[Elecraft] K2 cellphone interference
Our K2 appears inordinately sensitive to radiation from my GSM phone. It occurs with the audio gain turned all the way down so it appears that this problem might be with the audio amp stage. Have others experienced this and, if so, has anyone come up with a fix? -- Brian Lloyd Granite Bay Montessori brian AT gbmontessori DOT com 9330 Sierra College Blvd. +1.916.367.2131 (voice) Roseville, CA 95661, USA http://www.gbmontessori.com I fly because it releases my mind from the tyranny of petty things . . . — Antoine de Saint-Exupéry PGP key ID: 12095C52A32A1B6C PGP key fingerprint: 3B1D BA11 4913 3254 B6E0 CC09 1209 5C52 A32A 1B6C ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K2 cellphone interference
Yes, same thing here. It doesn't do it with the K3 at all however. I'm sure it's breakthrough directly into the audio stages. It's never bothered me too much as most of the time my cellphone is in the car or in a jacket pocket somewhere. Some judicious decoupling somewhere would no doubt help. 73 Stephen G4SJP On 08/06/2008 13:00, Brian Lloyd [EMAIL PROTECTED] wrote: Our K2 appears inordinately sensitive to radiation from my GSM phone. It occurs with the audio gain turned all the way down so it appears that this problem might be with the audio amp stage. Have others experienced this and, if so, has anyone come up with a fix? -- Brian Lloyd Granite Bay Montessori brian AT gbmontessori DOT com 9330 Sierra College Blvd. +1.916.367.2131 (voice) Roseville, CA 95661, USA http://www.gbmontessori.com I fly because it releases my mind from the tyranny of petty things . . . Antoine de Saint-Exupéry PGP key ID: 12095C52A32A1B6C PGP key fingerprint: 3B1D BA11 4913 3254 B6E0 CC09 1209 5C52 A32A 1B6C ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K2 cellphone interference
Brian, I have heard another report of this noise, but no analysis of the effect. Yes, the fact that the level does not depend on your audio gain setting indicates that it is confined to the AF Amplifier. Note that the K2 uses the LM380N BYPASS pin (8) for sidetone input. Normally, the BYPASS pin is used to increase the power supply rejection capabilities of the LM380 and many designs simply ignore it (nothing connected) because the power source is well filtered for ripple. As a quick test, try grounding the negative side of Control Board C32 with a very short (low inductance) wire and see if that improves the situation. If so, it could be that the power supply wiring in the K2 is picking up the radiation and feeding it to the audio amp. Understand that this is just a guess, but it would be nice to know if this is the source of the interference. Of course, the real problem is the phone - if it radiates that much, I wonder just how well it meets other radiation requirements. Note that your sidetone will not work with the test wire I suggested in place - I did say it was a test! If one would want to implement adding the bypass capacitor as a permanent fix, then a different method of sidetone injection would have to be created. A quick look at the K1 audio may provide some ideas (add an LM386 ahead of the LM380N for more AF gain and isolation of the AF stream from the sidetone - no, I do not have time to design and test and document it, it is just a thought). 73, Don W3FPR Brian Lloyd wrote: Our K2 appears inordinately sensitive to radiation from my GSM phone. It occurs with the audio gain turned all the way down so it appears that this problem might be with the audio amp stage. Have others experienced this and, if so, has anyone come up with a fix? -- Brian Lloyd Granite Bay Montessori ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K2 cellphone interference
On Sun, 8 Jun 2008 05:00:16 -0700, Brian Lloyd wrote: Our K2 appears inordinately sensitive to radiation from my GSM phone. This is a VERY common problem in the audio world. Cell phones and Blackberrys have interrupted hearings on Capitol Hill when they got into the sound systems in hearing rooms! Manufacturers of mics and other audio gear were caught with their pants down. I did some consulting work on that project. I've also consulted with mic mfrs on fixing their mic designs. GSM phones and Nextel phones are part of a broad group of phones that use TDMA -- Time Division Multiplex of the radio channel. That means that they transmit square waves with a very small duty cycle, because many conversations are sharing a channel, each transmitting for only 1-2% of their cycle. A phone with 50 mW average power out may have a peak RF output of 1-2 watts! To make matters worse, the repetition rate is 210 Hz, so the harmonics are right in the middle of the audio spectrum, making them VERY audible. So what you are hearing is that modulation (those square waves). About