HP 13255 Technical Information page 28

1j2~5
Cartridge Tape MOdule
13255-91032/26
Rev
AUG-Ol-7b
3.1.3.2
Refer to the truth table in section
3.1.1.2
for a summary of the fol-
lowing. When RE is high (read) and USO is low (DATA is a don't care),
Q4 and Q5
are tied together and go low. This drives
U17,
Pin
12
high
turning switches
1
and 2 on and Head
1
is connected to the preamp.
~hen
RE and
usa are high
(DATA is a don't care),
06
and Q7 are tied
together and go low. This drives Ul1, Pin 6 high turning switches 3
and
4
on and Head
0
is connected to the preamp.
3.1.4 HEAD
AMPLJFI~R
CIRCUIT - READ/WRITE PCA.
3.1.4.1
The read amplIfier circuit amplifies the input signal from either head,
differentiates the signal so that the peaKS of the flux transitions be-
come zero crossing,
and
then detects these zero crossings to produce a
TTL signal with transitions at eaCh zero crossings (flux transitions
peak).
3.1.4.2 the preamp (U5) is a differential input/differential output op-amp. Its
gain is determined
by R39
and R40 plus the
MOS
switCh on resistance in
one leg, and by R38 and R41 plus the
MOS
switCh on resistance in the
other leg. The differential gain is 40dB. R38 and
C36
on one side,
and
R39
and
C39
on the other side, roll off the response starting at
30
kHZ. C40 resonates the head at approximately 50 kHz, effectively
compensating for some loss in the head output.
3.1.4.3 Differentiator (U4) is also a differential input/differential output
op-amp. Its differentiating characteristics are determined
by C21,
R46, C16, and R34 in one leg, and
by
R41,
C22,
C19, and R35 in the
other leg. Its gain at the two frequencies of interest (4 kHZ and 8
KHz) is -2
d~
and +4 dB, respectively. C21 with R46, and C22 with R47
stop its gain rise at
+20
CB at 80 KHZ; and R34 with C16, R34 with C16,
R34 with
C16,
and R35 with
C19
start its roll oft at 160 kHZ. The pre-
amp outputs are connected to this ditferentiator Which translates each
amplitude peak of the preamp outputs to zero crossings.
3.1.4.4 The differentiator outputs are ac coupled at Pins 8 and 9 to comparator
Ul. The two inputs to U1 are lifted otf ground and reference to
+5V
through RtOtF and Rl0tG. This is necessary because this particular
comparator must not have its
inp~ts
allowed to go below -0.3 Vdc. This
comparator detects the zero crossings of the difterentiator outputs
and changes state each time, effectively squaring the ditferentiator
output waveform. A small amount of hysteresis is used to assure that