AR7030 filter calibration

The following describes the operation of the filter calibration
routines in the AR7030 which will explain the figures observed.

The Filter calibration system in the AR7030 serves three main purposes :-

1) to calibrate the filter centre frequency offsets required for LSB and USB
    operation so that the filters give consistent carrier attenuation and
    tonal quality despite widly varying characteristics.

2) to enable additional or changed filters to be incorporated easily, set up
    for optimum operation and identified and selected by the radio operator.

3) to select the appropriate IF tail filter to match the bandwidth of each
    user selected filter.

The system operates by feeding a variable frequency IF signal through each
filter in turn and monitoring the filter attenuation using the receiver's AGC
system. The filter is scanned below and above its nominal centre frequency
and the points where the signal is reduced by 8dB and 20dB are recorded.

The -8dB point is used, rather than -6dB, because the filter's transition
slope is steeper at this point and with the rather granular measurement
of the AGC system this provides a more consistant assesment of the filter's
bandwidth. The receiver software then applies a correction to the measured
-8dB bandwidth to produce a typical -6db figure and to allow for the effects
of the IF tail filter, which is outside the AGC measurement loop (see the
AR7030 block diagram).

The corrected -6dB bandwidth is used to select the most appropriate IF tail
filter bandwidth (4kHz or 10kHz) and to provide the entry in the filter
bandwidth table for user identification. The Filter table order is set by
sorting these bandwidth figures into ascending order. Note that the
calibration routine test the filters in the order in which they are installed
in the circuit board, and the filter numbers displayed are not necessarily
the same as the numbers shown when filters are selected in the FILTER menu.

The -20dB filter frequencies are used to establish the USB and LSB carrier
positions for each filter. The filter offsets are internally limited to +/-
3kHz because there is little need to apply a full USB or LSB offset to the
widest filters, but filters up to 4kHz bandwidth will be set to eliminate the
unwanted sideband, with a carrier attenuation of 20dB.

An average frequency between the -20dB points is used as the fiter centre
frequency for CW, DATA, AM and Sync modes. The PBS offset diplay is based
on this calculated centre frequency rather than the nominal 455kHz centre
of the filters. This allows for manufacturing tolerances and thermal
variations.

Because the calibration system shares many common parts with the PBS system
its frequency range is similarly limited to +/- 4.2kHz. In practice this is
not a problem because very acurate calibration is only needed for the narrower
filters used in SSB modes, and the wider filters, used mainly for AM and NFM,
will tolerate a few hundred Hertz centre offset without any noticable effect.

Because of these frequency limitations the maximum -8dB bandwidth that can be
measured is 8.5kHz, which will report as a -6dB bandwidth of 6.7kHz. Any
filters that are wider than this will all report the same bandwidth. The
standard 9.5kHz filter is a special case (which is why it cannot be changed)
and it will always have a bandwidth of 9.5kHz entered in the filter table.
