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Old 8th January 2013, 04:24 PM   #1491
RNMarsh is offline RNMarsh  United States
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Default LDR vs THD

I mentioned 2 things which I still think are important but got glossed over -- dual stage opamp for increased feedback to reduce distortion and the LDR/jFET doesnt seem to be the most important in reducing further thd.

Now I saw an oscillator in TMW that is -145db. It uses a LDR for level control (NSL32SR3)... so that kinda proves it isn't much of a thd limiting factor. And, two LME49710 in series. I would like to see if Victor can make a few pcb with the circuit. And, how hard would it be to do something similar in the 339A?

-RNMarsh
I copied the schematic from here:

www.tmworld.com/photo/297/297122-TMW_oscillator_fig1.jpg

Last edited by RNMarsh; 8th January 2013 at 04:31 PM.
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Old 8th January 2013, 05:39 PM   #1492
JensH is offline JensH  Denmark
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I have followed this thread for some time now. I have also looked at the one you link to RNMarsh. In fact I am working on a PCB for that one + the notch filter. It has been at a slow pace though, so I have not finished it yet. And you guys keep throwing in new ideas on how to do the feedback control

It may be worth trying though, so maybe I will also prepare the PCB for a FET based solution. Perhaps with and without the PV device.

There seems to be some errors in the schematic you linked to. Some parts are a bit difficult to read and some values are missing. You should go to

Download

and download the "bonus". This seems to be a corrected version and easier to read.

I plan to modify it to run at 1 kHz, but still with an option to mount the components for 2 kHz operation, just in case. I have the key components like the LME49710, styroflex capacitors and the LDR already, so I just need to finish the layout and mount it.
I will need to get some of the resistor values. I am doing most of it as SMD, but I am in doubt whether I should put in leaded resistors (or the option to use either SMD or leaded). The styroflex capacitors are of course leaded.
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Old 8th January 2013, 06:49 PM   #1493
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Quote:
Originally Posted by JensH View Post
I have followed this thread for some time now. I have also looked at the one you link to RNMarsh. In fact I am working on a PCB for that one + the notch filter. It has been at a slow pace though, so I have not finished it yet. And you guys keep throwing in new ideas on how to do the feedback control

It may be worth trying though, so maybe I will also prepare the PCB for a FET based solution. Perhaps with and without the PV device.

There seems to be some errors in the schematic you linked to. Some parts are a bit difficult to read and some values are missing. You should go to

Download

and download the "bonus". This seems to be a corrected version and easier to read.

I plan to modify it to run at 1 kHz, but still with an option to mount the components for 2 kHz operation, just in case. I have the key components like the LME49710, styroflex capacitors and the LDR already, so I just need to finish the layout and mount it.
I will need to get some of the resistor values. I am doing most of it as SMD, but I am in doubt whether I should put in leaded resistors (or the option to use either SMD or leaded). The styroflex capacitors are of course leaded.
One comment, you have divided the AGC range so low that a 100Hz or lower oscillator may be impractical to adjust.
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Old 8th January 2013, 07:01 PM   #1494
JensH is offline JensH  Denmark
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I only plan to have a fixed frequency, preferably 1 kHz with 2 kHz as an option. I also consider a possibility to lock it to an external oscillator with a precise frequency by using injection locking.
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Old 8th January 2013, 11:15 PM   #1495
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Default Replacing the LED fixed my 339A's E1 LDR

Quote:
Originally Posted by davada View Post
Hi John,

Not sure if those will work.

One thing you can try is to break the epoxy out on the LED side and find an LED to replace the one that's open. No doubt you can find some black epoxy at your local hardware store.

Cheers,
David, I got antsy and decided what the heck, the LDR I have is toast so why not try David's suggestion since the parts I ordered aren't here and won't be for a few days.

I used a bench grinder to work from the LED end of the LDR towards the VR cell. At about 1/8th inch in I could see, and remove, the clear lens of the original LED from the "tunnel" it resided in. I bought a 20mA, 2.1V, 1900mcd Yellow LED with a clear lens. I had to cut the lens diameter down to fit it into the "tunnel." I then checked and, indeed the device shifted rapidly away from its dark resistance as the LED was fed current. At this point I went ahead and used black RTV to make the assemblage into a single unit and installed it into the 339A.

It works! The shape and level of the residual appears on my scope as it did prior to the 339A failing.

Thanks for the inspiration. I'm delighted.

