[Elecraft] What value for R88 with the KN2B

Sat Mar 29 11:05:01 2003

Hi Larry,

You do need the 2.7K resistor for R88 to provide a DC path to ground for the leakage current of varactor diode D29 in the crystal
filter.  Without this resistor, D29 will not perform it's desired tuning function.  Since R88 is across a tuned circuit, there may
be some other useful effect, such as broadening the response of the circuit.

In general, you can't connect a diode to a capacitor without having a DC path to some voltage source (or ground).  Otherwise, the
diode leakage current charges the capacitor. This current is highly variable so it's better to just eliminate its effect with a
resistor. The resistor can  usually be very large compared to other impedances in the circuit because the leakage current is very
small; for example, it's less than 50na for D29.

73/ Bob - AG5Q

----- Original Message -----
From: "Larry Benko" <[email protected]>
To: <[email protected]>
Sent: Saturday, March 29, 2003 9:01 AM
Subject: [Elecraft] What value for R88 with the KN2B

> I posted a question yesterday and have received a couple of
> comments and questions but no definitive reply.  I don't think
> that I made the original question clear enough so I will try
> again.
>
> Specifically, Rev. E of the K2 schematic, dated Feb. 16, 2003,
> says on sheet 2 of 4 in the upper right corner that if the KNB2
> Noise Blanker is installed to remove R88 on the K2 RF board and
> makes no mention of putting any component back in its place.
>
> Rev. D of the Noise Blanker documentation, dated Aug. 20, 2002,
> says to remove R88 on the K2 RF board and to then install a
> 2.7k resistor in the R88 location.  The KNB2 kit does provide
> this 2.7k resistor.
>
> Which is correct?  Do I follow the instructions for the KNB2 or
> the NEWER schematic for the K2?
>
> Apparently the original components R88, R89, R90 make up a 5dB
> pad which also provides some impedance stabilization to drive the
> crystal filter.  The KNB2 has a bandpass filter which appears to
> do the same function perhaps at the same level or not.  The
> presence of R88 would change the level to the crystal filter
> slightly.
>
> Thanks again and 73,
> Larry Benko, W0QE
>
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