Overview
This document discusses the RF frequency calculation formulas for the CMT2300A, CMT2119B, and CMT2219B, aiding in further design and application based on these products.
The product models covered in this document are detailed in the table below:
| Product Model | Frequency Range | Modulation Type | Main Function | Configuration Mode | Packaging |
|---|---|---|---|---|---|
| CMT2300A | 126.33 - 1020 MHz | (G)FSK/OOK | Transceiver | Register | QFN16 |
| CMT2119B | 126.33 - 1020 MHz | (G)FSK/OOK | Transmitter | Register | QFN16 |
| CMT2219B | 126.33 - 1020 MHz | (G)FSK/OOK | Receiver | Register | QFN16 |
Before reading this document, it is recommended to consult the AN142-CMT2300A Quick Start Guide, AN184-CMT2119B Quick Start Guide, and AN161-CMT2219B Quick Start Guide for basic product information.
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RF Frequency Calculation
This section details the RF frequency calculation and manual configuration methods for the three products. Note that the RX part description is not applicable to the CMT2119B, and the TX part statement is not applicable to the CMT2219B.
For general RF frequency configuration, it is recommended to use the RFPDK (RF Parameter Design Kit) to generate parameters and write them to the registers in the frequency area. If separate TX and RX frequency configuration is needed without using the fast frequency hopping mechanism, users must understand the register configuration details and related value calculations. The registers in the frequency area are listed below:
| Register Address | Register Name | Parameter | Bit Range | Notes |
|---|---|---|---|---|
| 0x1C | CUS_RF5 | FREQ_RX_N | [7:0] | Located in Bit 7 |
| 0x1D | CUS_RF6 | FREQ_RX_K | [7:0] | Located in Bit 5 |
| 0x1E | CUS_RF7 | FREQ_RX_K | [15:8] | Located in Bit 5 |
| 0x1F | CUS_RF8 | FREQ_TX_N | [7:0] | Located in Bit 7 |
| 0x1F | CUS_RF8 | FREQ_TX_K | [7:0] | Located in Bit 6 |
| 0x1F | CUS_RF8 | FREQ_TX_K | [15:8] | Located in Bit 5 |
| 0x1F | CUS_RF8 | FREQ_PALDO_SEL | N/A | Located in Bit 4 |
| 0x1F | CUS_RF8 | FSK_SWT | N/A | Located in Bit 3 |
| 0x1F | CUS_RF8 | FREQ_DIVX_CODE | [2:0] | Located in Bit 2 |
| 0x1F | CUS_RF8 | FREQ_VCO_BANK | [2:0] | Located in Bit 1 (Default: 000) |
The value of FSK_SWT in the table is generated by RFPDK and does not depend on frequency. This value should not be changed when configuring other register bits.
Configuring RF Parameters of RX
To configure the frequency of RX, the following items need to be set:
- FREQ_VCO_BANK <2:0>
- FREQ_DIVX_CODE <2:0>
- FREQ_RX_N <7:0>
- FREQ_RX_K <19:0>
- AFC_OVF_TH <7:0>
Here, N represents the integer part of the frequency word, and K represents the fractional part. DIVX_CODE selects the PLL division factor, and VCO_BANK selects the VCO operating frequency range. The calculation proceeds as follows:
First, consult the table to determine the values for FREQ_VCO_BANK<2:0> and FREQ_DIVX_CODE<2:0> (both to be written to registers), and the DIVIDER (frequency dividing factor used for N and K calculation), based on the target frequency band.
Correspondence between PLL Analysis Parameters and Target Frequencies
| Target Frequency Band | FREQ_VCO_BANK<2:0> = 110 | FREQ_VCO_BANK<2:0> = 001 | FREQ_DIVX_CODE <2:0> | DIVIDER |
|---|---|---|---|---|
| 758 - 840 MHz | 840 - 1020 MHz | 000 | 2 | |
| 379 - 420 MHz | 420 - 510 MHz | 001 | 4 | |
| 189.5 - 210 MHz | 210 - 255 MHz | 010 | 8 | |
| 126.33 - 140 MHz | 140 - 170 MHz | 011 | 12 | |
| 252.67 - 280 MHz | 280 - 340 MHz | 101 | 6 |
Next, calculate the Local Oscillator (LO) frequency. In the formula below, FREQ_RF is the target RF frequency in MHz, and FREQ_LO is the calculated local oscillator frequency in Hz.
