NJ88C30 VHF SYNTHESISER
The NJ88C30 contains all the logic needed for a VHF PLL synthesiser and is fabricated on the GPS highper formance, small geometry CMOSprocess.The circuit contains a referenceoscillator and divider, a two-modulus prescaler and 4-bit control register, a 12-bit programmable divider, a phase comparator and the necessary data input and control logic.
- Low Power CMOS
- Easy to Use
- Low Cost
- Single Chip Synthesiser to VHF
- Lock Detect Output
- > 200MHz f
Crystal Oscillator and Reference Divider
The Reference oscillator consists of a Pierce type oscillator intended for use with a parallel resonant fundamental crystal. An external reference oscillator may be used by either capacitively coupling a 1V RMS sinewave into CRYSTAL IN (pin 6) or, if CMOS levels are available, by direct connection to CRYSTAL IN.
The reference oscillator drives a 4100 prescaler followed by a reference divider to provide a range of comparison frequencies which are selected by decoding the first three bits (DR2, DR1, DR0) of the input data.
To assist in trimming the crystal, an open drain output at one hundredth of the reference oscillator frequency is provided on CRYSTAL MONITOR pin 5
The programmable divider consists of a 415/16 two modulus prescaler with a 4-bit control register, followed by a 12-bit programmable divider. A 1V RMS sinewave should be capacitively coupled from the VCO to the divider input VCO pin (pin 10).
The overall division ratio is selected by a single 16-bit word (DF15 to DF0), loaded through the serial data bus. A lower limit of 240 ensures correct prescaler operation; the upper limit is 65535. The VCO frequency in a locked system will be this division ratio multiplied by the comparison frequency.
The phase comparator consists of a digital type phase comparator with open drain f
outputs and an open drain LOCK DETECT (LD) output. Open drain outputs from the reference divider and programmable divider are provided for monitoring purposes or for use with an external phase comparator. The phase comparator is linear over a ±2
range and if the phase gains or slips by more than 2
, the phase comparator outputs repeat with a 2
Once the phase difference exceeds 2
, the comparator will gain or slip one cycle and then try to lock on to the new zero phase difference.
Data Input and Control Register
To control the synthesiser a simple three-line serial input is used with DATA, CLOCK and DATA TRANSFER signals. The data consists of 19 bits; the first three, DR2, DR1 and DR0, control the reference divider while the following sixteen, DF15 to DF0, control the prescaler and programmable divider. Until the synthesiser receives the DATA TRANSFER pulse, it will use the previously loaded data; on receiving the pulse it will switch rapidly to the new data.
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