Optimize Wide Bandwidth Signal Performance with Channel Correction
2019-01-02 | 7 min read
When you use a signal generator to output a continuous wave (CW), the signal generator confirms its output amplitude accuracy at the RF output port. Even when the temperature increases with time, the signal generator has an automatic leveling control (ALC) circuit to monitor and adjust its output power.
Outside the signal generator, using flatness correction ensures that you continue to have flat output power even after the signal has passed through all the cables, switches, and splitters. The amplitude accuracy can move from the signal generator’s RF output to the device under test’s (DUT) input. Refer to the previous blog posts "What is Flatness Correction and Why it Matters" and “Overcome Power Drifts with External Leveling” for more information.
While the tactics mentioned above address a specified frequency point amplitude compensation, at a different frequency point, there is a different offset value to compensate for the amplitude flatness. When a signal is a modulated signal, it occupies a certain bandwidth. Applying a single offset value to the signal cannot correct the flatness effects of the entire signal bandwidth. The effects include amplitude flatness and phase flatness.
Using Internal Channel Correction
Most new vector signal generators have an internal calibration routine, also referred to as factory calibration, that collects correction data for both the baseband and RF magnitude, along with phase errors over the entire RF frequency and power level range. The correction data includes the parameters of the correction filter applied to baseband waveforms, and the correction processing is done by a digital signal processor (DSP) in real time.
Figure 1 is a 5G new radio (NR) signal and the signal bandwidth is up to 100 MHz. From the plot B, when the channel correction is off (default setting), you can see the signal’s spectrum has a slight decline from left to right. Using the Orthogonal Frequency-Division Multiplexing (OFDM) demodulation equalizer, you can clearly see the channel frequency response has a difference (maker 1 and 2) of 2.6 dB as shown in plot D.
Figure 1. A 5G NR signal with 100 MHz bandwidth and internal channel correction turned off
When the correction is on, the signal generator flattens the system magnitude and phase responses across the maximum bandwidth supported by the instrument (up to 160 MHz bandwidth for MXG N5182B). Figure 2 shows the same 5G signal, but plot B signal spectrum is flat now. The difference in the equalizer channel frequency response is down to 0.6 dB. In addition, the error vector magnitude (EVM) is improved from 0.44% to 0.36% as shown in plot C.
Figure 2. A 5G NR signal with 100 MHz bandwidth and internal channel correction turned on
Characteristics of the Internal Channel Correction
You may wonder why the default setting of the internal channel correction is off? General purpose signal generators are optimized for performance, measurement speed, and cost. Most test scenarios have narrow bandwidths, just-enough performance, or high measurement speed. Channel correction has few impacts on measurement results and is not necessary as it increases test time.
When the correction feature is on and the frequency is changed, the firmware will calculate a channel correction filter. Additional time is required to complete the whole process and the amount of time depends on the types of frequency switching.
Arbitrary Frequency Switching
When the internal correction feature is on, arbitrary frequency switching on the MXG N5182B will take up to an additional 3.3 ms (typical) to 6.8 ms the first time that frequency is specified. Related parameters will be cached. After that, switching to that frequency will take up to an additional 1.3 ms. Up to 1024 unique frequencies can be cached before the oldest cache is forgotten.
If a frequency sweep is activated, then the calculation and caching will occur upfront for the first 256 unique frequencies, while all additional unique frequencies will have the characteristics of arbitrary frequency switching.
User Channel Correction Calibration
The user channel correction calibration extends the signal generator’s performance to a new calibration plane – the user's DUT input port. You can use a USB power sensor to perform the calibration as shown in Figure 3. You need to specify the start and stop frequencies, configure the power meter, then execute the calibration. Run this calibration when the ambient temperature has varied by at least ±5 degrees Celsius from the ambient temperature at which the previous calibration was run.
Figure 3. Use a USB power sensor to execute user channel correction calibration
Modern signal generators provide not only wide modulation bandwidths but also ample flatness performance across the entire signal bandwidth for even the most demanding design tasks. They achieve this combination of bandwidth and accuracy through the use of a digital signal processing (DSP) for real-time signal processing and a factory-calibrated channel correction technique. Using these technologies and with the press of a button, you can minimize modulation errors caused by hardware flatness and provide high modulation accuracy with wide modulation bandwidth.