Datasheet TH7122 (Melexis) - 10

TH7122 27 to 930MHz FSK/FM/ASK Transceiver 3 Functional Description 3.1 PLL Frequency Synthesizer
The TH7122 contains an integer-N PLL frequency synthesizer. A PLL circuit performs the frequency synthe- sis via a feedback mechanism. The output frequency fVCO is generated as an integer multiple of the phase detector comparison frequency fR .This reference frequency fR is generated by dividing the output frequency fRO of a crystal oscillator. The phase detector utilizes this signal as a reference to tune the VCO and in the locked state it must be equal to the desired output frequency, divided by the feedback divider ratio N. VCC Charge External Loop Filter Reference Reference Phase-frequency Pump Oscillator Divider Detector f f LF f RO R VCO Feedback Voltage Controlled fN Divider Oscil ator Fig. 2: Integer-N PLL Frequency Synthesizer Topology The output frequency of the synthesizer fVCO can be selected by programming the feedback divider and the reference divider. The only constraint for the frequency output of the system is that the minimum frequency resolution, or the channel spacing, must be equal to the PFD frequency fR, which is given by the reference frequency fRO and the reference divider factor R: f f RO = . (1) R R When the PLL is unlocked (e.g. during power up or during reprogramming of a new feedback divider ratio N), the phase-frequency detector PFD and the charge pump create an error signal proportional to the phase difference of the two input signals. This error signal is low-pass filtered through the external loop filter and input to the VCO to control its frequency. A very low frequency resolution increases the settling time of the PLL and reduces the ability to cancel out VCO perturbations, because the loop filter is updated every 1/fR. After the PLL has locked, the VCO frequency is given by the following equation: fRO f = N ⋅ = N ⋅ f . (2) VCO R R There are four registers available to set the VCO frequencies in receive (registers RR and NR) and in trans- mit mode (registers RT and NT). These registers can be programmed using the Serial Control Interface in Programmable User Mode (PUM). In case of Stand-alone User Mode (SUM), the registers are set fixed val- ues (refer to para. 4.1.1). The VCO frequency is equal to the carrier frequency in transmit mode. While in receive mode the VCO frequency is offset by the intermediate frequency IF. This is because of the super-heterodyne nature of the receive part. 39010 07122 Page 10 of 44 Data Sheet Rev. 008 June/07 Document Outline Features Ordering Information Application Examples Pin Description General Description Document Content 1 Theory of Operation 1.1 General 1.2 Technical Data Overview 1.3 Note on ASK Operation 1.4 Block Diagram 1.5 User Mode Features 2 Pin Definitions and Descriptions 3 Functional Description 3.1 PLL Frequency Synthesizer 3.1.1 Reference Oscillator (XOSC) 3.1.2 Reference Divider 3.1.3 Feedback Divider 3.1.4 Frequency Resolution and Operating Frequency 3.1.5 Phase-Frequency Detector 3.1.6 Lock Detector 3.1.7 Voltage Controlled Oscillator with external Loop Filter 3.1.8 Loop Filter Receiver Part 3.2.1 LNA 3.2.2 Mixer 3.2.3 IF Amplifier 3.2.4 ASK Demodulator 3.2.5 FSK Demodulator 3.3 Transmitter Part 3.3.1 Power Amplifier Output Power Adjustment 3.3.3 Modulation Schemes 3.3.4 ASK Modulation 3.3.5 FSK Modulation 3.3.6 Crystal Tuning 4 Description of User Modes 4.1 Stand-alone User Mode Operation 4.1.1 Frequency Selection 4.1.2 Operation Mode 4.1.3 Modulation Type 4.1.4 LNA Gain Mode 4.2 Programmable User Mode Operation 4.2.1 Serial Control Interface Description 5 Register Description 5.1 Register Overview 5.1.1 Default Register Settings for FS0, FS1 5.1.2 A – word 5.1.3 B – word 5.1.4 C – word 5.1.5 D – word 6 Technical Data 6.1 Absolute Maximum Ratings 6.2 Normal Operating Conditions 6.3 DC Characteristics 6.4 PLL Synthesizer Timings 6.5 AC Characteristics of the Receiver Part 6.6 AC Characteristics of the Transmitter Part 6.7 Serial Control Interface 6.8 Crystal Parameters 7 Application Circuit Examples 7.1 FSK Application Circuit Programmable User Mode (internal AFC option) 7.2 FSK Application Circuit Stand-alone User Mode 7.3 FSK Test Circuit Component List (Fig. 14 and Fig. 15) 7.4 ASK Application Circuit Programmable User Mode (normal data slicer option) 7.5 ASK Test Circuit Component List (Fig. 16) 7.6 ASK Application Circuit Programmable User Mode (peak detector option) 7.7 ASK Test Circuit Component List (Fig. 17) 8 Extended Frequency Range 8.1 Board Component List (Fig. 18) 9 TX/RX Combining Network 9.1 Board Component List (Fig. 19) 9.2 Typical LNA S-Parameters in Receive Mode 9.3 LNA Input Impedances in Transmit Mode 10 Package Description 10.1 Soldering Information 11 Reliability Information 12 ESD Precautions Your Notes 13 Disclaimer