[ARC5] Smart People: IF Curve- What Am I Doing Wrong?

[AKLDGUY .]

> Here's the "set-up:"
> With a "fixed" IF, I'd align at the IF frequency.
> However, the ARC-2 has a variable IF.

Readers may need to know a little more about the ARC-2 before venturing an
opinion,
but there's very little technical data on it out there.
The ARC-2 covers 2-9 MHz in four bands:

A  2 to 3.016
B  3 to 4.525
C  4 to 6.033
D  6 to 9.050

Looking at just the receiver, am I correct in assuming that each band has a
fixed-tuned
(perhaps xtal-controlled) high frequency oscillator?

Am I correct in assuming that the IF stages are tuned manually by the
operator over a
1 MHz range on Band A, 1.5 MHz range on Band B, and so on?

If so, what range of frequencies applies for the IF on your band of
interest, Band B?

Here's the big question:
Is the IF tuning mechanically ganged to the RF stage(s) tuned circuits?

If the answer to the last question is yes, I suspect that by changing from
5 pF to 3 pF
you're destroying the linearity of the IF / RF tracking, and you're cutting
sidebands
because the RF stages are sufficiently selective to be a bandwidth factor
in their own
right.

If for example, the Q of the RF stage tuned circuits is 200 (quite
feasible), they by
themselves would have a BW of about 20 KHz at 3.890 MHz.

73 de Neil ZL1ANM


On Tue, Apr 8, 2014 at 2:10 AM, David Stinson <arc5 at ix.netcom.com> wrote:

> I've aligned hundreds of IFs and everything worked fine.
> But trying to draw a curve for the ARC-2 IF is "strange."
> Now, I don't believe what my figures are telling me.
>
> Here's the "set-up:"
> With a "fixed" IF, I'd align at the IF frequency.
> However, the ARC-2 has a variable IF.
> The manual procedure calls for aligning it with a Fo input.   The
> procedure says to set the rig on the lowest band at Fo 2.1 MC, AM mode, set
> the modulated signal generator level well below AVC action, meter the audio
> output and follow a step-by-step adjustment list for the "Main" slugs.
> Then move Fo to 2.9 MC and do the same for the "Trimmer" slugs.  I do all
> that, except I use a scope on the output rather than an audio level meter.
>
> I don't have a sweep generator, network analyzer or Star Trek Tricorder,
> nor am I likely to get any of these.
> To plot the curve, I set the rig Fo to 3.890 MC and
> leave it there.  Using an external freq standard (Icom R-70) to vary the
> signal generator frequency,
> I move the sig gen 1 KC at a time and note signal level on the scope,
> going  +/- 20 KC.
>
> Here's the problem:  My last attempt at "tightening" the IF bandwidth was
> to replace the 5 pFd IF coupling caps with 3 pFd and re-align.
> After doing so, the curve looked only slightly better, so I left the 3 pFd
> in place and  buttoned-up the rig.
> I turned it on this weekend to listen to some AM.
> Before, AM sounded sweet when the band wasn't crowded.  But now, it
> sounded "pinched;" just as you'd
> expect when using a "too-narrow" filters, yet the curve
> showed no real change in bandwidth.
> I put the 5 pFd caps back in service and the sweet,
> broad AM audio is back.
>
> OK, Smart People:  What's the really obvious mistake
> that I'm missing here?
>
> 73 DE Dave AB5S
>
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