd H spike from the notch circuit, which is what sets the min. THD floor for the unit to 10ppm (-100dB) at 1kHz. The notch null was around -115dB as I recall. I need to make some plots for Davada, so I'll post a couple when I take them. I have some old ones somewhere on this computer, but good luck finding any after several years....
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Old 25th November 2012, 06:41 AM   #873
RNMarsh is offline RNMarsh  United States
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Default Hopeful thd

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My HP 339A has significant 2nd H spike from the notch circuit, which is what sets the min. THD floor for the unit to 10ppm (-100dB) at 1kHz. The notch null was around -115dB as I recall. I need to make some plots for Davada, so I'll post a couple when I take them. I have some old ones somewhere on this computer, but good luck finding any after several years....
That is very hopeful news. The plots will be very helpful. Looking forward to them. Thx-RNMarsh
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Old 25th November 2012, 04:02 PM   #874
davada is offline davada  Canada
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Default 399a

Something I'm considering doing is tapping the output of the notch filter in the 339a.
This will bypass the auto set level in the analyzer which shoves the noise floor and distortion way up on the monitor output. Maybe add a connector somewhere on the rear of the chassis or a switch to route the monitor output there.

The auto set level takes the input level to the notch filter and puts it through a RMS convertor. The output of the RMS detector is compared to a reference and dual matched LDR is used to normalize the gain of the notch filter post amplifier to 1Vrms FS.

If the analyzer is set to a sensitivity of -80dBV FS, the notch is -115dB and the THD+N is near the notch the auto set level is applying a lot gain to normalize this to 1Vrms FS.
Furthermore this gain from the auto set level is provided with an LDR operating in a non ideal part of it's range which means a lot of distortion is added by the post amplifier as well as noise gain.

Additionally, it would be useful to tap the output of the auto set level's RMS detector to provide a level for scaling and this level is the input level to the notch filter.

These are simple mods that shorten the signal chain and provide a much more ideal sound card interface to the 339a.

I was able to get a notch to-125dB by replacing the notch filter op amp with a jfet input type.
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Old 25th November 2012, 04:26 PM   #875
davada is offline davada  Canada
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Default 399a

Something I'm considering doing is tapping the output of the notch filter in the 339a.
This will bypass the auto set level in the analyzer which shoves the noise floor and distortion way up on the monitor output. Maybe add a connector somewhere on the rear of the chassis or a switch to route the monitor output there.

The auto set level takes the input level to the notch filter and puts it through a RMS convertor. The output of the RMS detector is compared to a reference and dual matched LDR is used to normalize the gain of the notch filter post amplifier to 1Vrms FS.

If the analyzer is set to a sensitivity of -80dBV FS, the notch is -115dB and the THD+N is near the notch the auto set level is applying a lot gain to normalize this to 1Vrms FS.
Furthermore this gain from the auto set level is provided with an LDR operating in a non ideal part of it's range which means a lot of distortion is added by the post amplifier as well as noise gain.

Additionally, it would be useful to tap the output of the auto set level's RMS detector to provide a level for scaling and this level is the input level to the notch filter.

These are simple mods that shorten the signal chain and provide a much more ideal sound card interface to the 339a.

I was able to get a notch to-125dB by replacing the notch filter op amp with a jfet input type.
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Old 25th November 2012, 04:28 PM   #876
davada is offline davada  Canada
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Default I meant 339a

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type.
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Old 25th November 2012, 05:00 PM   #877
davada is offline davada  Canada
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Default Some corrections

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Originally Posted by davada View Post
Something I'm considering doing is tapping the output of the notch filter in the 339a.
This will bypass the auto set level in the analyzer which shoves the noise floor and distortion way up on the monitor output. Maybe add a connector somewhere on the rear of the chassis or a switch to route the monitor output there.

The auto set level takes the input level to the notch filter and puts it through a RMS detector. The output of the RMS detector is compared to a reference and a dual matched LDR is used to normalize the gain of the notch filter post amplifier to 3.162Vrms FS.

