[ARC5] ARC-2: Smart People Needed

[Geoff]

How much BCB feedthru is there?

Carl


----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: "ARC-5" <arc5 at mailman.qth.net>
Sent: Wednesday, April 02, 2014 12:52 AM
Subject: [ARC5] ARC-2: Smart People Needed


> I'm so glad I know people smarter than me.
> 
> So I'm trying to find a non-destructive way to tighten
> the IF bandwidth of the ARC-2.
> The IF tunes 1.0-1.5 MC as you tune the RF,
> making things a bit more "tricky."
> 
> Simplified circuit of the RT-91/ARC-2 receiver IF:
> 
> http://home.netcom.com/~arc5/ARC-2/ARC2IFCir.jpg
> 
> All the DC circuits (screen, fils) have been removed 
> for clarity.   Mixer plate circuit outputs to 1ST and 2ND
> if, which are tuned IF amplifiers, then to 3RD IF amp
> which is actually an untuned DC amplifier, then to 
> the 4th IF which has only a bandpass filter on the output.
> So I've omitted the 3RD and 4TH in the  diagram because
> the selectivity is in the Mixer plate, 1ST and 2ND IF.
> 
> My first thought was to reduce the coupling to the 
> tuned circuits, unloading them and thus increasing "Q."
> I changed the 800 pFd coupling caps first to 240 pFd,
> then down to 100 pFd.  The cap to the 3RD IF I
> changed to 50 pFd.   
> After each change, I realigned the IF according to 
> the procedure.  I noted no significant reduction in 
> stage gain, then plotted the bandwidth.
> To  my surprise, reducing these coupling caps
> actually made the IF circuit *broader." 
> A lot broader.
> 
> OK Smart People- I gotta be missing something
> here that's probably obvious to you.
> Why would reducing the coupling  here make
> the IF broader??
> 
> 73 DE Dave AB5S
> 
> 
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