[Elecraft] K2 cellphone interference

[Jim Brown]

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