If the analyzer is set to a sensitivity of -80dBV FS, the notch is -115dB then THD + notch level is near the noise floor. If the input level is less than the reference level the auto set level is applying gain at the post amplifier to normalize this to 3.162Vrms FS.

Furthermore this gain from the auto set level is provided with an LDR operating in a non ideal part of it's range (poor linearity) which means distortion is added by the post amplifier as well as noise gain.

Additionally, it would be useful to tap the output of the auto set level's RMS detector to provide a level for scaling and this level is the input level to the notch filter.

These are simple mods that shorten the signal chain and provide a much more ideal sound card interface to the 339a.

I was able to get a notch to-125dB by replacing the notch filter op amp with a jfet input type.
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Old 25th November 2012, 06:16 PM   #878
1audio is offline 1audio  United States
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I think you are saying that the auto set level is done with an LDR pair at the input to the notch filter that maintains the level there at 3.16V RMS. The LDR's will be a limiter for the distortion. You could upgrade them to the Silonex equivalent which are supposed to be better or use several in a series/parallel network to reduce the voltage drop across them which will reduce the distortion. The distortion is voltage (or maybe current, I'm not sure) dependent so a smaller signal is lower distortion.

A constant level in the notch is pretty important for the auto-notch circuits to work. If you tap before the level meter attenuator for an external FFT you will have a pretty predictable level/known distortion level on the monitor out. If your FFT has low enough noise and the notch circuit is low noise then you should have all you need.

The FET opamp probably has better common mode rejection than the original which will reduce the internal distortion.
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Old 25th November 2012, 06:19 PM   #879
davada is offline davada  Canada
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Default 339a mod

Some added notes.

The input range indicator of the 339a forces the user to set the input range to normalize a 3.162Vrms FS.

If anyone wants to explore the above mods I would suggest using resistive padding at the output of the notch filter to bring the FS from 3.162Vrms to 1Vrms. This is in range for a sound card. If padding is used it should match the input impedance of the sound card to avoid reflection in the coax cable from impedance mismatch. This is something that should be observed with instrumentation.
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Last edited by davada; 25th November 2012 at 06:27 PM.
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Old 25th November 2012, 07:33 PM   #880
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
I think you are saying that the auto set level is done with an LDR pair at the input to the notch filter that maintains the level there at 3.16V RMS. The LDR's will be a limiter for the distortion. You could upgrade them to the Silonex equivalent which are supposed to be better or use several in a series/parallel network to reduce the voltage drop across them which will reduce the distortion. The distortion is voltage (or maybe current, I'm not sure) dependent so a smaller signal is lower distortion.

A constant level in the notch is pretty important for the auto-notch circuits to work. If you tap before the level meter attenuator for an external FFT you will have a pretty predictable level/known distortion level on the monitor out. If your FFT has low enough noise and the notch circuit is low noise then you should have all you need.

The FET opamp probably has better common mode rejection than the original which will reduce the internal distortion.
Hi Demian,

I'm not referring to the LDR in the notch filter. The auto set level is done on the post side of the notch filter. It auto scales the output of the notch filter referenced to the input level into the notch filter. It raises the noise significantly when the input level is below the FS. If the input level is 1Vrms then the auto set level increases the gain a little more than 3 times above unity on the post side of the notch filter. There is a long chain of amplifiers, active filters and attenuators in the path before the meter's RMS detector. The monitor taps the meter's RMS detector input. In level mode the monitor is switched to the 339a input amplifier bypassing the analyzer and the noise in this mode is significantly lower.

The mod would eliminate a large part of the signal chain. The only thing effected by doing this would be the scaling and that we can control.

The noise floor from the monitor output is quite significant. I waiting for Dick's measurements to confirm this as I can't find the email with the screen shot I sent him last February and I'm using a different analyzer now which is still in beta test and I'm not convinced.

It seems the sound card FFT method benefits most by having a minimal signal chain.

The Jfet op amp I used doesn't have as good a CMRR as the HA2 2625 I replaced but does a better job of null. I suspect the input loading on the network is at effect here.
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Last edited by davada; 25th November 2012 at 07:37 PM.
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