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CIRCUIT DESCRIPTION
Front End/Notch Filters (p. 1/14)
The power for the active antenna (see page 14/14) is provided by Q101
and the bias network consisting of R102 and R103. Relay K101 selects
the antenna as the front end input during normal operation, and a 100
kHz calibration signal during unit self testing. To detect problems with
the antenna electronics, a VANT signal representing the current drawn
by the antenna system is sensed through divider R125 and R126 and
fed to the A/D converter. (see page 7/14)
The 100 kHz LORAN-C signal is filtered by the third-order Bessel filter
consisting of L113, L114, and L115, and C102, C114, and C115. The
front end filtering stages are designed to minimize phase distortion, and
NPO capacitors are used to minimize changes in filter parameters over
temperature. The filter is then followed by the first of four attenuators,
each of which consists of a resistive divider which is tapped based on
the setting of CMOS switches (here, U101A-U101D). The attenuator
stage is buffered by op amp U102 which provides about 22.5 dB of
signal gain.
Following the first attenuator are six individually tuned stages of notch
filters. Three of the filter stages can be tuned to notch out interfering
signals in the 110-220 kHz range, and the other three stages can be
tuned in the 40-90 kHz range. All of the notch filters are of the same
design, and the references in the following description apply to the first
notch filter. The filters are designed to provide less than 1 dB of
distortion in the 90-110 kHz band. Each filter is essentially made of two
RLC networks, with interstage unity gain buffers. R108, L101, and 1/2
of C116 make up one half of the first notch filter, and R109, L102, and
2/2 of C116 make up the second half. C116 is a ganged tuning
capacitor with integral 7 pf trimmer capacitors. Additional trimming is
provided by the two 47 pf capacitors C122 and C123. U103 provides
the interstage buffering. The following stages are identical, with
appropriate changes in part values for the 40-90 kHz filters.
LORAN Front End (p. 2/14)
Following the notch filters are three stages of filters, attenuators, and
22.5 dB gain stages. Each stage differs only in the attenuator step size
and in the center frequency of the filter. The three filters taken together
form a third-order Bessel filter. In the following description, references
are to parts in the first stage. The filter is made up of L116, C126, and
the total resistance of the attenuator, about 2 k ohms, divided between
R133, R134, R141, and R142. CMOS switch U109 selects taps of -0, -
16, -32, or -48 dB under micro-processor control for AGC. This stage is
buffered by U110 which provides about 22.5 dB of signal gain. The
following stages are similar. The next stage provides up to 16 dB of
