[Elecraft] What value for R88 with the KN2B

Fri Apr 4 23:04:02 2003

Hi Bruce,

You need the resistor so the xtal filter will work properly.  
It provides a path to ground for the leakage current through D29.

73/ Bob - AG5Q

----- Original Message ----- 
From: "Bruce D. McLaughlin" <[email protected]>
To: "'Bob - AG5Q'" <[email protected]>; "'Larry Benko'" <[email protected]>; <[email protected]>
Sent: Friday, April 04, 2003 7:48 PM
Subject: RE: [Elecraft] What value for R88 with the KN2B

> OK.  I am now very confused. (It doesn't take a lot to confuse me I'm
> sorry to say). I have K2 #3209.  I finished the radio a couple of days
> ago and now I'm also about done with the noise blanker.  So, do I
> install the 2.K resistor or not?  Although the schematic with my rig is
> Rev C, it appears to have the same instructions as Rev E.  That is R88
> and R90 removed and R89 replaced with a jumper.  There is no mention of
> installing any replacement resistor at R88 and yet, the earlier
> instructions that came with the NB say to install the 2.7K.  One reply
> to the earlier questioner said no and one said yes.  So . . . what
> should I do , i.e., is there a consensus?
> 
> Bruce - W8FU
> 
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of Bob - AG5Q
> Sent: Saturday, March 29, 2003 11:00 AM
> To: Larry Benko; [email protected]
> Subject: Re: [Elecraft] What value for R88 with the KN2B
> 
> 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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