VLA SIGNAL PATH

A VLA antenna can accept astronomical signals within the following ``0.9 times nominal'' frequency bands: 0.305-0.335 GHz in the 90-cm region, 1.240-1.700 GHz in the 20-cm region, 4.500-5.000 GHz in the 6-cm region, 8.080-8.750 GHz in the 3.6-cm region, 14.650-15.325 GHz in the 2-cm region, 22.000-24.000 GHz in the 1.3-cm region, and 40.500-44.500 GHz in the 7-mm region (see the ``VLA Observational Status Summary''). These astronomical signals are amplified and then mixed at the antennas with the first local oscillator at frequency $F_1$,² to up- or down-convert them to the intermediate frequency band at 4500-5000 MHz. $F_1$ settings for the standard VLA bands for VLBI are: 0.0 GHz for band VP, $-3.2$ GHz for band VL, 0.0 GHz for band VC, 13.0 GHz for band VX, 19.9 GHz for band VU, 17.5 GHz for band VK, and (51.6$-$13.0) GHz for band VQ. For observations at nonstandard frequencies in the 1.3- or 2-cm regions, $F_1$ can be set to any frequency between 17.0 and 20.0 GHz that satisfies the condition $F_1=17.1+(n \times 0.3) \pm 0.1$ GHz, where $n$ is some integer. After the $F_1$ mix, upper sideband is selected for all observations in the 7-mm region, plus the 90, 20, 6, and 1.3-cm regions; and lower sideband is selected for all observations in the 3.6 and 2-cm regions.

After amplification and further treatment, the signals at the antennas are converted down to the range 1000-1050 MHz by the L6 synthesizers at frequency $F_6$. The frontend filters are also inserted. $F_6$ can be tuned to any frequency between 2710 and 4010 MHz that satisfies $F_6=2400+(m \times 50) \pm10.1$ MHz, where $m$ is some integer.

The signals for band VP are treated similarly, but are upconverted to the range 1000-1050 MHz by a lower, fixed $F_6$ value. For band VP, there are only two valid $F_6$ settings, either $-689.9$ MHz or $-710.1$ MHz. The former is adopted for standard VLBI. For $F_6 =-689.9$ MHz a 50-MHz frontend filter would pass $F_6+1000.=310.1$ MHz to $F_6+1050.=360.1$ MHz centered at 335.1 MHz, but this is never allowed. Rather frontend filters of width 25 MHz must be inserted to avoid gain compression. These frontend filters pass $335.1\pm12.5$ MHz, or 322.6 to 347.6 MHz.

For all bands, the signals in the range 1000-1050 MHz from the four VLA IFs A, B, C, and D are then mixed with offset frequencies $F_{off}$ to place them in the 1200-1800 MHz band. For VLA IFs A, B, C, and D, $F_{off}$ is 300, 400, 550, and 650 MHz, respectively. If the frontend filters at the antennas have bandwidths $\Delta F_{front}$ in MHz, then each VLA IF will have its own unique band. These are $1300$ MHz to $1300+ \Delta F_{front}$ MHz for VLA IF A, $1400$ MHz to $1400+ \Delta F_{front}$ MHz for VLA IF B, $1550$ MHz to $1550+ \Delta F_{front}$ MHz for VLA IF C, and $1650$ MHz to $1650+ \Delta F_{front}$ MHz for VLA IF D. Usually, but not always, $\Delta F_{front}=50$ MHz for VLBI; see Table 2 and Section 8. These VLA IF signals, plus monitor data from all antennas, are transmitted via the waveguide system from the antennas to the Central Electronics Room in the VLA Control Building. During waveguide transmission, the VLA IF signals from each of the arm's antennas modulate a high waveguide carrier frequency that is different for each antenna, with the carrier frequencies being the same on each arm. Waveguide transmissions last for an average of $t_{to\ CB}=50473.33~\mu$s, and are separated by intervals of $t_{from\ CB}=1610~\mu$s during which antenna control and LO signals pass to the antennas from the Control Building. The consequent average waveguide duty cycle is $t_{to\ CB} / (t_{to\ CB} + t_{from\ CB}) =$ 96.9%, which must be taken into account when using VLA correlator data to calibrate the VLA for VLBI (see Section 10).

