For transient pulsed signals whose spectrum is unstable, or for TDD-based LTE and 5G-NR signals, engineers often need to observe the time domain and the frequency domain at the same time — and both views must be strictly synchronized with the frame period of the signal under test. The spectrogram (waterfall) display found on most instruments on the market offers neither fine time control nor a rigorous synchronized internal trigger. The TD-power+SA function, included as standard in QIBO TORI WaveEye real-time spectrum analysis software, was designed precisely for this type of measurement.
The original design goal of this function was very specific: to locate weak interference in the uplink interval of TDD signals. In a TDD system, uplink and downlink share the same frequency band and take turns over time; interference appearing inside an uplink slot is usually low in power and short in duration, which makes it very hard to separate from the strong downlink signal using a conventional spectrum display.
Since software version 20240908, the scope of this function has been widened. It now applies equally well to other pulsed wideband or narrowband signals, such as Wi-Fi, Bluetooth, radar, and the frequency-hopping signals of drone flight-control links. The rule of thumb is simple: as long as the signal is not continuous in time, this function can cut it open for a closer look.
The essential difference from a spectrogram lies in the triggering method. Triggered by an internal precision clock, the software acquires one continuous block of signal. The block length is set by Time Width, and the internal trigger period is always an integer multiple of that time width. As a result, the position of the uplink and downlink intervals in every capture is completely fixed relative to the trigger point (time zero at the left edge); the time-domain trace stays stable and does not drift, so results can be compared again and again.
Select TD-power+SA from the function selection menu to enter this function.
[Figure 1] Function selection menu — the TD-power + SA entry in QIBO TORI WaveEye
Once you enter the function, the display is divided into an upper and a lower part:
The upper part is the TD power trace, with time on the horizontal axis and signal level on the vertical axis, showing the pulse rhythm of the signal directly;
The lower part is the spectrum display. The light blue curve is the average spectrum over the whole time span, while the yellow curve is the instantaneous spectrum of the short segment marked by the yellow marker on the upper trace.
Click anywhere on the time-domain trace and the yellow marker moves to the new position, with the yellow spectrum below updating accordingly. If needed, the width of the yellow marker can also be changed, which alters the length of the time window used for calculation — precisely the fine control a spectrogram cannot provide.
Two parameters decide whether the measurement is valid:
Span: set it to a value that matches the bandwidth of the signal under test, otherwise the time-domain display will be disturbed by irrelevant out-of-band signals. In this example the span is 20 MHz, which corresponds exactly to the occupied bandwidth of the TDD-LTE signal.
Time Width: it should equal the frame period of the signal being analyzed. In this example it is 10 ms, which matches one LTE frame exactly, so the uplink and downlink intervals in the time domain stay consistently aligned.
The interface also shows the current status — center frequency 1895.000 MHz, resolution 61.035 kHz, reference level −40 dBm, RX gain 0 dB, aggregation width 10 and so on — so you can check at any time whether the settings match the system under test.
[Figure 2] Setup and display of the QIBO TORI WaveEye TD-power+SA function: upper trace is the power-versus-time plot (10 ms time width), lower trace is the spectrum (20 MHz span)
Figure 3 shows a typical uplink interference hunting case. Look at the upper trace first: the downlink interval is a regular series of high-level pulses, while the uplink interval, which should be flat, shows a "bulge" that does not fit the rhythm. Move the yellow marker onto this suspicious segment and the lower spectrum immediately gives its spectral signature — broadband noise, a single tone, or adjacent-channel leakage can be told apart at a glance.
This is the practical value of the function: use the time domain to find out "when", then use the spectrum to answer "what". Both steps happen on the same screen, with no need to switch back and forth between two instruments or two views.
[Figure 3] TDD uplink interference hunting: the suspicious segment in the upper time-domain trace and the abnormal signal in the spectrum below
Switch from Normal mode to Replay mode. The upper time-domain trace now comes to a standstill — repeating the same block of data continuously — while the lower spectrum keeps changing as you click. For clarity, the two spectrum curves are also reduced to a single one.
This mode suits post-capture review: the data from one capture is kept intact, so you can step through different time positions, compare when the interference starts and stops, how long it lasts, and what its spectral shape looks like. It also makes screenshots for archiving or discussion with colleagues straightforward.
[Figure 4] Replay mode: the time-domain trace is frozen; click any region and its spectrum is displayed below
Figure 5 shows a set of measured Wi-Fi cases. It can be seen that different active segments in the time domain correspond to different spectral content — in some segments the whole channel is fully occupied, in others only a few sub-carriers are hopping.
This also leads to a practical conclusion: if collisions with overlap in time and frequency exist in a Wi-Fi signal, this test method should be able to capture them accurately. The same approach transfers directly to the observation of Bluetooth, radar pulses and drone flight-control frequency-hopping signals.
[Figure 5] Measured Wi-Fi signals: different active segments in the time domain show different spectral content
1. What value should I use for Time Width?
Set it to the frame period of the signal under test so that the trigger lines up with the signal. For example, an LTE frame is 10 ms long, so set the time width to 10 ms. If the value has no relation to the signal, the time-domain trace will slide left and right, and the spectrum of the yellow marker loses its meaning.
2. What span is appropriate?
Match it to the occupied bandwidth of the signal; the TDD-LTE example here uses 20 MHz. Too wide a span brings in irrelevant out-of-band signals that bias the time-domain judgment; too narrow a span clips the sidebands of the signal itself.
3. The trace does not move in replay mode — has it frozen?
No. Replay mode is designed to freeze the time-domain trace and repeat the same block of data, so that you can click different time positions repeatedly and inspect their spectra. Switch back to Normal mode to exit.
4. Can this function replace DPX or the spectrogram?
No — it complements them. DPX and the spectrogram are good at answering "is there anything abnormal", with wide coverage and fast refresh; TD-power+SA is good at answering "in which slot does the anomaly occur and what does it look like", with higher precision and traceability.
5. Which software version is required?
TD-power+SA is a standard feature of the WaveEye software. Since software version 20240908, its scope has been extended from TDD interference hunting to all kinds of pulsed signals, including Wi-Fi, Bluetooth, radar and drone flight-control links. Contact us for upgrade instructions.
