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Low-distortion Audio-range Oscillator
Low-distortion Audio-range Oscillator
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Old 3rd August 2017, 05:53 PM   #6921
dimitri is offline dimitri  United States
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Quote:
I discovered that the LME49720 (at least in this circuit) seems to have a problem with EMI.
Thank you for confirming this, see here and here
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Old 3rd August 2017, 06:12 PM   #6922
mediatechnology is offline mediatechnology  United States
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The "hot spot" for the RF susceptibility is with the loop directly over the LME49720 IC package. FWIW my OPA1612s are on DIP adapters to permit easy swapping of TH and SMT parts.

The nearby Wima polypropylene caps do pickup hum and are easily shielded but they are not a source of ingress to RF.

I haven't yet found a way to prevent RF rectification with the LME49720 other than a thorough shield.
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Old 3rd August 2017, 06:42 PM   #6923
mediatechnology is offline mediatechnology  United States
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This is an FFT comparison of an LME49720 and OPA1612. The output of the notch filter has 40 dB gain. The oscillator output level is approximately +6 dBu. The OPA1612's 2nd and 3rd THD is in the range of -135 to -140 dBc.

My cell phone was on the other side of the room but a WiFi access point was about 3' from the prototype which has a bottom metal shield and a grounded top foil shield sandwiched between cardboard. Other than the 60 Hz and related components in the OPA1612 example there is not much benefit to the shield with that op amp. The LME49720 benefits greatly from the shield but not enough.

Low-distortion Audio-range Oscillator

LME49720 FFT of oscillator output after notch filter. +40 dB gain.

Low-distortion Audio-range Oscillator

OPA1612 FFT of oscillator output after notch filter. +40 dB gain.

The bursts seem to begin around 20 Hz and at first appear line-related. Triggering the 'scope from the 60 Hz powerlinline shows that the burst noise is asynchronous to the line frequency.

The NE5523, NJM2114, NJM2068, NJM2043, OPA2134, OPA2604, OPA1642 and MC33078 will all run in that circuit with various THD signatures higher than the OPA1612 but otherwise they look clean.

The LME49720 has lots of spurs.

Dimitri I'm not sure those links are to the RF problems. You might have had something different in your clipboard when you pasted them.
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Old 3rd August 2017, 07:16 PM   #6924
1audio is offline 1audio  United States
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Low-distortion Audio-range Oscillator
The spurs look to be multiples of 100 Hz. Not powerline so what else is running at that frequency?

The OPA1612 seems to be a valid alternative. The distortion with either seems exceptionally low.
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Old 3rd August 2017, 07:21 PM   #6925
1audio is offline 1audio  United States
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Low-distortion Audio-range Oscillator
The OPA1612 seems about twice the cost of the LME49710 but its 6db lower voltage noise and about the same current noise. I'll need to try it in more places. Is it as sensitive to input Z as the AD797 etc?
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Old 3rd August 2017, 08:16 PM   #6926
mediatechnology is offline mediatechnology  United States
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In this oscillator the OPA1612 is my second choice to a good (and shielded) LME49720. On a DIP adapter it is very expensive.

The 1612 has almost rail-to-rail output into 2K, decent 600R drive, low input voltage noise, OK current noise for an Ib compensated part. There's a lot to like about it. Its a Ge-Si part. The FET version is the 1642.

Take a look at the OPA1612 open loop output impedance vs. frequency curve. Looks odd to me.

I don't know about the OPA1612 THD with high impedance sources. Its not a good choice for MM preamps due to its current noise but is an excellent choice for a MC or mic preamp with external transistors.

The 100 Hz rep rate looks like something polling to me. I shut down two access points in the shop but it was still there and wasn't able to move all the cellular devices out of range. I also have a Zigbee-based smart meter and energy monitor bonded to it that I can't shut down and RF weather stations. Could be Ethernet-related. Most USB spurs I see are at 1 kHz.

The LME49720 seems to rectify ambient RF envelopes. It's response is relatively broadband.
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Old 3rd August 2017, 10:40 PM   #6927
simon7000 is offline simon7000  United States
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To tame the LME's EMI addiction wind ten turns of fine wire around a 1,000 ohm 1/4 watt resistor and put that right at the chip's input. Some folks us 10 ohms others 100 ohms. Doesn't really affect the audio noise level as the plain resistance should be well under an ohm.

Demian this was in the BT thread years ago.
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Old 3rd August 2017, 11:49 PM   #6928
JensH is offline JensH  Denmark
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Low-distortion Audio-range Oscillator
@mediatechnology

Do you have a DECT cordless phone base station in the room or close by? Or a 2.4 GHz DCT (DECT based technology)? Even when there is no connection, it will transmit a burst every 10 ms.
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Old 4th August 2017, 12:57 AM   #6929
mediatechnology is offline mediatechnology  United States
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Originally Posted by JensH View Post
@mediatechnology

Do you have a DECT cordless phone base station in the room or close by? Or a 2.4 GHz DCT (DECT based technology)? Even when there is no connection, it will transmit a burst every 10 ms.
Yes. I tried the DECT phone near the DUT this afternoon and it went crazy. Normally its about 8 feet away. I only see it when the remote phone is taken off-hook near the DUT and the base station link is active however. When the DECT link is idle there is no proximity effect.

We have so many IOT devices on our wireless network finding all of them is difficult. Our neighborhood's Smart Meter mesh network for data backhaul is in the ISM band.

I haven't tried input series inductors with the LME49710 but I did try adding a series resistors in the grounded non-inverting inputs which had no effect. Adding input inductance right at the input pins makes sense.

The LME49720 response to a grid dip meter sweep seems to show a broad peak between 50 and 100 MHz. The GDM maxes out at 250 MHz.

EDIT: I unplugged the DECT base station and the 100 Hz spurs disappeared in the LME49720 oscillator. Wow. Put a tape measure to it. The base is 8' 3" away from the LME49720. It looks like the DECT base station is constantly pinging the phones but the phones are only listening in low-power standby. That explains why there isn't a proximity effect near an idle phone. From a power conservation perspective it makes sense.

Odd that only the LME49720 responds to this.

Last edited by mediatechnology; 4th August 2017 at 01:17 AM.
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Old 4th August 2017, 02:31 AM   #6930
mediatechnology is offline mediatechnology  United States
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Low-distortion Audio-range Oscillator

LME49720 Oscillator Notched Output DECT 6 On

Low-distortion Audio-range Oscillator

LME49720 Oscillator Notched Output DECT 6 Off

Low-distortion Audio-range Oscillator

LME49720 Oscillator Notched Output DECT 6 Off With Top Shielded

The LME49720's EMI susceptibility extends all the way to 1.9 GHz. The average transmit power of a DECT 6.0 station is 4 mW maximum. The peak power is enough to cause 100 Hz ingress in an LME49720 from 8 feet away.

With a top shield for hum and the DECT phone off the LME49720 looks pretty good.

Thanks Jens for the heads-up.

Last edited by mediatechnology; 4th August 2017 at 02:35 AM.
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