CMX017 データシートの表示（PDF） - CML Microsystems Plc

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CMX017

UHF FM/FSK Transmitter

CML Microsystems Plc 

1.6 Application Notes

1.6.1 Generation of the Modulated RF Signal

The modulator on the CMX017 relies on the direct modulation of the RF Voltage Controlled Oscillator

(VCO), which is stabilised to the channel centre frequency by an external Frequency Synthesizer or

Phase Locked Loop (PLL). The user has control over the external PLL filter and VCO tank components

and choice of these components will have an impact upon the following PLL parameters:

Closed loop bandwidth, settling time and transient response

Modulation linearity

VCO phase noise

Three modulation requirements may be achieved as follows:

a) The minimum modulation signal frequency is above the PLL Closed Loop Bandwidth

In applications where the transmit channel frequency is fixed or occasionally changed, a narrow PLL

bandwidth can be chosen such that the minimum modulation frequency is in excess of the PLL

bandwidth. With the PLL active whilst the base-band signal is applied to the modulator there will be

negligible distortion of the modulated RF output signal. A benefit of this technique is that an output

phase noise improvement is achieved, compared to the free running VCO phase noise characteristic.

b) The minimum modulation signal frequency is within the PLL Closed Loop Bandwidth

In Frequency Hopped Spread Spectrum applications the frequency synthesizer must be agile.

Consequently the PLL bandwidth should be designed to achieve the required hop rate and, in most

systems, this bandwidth will exceed the minimum modulation frequency. With the PLL active during

modulation, the loop will 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 baseband signal, applied to the MODIN pin, then

modulates the free running VCO to generate the FM or FSK output signal.

c) 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.

© 1999 Consumer Microcircuits Limited

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D/017/3

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