[R-390] A vs non A

[Jim]

Someone needs their meds adjusted...

----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <r-390 at mailman.qth.net>; "2002tii" <bmw2002tii at nerdshack.com>; 
<donreaves at gmail.com>
Sent: Saturday, October 13, 2012 7:23 PM
Subject: Re: [R-390] A vs non A


> Oh boy, Here we go again.
>
> The *ONE* that *NEVER* did this for a living for Uncle Sam is once again
> making proclamations that Roger is wrong.
>
> Does no one else find this arrogant?
>
> N0DGN
>
>
> On 10/13/2012 3:26 PM, 2002tii wrote:
>> 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
>>
>>
>>
>>
>>
>>
>>
>>
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