John
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Old 9th January 2013, 12:59 AM   #1496
davada is offline davada  Canada
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Quote:
Originally Posted by johnfalc View Post
David, I got antsy and decided what the heck, the LDR I have is toast so why not try David's suggestion since the parts I ordered aren't here and won't be for a few days.

I used a bench grinder to work from the LED end of the LDR towards the VR cell. At about 1/8th inch in I could see, and remove, the clear lens of the original LED from the "tunnel" it resided in. I bought a 20mA, 2.1V, 1900mcd Yellow LED with a clear lens. I had to cut the lens diameter down to fit it into the "tunnel." I then checked and, indeed the device shifted rapidly away from its dark resistance as the LED was fed current. At this point I went ahead and used black RTV to make the assemblage into a single unit and installed it into the 339A.

It works! The shape and level of the residual appears on my scope as it did prior to the 339A failing.

Thanks for the inspiration. I'm delighted.

John
Hi John,

I'm glad it worked out for you.

I think this approach is probably less frustrating then trying to get a different LDR to work in there. HP was picky with some parts used in the 339a and they are listed as selected.

Maybe you can post the part number for the LED you used.

Cheers,

David.
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Old 9th January 2013, 01:19 AM   #1497
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
I mentioned 2 things which I still think are important but got glossed over -- dual stage opamp for increased feedback to reduce distortion and the LDR/jFET doesnt seem to be the most important in reducing further thd.

Now I saw an oscillator in TMW that is -145db. It uses a LDR for level control (NSL32SR3)... so that kinda proves it isn't much of a thd limiting factor. And, two LME49710 in series. I would like to see if Victor can make a few pcb with the circuit. And, how hard would it be to do something similar in the 339A?

-RNMarsh
I copied the schematic from here:

www.tmworld.com/photo/297/297122-TMW_oscillator_fig1.jpg
Hi Rick,

The amount of distortion contributed by a LDR/Jfet depends partly on it's involvement. If it's only handling a tiny amount of change in the loop gain then the distortion will be low.
This will work in a fixed frequency oscillator but there would be enough range for variable frequency oscillator like the 339a.

What's the value of R16 in the schematic you linked to?
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Old 9th January 2013, 03:19 AM   #1498
davada is offline davada  Canada
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Quote:
Originally Posted by davada View Post
Hi Rick,

The amount of distortion contributed by a LDR/Jfet depends partly on it's involvement. If it's only handling a tiny amount of change in the loop gain then the distortion will be low.
This will work in a fixed frequency oscillator but there would be enough range for variable frequency oscillator like the 339a.

What's the value of R16 in the schematic you linked to?
'but there would be enough range for variable frequency oscillator like the 339a."

This should read 'but there wouldn't be enough range for a variable frequency oscillator like the 339a'.
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Old 9th January 2013, 05:14 AM   #1499
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Hi Rick,

The amount of distortion contributed by a LDR/Jfet depends partly on it's involvement. If it's only handling a tiny amount of change in the loop gain then the distortion will be low.
This will work in a fixed frequency oscillator but there would be enough range for variable frequency oscillator like the 339a.

What's the value of R16 in the schematic you linked to?
18 Ohms. see the original and twin-T et al at:
www. janascard.cz

Thx-RNMarsh

Last edited by RNMarsh; 9th January 2013 at 05:17 AM.
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Old 9th January 2013, 09:20 AM   #1500
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Quote:
Now I saw an oscillator in TMW that is -145 dB.
I think I've commented on this design before--I would not say that "this oscillator has -145 dB distortion", I'd say that "the authors believe that this oscillator has -145 dB distortion". Their measurement setup is very questionable (they have rheostat-connected trimmers in the direct signal path of the notch filter, which is a substantial invitation for distortion at these levels), and they have not cross-validated their result with a different measurement setup. There is simply no establish method to reliably measure distortion below the -130 dB level, so any claims at and below this level must be looked at with highest scepticism.

Quote:
It uses a LDR for level control (NSL32SR3)... so that kinda proves it isn't much of a THD limiting factor.
It doesn't prove anything--because the distortion contribution from the multiplier element is not just a function of its inherent distortion performance, but equally as much of its decoupling. If you can stand very low multiplier authority (i.e. a lot of decoupling, and resulting long settling time), its quite easy to make a low distortion oscillator even with a mediocre multiplier (and level detector). The pudding is about making a low distortion oscillator with reasonable settling time (whatever this may be for a particular application--just saying that at 10 Hz, 1000 cycles are awfully long). Also see my rather long post back in this thread about the THAT1281-based multiplier.

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