[Elecraft] AGC behaviour

[Joe Subich, W4TV]

In addition to the toroid, make sure the shield of the USB 
connector in your computer is connected to the chassis (RF 
ground).  Many motherboards fail to ground the shield, allowing 
it to radiate the broadband noise generated by the high speed 
USB data between the computer and the USB->serial UART in the 
USB device.  



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
> Sent: Wednesday, September 24, 2008 12:45 PM
> To: Elecraft List
> Subject: Re: [Elecraft] AGC behaviour
> 
> 
> On Wed, 24 Sep 2008 07:59:36 -0400, Monty Shultes wrote:
> 
> >I have very high white noise in my K3 on 75 meters when connected to 
> >the
> >serial port computer interface. Removing this connection 
> greatly reduces the 
> >noise.  If you use HRD, LP-BRIDGE, or any control software, 
> you can also 
> >hear a pulsing noise related to polling the K3 over the 
> serial port.  I 
> >believe this can be lessened, but have not tried as yet.  
> Here in Florida, 
> >USA, the level of thunder storms creates a naturally high 
> noise level in the 
> >summer, but the effects described above are clearly 
> noticeable.  I use the 
> >Elecraft KUSB as serial interface.
> 
> I suspect that either the computer or the K3 are dumping RF 
> trash onto your 
> serial cable, which, like any antenna, is radiating it, and 
> your 75M antenna is 
> picking it up. I suggest that you wind the serial cable 
> around a toroid to form 
> a common mode choke tuned to 75M. Fair-Rite #31 is the best 
> material to use, and 
> a 2.4-inch o.d., 1.4-inch i.d. toroid is a convenient size. 
> Fair-Rite #43 will 
> work, but you'll need more turns. This size is commonly 
> called FT-240 in the ham 
> world. 
> 
> See http://audiosystemsgroup.com/RFI-Ham.pdf for tutorial 
> information about how 
> and why this works, as well as measured data for chokes with 
> 1 - 14 turns around 
> #31 and #43 toroids. Ferrite beads clamped onto the cable 
> will NOT fix this 
> problem on the HF bands. They WILL help on 2M. :) 
> 
> 73,
> 
> Jim Brown K9YC
> 
> 
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