UHF FM/FSK Transmitter

7

CMX017 Advance Information

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compensate for the signal frequency components within the loop bandwidth, introducing a frequency

dependant distortion of the modulated RF output signal.

To avoid the modulation distortion the carrier frequency is tuned by the external synthesizer chip, with no

modulating signal applied. The synthesizer control is then disabled, by setting the charge-pump output of the

synthesizer to tri-state. The DC coupled base-band signal, applied to the MODIN pin, then modulates the free

running VCO to generate the FM or FSK output signal.

5.1.3

A flat modulator response is required down to DC, whilst benefiting from the phase noise

improvements due to the PLL (Two-Point Modulation).

Unlike in the above two cases, where a fixed reference frequency is used in the PLL, the Two-Point

Modulation technique requires modulation inputs to both the PLL reference oscillator and to the RF VCO

circuits.

The modulation signal should be applied to both modulation inputs with suitable AC/DC levels and with the

correct phase to achieve cancellation of the loop’s feedback. This prevents the radio’s PLL circuitry from

counteracting the modulation process and so provides a clean flat modulation response down to DC.

5.2

Powersave and Zero-Power Modes

It is possible to power down each section of the transmitter independently. This feature may be useful when

the CMX017 is configured, with the CMX018, as a radio transceiver. It allows the transmitter to be powered

on and off, thereby saving power, during the sleep and standby periods.

In SLEEP mode (ENLMOD=’0’ and ENLPA=’0’) all sections of the device are powered down and the current

consumption is reduced to less than 10

µA. This is the zero-power mode.

In TRANSMIT-STANDBY mode (ENLMOD=’1’ and ENLPA=’0’) only the PA Driver and Power Amplifier

stages are powered down, while the Modulator and Buffered Oscillator remain active. This powersave mode

allows the external frequency synthesizer to stabilize the channel center frequency prior to the RF

transmission.

In TRANSMIT mode (ENLMOD=’1’ and ENLPA=’1’) both the Modulator and the Power Amplifier sections of

the device are fully operational.

Note: The device can be used as a stand alone “UHF Power Amplifier” by forcing (ENLMOD=’0’ and

connected to 3.0V.)

5.3

Lower Output Power Applications

For very short range applications the antenna drive can be taken from the modulator output, MODOUT, at

allows the supply current to be minimized and the Modulator output to be enabled or disabled using the

ENLPA input at pin 13.

5.4

Example Schematic and Layout

The following schematic (Figure 4) and printed circuit layout (Figure 5 and Figure 6) present a typical

application interface for the CMX017. To aid legibility, the schematic and layout are available electronically

from the MX-COM website http://www.mxcom.com.

Alternative components and component values are shown on the schematic. These should be selected

according to the intended application. The schematic uses the following ICs:

U2

IC Works

WB1315X

U4

Analog Devices

AD8532-SO8