The VLA delivers four intermediate frequency (IF) signals, called VLA IF A, VLA IF B, VLA IF C, and VLA IF D. VLA IF A and VLA IF B are right circularly polarized (RCP), while VLA IF C and VLA IF D are left circularly polarized (LCP). The VLA's AC and BD IF pairs can in principle be tuned to different frequencies but standard VLBI observations are done with these pairs at the same frequency. Dual-polarization VLBI programs can record both polarizations on a single tape. Many standard continuum setups for programs in VLBA formats use simultaneous dual polarization by default, although single-polarization setups are also available.
A VLBI T8 switch, described in detail in Section 14, was installed in 1997 August. This switch allows a one-to-one correspondence between the signals delivered from the VLA IFs A, B, C, and D to the VLBA IFs A, B, C, and D at the VLBI backend.
For standard single-antenna VLBI, the VLA now delivers signals from
all four VLA IFs to the VLBI backend; the VLA's AC and BD IF pairs are
set to identical frequencies within a bandwidth of up to 46 MHz; and
the VLBI backend selects RCP from VLA IF A 
VLBA IF A and/or LCP
from VLA IF C VLBA IF C. Nonstandard VLBI schemes are available
if signals from both VLA IF pairs are needed and/or signals spanning
more than 46 MHz are neeeed; contact Joan Wrobel or Greg Taylor (see
Section 17) for more information.
For standard phased-array VLBI, the VLA delivers analog sum signals
from all four VLA IFs to the VLBI backend. The VLA's AC and BD IF
pairs are set to identical frequencies within a bandwidth of up to
46 MHz. Only two of the four analog sum signals available are phased.
The VLA's IF A (RCP) and IF D (LCP) are usually used because they are
properly adjusted in both hardware and software for variations in the
instrumental delay (the ``round-trip'' phase correction). The VLA's B
and C IFs are not usually used for VLBI because their round-trip phase
corrections are slaved to those for the VLA's A and D IFs. The VLA's
C IF is slaved to the VLA's A IF, and the VLA's B IF is slaved to the
VLA's D IF. For this reason standard phased-array VLBI is done with
VLA IF A VLBA IF A selected for RCP and/or VLA IF D VLBA IF D
selected for LCP. Nonstandard schemes are available if it is
essential to phase only RCP signals (VLA IFs A and B) or only LCP
signals (VLA IFs C and D); contact Joan Wrobel or Greg Taylor (see
Section 17) for more information.