M5200A Channel Differencing for Differential I+jQ Measurements

While the Keysight M5200A digitizer does not support analog differential inputs, it is possible with the VSA software to use two of the inputs of the module and difference the data from each to get similar behavior to traditional differential inputs. This can then be used to do differential I+jQ measurements.

Configuration

Calibration for Best EVM Measurements

Configuration

To configure this mode, use the custom channel configuration dialog (Input > Channels > Custom...) in the VSA software and select the appropriate inputs. This is an example for differential I+jQ. When using this mode of operation it is important to match the delay between the channels being differenced. Since differences in cable lengths in the system as well as delay differences within the module can affect the delay, it is important to correct for this delay before differencing the data. But the exact delay to correct for is system dependent. Therefore, several input extension delay parameters are provided to allow for adjusting the delay between channels. The delay of each channel is relative to the overall acquisition trigger location.

Calibration for Best EVM Measurements

When using the differential baseband IQ channel configuration (Ch1-Ch3) + j*(Ch2-Ch4), it is important to calibrate the gain and delay between the channels so that mismatches between the gain and delay of the input channels on the module are compensated for and to minimize their contribution to the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. so that accurate EVM measurements of the DUT Device under Test: An acronym used to describe some type of electrical apparatus connected to test instrumentation. The apparatus can range from a single component to a complex subsystem such as a mobile phone, base station or MSC. can be made. Since the channel differencing (Ch1-Ch3) and (Ch2-Ch4) is being done before the data is input into the main VSA software, the normal User Correction feature in VSA cannot be used to make the delay and gain corrections for each channel (since the VSA user correction implementation is after the channel differencing has been done). Instead, the Delay and Gain Correction input extension parameters in VSA for the M5200A must be used to make these corrections. Use those parameters to compensate for delay and gain mismatches between channels when using a channel configuration that has channel differencing. As an example of the impact of this, the default setup EVM without calibration was observed in one case to be on the order of -30 dB when using a WLAN WLAN - Wireless Local Area Network: A system that includes the distribution system (DS), access points (APs), and portal entities. It is also the logical location of distribution and integration service functions of an extended service set (ESS). A WLAN system contains one or more APs and zero or more portals in addition to the DS. 11ax signal, but improved to around -50 dB after correcting for the delay and gain between the input channels due to cabling and the digitizer channel mismatches.

When using (Ch1-Ch3)+j*(Ch2-Ch4), if you want to change the delay or gain of both Ch1 and Ch3 by the same amount (or similarly Ch2 and Ch4 by the same amount), you can use the VSA User Correction feature for that. The VSA User Correction feature just can’t be used to change the individual parts of the channel differencing when using channel differencing with the M5200A.