DS8911 Datasheet | National Semiconductor

Description The DS8911 is a digital Phase-Locked Loop (PLL) frequency synthesizer intended for use as a Local Oscillator (LO) in electronically tuned radios The device is used in conjunction with a serial data controller a loop filter some varactor diodes and several passive elements to provide the local oscillator function for both AM and FM tuning The conventional superheterodyne AM receiver utilizes a low IF or down conversion tuning approach whereby the IF is chosen to be below the frequencies to be received The DS8911 PLL on the other hand utilizes an up-conversion technique in the AM mode whereby the first IF frequency is chosen to be well above the RF frequency range to be tuned This approach eliminates the need for tuned circuits in the AM frontend since the image half IF and other spurious responses occur far beyond the range of frequencies to be tuned Sufficient selectivity and second IF image protection is provided by a crystal filter at the output of the first mixer A significant cost savings can be realized utilizing this upconversion approach to tuning Removal of the AM tuned circuits eliminates the cost of expensive matched varactor diodes and reduces the amount of labor required for alignment down from 6 adjustments to 2 Additional cost savings are realized because up-conversion enables both the AM and FM bands to be tuned using a single Voltage Controlled Oscillator (VCO) operating between 98 and 120 MHz (The 2 to 1 LO tuning range found in conventional AM down conversion radios is reduced to a 10% tuning range 9 94 MHz to 11 02 MHz) Up-conversion AM tuning is acplished by first dividing the VCO signal down by a modulus 10 to obtain the LO signal This LO in turn is mixed on chip with the RF signal to obtain a first IF at the MIXER output pins This first IF after crystal filtering is mixed (externally) with a reference frequency provided by the PLL to obtain a 450 k Hz second IF frequency The DS8911 derives the 450 k Hz second IF by mixing an 11 55...