Unmodulated carrier and modulated carrier for the center frequency point

[zcmm]

Hi, for carrier and modulation carrier signal is confusing.

1. What is the difference between their center frequency point for same frequency? Or modulated carrier will change center frequency? if it is,why?

2. What is the difference between tx power?? Or modulated carrier will change tx power? if it is,why?

PCA1004 Dk SMA connect with the attenuator( attenuation value is -14dbm),then i use constant carrier and PRBS9,the tx power is as follow: why tx power is weak? constant carrier picture:

PRBS9 picture:

i use SMS connector,the result as follow:

[Jørn]

Hello zcmm

1. Center frequency is the frequency of the unmodulated signal. This signal will typically be close to a pure sine wave. An unmodulated signal cannot convey information though as it, ideally, has no bandwidth.

Amplitude, frequency and phase can be changed to modulate the carrier so it can convey information. Any change to the signal will increase the signals bandwidth.

In BLE a modulation scheme called Gaussian Frequency Shift Keying (GFSK) is used. The frequency is shifted up or down, compared to the unmodulated center frequency, to represent 1's or 0's respectively. It is filtered to shape the frequency spectrum, ensuring it stays within its allowed bandwidth and to reduce the sideband power. The filter used is called a Gaussian filter, hence the name GFSK.

The shift in frequency should be identical in both directions around the center frequency, as such the modulation scheme does not change the center frequency, but it does change the instantaneous frequency of the signal. If you look at the spectrum of the modulated signal the center frequency should, ideally, remain the same.

2. Frequency shift keying will typically not alter the total output power, however due to the increase in bandwidth the power would be more spread out in the frequency spectrum. Filtering can of course reduce output power somewhat, however ideally it should simply smoothen the transients.

EDIT:

When you measure RF signals you need to take great care with the circuitry. The soldering you have done can greatly affect the signal. The conductor you have soldered to the connector is very long, which adds a series inductor to the circuit. In addition to this the connector you have soldered onto will act as a stub which will affect the impedance due to reflections. All of these things can cause further attenuation to the signal you measure. I would recommend you use the SMA connector that is already present on the board for your measurements.

Best regards

Jørn Frøysa

[zcmm]

hi Jørn Frøysa, i use SMA connector,there is a little improvement.how to slove it?is my frequency spectrum question?