[Milsurplus] ARC-2: Smart People Needed

[Joe Connor]

Smart people, huh? Well, I guess I'll have to sit on the bench for this one.

Joe Connor
On Wednesday, April 2, 2014 11:25 AM, Ian Wilson <ianmwilson73 at gmail.com> wrote:
 
Hi Dave,
>
>Are the IFT primaries and secondaries coupled by both magnetic (proximity)
>coupling and capacitive coupling (the 5pF caps) or only by the caps?
>
>73, ian K3IMW
>
>
>
>On Tue, Apr 1, 2014 at 9:52 PM, David Stinson <arc5 at ix.netcom.com> wrote:
>
>> 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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