ATA5724-DK Microchip

APPLICATION NOTE Application Hints for ATA5723/ATA5724/ATA5728 ATA5723/ATA5724/ATA5728 Introduction The UHF receiver ATA5723, ATA5724, and ATA5728 are designed specially for automo- tive application like Remote Keyless Entry as well as Tire Pressure Monitoring System. These receivers are upgrades of ATA5743 and ATA5760. They have compatible pinning that allows the development of one layout for applications in three different ISM (industrial Scientific Medical) frequency bands. The frequency ranges are 312.5MHz to 317.5MHz, 431.5MHz to 436.5MHz and 868MHz to 870MHz. Another benefit of the receiver is the reuse of the software from Atmel s older receiver, the ATA5743. Additional features include the RSSI output, the pierce crystal oscillator achieving a better oscillation margin, an improved sensitivity as well as the image rejection. The purpose of this application note is to give a designer some hints how to start develop- ing a receiver module with ATA5723/ATA5724/ATA5728. Calculating the Required Crystal Frequency Figure 1. System Block Diagram for the Receivers Containing the Internal Frequency Generation and Processing (Synthesizer) f XTAL Loop XTO Filter f XTO f f VCO LC-VCO :2 or :3 f IF f Ref f f :2 :128 f f LO LO or :4 or :64 f RF 9118C-AUTO-05/15A reference frequency (f ) in the receiver is based on the crystal oscillator s frequency (f ), the loaded crystal resonance Ref XTO frequency (f ) respectively. The defined reference frequency is: XTAL f XTO For ATA5723: f = ----------- Equation 1 Ref 3 f XTO ----------- For ATA5724: f = Equation 2 Ref 2 f XTO ----------- For ATA5728: f = Equation 3 Ref 2 The fixed intermediate frequencies (IF) for the receivers are: IF of ATA5723 is 987kHz IF of ATA5724 is 987kHz IF of ATA5728 is 947.8kHz The local oscillator frequency (f ) can be calculated as: LO f = f f Equation 4 LO RF IF The correlation between local oscillator frequency (f ) and the Voltage Oscillator frequency (f ) is: LO VCO f VCO ------------ For ATA5723: f = Equation 5 LO 4 f VCO For ATA5724: f = ------------ Equation 6 LO 4 f VCO For ATA5728: f = ------------ Equation 7 LO 2 The reference frequency (f ) will be compared with the local oscillator frequency divided by a factor of 64 or 128. Ref f LO -------- For ATA5723: f = Equation 8 Ref 64 f LO -------- For ATA5724: f = Equation 9 Ref 64 f LO For ATA5728: f = --------- Equation 10 Ref 128 2 ATA5723/ATA5724/ATA5728 APPLICATION NOTE 9118CAUTO05/15