Rohde & Schwarz ZVB 14, Service Manual

Page: 82 / 198

Instrument Construction and Function Description R&S ZVB
1145.1078.82 3.14 E-4
Frequency reference
The
frequency reference board generates the highly stable and spectrally pure clock signals, required
by the R&S ZVB, which can be phase-locked to external synchronisation signals.
The various function blocks are:
The 128 MHz VCXO (voltage-controlled crystal oscillator) which generates a stable, low-noise
reference frequency for the synthesisers, for the A/D converters and for digital signal processing.
The PLL for phase locking the VCXO signal to an external reference signal or to a 10 MHz OCXO
(oven-controlled crystal oscillator) option.
The VCO and PLL which generate the clock for the netcon digital board (locked to the 128 MHz VCXO).
The frequency can be varied from 75 MHz to 86 MHz. The VCO frequency is programmable;-the
nominal clock frequency is 80 MHz.
A reference frequency of 10 MHz is standard. If the OCXO is fitted, the OCXO signal is brought out at
the ZVAB’s rear panel (10 MHz REF) so that further instruments can be synchronised.
The free-running VCXO (no OCXO, no external reference) can be calibrated using a pre-tune voltage.
If no OCXO is fitted, a 10 MHz signal is still output at the instrument’s rear panel. It is derived from the
80 MHz signal which is divided down to 10 MHz by the divider for the OCXO.
The following are also accommodated on the board:
- A control-CPLD to act as an interface between the serial bus and the board,
- Register for storing divider values,
- D/A converter for pre-tuning the VCXO and OCXO
- An on-board EEPROM for storing board-specific data
- Selftest facilities
OCXO reference (option B4)
As an option, the frequency reference board can be fitted to an OCXO (oven-controlled crystal
oscillator) which considerably improves the phase noise of the reference signal close to the carrier,
short-term stability and long-term stability.