HP 8340A, Service Manual

Page: 81 / 326

8-108
Model B340A - Service
Rl, R2, R3, Rll, R54, and Ll determine the bias and gain of Q5.
TP3 should be at approximately -14.B Vdc for correct bias.
100 MHz Buffer Amplifier B
Transistors Q9 and QB function as buff er amplifiers to isolate the
vcxo from variations in load which would otherwise cause frequency
pulling of the oscillator. Q9 is an emitter-follower amplifier
which presents a high impedance to the VCXO output. QB buffers the
output of Q9. Components L6, Cl5, and Cl6 form a tuned circuit to
match the impedance of the collector output of QB to 50 ohms. This
signal, 100 MHz OUT, is sent to the AB 3.7 GHz Oscillator.
The signal at the emitter of QB, equal to the input signal
(emitter�follower configuration), is input to the emitter of Qll,
a common-base buffer amplifier. The output at the collector of
Qll, through the tuned circuit of L7, Cl9, and C20, is applied to
the power splitter, T3, which splits the signal into two paths.
In one path, the signal from·T3 goes to U6 through a 3 dB pad. Q6
is a common-emitter amplifier with emitter degeneration (R27) for
gain stability. The outp�t at the collector of Q6, through the
tuned circuit of LB, C27, and C2B, is applied to the power
splitter, T2, which splits the signal into two paths. In each
path, the signal goes through a 3 dB pad and a tuned LC circuit
and becomes the 100 MHz OUT signal sent to the A29 Reference Phase
Detector and the A39 PLL3 Upconverter.
In the second path from power splitter T3, the signal goes to Q7,
a common-emitter buffer amplifier. The output at the collector of
Q7, through the tuned circuit of L9, C32, and C33, is applied to
the Quadrupler, Block c.
Quadrupler C
The 100 MHz signal from Q7 in Block B is applied to power splitter
Tl. The two outputs of Tl are input to transistors Q3 and Q4,
which form a Class C amplifier, full-wave rectifier circuit.
Positive half-cycles of the 100 MHz signal into Tl turn on Q3
while negative half-cycles turn on Q4. The collector outputs are
connected together to provide a fullwave rectified signal that is
rich in even harmonics. The tank circuit of C3 and LlO is tuned to
pass the 4th harmonic, 400 MHz. This signal is applied to the 400
MHz Amplifier, Block D.
400 MHz Amplifier o·
Q2 and Ql are both common-emitter amplifiers which have .
approximately 20 dB gain each.
Both have tuned outputs to filter
Scans by HB9HCA and HB9FSX