[R-390] Stagger Tuning and IF craziness

Mon Feb 18 09:38:01 EST 2013

Hi

The loaded Q of the driver coils is indeed in the vicinity of 33 (input resistance 100K ohms, resonating cap 120 pf, 120 pf = 2916 j ohms at 455 KHz). Since the 3 db bandwidth is Fc / Q you get 455 / 33 = 13.8 KHz. That's just for the driver coil on one end. The Q of the coil it's self, unloaded by the filter is significantly higher. 

In order to be "much wider" than 16 KHz, you might want 160 KHz on both ends. 455/160 gives you a Q of 2.8. For an input resistance of 100K ohms, that would be a resonating capacitance of ~ 10 pf. In order to hit something like that you would need to tune the input to each filter. That's not something the Collins guys wanted to do. Eight more trimmers cost money, and take up space. 

Bottom line - yes the coils do get into the bandwidth of the IF filters to a greater or lesser degree. The response is a composite of the driver coils and the disks. You don't want to do anything dramatic to those coils. You will certainly mess up the passband of the wider filters if you do. The whole filter was calculated / designed with the impact of the driver coil Q's taken into account. 

Bob

On Feb 18, 2013, at 12:44 AM, 2002tii <bmw2002tii at nerdshack.com> wrote:

> Tisha wrote:
> 
>> It turns out that the mechanical filter coils have a bandwidth of right
>> around 20 kHz (awful convenient) but it also explains a discrepancy I found
>> too where some filters use something other than #41 wire. By tweaking
>> things I was able to get the bandwidth of the mechanical filter down to
>> 12.85 kHz with a different wire gauge and turns ratio (with less parallel
>> capacitance to boot).
> 
> The response of the driver assembly (coil + magnetostrictive wire) has next to nothing to do with the ultimate filter response -- that is set by the vibration of the disks.  The driver assembly should be flat to substantially greater than the filter BW, to ensure a flat mechanical drive impedance to the disk resonator assembly.
> 
>> Each stage of the IF deck has a bandwidth and stagger tuning allows you
>> to flatten out the bandwidth response to where the wider mechanical filters
>> are not clipped off at the edges.
> 
> But, as stated in the technical manual and as many of us have documented, even when peaked at 455 kHz the IF transformers are plenty wide not to interfere with the transition zone of the 16 kHz filter (there are several sets of IF response curves on the web, taken through a complete IF assembly).  The only possible benefit of stagger tuning would be to remove whatever very slight general rounding there may be in the middle of the 16 KHz passband, and as I stated previously: (1) any such rounding would be a blessing, not a problem, in today's band conditions, and (2) the amount of any rounding present would be dwarfed by the large passband ripple due to the mechanical filter.  (Again, look at the IF response curves on the web.)
> 
>> The traditional response is to set everything to 455 kHz and peak away. If
>> you do not care about the wideband response at 16 kHz you could either
>> stagger tune to narrower values (like was mentioned setting them all the
>> same and ending up with maybe 4 kHz of BW, it sort of invalidates the
>> reason to have the 8 and 16 kHz mechanical filters) or you could still
>> leave them stagger tuned but set them closer.
> 
> I believe you misunderstand.  The three tuned IF transformers, all peaked at 455 KHz, have an aggregate response that is already wider than 16 kHz and does not interfere with the transition zone of the 16 kHz filter.  Stagger tuning can flatten the gentle peaking the transformer stages may exhibit *within* the passband of the 16 kHz filter, at the expense of widening their response to much wider than 16 kHz and thereby sacrificing the transformer stages' ability to add very useful close-in stopband attenuation (where the mechanical filter exhibits zeroes and a re-entrant response).  Some quick figuring suggests that if they are stagger-tuned "by the book" at +/- 12 kHz, the response of the transformer stages at the -6 dB points would increase from about 20 kHz to more than 50 kHz.  As Charles said in the post I quoted, think of the transformer stages as a "roofing filter."  Not a good idea to widen your roofing filter from 20 kHz to over 50 kHz.  If someone insists on stagger-tuning, by bringing the offsets down to +/- 4 or 5 kHz they will at least minimize the damage and end up with a "roofing" response in the 30 kHz area rather than > 50 kHz.
> 
>> I suspect the early idea of "leave these settings alone" was based upon
>> what would be the perceived difficulties of field aligning the IF's.
> 
> No, it was for the reason stated in the manual -- they are low-Q circuits that can generally be relied on not to drift out of spec (they fail from time to time, but if not failed they will be good to go).  The procedure is given in the manual, and the only thing difficult about it is swapping the "test" IF housing in and out.  I'm sure Roger can vouch that the military would not hesitate to ask radiomen to do that (and much more).  As an aside, the many, many IF stages in the Marantz 10B FM tuner -- arguably the best FM tuner ever made, and the precision linear-phase IF circuits were largely the reason -- were "tuned for life," and I have never yet seen one that wasn't spot on if it hadn't been monkeyed with.
> 
>> the easy way is to tune to a known signal, put a voltmeter on the
>> diode lead and adjust for maximum value. If they are stagger tuned at the
>> factory then by letting the field folk do the adjustments might mean that a
>> receiver had hot performance but would not hold a lock on a RTTY signal.
> 
> Again, I believe you misunderstand.  Putting a voltmeter on the diode load and adjusting for maximum value IS EXACTLY how those stages are stagger-tuned.  See the manual procedure cited in my earlier post.  The only difference is you don't do all of the sections at 455 kHz, you do 2 at 455, 2 at a + delta, and 2 at a - delta.  As mentioned, the "book" delta is 12 kHz, but I've argued that -- if you feel you absolutely must stagger-tune the transformer stages -- a delta of 4 or 5 kHz would be more appropriate.  If you do this, pay close attention to which transformer windings get tuned to each of the three frequencies (center, + delta, and - delta).
> 
> Best regards,
> 
> Don
> 
> 
> Copyright (c) 2013.  Not for redistribution
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