5 FREQUENCY BANDS & PERFORMANCE

The nominal frequency ranges for VLBA receiver systems are shown in Table 3. Actual frequency ranges are broader; consult the measurements reported by Hronek & Walker (1996) for details. Updates on frequency-dependent performance across VLBA bands is available at http://www.vlba.nrao.edu/cgi-bin/wbd_dir.pl. These actual ranges may be especially important for avoiding radio frequency interference (RFI), and for programs involving extragalactic spectral lines, rotation measures (Cotton 1995b; Kemball 1999), and multi-frequency synthesis (Conway & Sault 1995; Sault & Conway 1999).

Table 3: Receiver Frequency Ranges & Performance

Receiver	Nominal	Typical	Center	Typical	Baseline	Image
Band	Frequency	Zenith	Frequency	Peak	Sensitivity	Sensitivity
Designation	Range	SEFD	for SEFD	Gain	$\Delta S^{\rm 512, 2 m}$	$\Delta I^{\rm 512, 8 h}_{\rm m}$
	[GHz]	[Jy]	[GHz]	[K Jy$^{-1}$]	[mJy]	[ $\mu{\rm Jy~beam}^{-1}$]
90 cm	0.312 - 0.342	2227	0.326	0.097	(g) 29	(i) 196
50 cm (a)	0.596 - 0.626	2216	0.611	0.088	(g) 81	(j) 553
21 cm (b)	1.35 - 1.75	296	1.438	0.096	1.9	18
18 cm (b)	1.35 - 1.75	303	1.658	0.100	2.0	19
13 cm (c)	2.15 - 2.35	322	2.275	0.093	2.1	20
13 cm (c,d)	2.15 - 2.35	337	2.275	0.090	2.2	21
6 cm (*)	3.9 - 7.9	210	4.990	0.120	1.3	13
4 cm	8.0 - 8.8	307	8.425	0.113	2.0	19
4 cm (d)	8.0 - 8.8	407	8.425	0.106	2.6	25
2 cm	12.0 - 15.4	550	15.369	0.104	3.6	34
1 cm (e)	21.7 - 24.1	502	22.236	0.107	3.3	31
1 cm (e)	21.7 - 24.1	441	23.799	0.107	2.9	28
7 mm	41.0 - 45.0	1436	43.174	0.078	(g) 13	90
3 mm (f)	80.0 - 90.0	4000	86.2	0.025	(h) 52	(k) 447

Notes:
(a) User-selectable filters available to restrict frequencies to 608.2-613.8 MHz.
(b) Different ranges within the same 20 cm receiver.
(c) Filters at NL, LA, and OV restrict frequencies to 2200-2400 MHz.
(d) Using 13/4 cm dichroic.
(e) Different ranges within the same 1 cm receiver. Continuum performance is better at 23.8 GHz, away from the water line.
(f) See Table 4 for individual station details.
(g) Fringe-fit interval 1 minute.         (h) Fringe-fit interval 30 seconds.
(i) Data rate 256 Mbps.                 (j) Data rate 32 Mbps.
(k) 8-station array; 4-hour integration.

(*) These receivers are being upgraded substantially. This table now shows the performance of the new systems. See Section 2 for information on the upgraded receivers, including the current installation schedule.

Also shown in Table 3 are parameters characterizing the performance of a typical VLBA station for the various frequency bands. Columns [3] and [5] give typical VLBA system-equivalent-flux-density (SEFD) values at zenith and opacity-corrected peak gains, respectively. These are means over measurements in both polarization at all ten antennas, at the frequencies in column [4].

The typical zenith SEFD can be combined with the aggregate recorded data rate and appropriate integration times to estimate the root-mean-square (RMS) noise level on a single VLBA baseline ($\Delta S$; see Equation 3 in Section 9), and in a VLBA image ( $\Delta I_{\rm m}$; Equation 4 in Section 10). Characteristic values tabulated in columns [6] and [7] are computed assuming, for most cases, the VLBA's current 512-Mbps standard recording rate for continuum observations; a typical fringe-fit interval of 2 minutes; and a total on-source integration time of 8 hours. Exceptions, indicated in the table notes, apply to the fringe-fit intervals at the lowest and highest frequency bands, where shorter intervals are often required; for the recording rate limits imposed by the available RF bandwidth at the lowest frequency bands; and for most parameters at the extreme 3-mm band. Performance may be worse than the tabulated estimates on some baselines due to poor primary or subreflector surfaces or poor atmospheric conditions.

Table 4: Typical Performance Parameters at 86.2 GHz

Antenna	Nominal	Typical	Typical	Typical	Baseline
	Frequency	Zenith	Peak	Zenith	Sensitivity
	Range	SEFD	Gain	$T_{\rm sys}$	$\Delta S^{\rm 512, 30 s}$
	[GHz]	[Jy]	[K Jy$^{-1}$]	[K]	[mJy]
BR	80.0 - 90.0	3500	0.039	135	55.
NL	80.0 - 96.0	4900	0.055	270	65.
FD	80.0 - 96.0	3600	0.034	120	55.
LA	80.0 - 90.0	3100	0.051	160	$\cdots$
PT	80.0 - 96.0	4100	0.024	100	55.
KP	80.0 - 96.0	4600	0.025	110	60.
OV	80.0 - 96.0	5800	0.020	100	65.
MK	80.0 - 96.0	4100	0.023	100	55.

The 3 mm band extends beyond the design specification for the VLBA antenna, and is challenging for the panel-setting accuracy of the primary reflectors, the figure of the subreflectors, and the pointing of the antennas. In addition, performance in this band is highly dependent on weather conditions. Table 4 gives the approximate current performance at 86 GHz for each antenna, as well as the RMS noise in 30 seconds (at 512 Mbps) on a baseline to LA, which is one of the most sensitive 3 mm antennas.