A2 Front Panel Board - HP 5334B Service Manual

8-233. The Attenuator consists of resistors R321 through R325 and attenuates the input signal by 6 dB so that
when diode quad limiter CR303
is
conducting,
an
approximate 50 Ohms input impedance
is
developed. The 6
dB attenuation also keeps the diodes of CR303 from blowing if a 7V or greater peak voltage source is connected
to the input.
8-234. The Limiter consists of diode quad CR303 that limits the voltage sent to the input of U305 (pin 3), first
amplifier of the Amplifier stage. CR303 has two diodes of each polarity in parallel, to provide good limiting (to
make sure the input voltage does not exceed U305's maximum input specification) and good power handling of
the voltage entering amplifier U305.
8-235. AMPLIFIER STAGE. Amplifier stage consists U305, U304, and U303 and associated components. Pull-
up resistors R316 and R317 force the output of U305 (pin 1) to
+
5V. R316, R310, and R306 are swamping resis-
tors used to limit the gain of the amplifiers at low frequencies. They shunt the output thereby reducing the gain.
8-236. DIVIDER STAGE. U302 divides by 64. The input (pin 1) is internally biased and capacitively coupled.
R327 is a pull-down resistor. Pin 8 of U302 is the bias point with external decoupling (C305). The output of
U302 is 1.6V peak-to-peak at pin 4. Resistors R305 and R304 decrease this output voltage to the standard ECL
,
amplitude of .8V p-p. R304 also provides
-
1.2V Vbb bias via U301 pin 1 1 . U301 is a ECL line receiver used as
a buffer and line driver to send the divide-by-64 signal to MRC U20, of the Measurement Block. R302 is a pull-
down resistor and R301 establishes a 1000 source impedance to match the characteristic impedance of the long
trace to U20. R326, located near U20 in the Measurement Block, terminates the trace and provides 2.7V bias for
MRC U20 input.
8-237. PEAK DETECTOR. The purpose of the Peak Detector is to detect when an input signal is present or
not present. The input power to pin 1 of U302 is sensed by peak detector CR302. It is coupled in through RC
network C310 and R309. This network attenuates low frequencies to compensate for the rolloff in the gain at
high frequencies in the Amplifier circuit. Only two of the four diodes in CR302 are used. The diode connected
to R309 rectifies the RF (Radio Frequency) to generate a signal corresponding to the peak voltage. The diode
connected to R308 generates an equal amplitude but opposite polarity signal, which is used to cancel tempera-
ture drift. No RF is applied to this diode. The RF diode is biased by applying 12V to R330. This biases CR304 to
about 0.W. R315 and R309 establish about .lma current in CR302, which should be at
+
0.2V when no RF is ap-
plied. The voltage on CR304 and CR302 both drift at the same rate so that the voltage across R315 and hence
the bias current is independent of temperature. R329, CR305, R312, and R308 provide a similar bias to the com-
pensation diode in CR302, which should have
-
0.2V on it. The two diode signals are summed by R313 and
R314 to produce a voltage of within a few millivolts of zero, when no RF is applied or a voltage higher than
5mV, when RF is applied; then is sent to the input of Threshold Comparator U306 (pin 2).
8-238. THRESHOLD COMPARATOR. Threshold Comparator U306 goes high when sufficient dc is
generated from the incoming RF to exceed the threshold established by the voltage-divider formed by Threshold
Control, potentiometer R328 and resistor R319. Resistor R318 provides one dB of hysteresis. The output com-
parator U306 (pin 1) is a open collector. Thus, when no signal is present, it goes LOW and biases pin 4 of U301
(ECL Receivermuffer) to -5V, disabling it, and hence preventing the counter from trying to count noise when
insufficient signal is present. When the threshold is exceeded, the comparator output is a high impedance, and
R303 biases pin 4 of line receiver U301 to
-
1.2V Vbb voltage, thus allowing the Counter to count.
8-239. A2 FRONT PANEL BOARD
8-240. The Front Panel Block contains the display and keyboard circuitry. The front panel provides push-but-
ton control of all counter functions, and displays measurement results and diagnostic information, such as error
and failure codes. The front panel circuitry is mounted on the
A2
board and communicates with the rest of the
Counter through the Executive MCU.
HP 5334B
-
Service Manual
8-31