Application Hint 37l Micrel Application Hint 42 QwikRadio™- Calculating the CTH Capacitor by Carlos Ribeiro The following application hint refers to the MICRF002, MICRF007, MICRF011 and MICRF022 QwikRadio™ Receivers. The calculations apply to data-patterns with 50/50 and 33/66 duty cycles. The CTH capacitor value is dependent on several factors. It is important to calculate the right capacitor value for each specific application. A wrong CTH capacitor value will deteriorate the maximum range in the RF link. The example below shows how to calculate it and a table is provided for other frequencies. Example No. 1: 1.2kbps, 50% Manchester When a 50% duty cycle data pattern with a preamble is used, (see Figure 1) use the following formula to calculate CTH, BIT PERIOD = 1/DATA RATE PREAMBLE DEAD TIME DATA 50% MANCHESTER Figure 1. Data Pattern, 50% Manchester Table 1. Freq. 315 MHz, 50% Duty Cycle, Manchester encoding, no squelch, REFOSC=4.897MHz, pulse with preamble, fixed mode. Baud Rate bps 1200 2400 4800 9600 5 DATARATE (Equation 1) C TH = 578200 REFOSC Fixed Mode, SEL0 = 1, SEL1 = 0 Carrier Frequency: 433.92 MHz. Reference Oscillator: 6.7458 MHz, up conversion Plugging in the values at Equation 1, we have CTH CAGC SEL0 SEL1 39nF 18nF 8.2nF 4.7nF 2.2µF 1µF 0.47µF 0.22µF Table 1. 1 0 1 1 0 1 1 1 Table 2. Freq. 433.92 MHz, 50% Duty Cycle, Manchester encoding, no squelch, REFOSC=6.7458MHz, pulse with preamble, fixed mode. 5 1200 C TH = 578200 6.7258 Baud Rate bps 1200 2400 4800 9600 C TH = 48.6nF The closest commercial value is 47nF. Notice that the CTH value is dependent mainly on the baud rate (data pattern) and the reference oscillator frequency. See Table 1 and 2 for other baud rates and frequencies. CTH CAGC SEL0 SEL1 47nF 27nF 12nF 5.6nF 2.2µF 1µF 0.47µF 0.22µF Table 2. 1 0 1 1 0 1 1 1 Note 1: The AGC capacitor values are suggested values and will depend on how much dead time there is between preamble and data and how fast the receiver needs to be in stead state. Note 2: For 33/66 coding scheme, use the same equation 1 to calculate CTH. Note 3: Application note 22 has further information about the CTH pin. MICREL INC. TEL 1849 FORTUNE DRIVE SAN JOSE, CA 95131 + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 2001 Micrel Incorporated Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com November 2001 1 Application Hint 42