Chapter 13:
Theory of Operations
of SPI_PORTAL. Reads or writes to other registers will not interrupt the
programming process, as is the case with normal SPI accesses. When the entire
programming data has been written, verify that the INIT and DONE signals are
correct. After pulsing the FPGA_RST signal, wait for the CLOCK_OK signal to
become valid, indicating that the FPGA and clocks are functional. The SPI portal and
bypass registers should now work as described.
If a change in the sample rate is written to the FPGA, the CLOCK_OK signal will go
inactive for approximately 10 milliseconds, indicating that the clocks are not stable.
Care must be taken not to write to the SPI or bypass relays during this period. It is
recommended that the sample rate be written to the FPGA immediately after the
FPGA_RST signal has been pulsed. If the sample rate changes, the SPI devices must
be reconfigured.
Calibration
The calibration for the DPS-575 audio card involves two functional blocks: the
analog output drivers and the analog input receivers. The functional blocks may not
be calibrated independently, due to the nature of the system. The operator will require
an Audio Precision test unit, and the DPS-575 main chassis for interface control. The
analog processing sections have been designed such that the trimmer is just sufficient
to compensate for all the cumulative tolerances of the components. If any of the
measurements can not be brought into spec, it indicates that the circuit may have a
faulty or out of tolerance component.
Analog Output Driver
The analog output drivers must be calibrated first. The procedure is to set the analog
output channels to use a 1kHz audio test pattern, with amplitude of -18dBFS. The
output operating level should be set to 0dBu, with +18dB headroom. The Audio
Precision input should be set for 600Ω balanced termination. The analog output
should be adjusted using the appropriate trimmer until the Audio Precision reads an
input level of 0dBu. Repeat the procedure for the remaining three analog output
channels.
Analog Input Stage
The analog input drivers are calibrated after the analog output drivers. The procedure
is to connect the Audio Precision outputs to the analog card inputs. The input card
should be set for high impedance balanced termination. The Audio Precision outputs
should be set to 0dBu, balanced, with 40Ω impedance. The input operating level
should be set to 0dBu with +18dB headroom. The Auto-Track feature should be
disabled. With the same output configuration as in the previous analog output driver
configuration, make the same measurements. Adjust the input trimmer until the Audio
Precision reads an input level of 0dBu. Repeat the procedure for the remaining three
channels.
99
To Next Page
Loading...