four years ago, I published an AES paper showing how the cell phone can be used as a simple injection probe to find the path the RF is taking into the victim equipment. While listening to the output of the victim, put the cell phone in transmit mode and move it slowly along each individual wire or cable that is connected to the radio. Since the cell phone is operating in the 800-900 MHz range, you will see wavelength-related effects and find hot spots along the cable(s) that is(are) doing the coupling. Suspect the mic cable and the headphone cable. The most common cause of GSM interference is a pin 1 problem. Another common cause is coupling around the feedback loop of the output stage that drives the headphones or an external speaker. In both cases, the fix is to either correct the pin 1 problem by properly connecting the cable shield to the chassis, not the circuit board, or clamping one or more UHF ferrites onto the cable very close to the point of entry. Fair-Rite #61 is the weapon of choice at cell phone frequencies. These coupling mechanisms are described in detail in a tutorial on my website. It's a free pdf download, no cookies. http://audiosystemsgroup.com/RFI-Ham.pdf Jim Brown K9YC Chair -- Technical Committee on EMC Audio Engineering Society ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] K2 cellphone interference
On Jun 8, 2008, at 8:14 AM, Jim Brown wrote: On Sun, 8 Jun 2008 05:00:16 -0700, Brian Lloyd wrote: Our K2 appears inordinately sensitive to radiation from my GSM phone. This is a VERY common problem in the audio world. I know. I used to make a living designing high-end audio preamps for audiophiles. EMI through the phono stage was always a problem and dealing with it with an impact on sound quality became an art. Being a ham also meant that my designs were exposed to RF from the get-go so I tended to fix things to my own satisfaction before they every saw the light of day. About four years ago, I published an AES paper showing how the cell phone can be used as a simple injection probe to find the path the RF is taking into the victim equipment. While listening to the output of the victim, put the cell phone in transmit mode and move it slowly along each individual wire or cable that is connected to the radio. Since the cell phone is operating in the 800-900 MHz range, you will see wavelength-related effects and find hot spots along the cable(s) that is(are) doing the coupling. Suspect the mic cable and the headphone cable. Except the problem occurs with mic, external speaker, and antenna disconnected. That leaves either improper shielding or ingress on the power cable. The most common cause of GSM interference is a pin 1 problem. Another common cause is coupling around the feedback loop of the output stage that drives the headphones or an external speaker. I hate to sound stupid but, what is a pin 1 problem? I can understand an improper electrostatic shield problem or a common mode RF current problem but the reference to pin 1 leaves me confused. Ah, never mind. All I needed was to read your paper listed below. Thank you. OTOH, it is using a term from the audio engineer's lexicon, one that might not be obvious to others outside that discipline. (It certainly was to me and I was already aware of the problem.) In both cases, the fix is to either correct the pin 1 problem by properly connecting the cable shield to the chassis, not the circuit board, or clamping one or more UHF ferrites onto the cable very close to the point of entry. Fair-Rite #61 is the weapon of choice at cell phone frequencies. These coupling mechanisms are described in detail in a tutorial on my website. It's a free pdf download, no cookies. http://audiosystemsgroup.com/RFI-Ham.pdf I guess that people forget that, in shielding equipment, they are building a Faraday cage around it. That means that you need to terminate your shield at the OUTSIDE of the equipment, not inside. One wants to continue the Faraday cage all the way out to the input device. This means that the shield of any wire needs to be attached to the chassis externally. That isn't hard to understand. I know that I solved the problem in my designs by using shielded twisted-pair for phono cartridge input and tying the shield to the chassis. Now, having said that, it doesn't appear to be coming in on the mic cable or the external speaker cable. I already use my cell phone as a probe and the problem seems to be a function of proximity to the radio on any side. Pickup occurs as much as 8' away from the radio with everything (except power) disconnected. Jim Brown K9YC Chair -- Technical Committee on EMC Audio Engineering Society ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com -- 73 de Brian, WB6RQN Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
