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Low-distortion Audio-range Oscillator
Low-distortion Audio-range Oscillator
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Old 6th August 2017, 05:12 AM   #6951
chris719 is offline chris719  United States
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Low-distortion Audio-range Oscillator
I use a GSM radio evaluation board in test mode (continuous TDMA transmit at 30 dBm) up close to provoke bad behavior from electronics.
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Old 6th August 2017, 07:01 AM   #6952
JensH is offline JensH  Denmark
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Low-distortion Audio-range Oscillator
Quote:
Originally Posted by vicnic View Post
And how about the differential input impedance?:
AD797 - R= 7,5 kohm C=20pF
OPA1612 - R= 20 kohm C= 8pF
LME49720 - R= 30 kohm C= unknown
It is difficult to say without knowing the details of the chip design.
Maybe Scott Wurcer can provide some info regarding the AD797?

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And I am sure you make that test in a shielded room to avoid problems with the FCC
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Old 6th August 2017, 07:51 AM   #6953
jcx is online now jcx  United States
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I believe Walt already did some RFI susceptibility tests and the AD797 came off very good
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Old 6th August 2017, 02:55 PM   #6954
mediatechnology is offline mediatechnology  United States
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I do recall seeing the late bcarso comment in the JCB thread regarding possible rectification in the LME479720/LM4562 input bias current cancellation stage or input stage.

I'm very suspicious of that Ib-correction stage as it relates to Vicktor's burst noise. I have zero science to back that up. Just a hunch.

The bias current patent - which doesn't provide any clues but may be of interest - is here: http://www.waynekirkwood.com/images/...dio_Op_Amp.pdf

Someone else mentioned pad/ESD diode resonance causing rectification. At the US DECT frequency that appears to be resonant, 1.9 GHz, the dimensions need to be small.

A Wifi acess point is within a couple of feet and it does not ingress. My Verizon phone, which sometimes operates in the 700 MHz(?) band doesn't ingress either. Somewhere around 1.9 GHz there's a peak.

It's been awhile since I did out-of-band sweeps >100 kHz <3 MHz with the LME49720 in SR limit but I recall it having a pretty big Vos shift compared to some other dual parts. The LME49720 likes to rectify even at low-ish frequencies when the inputs are deliberately driven. Having said that I've never seen any AM band issues where it rectifies things "in the air."

John Dunn wrote about a similar DC Vos test in 1993:

Click the image to open in full size.
"Check For Op Amp DC Offset Shift," John Dunn, Electronic Design, March 4, 1993.

PDF here: "Check For Op Amp DC Offset Shift," John Dunn, 1993. - Pro Audio Design Forum

There may be zero correlation between the LME49720s Vos vs frequency shift and its ability to demodulate DECT traffic.

Last edited by mediatechnology; 6th August 2017 at 02:57 PM.
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Old 6th August 2017, 04:09 PM   #6955
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by JensH View Post
It is difficult to say without knowing the details of the chip design.
Maybe Scott Wurcer can provide some info regarding the AD797?

@chris719
And I am sure you make that test in a shielded room to avoid problems with the FCC
These numbers for a bipolar reflect device size (Cin) and operating current (the open loop Rin is related to the gm (Ic) and beta). The lower number on Rin reflects higher operating current (797 is 900uA per side), I suspect simply the ratio of EMI incursion to standing current comes into play. I know that flea power op-amps can be horrible for EMI. There is nothing special in the 797 for EMI, comparing it to the LT1115 would make sense the operating currents are very similar but the Ib comp is very different. The Ib comp circuit in both cases is operating at tiny currents and could be susceptible to EMI.

