Swann CUBIC ASTRO 102 BX Service Manual page 38

_CUBIC
COMMUNICATIONS
S""AIII
DIVISIDN
FREQUENCY COUNTER
The ASTRO l02BX uses an accurate, crystal controlled time base and a
frequency counter to provide the operator with a digital display of the
transceiver's operating frequency. This counter measures the local oscil-
lator frequency, corrects for the fact that this is 9 MHz above that which
the transceiver is receiving or transmitting, and displays the results.
The incoming local oscillator signal, taken from the emitter of Ql14
on the exciter board, is amplified and processed by the counter input amp-
lifier. RI02, RIO) and Rl04 form an attenuator network, used to reduce
any
spurious signals generated in the counter, preventing them from being coup-
led back into the receiver circuitry. The overall design of the counter in-
cludes extensive planning to prevent "counter birdies"--an unfortunate phe-
nomenon in all digital dial radios. This includes the solid shielding box
and cover that the counter is enclosed in, feedthrough capacitors on leads
penetrating this shield, high isolation amplifiers, input attenuator, etc.
The results are that the unavoidable spurs are reduced to insignificant
levels. The technician is cautioned to exercise particular care in main-
taining the integrety of this shielding.
CI04 couples the signal to the base of the first of two devices con-
nected as a compound pair, QlOl and Ql02. Bias for QlOl is provided by con-
ventional voltage divider RI05-RI06, and the drop across its emitter resis-
tor, RIO?, provides bias for QI02. The emitter resistance of Ql02 is div-
ided into two parts, RIo8 and RI09, with the latter bypassed by CI05. This
arrangement compensates for the inherent 6 dB-per-octave rolloff in trans-
istor gain, thus levelling its response over the range desired. Output from
Ql02 is taken via the collector, coupled through clo6 and RIl), and applied
to the base of QlO). This stage is conventional in all respects, with Rl14
us.
C>-'V'IV-.
L. • •
O"'o/V..,.
CL.
It.
x..
It It'T
'"~OUENCY COUNT~"
1Il30
lil 11 I
1'140
1t11l2
11111 I0Il
6-1