ISOLATION AMPLIFIER
The Band 2 input signal enters on J4. This RF signal
is
terminated in 50 ohms by the combination of R1
and the input impedance of the amplifier. The input signal level
is
detected by CRI, filtered by C3, and
applied to one input of the Band 2 lock detector
(Ul).
The isolation amplifier
is
a
common base amplifier with
a
gain of -10 dB. An input signal range of +lo to
-20dBm
is
translated to
a
0 to -30dBm range into the mixer so the mixer will be in
its
linear range for
all signal input levels. The amplifier peaks slightly near 1
GHz to overcome an increase in mixer conversion
loss
at
these frequencies.
MIXER OPERATION
The local oscillator (LO)
is
applied to the IF terminal and the IF
is
removed from the LO terminal. This
swap allows the mixer
(MX1) to be unbalanced and act
as
a
low loss attenuator for signals between lOMHz
and 200MHz where no mixing
is
necessary. The mixer has
a
nominal 400MHz LO for signals between
200MHz and 6OOMHz; and has
a
nominal 800MHz LO for signals between 6OOMHz and IGHz. A 980MHz
LO allows operation with input signals to 116OMHz.
The output of the mixer drives an IF amplifier through
a
7
section, 200MHz low-pass filter. The IF amp-
lifier
is
a
"feedback pair" amplifier whose gain
is
stabilized by feedback, to be equal to 24dB. Inductor
L6
is
used to extend the high frequency response to 200MHz. The 1 pF capacitor ((226) between R34 and
R35
is
a
low pass filter to reduce the 1200 to 1500 MHz LO harmonics that reach the IF amplifier.
BAND
2
LOCK DETECTOR
The IF amplifier output goes to the signal select circuit and to the Band 2 Lock Detector. The Band 2 Lock
Detector has
a
voltage proportional to the
IF
level on the positive input, and
a
voltage proportional to the
RF signal on the Negative input. The conversion gain from RF input to IF amplifier output
is
a +6dB for
all valid signals, and less than -6dB for
all
spurious signals. The output of U1
is
positive only when
a
valid
IF signal
is
present. A small offset
is
added by R12 and R13 to guarantee
a
non lock condition when no
signal
is
present. Resistor R90 provides about 1dB of positive feedback to prevent eratic output from
noise
at
the point of threshold.
LO BUFFER
b
The VCO signal from the Band 3 Converter(A201A, J2) enters on J1. The signal goes through a 6 dB
attenuator
(R111, R112, R114), and
a
low pass filter (L1, C63, C64
to attenuate high order harmonics),
and
is
terminated by
a
51 ohm resistor (R16). Two high input impedance signal splitters ((32, Q3) get
their input signals from R16. Q2 and
Q3 operate on the same basic principal. One output
is
taken from
the emitter (acting
as
an emitter follower) which provides unity gain for the input signal. The AC terminat-
ing impedance on the emitter
is
adjusted to be 50 ohms so the amplifier will act as
a
unity gain amplifier
for the 50 ohm load which terminates the collector when a coax cable
is
connected. U2 has an additional
transformer
(TI) in
its
collector lead to increase the signal output to J3 by about 4 dB.
Scans by ArtekMedia © 2007
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