November 13, 2017
Frequency repack - monitoring group delay errors
The frequency reallocation will require stations to re-tune or even replace the output mask (band-pass) filters. Group delay measurements can help to set up and monitor the performance of the newly set transmission chain.
It is a well-known fact that frequency response and group delay errors within ATSC1.0/3.0 DTV signal bandwidth affect the signal quality by reducing its signal-to-noise ratio (SNR). Such errors characterize the transmission path between the transmitter output and the receiver input and may result in signal decoding issues for the end-users, TV viewers.
A typical source of the errors, especially of the group delay error, is a band-pass (mask) filter at a transmitter system output.
Though today, transmitter and modulator manufacturers supply systems with integrated adaptive digital pre-distorter (pre-correction) modules that automatically control and modify the signal amplitude and phase parameters, the group delay should be constantly monitored during the transmitter normal operation due to possible filter aging effects or the pre-distorter performance.
Since the group delay is related to the filter phase and amplitude response, it cannot be seen or measured using an ordinary spectrum analyzer.
Using AVQ1022 - RF Layer Monitoring Receiver and Signal Analyzer for Group Delay monitoring
AVQ1022 with two RF inputs provides a convenient solution for sampling RF signal before and after the band-pass mask filter of a transmitter system:
AVQ1022 Application Block Diagram
Screenshots of Group Delay and Amplitude and Phase response plots:
AVQ1022 Amplitude and Phase Channel Response Plot
AVQ1022 Group Delay Plot
AVQ1022 can be used not only during transmitter system installation or servicing but also for 24/7 day-by-day remote monitoring to assist with maintaining good signal quality and in-depth troubleshooting in case an issue arises.