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Old 31st March 2013, 08:56 AM   #11
JMFahey is offline JMFahey  Argentina
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Quote:
I've been having persistent issues with RFI from my 4g cell phone when it's transmitting or receiving data within approximately 1 foot of either headphone amp.
Short answer: keep your cell phone further away.

Monster RF interference makers, which by the way is to be expected, they are 2 way powerful RF transceivers, what else do you expect?

Mine gets into the PC, the phone, the TV set, *everywhere* ... and it's fine.

If it's communicating to some 100 to 200 ft high RF tower which may be up to a couple Miles away, is it strange that some of that energy causes interference 1 ft away?
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Old 31st March 2013, 09:20 AM   #12
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Originally Posted by JMFahey View Post
Short answer: keep your cell phone further away.
While I can appreciate the ease of this fix I'm not sure it's possible since this is a portable headphone amp likely to be in a pocket living either alongside the cell phone or very nearby.

I would like to write this off as "not possible" and I know cell phones are monster RF emission machines by design but there are some retail amps that are apparently immune or at least very well behaved strapped right onto a cell phone. FiiO E6 comes to mind.

I have a number of devices that experience RF bleed from my cell phone and living in a low signal area doesn't help one bit (the phone has to ramp up power to connect) but that doesn't mean I shouldn't try to do it better and thought perhaps I was missing something obvious or at least something more seasoned forum members may have battled with and won.
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Old 31st March 2013, 03:41 PM   #13
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Okay, PS decoupling. throw some as-large-as-you-can-find X7R MLLC caps, something in 1206-1210 size ans 4.7-22uF capacity, right at opamp's supply pins looking towards the middle of the board at the GND layer. this way we do as good decoupling as possible.

Then, given the ground fill and tiny circuit which should work against EMI and they dont - i'd bet on the improper ferrites use (btw, which are these? there are lots of these each of which are kinda different...)

The improer way is... EMI thing says - put filtering on _every_ wire going in/out of the circuit. You've missed the GND wire and PS section. Given the largish length of the connected wires and leading towards another EMI mistake ("put all the connectors and wires on same side, same spot").. I'd put ferrite beads on every connected cable and see whether it fix something...

A bit of workaround would be use of FET-input opamp just to check whether it helps... Their input stage isn't prone to RF demodulation thing.


It's great that youv'e put up this issue, as it seems it is both really common AND really ignored especially at DIYers community.

Oh, and it's a nice puzzle to solve
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Last edited by s3tup; 31st March 2013 at 03:44 PM.
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Old 31st March 2013, 04:39 PM   #14
Ruwe is offline Ruwe  Europe
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Hi,
I assume that your kit is in some sort of metl or aluminum box, which is grounded. If that's the case the RFI can enter most likely from the inputs and the outputs. The input seems high impedenace to ground, but your output is not.
The first thing I would try is to short the output coils or to replace them with some small resistor (may be 1-2 ohms for headphones output is OK). The coils form a sort of tank circuit with your feedback capacitors. I don't see the values of your coils, so can not calculate the frequency, but it's possible that your output cable works like a nice antenna at high frequencies.
Try that first, if you still have problems short the input coils too.
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Old 31st March 2013, 05:33 PM   #15
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Quote:
Originally Posted by s3tup View Post
Okay, PS decoupling. throw some as-large-as-you-can-find X7R MLLC caps, something in 1206-1210 size ans 4.7-22uF capacity, right at opamp's supply pins looking towards the middle of the board at the GND layer. this way we do as good decoupling as possible.
I didn't put larger decoupling caps at the op amp because in the actual amp (not the test circuit) literally 1/2 inch away are 47uf X7R caps coming from the DC/DC converter. They aren't the cheapest of components and since they are really close in the full amp layout and still EFI problems I just didn't place them on the test board.

Quote:
Then, given the ground fill and tiny circuit which should work against EMI and they dont - i'd bet on the improper ferrites use (btw, which are these? there are lots of these each of which are kinda different...)

The improer way is... EMI thing says - put filtering on _every_ wire going in/out of the circuit. You've missed the GND wire and PS section. Given the largish length of the connected wires and leading towards another EMI mistake ("put all the connectors and wires on same side, same spot").. I'd put ferrite beads on every connected cable and see whether it fix something...
The ferrite beads I'm using are these
LI1206H151R-10 Laird-Signal Integrity Products | 240-2402-1-ND | DigiKey
I've only just started learning about EFI/EMI suppression (crash course really). I checked around for material on calculating the proper type of ferrites and didn't come away with anything solid so I just got those to try last order. If you think they are usable I'll go ahead and add more to the other lines.

