Unmodulated carrier and modulated carrier for the center frequency point

RE: Unmodulated carrier and modulated carrier for the center frequency point

Jørn — Thu, 06 Jul 2017 12:03:38 GMT

Are the your cables and connectors specified for 50 Ohm? You should be getting around 0dBm, possibly a little bit lower but not much. It might be a good idea for you to ensure the spectrum analyzer is operating as it should by measuring a source with known output power. That way you can either exclude your measurement setup as a source of error, or find any fault that may lie with it.

RE: Unmodulated carrier and modulated carrier for the center frequency point

zcmm — Thu, 06 Jul 2017 09:31:30 GMT

i never do not configured the output power,the default value is 0dbm

RE: Unmodulated carrier and modulated carrier for the center frequency point

Jørn — Thu, 06 Jul 2017 09:29:24 GMT

What output power have you configured the chip for?

RE: Unmodulated carrier and modulated carrier for the center frequency point

zcmm — Thu, 06 Jul 2017 09:09:35 GMT

it is HP8594E

RE: Unmodulated carrier and modulated carrier for the center frequency point

Jørn — Thu, 06 Jul 2017 08:57:20 GMT

What is the name of the spectrum analyzer you are using?

RE: Unmodulated carrier and modulated carrier for the center frequency point

zcmm — Thu, 06 Jul 2017 08:16:53 GMT

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

RE: Unmodulated carrier and modulated carrier for the center frequency point

Jørn — Fri, 30 Jun 2017 12:01:24 GMT

I have updated my answer to answer your question.

RE: Unmodulated carrier and modulated carrier for the center frequency point

zcmm — Fri, 30 Jun 2017 10:33:35 GMT

thanks Jørn Frøysa.you slove my question.Can you tell me why the tx power is weak for spectrometer ? is wrong spectrometer setting ? or other?

RE: Unmodulated carrier and modulated carrier for the center frequency point

Jørn — Thu, 29 Jun 2017 13:32:13 GMT

Hello zcmm

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.

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