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Old 26th September 2012, 01:13 AM   #481
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Quote:
Originally Posted by zigzagflux View Post
I use a Hammond aluminum enclosure, with surface mount terminal block going through holes in the side of the enclosure. +/-VDC power supply, +/- signal output, and an earth terminal which is bonded to the enclosure. Victor's oscillator is supported inside with standoffs epoxied to the enclosure (JB Weld) and then the PCB is glued to the standoffs in two places where there are no electrical connections to the circuit. That way the oscillator is securely mounted inside a metal enclosure, and the I/O is isolated from ground for a more or less balanced output. Works very well for low cost.

To keep the enclosure small, I did replace the carbon pot with a Bourns multiturn pot. That is also glued to the pcb with the adjustment screw pointing up. I remove the cover if I need to make adjustments (which I don't, it's just set for 2V output).

I can provide pictures if you need a better description. A little effort, but the oscillator operates so well it was hard not to make it work.
Thanks, but I have a different dream. I'm glad yours works well for you.
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Old 26th September 2012, 03:35 PM   #482
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Can someone help me with basic math please? I calculate that the highest frequency for this oscillator is 7,962Hz but they list the range as 1,500 to 15,000Hz. When the 2kohm pots are at zero, I used 200 ohms and 0.1uF and got this result. sump'in ain't right.

I calculate that I need 100 ohm resistors with 1kohm pots to get the range 1.5kHz to 15kHz. Is that right? I cheated and used this online calculator:
http://mycircuits9.blogspot.com/2012...alculator.html

I also need a recommendation on a stepped attenuator for the 2 gang 2k (1k?) pots.
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Last edited by dirkwright; 26th September 2012 at 03:59 PM.
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Old 26th September 2012, 04:56 PM   #483
coluke is online now coluke  Italy
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With the shown values it should cover the range 720 Hz - 7960 Hz.

L.
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Old 26th September 2012, 05:32 PM   #484
davada is offline davada  Canada
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Quote:
Originally Posted by dirkwright View Post
Can someone help me with basic math please? I calculate that the highest frequency for this oscillator is 7,962Hz but they list the range as 1,500 to 15,000Hz. When the 2kohm pots are at zero, I used 200 ohms and 0.1uF and got this result. sump'in ain't right.

I calculate that I need 100 ohm resistors with 1kohm pots to get the range 1.5kHz to 15kHz. Is that right? I cheated and used this online calculator:
WEIN BRIDGE OSCILLATOR CALCULATOR | MY CIRCUITS 9

I also need a recommendation on a stepped attenuator for the 2 gang 2k (1k?) pots.
It's f = 1 / 2(PI)RC. But you may have to account for some input resistance of the op amps. This will cause a deviation from the nominal. Stray capacitance will cause a shift as well.

I don't think you want an attenuator here just to switch in different resistor values for desired frequencies. You can use the above formula to find the resistor values by rearranging to... R = 1 / 2(PI)fC. Likewise C = 1/2(PI)fR.

You may need to trim the resistance to bring the frequency closer to what you want.
Most designers put a frequency vernier pot in somewhere but some believe this pot can cause additional distortion and for good reason, it can.

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Last edited by davada; 26th September 2012 at 05:42 PM.
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Old 26th September 2012, 05:38 PM   #485
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Quote:
Originally Posted by davada View Post
It's f = 1 / 2(PI)RC. But you may have to account for some input resistance of the op amps. This will cause a deviation from the nominal. Stray capacitance will cause a shift as well.

I don't think you want an attenuator here just to switch in different resistor values for desired frequencies. You can use the above formula to find the resistor values by rearranging to... R = 1 / 2(PI)fC.

You may need to trim the resistance to bring the frequency closer to what you want.
Most designers put a frequency vernier pot in somewhere but some believe this pot can cause additional distortion and for good reason, it can.

David.
OK, so you're suggesting that I make this a fixed frequency oscillator? I think a variable oscillator would be more useful which is why I'm pursuing this. It's really hard to get a dual gang pot to track exactly unless you use a stepped pot with precision resistors. I think that's where the distortion that you mentioned comes from.
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Old 26th September 2012, 05:40 PM   #486
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Originally Posted by coluke View Post
With the shown values it should cover the range 720 Hz - 7960 Hz.

L.
thanks. I wonder why the screwed that up so badly? I guess the need an editor at Linear Technology to verify their application notes.

I should look up what Walt Jung has to say about oscillators. He seems to be a reliable source, among many of course.
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Old 26th September 2012, 06:39 PM   #487
davada is offline davada  Canada
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Quote:
Originally Posted by dirkwright View Post
OK, so you're suggesting that I make this a fixed frequency oscillator? I think a variable oscillator would be more useful which is why I'm pursuing this. It's really hard to get a dual gang pot to track exactly unless you use a stepped pot with precision resistors. I think that's where the distortion that you mentioned comes from.
That's not what I'm saying at all.

An attenuator is for attenuating. What I'm saying is to make up you own switched resistor network, like an attenuator, but customized it for the frequencies you desire.
A stepped attenuator won't work properly for frequency adjustment.

I think your intention is, is to replace the gang pot with a switched network.

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Last edited by davada; 26th September 2012 at 06:46 PM.
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Old 26th September 2012, 06:52 PM   #488
richiem is offline richiem  United States
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@dirkwright -- the output level can be adjusted (within limits) by altering the reference DC voltage for the AGC loop provided by the 5V zener diode -- for lower DC level and lower output level, an LED works very well as a zener -- a red LED is around 2V, green up toward 3V; white, a bit more. Less good, but lower DC references would be series 1N4148s -- say two of them, but their knees are not sharp.

Do not change the ratio of the 10k and 14k resistors -- the DC through the 14k corresponds in heating to the heating value of the oscillator signal through the 10k. Putting the two diodes in close proximity (VERY close -- I would wrap them together in some insulation) provides first-order temperature compensation for the signal level. These resistor values are chosen to keep currents through the diodes moderate but high enough to provide some heating. So if you're going to try for lower output, try just reducing them by 1/2 to start.

Some Wien bridge tuning resistor (or capacitor) mismatch doesn't generally lead to an increase in distortion, just a shift in frequency -- the effect on gain is too small; and most of the distortion comes from the amps and the AGC loop anyway.

My opinion is that this oscillator leaves a lot to be desired, especially compared to Victor's design.

RE editing of the Linear app notes, yeah that needs to be better -- note that in the app note this oscillator came from, figures 47 and 48 are reversed with the respect to the descriptions...
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Old 27th September 2012, 12:43 AM   #489
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Quote:
Originally Posted by davada View Post
That's not what I'm saying at all.

An attenuator is for attenuating. What I'm saying is to make up you own switched resistor network, like an attenuator, but customized it for the frequencies you desire.
A stepped attenuator won't work properly for frequency adjustment.

I think your intention is, is to replace the gang pot with a switched network.

David.
Um yeah, I just don't know the proper lingo. Thanks.
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Old 27th September 2012, 12:50 AM   #490
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Originally Posted by richiem View Post

My opinion is that this oscillator leaves a lot to be desired, especially compared to Victor's design.
Thanks for your very helpful comments.

Well, I don't even remotely understand Victor's design, so I'm at a loss to do anything with it. It doesn't appear to be a Wein bridge, and it's not a state variable type so I don't have a clue. I don't think I want a fixed frequency oscillator after all.
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