What You Need to Know about Wideband Signal Analysis
Exponential growth in demand for faster data rate applications has triggered the need for new technologies capable of wide signal bandwidth. Cellular communication is transitioning from 4G to 5G to enable extreme data throughputs. 5G NR uses existing and new technologies to achieve the anticipated extreme data throughputs, including wider channel bandwidths, carrier aggregation, a high modulation density, and multiple antennas. The 5G NR maximum channel bandwidth is 400 MHz for the frequency range 2 (FR2), and the maximum aggregated channel bandwidth (contiguous) goes up to 1.2 GHz.
Satellite communication providers are building networks in space to enable high-speed communications from anywhere. High-throughput satellites now use transponders with bandwidths up to 2 GHz to achieve required data rates. Wide bandwidths enable high-throughput data, range resolution and accuracy, and low latency but also introduce more noise. The nose increases test complexity and measurement uncertainties.
More bandwidth guarantees a higher data rate?
Millimeter-wave frequency bands provide wider available bandwidths. Wide bandwidths enable high-throughput data, range resolution and accuracy, and low latency but also introduce more noise. Noise is part of all communications channels. A transmit signal needs to compete with the channel’s noise floor to get better sensitivity at a receiver. The Shannon-Hartley theorem below tells you the maximum rate at which information can be transmitted over a communication channel within a specified bandwidth with the presence of noise.
C = B*log2 (1+S/N)
C is the channel capacity in bits per second (bit/s).
B is the signal bandwidth in Hz.
S is the average received power over the bandwidth in watts.
N is the average power of the noise over the bandwidth in watts.
The wider bandwidths (B) promise dramatic gains in channel capacity. Unfortunately, these bandwidths gather up more noise, and that limits real-world capacity and spectral efficiency.
Similarly, increasing analysis bandwidth introduces more noise to a signal analyzer. The noise reduces the SNR in measurements and makes accurate millimeter-wave measurements more difficult. Figure 1 shows QPSK signals with 10 MHz (red) and 100 MHz (yellow) symbol rates in the frequency domain. Both signals have the same output power. The wider bandwidth introduces more noise over the bandwidth.
Figure 1. Narrow and wide bandwidth signals with the same output power
Using higher-order modulation schemes (for example, 256QAM) can achieve faster data rates, but the symbols are closer and sensitive to noise. Wideband noise and excess path loss at millimeter frequencies between signal analyzers and DUTs result in a lower SNR for the digitizer. The low SNR causes the transmitter measurements to have a poor EVM and adjacent channel power ratio performance, which does not represent the DUT’s performance. So, selecting a signal analyzer with the lowest system noise floor is critical for wideband vector analysis.
Improve Signal Condition
Figure 2 is a simplified block diagram of a vector signal analyzer. When making EVM measurements, you need to set optimum levels for the signal analyzer’s input mixer, the phase noise configuration of the LO, and the digitizer to achieve the best results. Each of these components has its constraints and use cases.
Figure 2. A vector signal analyzer block diagram
Optimize input mixer level
The input mixer-level setting is a trade-off between distortion performance and noise sensitivity. You can achieve a better SNR with a higher input mixer level or better distortion performance with a lower input mixer level. The best mixer-level setting depends on the measurement hardware, characteristics of the input signal, and specification test requirements.
You can also apply an external low-noise amplifier (LNA) at the front end, with or without the internal preamplifier, to optimize the input level of the mixer. Keysight N9042B X-series signal analyzer provides a built-in LNA and preamplifier for various test scenarios, as shown in Figure 3. The two-stage gain delivers greater flexibility to balance noise and distortion for optimizing the best low-input-level measurement performance.
Figure 3. The built-in LNA drives down the noise, and the two-stage gain delivers greater flexibility to balance noise and distortion
Figures 4 and 5 show demodulation analysis of a 5G NR signal with a low input level at about -40 dBm. Figure 4 is the result of turning LNA off and Figure 5 is on. The LNA provides a better SNR for the demodulation analysis.
Figure 4. The signal analyzer cannot well demodulate a low-level 5G NR signal
Figure 5. Turn on LNA to achieve better SNR for demodulation analysis
Optimize SNR for an IF digitizer
The system IF noise of a signal analyzer must be low enough to get the best EVM measurement results. At the same time, the input signal to the digitizer must be high enough without overloading the digitizer. This balance requires a combination of RF attenuator, preamplifier, and IF gain value based on the measured signal peak level.
Accelerate Your Wideband Measurements
Keysight X-series signal analyzers let you press a single key to optimize these hardware settings, improving SNR and avoiding digitizer overload. The optimization processing requires measuring the signal peak level and setting up the analyzer. However, the measured period may not represent the complete power characteristics of the input signal. A user can manually tweak the settings, such as IF gain and RF attenuators, to achieve the best measurement results.
Keysight can help you be certain of your device’s performance with test solutions that provide greater visibility, accuracy, and repeatability, so you can focus on your next breakthrough.
Table 1. Keysight wideband signal generation and analysis solutions
1. Support maximum analysis bandwidth with a wide IF output option and an external digitizer.
2. Get up to 4 GHz of RF bandwidth with wideband external differential I/Q inputs.
3. Get up to 4 GHz of RF bandwidth with dual-channel bonding.
4. Modular form factor