Quote:
A bit of workaround would be use of FET-input opamp just to check whether it helps... Their input stage isn't prone to RF demodulation thing.
Are there any you can suggest with comparable output current to the NJM4556AD I'm currently using? If need be I can probably squeeze output buffers in alongside a lighter opamp. I have some AD8397's on the way currently but they are standard bipolar I believe.



Quote:
Originally Posted by Ruwe View Post
Hi,
I assume that your kit is in some sort of metl or aluminum box, which is grounded. If that's the case the RFI can enter most likely from the inputs and the outputs. The input seems high impedenace to ground, but your output is not.
The test circuit isn't but the actual amp that I am troubleshooting is in a full aluminum casing grounded very well to the board but not earth ground (portable battery powered).

Quote:
The first thing I would try is to short the output coils or to replace them with some small resistor (may be 1-2 ohms for headphones output is OK). The coils form a sort of tank circuit with your feedback capacitors. I don't see the values of your coils, so can not calculate the frequency, but it's possible that your output cable works like a nice antenna at high frequencies.
Those are just SMD chip ferrite beads. The software I'm using (diptrace) doesn't have ferrite bead symbol in the library so I just used the standard inductor symbol to represent them.
I have already tried removing them in pairs, outputs, input and both installing jumpers across the pads. Nothing seemed to change the amount of RF signal bleed. In fact I can remove all filtering including the DC blocking input caps and the RFI doesn't seem to get either better or worse.. It's just being stubborn
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Old 31st March 2013, 07:18 PM   #16
agdr is offline agdr  United States
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Quote:
Originally Posted by Amperage View Post
Are there any you can suggest with comparable output current to the NJM4556AD I'm currently using?
Try the OPA2132:

http://www.ti.com/lit/gpn/opa2132 (opens PDF)

which is FET input rather than bipolar. From the chart on page 6 of the datasheet it current limits at around 30-40mA with a 10V swing, depending on temperature, a bit less than half the NJM4556A's 70mA. The 2132 is pin compatible with the NJM4556A and has nearly the same operating voltage range.

Any non-linearities in the circuit or chip, like the diode (base-emitter) junctions in the bipolar inputs and outputs, can de-modulate the RF.

To help track down where the RFI is getting in, temporarily tack solder a short on each NJM4556A input under the board. Just a tiny wire from the socket pin to the nearest ground point. If that doesn't stop the problem then your input circuit is OK.

A 2500Mhz cell phone's wavelength is around 12cm, way too long for the tiny bonding wires and die inside the chip to be an effective antenna. Has to be something outside the chip conducting the RFI to the chip and/or demodulating the RF.

If shorting the inputs doesn't stop it, try making a "T" network out of two 10 ohm resistors soldered in series with a 150pF-or-so ceramic cap (with leads on the resistors and cap cut as short as possible) soldered at the junction of the two resistors. One resistor free end goes to an output under the board on the NJM4556A, the other free resistor end to one headphone channel, and the free end of the cap to a ground point near the output pin. If this filter reduces the problem then the RFI is conducting in through the output lead and getting demodulated in the chip.

If neither the input or output test nails it, that leaves the feedback loop and power supply feed.

BTW, welcome to the forum! That is a great user name. Amazing that on a forum like this nobody had thought to use "Amperage" before.

Last edited by agdr; 31st March 2013 at 07:34 PM.
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Old 31st March 2013, 07:52 PM   #17
qusp is offline qusp  Australia
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Amperage, I can highly recommend both the parts and reference materials at the Murata site regarding common mode and differential filters. you will likely need a bit of both. Noise countermeasures.

they have some very good compound filter parts with common mode and differential filters (C || L || CRC type stuff) SMD filters. the selection guide on the site is quite good also. common mode noise is not the sort of thing you can really do a broad solution for, its more a build and measure thing, or at least model, you need to know the frequencies of interest, load and source impedance etc. not easy to know beforehand.

