[Elecraft] K2 cellphone interference

[Brian Lloyd]

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
>
>
>
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--

73 de Brian, WB6RQN
Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com