Synthesizer
A 900 MHz RX synthesizer is used as first LO. The TX synthesizer is modulated and working at a fixed frequency of 90 MHz. The TX frequency is obtained by mixing the frequencies from the two synthesizers. The main synthesizer uses fractional-N to achieve lower phase noise and faster switching. In most applications the phase noise is proportional to the overall division ratio and since fractional-N uses a higher comparison frequency the phase noise will be lower. The auxiliary synthesizer is a conventional loop without fractional-N and the comparison frequency is 75 KHz. The RX VCO is a module which operates between 914.040 MHz and 938.970 MHz. The MALIN chip contains the TX VCO , TX buffer stage, TX mixer, RX buffer stage and the RX prescaler. The TX PLL, RX PLL and the lock detect logic are part of the BERTINDY chip.
Programming
The name of the PLL circuit is BERTINDY, which is a combination of the two chips Bertram and Cindy. A three line bus is used to program the synthesizer with the help of a clock frequency. The RX synthesizer is operating at 45 MHz above the required RX frequency and a 14.85 MHz reference frequency is used.
Power
The synthesizer part in BERTINDY is put in power down mode by sending a command to the chip. The auxiliary and main synthesizer can be powered down separately and the RX-VCO is controlled by BERTINDY. The prescaler and the 90 MHz VCO of MALIN is controlled by sending power up/down commands.
Lock Detect
A signal showing when the synthesizer is locked is implemented in BERTINDY. The lock detect circuit operates with both the main and the auxiliary synthesizers. If both synthesizers are enabled, the lock detect output becomes active when they are both locked. If only one synthesizer is enabled, the lock detect output becomes active when that one is locked.
