Talking about the TH7122x Transceiver of External Microprocessor Reference Oscillator (RO)

This application note describes the TH7122x transceiver using an external microprocessor reference oscillator (RO). It proposes a typical microprocessor clock oscillator based on the Pierce topology and the typical level required for the reference signal.

This application note describes the TH7122x transceiver using an external microprocessor reference oscillator (RO). It proposes a typical microprocessor clock oscillator based on the Pierce topology and the typical level required for the reference signal.

TH7122x has programmable R and N dividers, and the chip does not provide any data synchronization or clock recovery signals. Therefore, only their range limits the setting of the frequency divider, the frequency consideration of the channel spacing, and the maximum reference frequency. EVB7122x evaluation boards usually use a 7.1505 MHz crystal oscillator as the reference oscillator, because this is a good compromise for tuning 315, 433.92, 868.3, and 915 MHz in transmit and receive using the integer values ​​of the R and N counters. This gives it a unique function, that is, these frequencies can be set via jumpers on the board.

In the programmable mode using the internal reference oscillator, the reference frequency can be from 3 MHz to 12 MHz. Since the range of the R counter is 4 to 1023, the frequency must have the following restrictions:

4leq fracturing{f_ {RO}}{f_ {R}}leq 1023

Where fR is the channel interval or reference frequency, and fRO is the frequency of the crystal oscillator. In addition, remember that the frequency of the VCO can be expressed as:

f_ {VCO}=Ncdot f_ {R} = N cdot frac{f_ {RO}}{R}

If an external reference is provided to the RO input (pin 10), the frequency range is 1 MHz to 16 MHz. This is completely within the reference range of almost all microprocessors, so the microprocessor and TH7122x can use the same crystal. In many applications, this can result in significant cost savings. It is important that the crystal tolerance must be adapted to the frequency requirements of the transceiver application. If used in ASK mode, tolerances are usually not important, in most cases a crystal of 50 ppm or 100 ppm should be sufficient. However, for FSK operation, the center frequency must usually be in the range of 10 kHz to 20 kHz of the center frequency, so the tolerance should be at least 20 ppm. Most standard microprocessor crystals are specified as 50 ppm.

A typical microprocessor clock oscillator based on the Pierce topology is shown in Figure 1:

Talking about the TH7122x Transceiver of External Microprocessor Reference Oscillator (RO)

The output of the inverter swings from ground to VCC, and the input is usually the same. The RO pin of TH7122x can be directly connected to the oscillator output, but the level is much higher than the level required by the transceiver. If they are separated by a certain distance on the PCB, the high-level clock signal may radiate harmonics or couple the clock signal Enter the nearby trace.

A simple way to provide reference signals to TH7122x is to use the circuit in Figure 2:

Talking about the TH7122x Transceiver of External Microprocessor Reference Oscillator (RO)

The C2/C3 voltage divider provides AC coupling to pin 10 of TH7122x and is selected to provide the required level. The series combination of C1, C2 and C3 determines the load on the crystal.

When TH7122x is used with an external reference, the ROMIN and ROMAX current settings in word B should be set to 0. This will turn off the reference oscillator amplitude adjustment.

The Links:   CM200YE4-12F 6MBP150RA060

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