### ETC APPLICATIONHINT42

```Application Hint 37l
Micrel
Application Hint 42
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
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
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.