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: [email protected] 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
