[R-390] Please spare us

[David Wise]

> From: Dan Merz [mailto:djmerz at 3-cities.com]
> 
> Hi,  the crystal filtering in the radios you mention is of 
> the type that
> dates back to the early thirties and is not the passband type 
> of filter with
> a relatively flat top that passes a range of frequencies 
> uniformily.  Rather
> the top is very sharp and the passage of some range of frequencies is
> further down on the skirts of the response.  This was early 
> on called a
> "signal signal response" filter because the bfo could be 

Typo.  "Single signal".  You leave out the important fact that
the crystal had not only a sharp peak, it also had a sharp notch.
The Phasing control affected the spacing between them, so you
could peak up on the desired signal and simultaneously null out
an interfering signal nearby.  There must have been some
shortcomings or side-effects, because later crystal filter designs
(such as in the R-390x) make a point of doing away with the
notch, by means of a neutralization adjustment.

[snip]

> very close.  I added a 500 khz ssb mechanical filter to my 
> 388 to see the
> difference;  it helped some but wasn't world shaking.  Though it was a
> Collins filter,  it may not have been as good as the ones 
> used in the 51J4.

It depends a lot on how you install it.  "Blowby" is a big issue,
and you have to take a lot of care with shielding to get anywhere
near the out-of-band rejection the filter is capable of.  That's
why the R-390A filters are in those double-ended flange-mount cans.
This is discussed in the Cost Reduction Report.

73,
Dave Wise