richard allen wrote:
In our military system tests we acquire 10-20 impulses and sum them
in the frequency domain. If the ripple is actually in the system
then it will stay in. This was what the generals all wanted. In our
seismic stuff, where the geophysicists seem to think it is needed to
find oil, we require the < 1 dB flatness in the frequency domain
plateau of a single impulse.
In the VLBI world, we continuously inject weak pulses at a 1 MHz rate (1
usec between pulses) which have rise times in the 10s of psec range.
These pulses, injected into the radio astronomy receiver front ends,
manifest themselves as a frequency rail every 1 MHz. To achieve a very
wide (upwards of 1 GHz) flat passband, these "phase cal" signals are
processed to determine the phase offset needed to bring them all into
line. The pulse generator that Gerald built into the the SDR-1000 is my
design, derived from the same concept.
In the VLBI world, we adjust the pulse amplitude so that it only
increases the wideband system temperature by only 1-2%, but the pulses
are seen with good SNR as frequency rails in FFT analyzers running a few
Hz bandwidth.
73, Tom
