[R-390] A vs non A

[Nick England]

Thanks, Don - that's a well-stated and useful engineering explanation.
Nick K4NYW
www.navy-radio.com

On Sat, Oct 13, 2012 at 3:26 PM, 2002tii <bmw2002tii at nerdshack.com> wrote:
>
> Actually, if you want to adjust 390 IFs to factory alignment (linear
> phase, maximally-flat, constant group delay), sweep alignment really
> is a practical necessity.  The reason is that 390 IFs are not
> strictly "stagger tuned," as that term is conventionally used by
> filter designers.  Stagger tuning (strictly defined) refers to
> broadening the passband of a multi-stage filter (like an IF) by
> peaking various stages at slightly different frequencies.  Imagine
> drawing a single, peaked response curve at the IF center frequency,
> then drawing two more, offset to the left and right so that the upper
> -3 dB point of each filter coincides with the lower -3 dB point of
> the next higher filter.  Now, imagine the overall response of the
> chain -- it will more or less follow the skirts of the upper and
> lower offset filters, and the top will be a wavy line that averages
> the three filter responses.  (I just used coinciding -3 dB points as
> an example.  In practice, the offset frequencies could be chosen so
> that the -1 dB, or -0.5 dB, or other close-in attenuation points
> coincide, or so the -1 dB points of the offset filters coincide with
> the -3 dB points of the center filter, or ... on and on.)  Some
> people stagger tune 390A IFs to make sure the overall response is
> wider than the widest mechanical filter, and instructions have been
> published and are available for doing that.
>
> This "true" stagger tuned IF can be tuned (to a reasonable
> approximation) with a sig gen and voltmeter, because each stage is
> peaked at some frequency.  If you know what frequency each stage is
> supposed to be tuned to, you can change the sig gen frequency to each
> of these frequencies in turn and peak the appropriate
> stage.  However, this only gets you to an approximation because it
> assumes that the Q of each stage is the same as the design value.  If
> it isn't (and it probably won't be, at least not to high precision),
> you won't get the -3 dB points (or whatever alignment points it was
> designed for) to exactly coincide, so the passband ripple will not be
> optimized.  To optimize the passband ripple, sweep tuning is a
> practical necessity even for a "true" stagger-tuned IF.
>
> Note the term "passband ripple."  Therein lies the limitation of
> "true" stagger tuning -- it results in passband ripple, not a true
> linear-phase, constant group delay response.
>
> The linear-phase, constant group delay response of the 390 IFs is not
> achieved this way.  Rather, each stage is centered on the nominal IF
> frequency, but some stage pairs are undercoupled for a peak response,
> and others are overcoupled for two peaks with a valley at the center
> frequency.  Perhaps this should be called "stagger coupled."  By
> adjusting the coupling, a maximally flat, linear-phase, constant
> group delay response can be achieved.  Because all of the filters are
> tuned to the center frequency, and some are not peaked, but rather
> overcoupled with two peaks and a valley, there can be no list of
> frequencies where each stage should be peaked.  One might think that
> you could instead specify the depth of the valley of the overcoupled
> stages instead, and you can -- but you can only measure this with
> those two stages in isolation, not with the IF strip in the radio,
> because the response of the other stage couplings makes the overall
> measurement worthless for tuning the individual stage pairs.  (Even
> with the stages isolated, this would be very tedious with a sig gen
> and voltmeter -- you would need to find and measure the
> center-frequency response, find and measure each peak, adjust the
> coupling, repeat, repeat, repeat, ....)
>
> So, one may call the 390 IF "stagger tuned" using that term loosely,
> but it is not truly stagger-tuned in the sense that each stage is
> peaked at a certain frequency.  And there is no practicable procedure
> for tuning "stagger coupled" filters, like the 390 IF, without
> sweeping them.  This is presumably why the 390 manuals caution you
> not to undertake the procedure they provide unless the IFs are AFU
> and you are desperate -- it will not return the IF strip to its
> proper maximally flat, linear-phase, constant group delay response,
> but it may get the radio back on the air.
>
> Best regards,
>
> Don
>
>
> Copyright (c) 2012.  Not for redistribution
>