[Elecraft] Re: Elecraft info [noise blanker technical detail]

[wayne burdick]

Hi Phil,

I designed the KNB2 before the KNB1, and the KNB2 is the more versatile 
the two blankers. It also has more dynamic range, compatible with the 
K2's receiver.

If your friend wants to use the KNB2 in his own design, he'll need to 
substitute discrete control signals for those that are normally decoded 
and driven by the KNB2's 8-pin PIC. I think it is clear from the 
schematic what the PIC's pins do, but you could also probe them as you 
activate the unit from the front panel of the K2, or I can give you 
more technical detail as needed. The reason we used a PIC here is so 
the blanker could be controlled over the auxBus, like other options in 
the K2.

Regarding center frequency: 39 MHz may a little high for the KNB2, so 
he may if fact want to mix down to the K2's I.F. (which is 4.914 MHz, 
by the way, not 4.014). Still, it may be worth experimenting with 
component changes on the KNB2 to make it work at the much higher 
KWM-380 I.F. This would certainly be simpler. Also, a mixer could 
compromise his receiver's dynamic range unless carefully designed.

The remaining issue is input/output impedance. The KNB2's native 
Zin/out is about 150 ohms, which matches the K2's mixer post-amp and 
the following crystal filter. I'm not sure what the KWM-380's impedance 
is at this point in the circuit, but the KNB2's tuned circuit could be 
redesigned to match just about anything. One thing he may want to do is 
eliminate the resistive divider at the input to the first tuned circuit 
in favor of a capacitive divider. This will improve delay through the 
filter, which could help with some types of noise. (I had to use a 
resistive divider here in order to preserve the K2's high dynamic 
range; the resistors serve essentially the same function as the 5-dB 
pad that is removed when the KNB2 is installed in the K2.)

I wish him good luck with his project.

73,
Wayne
N6KR


On Feb 10, 2005, at 8:20 PM, Philip Westover wrote:

> Hi Everyone,
>
> A good friend of mine wrote the following to me, and I was hoping I 
> might get some comments from this list.  He is working on 
> incorporating an Elecraft noise blanker into his old KWM-380....
>
> I'd appreciate any input I could get to help my friend.
>
> - Phil, WA7URV

---

http://www.elecraft.com