EDIT - The test circuit above probably has little to do with EMI problems. At G = 100 and forced 10V output you are driving some of these amplifiers inputs into seriously non-linear regions.
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Last edited by scott wurcer; 6th August 2017 at 04:19 PM.
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Old 6th August 2017, 05:19 PM   #6956
mediatechnology is offline mediatechnology  United States
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Quote:
Originally Posted by scott wurcer View Post
The test circuit above probably has little to do with EMI problems. At G = 100 and forced 10V output you are driving some of these amplifiers inputs into seriously non-linear regions.
True. I didn't use that test circuit. But when I did drive the LME49720 into some serious SR-limiting (using a different test circuit IIRC unity gain inverting) it had an incredibly sharp knee more than one would expect from SR asymmetry alone. Its almost as if the DC it produced fed on itself or the Ib-comp went haywire.

An OPA1612 and other high-ish BW duals didn't demonstrate the radical Vos shift.

As I said the Vos shift vs frequency may have zero correlation to EMI susceptibility but it shows me the LME49720 tends to rectify out-of-band signals more than most similar BW op amps.

As good as the part is it has some warts to work around (1) satisfying start-up bias current (2) envelope detection of DECT/EMI susceptibility and (3) the major problem of large numbers of noisy parts.
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Old 11th August 2017, 02:23 PM   #6957
brianmk is offline brianmk
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Originally Posted by arb View Post
For info, recent project write-up by Tom McKay and Rod Elliott (Project 174 - Ultra-Low Distortion Sinewave Oscillator) implementing a sample and hold agc with Linsley Hood's two-stage Wien oscillator.
Project 174
I recently breadboarded the sample & hold described in this project with a view to replacing the full wave detector in an oscillator I built a while back.
The existing detector has a small amount of ripple that contributes to the harmonic distortion. Attempts to increase the time constant makes the settling time too long and can result in squegging.

The oscillator in the original project write up only operates up to 10kHz. My oscillator covers 15Hz->150kHz in 4 switched ranges.
My tests show that the S&H works up to about 28kHz.
Above that, the sampling pulses stop.
For operation at higher frequencies, C9 and C10 need to be reduced to give narrower sampling pulses.
However, this stops it working correctly at lower frequencies.
The only answer I could come up with is to switch C9 and C10 according to the frequency range.
I tried using a 4066 analogue switch to do this.
It meant reducing the +/- 12V supplies on the S&H down to +/-9v.
I was also concerned about the spikes on the input to pin 6 of the comparator.
These exceed Vcc which could damage a 4066. It needs an added protection diode between pin 6 and Vcc.
I found that using values of 1nF for C9 and 56pF for C10, the S&H can be made to operate up to about 100kHz.
Above that, the delay in the sampling pulses relative to the peak of the sinewave cause the the AGC control output to drop.
Replacing the LED 'D3' with three diodes in series helps by widening the clipped top of the sinewave. However, this reduces the AGC output voltage somewhat.

Another problem with the S&H is the unwanted offset in the AGC control output due to the FET biasing resistor R15. This is easily corrected by replacing the FET with a voltage follower using the unused half of the second TL072.
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Old 12th August 2017, 11:28 PM   #6958
1audio is offline 1audio  United States
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I think that's pretty impressive for a circuit with simple parts.

You can switch to a simple AGC at higher frequencies where the settling time is not an issue (Optimation did that).

Or the S/H circuit for the Boonton 1120 works to 140 KHz. One part has been discontinued from that design but ADI has newer parts that should work Sample/Track and Hold Amplifiers | Analog Devices .

The manual for the current instrument is here with details on the circuit and parts lists etc.: http://www.boonton.com/~/media/Boont...%20Manual.ashx
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File Type: png Boonton S+H.PNG (80.4 KB, 141 views)
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Old 12th August 2017, 11:36 PM   #6959
1audio is offline 1audio  United States
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Low-distortion Audio-range Oscillator
You might be able to use 2 AD873's as a solution. Or Davida's ADC+DAC which is quite original and simple.
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Old 12th August 2017, 11:42 PM   #6960
arb is offline arb
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Quote:
Originally Posted by 1audio View Post
You might be able to use 2 AD873's as a solution. Or Davida's ADC+DAC which is quite original and simple.
AD783?
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