Optimize the MAX2309 PLL Divider Values in Dual-Mode Cellular Phones

Category: Telephone, cellular phone and intercom
Manufacture: Maxim Integrated Products
Datasheet: Download this application note

Maxim > App Notes > WIRELESS, RF, AND CABLE Keywords: PLL, AMPS, IF, TDMA, IS136, pll, analog amps, if, cellular, digital tdma, is-136
Optimize the MAX2309 PLL Divider Values in Dual-Mode Cellular Phones
Abstract: Describes method to set the PLL divider values in dual mode (analog AMPS and digital TDMA) cellular phones. IS-136 dual mode phone frequency plan is outlined, using a first IF of 119.64MHz, and an AMPS IF of 455kHz. Tradeoffs are needed between PLL phase detector frequency, loop bandwidth and spurious outputs. Brute force solution must switch loop filter components to control stability. A 50Hz error in AMPS mode allows PLL to use same filter components. Additional Information:
Wireless Product Line Page Quick View Data Sheet for the MAX2309 Applications Technical Support
The original cellular telephone system in the USA is based on analog frequency modulation (FM) technology and is called advanced mobile phone system (AMPS). Many telephones still attempt to support the old standard along side the modern digital systems. This is not a tremendous technical challenge when the baseband processor and RF (radio frequency) transceiver are designed for this support. Some modern baseband processors have opted to abandon AMPS support, forcing a telephone handset design engineer to "graft on" the needed circuitry for an analog implementation of AMPS. This application note presents a typical frequency plan for a cellular telephone handset that supports both time division multiple access (TDMA) IS-136 digital mode, and analog AMPS mode. A method is given to select phase lock loop (PLL) divider values to maintain fast response time in the 2nd local oscillator (LO). Using a temperature compensated crystal oscillator (TCXO) of 19.44MHz, a first IF (intermediate frequency) of 119.64MHz, and an AMPS IF of 455kHz, it is possible to operate the 2nd LO phase detector at ~40kHz for both digital and analog modes. A Mathcad worksheet is presented which is easily modified to support other 1st IFs and TCXOs. Figure 1 shows a partial block diagram for a typical cell phone which supports both IS-136 TDMA and AMPS operation. The front end is the MAX2338 and the required RF filtering. The 1st LO drives the RF mixer, and is controlled by the phase locked loop, referenced to the TCXO. The output of the MAX2338 is the IF of 119.64MHz, and is conveyed through a differential IF filter to the IF processing circuits. The digital IF is accomplished in the MAX2309 RXIF. This IC contains the quadrature demodulator which mixes the IF down to baseband I and Q outputs. The MAX2309 also contains the PLL and voltage controlled oscillator (VCO) needed to implement the 2nd LO for the radio. The PLL is programmed through a three-wire serial bus familiar to most engineers. The feedback and reference dividers are actually pairs of registers. This allows the divider values for digital mode operation and analog (FM-AMPS) operation to be loaded once at system initialization, then it is simple to select the correct pair of registers to change PLL modes.
Figure 1. A dual mode receiver for TDMA with analog AMPS support. In the system under question, the baseband processor does not have the proper support to allow the AMPS FM signal to be digitally demodulated. A small FM IF section is added, using common 455kHz IF filters which are lowcost and readily available. The needed LO to mix the 1st IF of 119.64MHz down to 455kHz is borrowed from the MAX2309. This is possible because when the phone is in AMPS mode, the I/Q outputs are not needed for digital processing.
The Second LO for Digital Mode
The 2nd LO to allow TDMA digital operation needs to be 119.64MHz. This exactly mixes the IF to baseband to allow digital to analog conversion. The digital signal processor (DSP) then takes over and completes the needed error detection and correction, etc. The VCO in the MAX2309 that produced the 2nd LO needs to run a twice the required frequency, since there is an internal divide by 2. This divide by two improves performance by: 1. Moving the VCO away from the 1st IF. 2. More accurately obtaining 50% duty cycle. 3. Simplifies the 90 degree circuitry in the quadrature demodulator. To produce the 2nd LO used for TDMA operation, the internal registers need to be programmed with values that will run the VCO at 239.28MHz. Several solution sets exist for this:
R = 486, M = 5982 fREF (phase detector operation rate) = 19.44MHz/486 = 40kHz R = 243, M = 2991 fREF = 80kHz
Both solutions presented above for the 2nd LO digital mode appear to be acceptable, as the phase detector is at

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