DS8911

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 accomplished 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 kHz second IF frequency. The DS8911 derives the 450 kHz second IF by mixing an 11.55 MHz first IF with a 12.00 MHz reference frequency.

FM and WB (weather band) tuning is done using the conventional down conversion approach. Here the VCO signal is buffered to produce the LO signal and then mixed on chip with the RF signal to obtain an IF frequency at the MIXER output pins. This IF frequency is typically chosen to be 10.7 MHz although placement at 11.50 MHz can further enhance AM mode performance and minimize IF circuitry. The PLL provides phase comparator reference frequencies of 10, 12.5, 25, and 100 kHz. The tuning resolutions resulting from these reference frequencies are determined by dividing the reference by the premix modulus. Table II shows the tuning resolutions possible.

The DS8911 contains the following logic elements: a voltage controlled oscillator, a reference oscillator, a 14-bit programmable dual-modulus counter, a reference frequency divider chain, a premix divider, a mixer, a phase comparator, a charge pump, an operational amplifier, and control circuitry for latched serial data entry.

The DS8913 includes all the above logic elements except that it requires a 10 MHz reference frequency instead of 12 MHz.