[R-390] A vs non A

[2002tii]

Roger wrote:

>R390 IF's are no more complex than the R390/A
>Either can be done very well with a simple signal generator and volt meter.
>
>The Sweep generator myth is in with moster gold plated Audio cable.
>Can I sell you some old bridge stock? I have original paper certificates.

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 390A 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