f

=

CARRIER

f

2

XOSC

16

(

f

2

XOSC

16

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Frequency Correction

Use these general parameters to perform tests in RX mode when using LabVIEW:

•

Set the DUT to RX mode using SmartRF Studio 7.

•

Supply/temperature are set by LabVIEW.

•

The signal analyzer is configured by LabVIEW to transmit data continuously or in packets that adhere

to standards.

•

SmartRF Studio 7/LabVIEW captures the data from the DUT.

•

This collected information then can be interpreted either in LabVIEW or exported to other PC-based

software.

3

Frequency Correction

Electronic circuits often use the mechanical resonance of a vibrating piezoelectric crystal to create an

electrical signal with a very precise frequency. This frequency is commonly used to provide a stable clock

signal for digital integrated circuits and to stabilize frequencies for radio transmitters and receivers.

Environmental changes in temperature, humidity, pressure, and external vibration can change the

resonant frequency of a crystal. The age of a crystal also adds inaccuracies to the crystal over time.

Because there is always some inaccuracy in the crystals used with radios, one way to correct for this error

is required in order to obtain an accurate measurement of sensitivity and other parameters.

The carrier frequency in the chip is mathematically related to the crystal frequency. For example, for the

CC2500 the carrier frequency is calculated as shown by Equation 1:

(1)

Where FREQ[23:0] is the base frequency for the frequency synthesizer in increments of

However, the actual crystal frequency is not the same as the stated crystal frequency as a result of the

inaccuracies noted earlier. Consequently, we must calculate the actual crystal frequency.

After putting the device into unmodulated, continuous TX mode with the settings found using SmartRF

Studio, use a spectrum analyzer to measure the exact carrier frequency coming out of the chip.

This measured frequency is then put into Equation 1 from the product data sheet, and one solves for f

This result is the actual crystal frequency for the specific DUT that can then be used to determine the

exact carrier frequency across the band.

In the CC253x/CC254x devices, the FREQTUNE register is used to tune the crystal oscillator. The default

setting '1111' leaves the XOSC not tuned. Changing the setting from default switches in extra capacitance

to the oscillator, effectively lowering the XOSC frequency. As a result, the final crystal frequency can be

controlled by adjusting the value of the FREQTUNE register in these devices.

9

SWRA370

– August 2011

Basic RF Testing of CCxxxx Devices

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