[R-390] Stagger Tuning and IF craziness

[Tisha Hayes]

Oddly enough I was churning through different combinations of coil wire
gauges, winding counts and things with the mechanicals (wonder what got me
going on that) and I found that the best info we had on the mechanical
filters was for 610 turns of #41 on a 1.1mm bobbin, 2.515mm long. I ran the
numbers and figured out the inductance, opposed mutual inductance, k
factor, M factor, value of the parallel cap when resonant at 455 kHz and
ended up with the bandwidth of the RLC of each side of the filter.

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).

Just like the different stages in the IF deck, the mechanical filter is
nothing more than a very unique IF transformer. 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.

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.

One of the tech documents actually called for the IF covers to be removed
and drilled out for access (no more of that "no user serviceable parts
inside approach"). There are ways of aligning the IF's without high end
test equipment but it would be "so" much easier with a sweep generator and
a spectrum analyzer.

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. We all
know 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.

Of the things we can "do" to a receiver (snipping out parts and other evil
activities) the idea of optimizing the IF settings to peak for your type of
listening is benign and so easily reversed with a tuning tool.

-- 
Ms. Tisha Hayes/ AA4HA


*"neque est alter hujus universi locus quam anima"*
*('For there is for this universe no other place than the soul or mind')*
*-Arthur Schopenhauer*