BVS BumbleBee Tablet Simplifies Radio Frequency
Power Measurements
By Sandeep Natekar, Software Engineer, Berkeley Varitronics Systems, Inc

Spectrum Analysis has achieved a whole new dimension with low-cost portable hand-held spectrum analyzers making their foray in the RF test and measurement industry. These devices offer

some of the most important spectrum analysis features and functions for use by engineers and scientists in laboratories as well as field technicians and other mobile RF professionals. Most of these handheld spectrum analyzers are packaged with PDAs or PocketPCs for running software applications for displaying the spectrum or processing data from the handheld receiver. While such a packaged product is found to be attractive by a lot of RF professionals, there is always a need for leveraging the limitation of the handheld computer with respect to its display, processing power, memory and battery life. Even the newest of PocketPCs and IPAQs offer only up to 64 MB of RAM and the fastest these operate is at 400 MHz. These computing credentials severely constrain the capabilities of the spectrum analysis software which runs on them.

To address these limitations of its handheld spectrum analyzer products, Berkeley Varitronics Systems has now come out with a new product in its BumbleBee family of handheld spectrum analyzers. BumbleBee Tablet offers greater processing power, better display resolution, a lot more memory and a much longer battery life compared to it’s PDA based siblings. The BVS BumbleBee Tablet comes with the BVS BumbleBee handheld wireless receiver packaged with a Samsung Q1 Ultra Mobile PC (UMPC). The Samsung Q1 UMPC comes with a 1 GHz Pentium 4 processor, 1GB RAM and a battery life of about 4 hours. This provides additional horsepower to the BumbleBee Tablet Spectrum analysis software, which runs on the UMPC to provide faster spectrum sweeps along with measurement capabilities such as Channel Power Measurement, Co-channel interference and Peak to average signal power measurement. The new spectrum analysis software also provides better spectrum averaging options to eliminate noise in a spectrum under measurement. Moreover, the BumbleBee Tablet lets the user record real-time spectrum information in the form of a Spectrum Data Log file, which can be later replayed for analysis. This article discusses some of the useful features of the BVS BumbleBee Tablet and highlights it’s advantages to RF professionals.

Power Trigger Mode
The Swept Spectrum Mode can be slow to detect and measure bursts of signal energy. This is because when the receiver scans a certain portion of the spectrum, it will miss a burst of signal energy such as a frequency hop of a Frequency Hopping device on a nearby frequency. In the Swept Spectrum Mode, the only time the receiver will detect and measure signal energy is when the burst of signal energy occurs at the frequencies the BumbleBee Tablet receiver is currently scanning. This problem can be particularly compounded if the Sweep Span is large and the resolution bandwidth is small. As a solution to this problem, the BumbleBee Tablet Receiver has a Power Trigger Mode. In this mode, the user can set a power threshold for the receiver to trigger on, every time the channel power exceeds this threshold. This way, the receiver captures bursts of signal energy occurring over a 20 MHz channel bandwidth whenever the Channel Power within this 20 MHz channel exceeds the Power Threshold. In addition to this, the user can set a delay so that the receiver will measure the channel power only after the amount of specified delay following a trigger. This setting ensures that the RF energy of the desired portion of the data packet is measured at all times.

Spectrogram Display
A Spectrogram can be a good indicator of power variations over a period of time for a given span. A spectrogram can help in differentiating between Frequency Hopping and Direct Sequence Spread Spectrum Transmission, CW signals and Narrow Band Data transmissions. In Figure 2, the Spectrogram displays signal between the maximum (-20 dBm) and the minimum (-91 dBm) for the last 100 spectral sweeps using the defined color gradient. The region having Black color represents power values below the minimum value, while White is the color used to represent power values above the maximum. The Orange-Yellow color shows Spread Spectrum Signal transmissions.

Accurate Channel Power Measurements
Measuring channel power or power in a span of interest can be done by the BumbleBee Tablet by simply dragging the stylus across the screen, high-lighting the span whose power measurement is desired.

Figure 3 shows a Wideband Frequency Modulated signal with a center frequency of 2451 MHz and a span of approximately 11 MHz. The waveform (Figure 3) is obtained with the help of a Signal Generator. The output of the signal generator has been connected as an input to the BumbleBee Tablet Receiver. Upon turning the Frequency Modulation off, it can be seen that the spectrum consists of a single peak (see Figure 4) at the center frequency of the previous Frequency Modulated signal. The Red Marker on the Peak indicates a Peak power of -31 dBm. This is in agreement to the principles of modulation that the power in the modulated signal is the same as the power in the unmodulated signal.

Markers/Delta Markers
The BumbleBee Tablet provides marker functions, which can greatly help in power measuring capabilities. Figure 5 shows a Frequency Hopping Spread Spectrum Signal (FHSS). It must be noted that the Yellow trace is the Peak trace, which shows the Frequency Hopping signal. A marker has been positioned on one of the peaks of the Frequency Hopping signal. This happens to be at 2409.90 MHz and shows a power level of -66 dBm. The Delta marker has been positioned on the peak trace at a frequency of 2409.15 MHz and the delta power happens to be -87 dBm. The Marker Legend on the lower right of the grid shows that the Span between the marker and the delta is 0.75 MHz while the power difference between the two is -21.00 dBm. It also shows that the total channel power between the marker and the delta is -61.71 dBm. Markers and Delta markers provide an elegant way of measuring adjacent channel power measurement in addition to measuring total channel power measurement functions.

Averaging Features
The BumbleBee Tablet provides useful waveform averaging features to eliminate rapid fluctuations due to noise in the observed spectrum in order to obtain a waveform, which reflects the desired signal power. The BumbleBee Tablet provides multiple trace averaging and adjacent point averaging for this purpose. Multiple trace averaging averages multiple traces to produce a waveform, which reflects the true characteristic of a signal under test, which is free from noise. Noise variations over the past N traces get averaged out and the signal stands out in the spectrum.

As can be seen in Figures 6 and 7, a noisy spectrum waveform can be smoothed by averaging N (= 11) traces to produce the smooth waveform shown in Figure 7. In the absence of averaging and in the presence of noise, the delta is seen to be 45 dBm below the marker. However, upon averaging 11 consecutive traces, it can be seen that the delta is steady and 21 dBm below the peak. Similarly, adjacent point averaging can remove rapid fluctuations by averaging consecutive points of a waveform to show the true signal envelope. This can be seen in Figure 8 and Figure 9.

Conclusion
The BVS BumbleBee Tablet uses high processing power and memory to perform advanced spectrum analysis functions and provides greater accuracy and display resolution of the measured RF signals, while maintaining highly portable analysis suitable for the field. The BVS BumbleBee Tablet software typically displays a span of 50 MHz at 50 KHz resolution bandwidth in 230 milli seconds. This is a major performance enhancement when compared to other products in the same league. Moreover, since 1 GB of memory is available, the BumbleBee Tablet software can perform unhindered (multitask) in the presence of other software applications running on the Tablet. A lot more data can be logged in real time and this can be post processed on the unit itself, without the need for exporting the logged data to a high performance computer like a desktop or a laptop PC. The extended battery life (~ 4 hours) of the unit coupled with the high-resolution display screen provides greater analysis capabilities for the spectrum being observed. All these factors coupled with the low cost of the product make it an attractive solution for RF professionals looking for a portable alternative to the expensive and bulky bench top lab spectrum analyzers.

BERKELEY VARITRONICS
www.bvsystems.com
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