In my previous post about the roadtest review of Agilent/Keysight N9322C Spectrum Analyzer and Texas Instruments Value Line Development Kit CC11XLDK-868-915 I showed how I got started with this work, how I setup an RF communication link, and how I started to measure various characteristics of the transmitter. In this post I will continue with the characterization of the transmitter and the communication link by looking at phase noise, spurs in TX frequency spectrum, and Doppler frequency shift.
Measuring phase noise
Phase noise is a figure of merit of oscillators that relates to frequency stability. In transmitters, like the CC115L and CC110L that are part of the TI development kit, the phase noise can be used to quantify the stability of the carrier frequency. To further analyze the phase noise of CC110L carrier frequency, I have setup the experiment shown in the following figure.
In this experiment I have modified the CC110L module by disconnecting the built-in antenna and mounting an SMA connector for an external antenna or for connecting directly to the N9322C spectrum analyzer, like I am showing in the picture. Next step, I setup the transmitter mode in SmartRF Studio control panel to send only the carrier frequency unmodulated. After setting up this experiment and preparing to measure phase noise, I discovered that N9322C does not do phase noise measurement. I searched the user manual and then I moved my search on the Internet until I found this comparison chart of Agilent/Keysight Spectrum Analyzer Series.
But I didn't get discouraged and I continued to look into a method of measuring phase noise. So next I tried to use the direct spectrum technique, since N9322C has all the features I need for this measurement method. To start this method I setup the N9322C to display the frequency spectrum of the carrier signal, as I am showing in the following figure.
I have annotated on this picture what an ideal (noiseless) signal would look like. The spectrum of an ideal signal has only one spectral component at the carrier frequency value and nothing else. So intuitively, phase noise should characterize all the “deviations” of the real carrier frequency spectrum from the ideal signal spectrum. Since the deviations are symmetrical around the carrier frequency, phase noise uses only one side, also called sideband phase noise. The following figure shows the components of the spectrum used in measuring the phase noise.
The sideband phase noise at a frequency offset from carrier can be calculated as area of 1Hz bandwidth at that offset frequency divided by the total area under the curve. This is a little hard to compute from the measured spectrum without exporting the data and post processing it in Excel or Matlab (or equivalent), so instead I found out that I can use a simplified techique. The simplified technique calculates phase noise as the ratio between the noise power in a 1Hz bandwidth at the measured frequency offset from the carrier frequency (marked as fc+fm in the figure above) and the power of the carrier signal (marked as P_fc in the figure above). So the phase noise at a frequency offset of fm can be calculated as:
P_fc(dBm) can be easily read from the frequency spectrum measured with the N9322C spectrum analyzer using the peak-detect function or just a marker. In this experiment P_fc is equal to –14.33dBm. The challenge comes now from determining the P_fm(dBm/Hz), since I needed to read the level and then integrate over 1Hz bandwidth. So next I setup the N9322C spectrum analyzer in average mode to make easier the reading of level values. The picture below shows the average version of the same frequency spectrum in the above picture.
Then I turned on all six markers available in N9322C spectrum analyzer and I placed them on one side of the frequency spectrum. The spacing can be adjusted to the desired frequency offset where we are interested to measure phase noise. Here is a picture showing the six markers and the markers table at the bottom of the screen. The markers table is a nice feature available in N9322C spectrum analyzer since it makes easier the reading of all marker values.
So the N9322C spectrum analyzer can be used to determine the carrier power. P_fc(dBm) and P_fm(dBm) at any offset frequency from the carrier. For simplicity I assumed that the marker read values are constant over 1Hz bandwidth, so this simplified the integration step; it may introduce some error but it allows me to calculate the phase noise easily only from values read directly on the Agilent/Keysight N9322C spectrum analyzer.
TX Spurs Analysis
Ideal oscillators would have only one frequency spectral component; however, real oscillators have additional frequency components named spurs. A lot of effort is made during design to implement techniques to suppress spurs. The measured frequency spectrum of the CL110L on the Texas Instrument evaluation board does not show any visible spurs. However, I wanted to further analyze the spectrum and search for spurs. I started by first identifying the mechanisms of generating spurs from the schematic diagram of the evaluation board, and I selected to look into modulation type coupling of the local oscillator into the carrier frequency, and local oscillator feed-through. The following figure shows the measured frequency spectrum of the CL110L with my annotations of the locations where I could find the local oscillator feed through component (at 26MHz) and the modulation coupling spurious components (carrier frequency + 26MHz, and carrier frequency – 26MHz).
The picture below shows the local oscillator feed through into the output signal of the CL110L.
So indeed I found a frequency component at the local oscillator frequency value. The amplitude is very small, -100.6 dBm, (so small that it does not create any significant issue), but finding it matches my expectations that there is some feed-through from the 26MHz local oscillator. Also, I liked that the N9322C spectrum analyzer was enough sensitive to detect this component.
Next I looked for modulation type spurs from the local oscillator. I expected these spurs at two locations left and right from the carrier signal and separated by 26MHz offset. The picture below shows the spur at 893.97MHz, which is equal to the carrier frequency of 867.97MHz + 26MHz.
The spur amplitude is very small, -86.93dBm, but I was happy to find it where I expected based on my analysis of the evaluation board circuit. The spur on the other side of the carrier, at 867.97MHz - 26MHz = 841.97MHz, is shown in the following figure.
The amplitude of this spur is –86.79dBm,which is close to the other spur (we expect these two spurs to have the same amplitude).
Doppler shift is a phenomenon that results in frequency shift at the receiver when the transmitter moves. Some of you may have noticed or hear in a movie the whistle of a train approaching, passing by, and then moving away. The sound of the whistle has a higher frequency pitch when traveling towards us and a lower frequency pitch when moving away from us. This is due to the phenomenon called Doppler frequency shift or Doppler effect.
So I wanted to use the Agilent/Keysight N9322C spectrum analyzer to try to measure the Doppler frequency shift of the RF signal transmitted by the TI CC115L module mounted on the SmartRF evaluation board. This board is small and easy to manipulate, so it makes a good candidate for picking up in my hand and moving it while observing the transmitted frequency spectrum with the N9322C spectrum analyzer.
The experiment was successful in detecting the Doppler frequency shift, and I have captured it in the video below. As I move the CC115L transmitter towards the receiving antenna, the carrier spectral component moves towards the right side on the N9322C screen, which shows that the received frequency has increased. This is equivalent to the train whistle having higher pitch when approaching. Then, when I move the CC115L transmitter away from the receiving antenna, the carrier spectral component moves towards the left side of the N9322C screen, which shows that the received frequency has decreased. This would be similar to the train whistle lowering the pitch when moving away.
This concludes the second set of measurements that I have done with the Agilent N9322C spectrum analyzer on the Texas Instruments CC11XLDK-868-915 development kit. I will come back with additional blog posts as I run more experiments.