KRX3 roofing filter matching for diversity receive

[Chuck Guenther]

Stan, you asked a good question-- one I asked about some time ago.
When I worked in the aerospace industry, I designed and used analog
and digital filters for measurement applications.  Most textbook design
formulas and procedures are based on 3 dB bandwidths, although sometimes
our measurement requirements demanded 0.5 dB bandwidth.

Amateur radio IF bandwidths seem to have been standardized at the 6dB
points.  If the filter designers are reporting 3 dB points, this might
account for the
difference you asked about.  BTW, I just checked the INRAD website and
looked
for the filter curves for the K3 roofing filters and couldn't find
them.  They used to
be available there.

73,
Chuck  NI0C
K2-10 s/n 5853  K3 s/n 1061


Stan Rife, W5EWA, wrote:
"I'm not sure why a filter would be advertised as a 250hz filter but
actually have
a (usable?) bandwidth of 370hz. Is this just a difference in how it's
measured? "
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[Richard Ferch]

W5EWA wrote:

> So is the information in the manual just the easy way to set it up,
> and you can actually put them in whatever slots you want?
>>From reading the manual I understood that they had to be in decending order
> starting from FL1, and the filters in both receivers had to match for
> diversity receive.

You should always follow the manual carefully. The manual says that
filters should be in descending order of width, but they do not have to
be in consecutive slots.

For example, in my main RX I have a 2.7 kHz in slot 2, a 500 kHz in slot
4, and a "250 Hz" (configured to switch in at 350 Hz) in slot 5. Slots 1
and 3 are open. In the sub-RX I have 2.7 kHz and 500 Hz filters. I could
have installed the sub-RX filters in slots 1 and 2, or 4 and 5, or 1 and
5, or ..., but I chose to install them in slots 2 and 4 simply to reduce
confusion.

For diversity receive, you should have similar filters in both
receivers, e.g. matched pairs at the bandwidths you usually use during
diversity receive. If the filters are 5-pole filters, the offsets should
be matched as well. Note that during diversity receive, if the filters
in each receiver are not identical (as in my case below 350 Hz widths)
you may hear phase modulation, although in my case I can't say that this
has ever been a serious problem - to me it sounds rather similar to the
AFX feature.

There is one other restriction: if you plan to transmit in FM, you must
have an FM filter in slot 1 on the main board.

73,
Rich VE3KI
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