FREQ_LO = FREQ_RF x 10^6 + 26 MHz/92
Then, calculate the frequency word N.K:
N.K = FREQ_LO x DIVIDER / 26 MHz
Obtain the integer part of N.K and convert it to binary to get the value for FREQ_RX_N <7:0>. The fractional part is multiplied by 2^20 and rounded off to obtain the value for the register FREQ_RX_K <19:0>.
The final step is to obtain the value for AFC_OVF_TH <7:0>. This register is not in the frequency area. It is a key parameter for the receiver's Automatic Frequency Control (AFC) algorithm and is calculated based on receiver parameters such as RX RF frequency, data rate, deviation, and crystal PPM. The result is displayed on the RFPDK screen.
RFPDK Screenshot Description
The RFPDK (RF Parameter Design Kit) interface displays various chip parameters and operation settings. Key sections include "Chip Parameters" with fields for Frequency, Data Rate, Xtal Frequency, Tx Power, and TRx Matching Network Type. "Operation Settings" cover Bandwidth, AFC, (De)Modulation, and PA Ramp. The "AFC_OVF_TH Screen of RFPDK" figure shows specific values being set, such as AFC_OVF_TH = 10.
Users must input the desired RF frequency for RX manual configuration in RFPDK and then retrieve the AFC_OVF_TH <7:0> value. This value should be written to register 0x27 CUS_FSK4 to ensure normal receiver operation. Failure to do so may result in receiver errors during AFC, potentially causing reception failure.
For example, if the target RF frequency for RX to be configured is 433.92 MHz:
- FREQ_VCO_BANK <2:0> is 001.
- FREQ_DIVX_CODE <2:0> is 001, and DIVIDER is 4.
- FREQ_LO is 434202608.7.
- N.K is 66.80040135.
- FREQ_RX_N <7:0> is 66, with a corresponding binary value of 01000010.
- FREQ_RX_K <19:0> is 839282, with a corresponding binary value of 11001100111001110010.
- AFC_OVF_TH <7:0> is available in RFPDK.
Configuring RF Parameters of TX
The following parameters need to be configured for TX:
- FREQ_VCO_BANK <2:0>
- FREQ_DIVX_CODE <2:0>
- FREQ_TX_N <7:0>
- FREQ_TX_K <19:0>
- FREQ_PALDO_SEL
FREQ_VCO_BANK<2:0> and FREQ_DIVX_CODE<2:0> are obtained similarly to RX. TX and RX share these values. If the target frequency bands differ, recalculation and writing to these registers are necessary for each TX or RX configuration.
Calculate the LO frequency, which differs from the RX LO frequency. In the formula below, FREQ_RF is the target RF frequency in MHz, and FREQ_LO is the calculated local oscillator frequency in Hz.
FREQ_LO = FREQ_RF x 10^6
Calculate the frequency word N.K, using the same formula as for RX:
N.K = FREQ_LO x DIVIDER / 26 MHz
Finally, consult the table below to determine the value for FREQ_PALDO_SEL:
| TX Frequency | FREQ_PALDO_SEL |
|---|---|
| < 500 MHz | 0 |
| >= 500 MHz | 1 |
For example, if the target RF frequency for TX to be configured is 433.92 MHz:
- FREQ_VCO_BANK <2:0> is 001.
- FREQ_DIVX_CODE <2:0> is 001, and DIVIDER is 4.
- FREQ_LO is 433920000.
- N.K is 66.75692308.
- FREQ_TX_N <7:0> is 66, with a corresponding binary value of 01000010.
- FREQ_TX_K <19:0> is 793691, with a corresponding binary value of 11000001110001011011.
- FREQ_PALDO_SEL is 0.
Revise History
Revision history records:
| Version No. | Chapter | Description | Date |
|---|---|---|---|
| 0.8 | All | Initial version | 2018-10-10 |
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