Time Base Block - HP 5334B Service Manual

8-196. TIME BASE BLOCK
8-197. The Time Base Block provides a buffered 10 MHz reference signal against which the unknown signal is
measured. The 10 MHz signal is generated by either the standard oscillator circuit on the A1 Main Board or the
Option 010 ovenized high stab'ity oscillator. In addition, the Counter accepts an external 10 MHz signal through
the rear panel TIME BASE BNC connector. Regardless of the source, the reference signal is buffered before
going to the Measurement Block.
8-198. Time Base Block Circuit Description
8-199. The Time Base provides the buffered 10 MHz reference signal for MRC U20, and MFC U14, in the
Measurement Block.
8-200. The operation of the T i e Base Block centers on the triple differential amplifier U11. Quartz crystal Y1
works with amplifier U l l B to generate the standard 10 MHz signal, which is output to U l l A through the Inter-
nrnxternal T i e Base switch (Sl) located on the rear panel of the Counter. Schmitt trigger U l l B receives the
10 MHz signal from either the standard, option, or external time base, and outputs a square wave to drive UllC,
which buffers the signal going to the Measurement Block.
8-201. The output of U l l A also goes to the square wave-to-sine wave converter circuit made up of Q1, Q2, and
associated components configured as a differential pair. Current through the pair is set by R14. The output is
taken single-ended off the collector of Q2 where the signal is filtered through L1, C16, and R7 for the 10 MHz
sine wave. When the rear panel TIME BASE switch is set to INT, this circuit outputs a 10 MHz sine wave
through coupling capacitor C6 to the rear panel BNC connector for use by other instruments as a time base
reference. When the Counter is set to EXT, the same connector is used as the input jack for an external 10 MHz
source and the signals from the converter and internal 10 MHz circuit are disconnected. The external signal is
input directly to Schmitt trigger U l l A through current limiting resistor R2 and coupling capacitor C17. Diodes
C R l l and CR12 limit the signal amplitude to 1.5 volts peak-to-peak.
8-202. The standard oscillator consists of U l l B and quartz crystal Y1. Feedback resistors R12C and R12D bias
U l l B for oscillation. Capacitors C1 and C5 tune the crystal for series resonance at 10 MHz. Pull-down resistor
RlOD provides current to the ECL output. Resistor R15 damps the fast ECL transitions to prevent RFI (Radio-
Frequency Interference). The output signal is ac coupled through C8. Note that this capacitor must be removed
if the Option 010 oscillator is installed.
8-203. The standard, option, or external 10 MHz signal is coupled through C17 to U l l A which acts as a
Schmitt trigger. Resistors RlOC, RllC, and R l l D bias U l l A for approximately 100mv of hysteresis. Resistors
RlOA and RlOB sink current from U1lAYs complimentary ECL outputs.
8-204. U l l C is driven single-ended with input pin 12 biased to Vbb at pin
1 1 .
Resistors R l l A and R l l B
provide current to U11C's complimentary outputs and resistors R12A and R12B damp the ECL transitions. The
signal is ac coupled through C29 and C30 to provide an ECL signal for the MRC external reference input and
the MRC oscillator input.
8-205. For the Option 010 oscillator, the Main Board unregulated
+
15 volt power supply is input directly to
the oven oscillator assembly. The
+
15 volt supply from the Main Board is regulated by U205 to provide
+
12 volts to the oscillator circuit. Coupling capacitor CS must
be
removed, and coupling capacitor ClOO must
be
added when the Option 010 time base is installed.
HP 5334B
-
Service Manual
8-28