In the Central Electronics Room, the VLA's A and B IFs are mixed with 1200 MHz and the C and D IFs are mixed with 1800 MHz. All signals are then mixed down to base band by frequencies derived from the VLA's Fluke synthesizers. For VLA IF A, the nominal Fluke synthesizer value $F_A$ should be in the range 100 to 150 MHz. For VLA IF B, $2 \times F_B$ should be in the range 200 to 250 MHz. For VLA IF C, $-2 \times F_C$ should be in the range $-250$ to $-200$ MHz. For VLA IF D, $-FD$ must be in the range $-150$ to $-100$ MHz. To facilitate polarization measurements, $F_A$ and $F_C$ are slaved by $F_A+(2 \times F_C)=350$ MHz, and $F_B$ and $F_D$ are slaved by $(2 \times F_B)+F_D=350$ MHz. These force IFs A and C to have the same sky frequencies, and IFs B and D to have the same sky frequencies, Finally, the signals go to the T5 module of each antenna, where they are filtered with backend base band filters of width $\Delta F_{back}$ selected according to the bandwidth code on the source request card in the VLA observe file; see Table 2 and Section 16.2. These filters have bandwidths ranging between 0.2 and 50 MHz. For single-antenna VLBI, four T5 outputs, one for each of the antenna's VLA IFs, are sent to the VLBI T8 switch in the VLBI equipment racks (see Section 14). For phased-array VLBI, the T5 signals from all active antennas and all IFs are sent to the VLA correlator, where they are combined to yield an analog sum for each VLA IF. These four sums are sent to the VLBI equipment racks, where they are buffered, equalized, and then sent to the VLBI T8 switch (see Section 14).

Let $F_{VLA}$ be the frequency delivered by the VLA to DC at the input to the 600 MHz VLBI upconverter (see Section 14).

For observations at 0.33, 1.7, 5.0, and 22 GHz, the final VLA IF signals sent to the VLBI hardware are net upper sideband, and $F_{VLA}$ in MHz can be calculated from the following formulae for the VLA's AC IF pair

$$F_{VLA} = F_1 + F_6 + 900. + F_A = F_1 + F_6 + 1250. - (2 \times F_C)$$ (1)

and for the VLA's BD IF pair

$$F_{VLA} = F_1 + F_6 + 800. + (2 \times F_B) = F_1 + F_6 + 1150. - F_D.$$ (2)

For observations at 43 GHz, $F_1$ is the difference between two frequencies, called $F_{1hi}$ and $F_{1lo}$. Then the final VLA IF signals sent to the VLBI hardware are net upper sideband, and $F_{VLA}$ in MHz can be calculated from the following formulae for the VLA's AC IF pair

$$F_{VLA} = F_{1hi} - F_{1lo} + F_6 + 900. + F_A = F_{1hi} - F_{1lo} + F_6 + 1250. - (2 \times F_C)$$ (3)

and for the VLA's BD IF pair

$$F_{VLA} = F_{1hi} - F_{1lo} + F_6 + 800. + (2 \times F_B) = F_{1hi} - F_{1lo} + F_6 + 1150. - F_D.$$ (4)

For observations at 8.4 and 15 GHz, the VLA's $F_1$ mix is lower sideband. Thus at these two wavelengths, the final VLA IF signals sent to the VLBI hardware are net lower sideband, and $F_{VLA}$ in MHz can be calculated from the following formulae for the VLA's AC IF pair

$$F_{VLA} = F_1 - F_6 - 900. - F_A = F_1 - F_6 - 1250. + (2 \times F_C)$$ (5)

and for the VLA's BD IF pair

$$F_{VLA} = F_1 - F_6 - 800. - (2 \times F_B) = F_1 - F_6 - 1150. + F_D$$ (6)

For the standard VLA bands VP, VL, VC, VX, VU, VK, and VQ, the values for $F_{VLA}$ are 322.60 MHz, 1639.90 MHz, 4960.10 MHz, 8439.90 MHz, 15389.90 MHz, 22210.10 MHz, and 43110.1 MHz, respectively. Let $F_{VLA50}$ be the frequency delivered by the VLA to 50 MHz at the input to the 600 MHz VLBI upconverter (see Section 14). Then for the VLA bands VL, VC, VX, VU, VK, and VQ, $F_{VLA50} =$ 1689.90 MHz (VLA net USB), 5010.10 MHz (VLA net USB), 8389.90 MHz (VLA net LSB), 15339.90 MHz (VLA net LSB), 22260.10 MHz (VLA net USB), and 43160.1 MHz (VLA net USB). For VLA band VP, 25-MHz backend filters are used so the corresponding $F_{VLA25}$ is 347.60 MHz; this provides a perfect match between the frequencies passed by the frontend filters (see above) and the backend filters.