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MICRF501BLQ Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MICRF501BLQ Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 18 page  March 2003 9 MICRF501 MICRF501 Micrel Applications Information VCO and PLL Section The frequency synthesizer consists of a VCO, crystal oscilla- tor, dual-modulus prescaler, programmable frequency divid- ers, phase-detector, charge pump, lock detector and an external loop filter. The dual-modulus prescaler divides the VCO-frequency by 32/33. This mode is controlled by the A- divider. There are two sets of M, N and A-frequency dividers. Using both sets in transmit mode, FSK can be implemented by switching between those two sets. The phase-detector is a frequency/phase detector with back slash pulses to mini- mize phase noise. The VCO, crystal oscillator, charge pump, lock detector and the loop filter will be described in detail below. Voltage Controlled Oscillator (VCO) VDD Pin 5 Pin 6 C13 D1 MA4ST350 C35 2.2p R7 3.6k 15p L1 47nH R8 47k loopfilter_output Pin 7 OSCOUT Figure 3. VCO The circuit schematic of the VCO with external components is shown in Figure 3. The VCO is basically a Colpitts oscilla- tor. The oscillator has an external resonator and varactor. The resonator consists of inductor L1 and the series connec- tion of capacitor C13, the internal capacitance, the capaci- tance of the varactor and C35 in parallel with D1. The capacitance of the varactor (D1) decreases as the input voltage increases. The VCO frequency will therefore in- crease as the input voltage increases. The VCO has a positive gain (MHz/Volt). C35 is added, if necessary, to bring VCO tuning voltage to its middle range or VCC/2, which is measured at Pin 9 - CMPOUT. If the value of capacitor C13 and C14 become too small the amplitude of the VCO signal decreases, which leads to lower output power. The layout of the VCO is very critical. The external compo- nents should be placed as close to the input pin (Pin 6) as possible. The anode of the varactor D1 must be placed next to pins 7 and 8. Ground vias should be next to component pads. Crystal Oscillator The crystal oscillator is the reference for the RF output frequency as well as for the LO frequency in the receiver. The crystal oscillator is a very critical block since very good phase and frequency stability is required. The schematic of the crystal oscillator with external components for 10MHz is shown in Figure 4. These components are optimized for a crystal with 15pF load capacitance. DIFVDD C21 47p C20 2-6p C36 1n 10MHz C22 5.6p Pin 12 Pin 13 XOSCOUT DIGGND Figure 4. Crystal Oscillator The crystal oscillator is tuned by varying the trimming capaci- tor C20. The drift of the RF frequency is the same as the drift of crystal frequency when measured in ppm. The total differ- ence in ppm, ∆f(ppm), between the tuned RF frequency and the drifted frequency is given by: ∆f(ppm) = S T × ∆T + n × ∆t where: • S T is the total temperature coefficient of the oscillator frequency (due to crystal and components) in ppm °C. • ∆T is the change in temperature from room temperature, at which the crystal was tuned. • n is the ageing in ppm/year. • ∆t is the time (in years) elapsed since the transceiver was last tuned. The demodulator will not be able to decode data when ∆f(Hz) = ∆f(ppm) × f RF is larger than the FSK frequency deviation. For small frequency deviations, the crystal should be pre-aged, and should have a small temperature coeffi- cient. The circuit has been tested with a 10MHz crystal, but other crystal frequencies can be used as well. The circuit has been tested with a 10MHz crystal, but other crystal frequencies can be used as well. Prestart of XCO The start-up time of a crystal oscillator is typically some milliseconds. Therefore, to save current consumption, the MICRF501 circuit has been designed so that the XCO is turned on before any other circuit block. During start-up the XCO amplitude will eventually reach a sufficient level to trigger the M-counter. After counting two M-counter output pulses the rest of the circuit will be turned on. The current consumption during the prestart period is approximately 300 µA. Lock Detector The MICRF501 circuit has a lock detector feature that indi- cates whether the PLL is in lock or not. A logic high on Pin 15 (LOCKDET) means that the PLL is in lock. The phase detector output is converted into a voltage that is filtered by the external capacitor C23, connected to Pin 14, LDC. The resulting DC voltage is compared to a reference window set by bits Ref0 – Ref5. The reference window can be stepped up/down linearly between 0V, Ref0 – Ref5 =1, and Ref0 – Ref5 = 0, which gives the highest value (DC voltage) of the reference window. The size of the window can either be |
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