Last edited by qusp; 31st March 2013 at 07:56 PM.
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Old 31st March 2013, 09:02 PM   #18
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by Amperage View Post
I have already tried removing them in pairs, outputs, input and both installing jumpers across the pads. Nothing seemed to change the amount of RF signal bleed. In fact I can remove all filtering including the DC blocking input caps and the RFI doesn't seem to get either better or worse.. It's just being stubborn
Then, as agdr already suggested, probably the RF enters through the power supply or you are just exciting some feedback loop oscillations with your phone.
By the way, why are you using the 150pF capacitors in the feedback? The OpAmp should be stable at gain of 11 without them. Also those capacitors may be working together with the 150pF ones at the inputs and add to the problem. Have you tried to disconnect the 150pF in the feedback loop?
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Old 31st March 2013, 09:34 PM   #19
gootee is offline gootee  United States
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Quote:
Originally Posted by Amperage View Post
I didn't put larger decoupling caps at the op amp because in the actual amp (not the test circuit) literally 1/2 inch away are 47uf X7R caps coming from the DC/DC converter. They aren't the cheapest of components and since they are really close in the full amp layout and still EFI problems I just didn't place them on the test board.
You will want to put some small-size (not necessarily small value) caps RIGHT AGAINST the power pins. It's not optional. There is a positive feedback loop for high frequencies, through the power rails (for almost every transistor-based amplifier). You must short-circuit the HF to ground. It's even possible that the RFI is much worse than it would be, because it's exciting that feedback mode. A half-inch away is useless, for this. Even two mm is too far away.

Use physically-small caps, the smallest you can manage, but use the largest value that comes in that size.

That might not solve your immediate problem but it has to be done, anyway. You will also want to tighten up your layout. Every component that connects to an IC pin should be soldered as close as possible to the pin. As close as possible doesn't mean a few mm away. Any open lenth of trace connected to a pin will be presenting parasitics to the pin, which should be avoided.

Also, look for any loops that enclose geometric area. They will make antennas (See Faraday's Law.).
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Old 31st March 2013, 11:22 PM   #20
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Originally Posted by agdr View Post
Try the OPA2132:

http://www.ti.com/lit/gpn/opa2132 (opens PDF)

which is FET input rather than bipolar. From the chart on page 6 of the datasheet it current limits at around 30-40mA with a 10V swing, depending on temperature, a bit less than half the NJM4556A's 70mA. The 2132 is pin compatible with the NJM4556A and has nearly the same operating voltage range.
That's a pretty sharp cut in max power output but if nothing else I try helps that's something not too difficult or expensive to try. If the output is clear of interference I can add a 2nd stage unity gain op amp/buffer for increased power handling.

Quote:
To help track down where the RFI is getting in, temporarily tack solder a short on each NJM4556A input under the board. Just a tiny wire from the socket pin to the nearest ground point. If that doesn't stop the problem then your input circuit is OK.
This is a great suggestion and what I'm going to try next. Easy, quick and free way to eliminate the inputs as the possible source.

Quote:
If shorting the inputs doesn't stop it, try making a "T" network out of two 10 ohm resistors soldered in series with a 150pF-or-so ceramic cap (with leads on the resistors and cap cut as short as possible) soldered at the junction of the two resistors.
I had considered doing this before and it's easy enough to patch in quick but I avoided it because of all the negative information I've read about op amp stability problems driving capacitive loads.

Quote:
Originally Posted by qusp View Post
Amperage, I can highly recommend both the parts and reference materials at the Murata site regarding common mode and differential filters. you will likely need a bit of both. Noise countermeasures.
Thanks for the link. I'll have to spend some time tonight checking it out

Quote:
Originally Posted by Ruwe View Post
By the way, why are you using the 150pF capacitors in the feedback? The OpAmp should be stable at gain of 11 without them. Also those capacitors may be working together with the 150pF ones at the inputs and add to the problem. Have you tried to disconnect the 150pF in the feedback loop?
They are there simply cause it was in some of maxim's white papers for anti-RFI measures. the RFI existed without the feedback caps. Those were one of the later additions when I started messing around trying to clean this up. I added the input 150pf caps, then the feedback caps and finally the ferrite beads on the inputs and outputs.

Quote:
Originally Posted by gootee View Post
You will want to put some small-size (not necessarily small value) caps RIGHT AGAINST the power pins.
I'll tack some on when I add above mentioned shorting jumpers across the input pins. I'll see if I can rework the layout a bit tonight. to speed things along I'm making all the prototype boards at home so I'm pretty much limited to double layer PCBs so I can't get too fancy with independent signal and ground planes. Sometimes I have no choice but to route traces a few mm around something else in the way.

Thanks again for the help everyone. I'll try some of these suggestions and come back later tonight with the results.
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