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VLA Frequency Page
This page allows specification of the details of the VLA LO chain in
order to obtain the required sky frequency. A frequency can be specified
as either a total sky frequency, a frequency offset or velocity (using a
line rest frequency from the LO Options Page),
explicit Local Oscillator settings or some combinations of these.
NB: Not all frequency/bandwidths are possible with the VLA and
not all combinations of frequencies that can be specified on this page
will work. The Frequency Page will refuse to display itself if the
Synthesizer setting option on the LO Options Page is set to "no change".
Unless you really need to specify the LO settings, don't. If these are
unspecified then JObserve will compute "optimal" settings as per G. Hunt.
In order to indicate which frequency parameters are set and which are
computed and to allow some control, buttons are displayed under the
relevant Text boxes. If the button is depressed and is labeled "set" then
the parameter in the above box has been explicitly set. The button can
be set or unset to indicate whether or not you want this parameter fixed.
The current combination can be checked by hitting the "Verify" button on
the button bar. Alternately, modifying a value in a Text Field and then
hitting Return (Enter) on the keyboard will cause the value to be read,
checked and other entries to be updated. Any warnings or error messages
will be displayed in the status line.
Band Code
The frequency "Band code" can be specified for each of the VLA's 4 IFs (A,
B, C, D). The most common case is for all IFs to be at the same band.
For this case, select the band for IF A; all other IFs will be modified
to be at the selected band. The VLA hardware constrains IFs A and C to
be at the same frequency; similarly B and D. To choose a band for IFs A
and C only, select a new band for IF C; to choose a band for IFs B and D
only, select one of these. A blank Band Code means no band is specified.
Possible band codes with standard frequency limits are:
| Code | Low Frequency | High Frequency |
| 4 | 48 MHz | 96 MHz |
| P | 306 MHz | 340 MHz |
| L | 1.34 GHz | 1.73 GHz |
| C | 4.50 GHz | 5.00 GHz |
| X | 8.00 GHz | 8.80 GHz |
| U | 14.4 GHz | 15.4 GHz |
| K | 22.0 GHz | 24.0 GHz |
| Q | 40.0 GHz | 50.0 GHz |
If one IF pair has a Band Code of 4 or P, then the other pair may
be at any band. Any other combination of Bands is not possible in
the VLA electronics. The band codes may also be specified on the Source Page.
Bandwidth (Bw)
The total bandwidth in MHz for each IF is specified in the
appropriate Choice. The bandwidth may also be specified on the Source Page.
For the Spectral Line cases, both the total number of channels produced by
the correlator and the channel widths available depend upon the selection
of the Correlator Mode,
and the Hanning Smoothing option
(both on Correlator Page). Both of
these options should be correctly set before selecting the bandwidth.
In the case of spectral line observations, the Bandwidth display shows
both the maximum number of channels and the minimum channel bandwidth.
The 8 MHz filter is only selectable in Continuum mode. This mode
is the normal default; it is, however, only true when the Correlator Mode is blank
or set to Continuum.
When a new bandwidth is selected for an IF, OBSERVE will
make checks on the validity of the current values of the Channel Count and the Starting Channel, both of
which are set on the Correlator Page. If they are invalid, they will be
marked as not present. WARNING! If you have already selected
values on the Correlator Page then you should remember to check these
values for validity after making the change here.
Front End LO
The Front End Local Oscillator setting in GHz. To be able to make
an appropriate selection from the choices here, the Bands must be
correctly set first. At 4, P, L, and C band there is a single choice
for the Front End Local Oscillator. In these cases, OBSERVE will select
the correct value, At X, U, K, and Q band there are several choices.
To decide among them, you may need to run LOSER before entering OBSERVE.
If all IFs are at the same Band, then the VLA hardware constrains all
Front End LO values to be identical. In this case, entering a value
for one IF updates all others. If the IFs are not at the same Band,
then the VLA hardware constrains IFs A and C to be at the same frequency;
similarly B and D. Selecting a Front End LO setting for IF A therefore
also selects the same setting for C and vice versa; similarly B and D.
If an explicit value is not specified, then this field is computed to give
an allowed value that will produce the desired frequency. Note: there
are obscure constraints on this value which depend on the frequency band.
Unless you know what you're doing and really need to specify this
value, DON'T.
The button under the Text Field is depressed and labeled "set" if the
corresponding value is fixed. Otherwise, the value is computed and may
change if other fields are modified.
2-4 GHz LO
The 2-4 GHz Synthesizer Local Oscillator setting in MHz. To be able to
select from the choices here, the Bands must be correctly set first.
The VLA hardware constrains IFs A and C to be at the same frequency;
similarly B and D. Selecting a 2-4 GHz LO setting for IF A therefore
also selects the same setting for C and vice versa; similarly B and D.
If an explicit value is not specified, then this field is computed to give
an allowed value that will produce the desired frequency. Note: there
are obscure constraints on this value which depend on the frequency band.
Unless you know what you're doing and really need to specify this
value, DON'T.
At 4 and P band, the selections are negative indicating an upconversion
to C band. To decide among the many choices, you may need to run LOSER
before entering OBSERVE.
The button under the Text Field is depressed and labeled "set" if the
corresponding value is fixed. Otherwise, the value is computed and may
change if other fields are modified.
Freq/Vel
The meaning of this value depends on the Frequency/Velocity Option
on the LO Options Page and is
either the Baseband Local Oscillator setting ("Absolute Frequency"),
a frequency offset ("frequency offset") or the velocity at which the
line is to be observed ("radio velocity" or "optical velocity"). If the
Frequency/Velocity Option is blank, Absolute Frequency, or Frequency
Offset, then this field is in MHz, otherwise it is a Velocity in km/sec.
The VLA on-line hardware constrains IFs A and C to be at the same
frequency; similarly B and D. Selecting a value for IF A therefore also
selects a corresponding value for C and vice versa; similarly B and D.
When entering the value, include the sign if negative; commas are
optional.
In all cases, only the values for IFs A and B are written to the OBSERVE
file. If either of these is left unspecified, it is written to the
OBSERVE file as such. This will be interpreted as zero.
In the majority of continuum cases, the Frequency/Velocity Option will
be left blank or set to Absolute Frequency. In this case, the value
for the different IFs must be in the ranges (MHz) as follows:
| IF | Low (MHz) | High (MHz) |
| A | 100 | 150 |
| B | 200 | 250 |
| C | 200 | 250 |
| D | 100 | 150 |
The sums (A+C) and (B+D) are constrained to be exactly 350.
If the Frequency/Velocity Option on the Frequency Page is set to
"Frequency Offset", then the range of this field is +/-99.999999 MHz.
This is added to the Line Rest Frequency from the Frequency Page to
determine the frequency of observation. At present, the on-line system
is not smart enough for this to be sufficient. Correct settings for
the Front End L.O. and the 2-4 GHz L.O. must also be supplied.
If the Frequency/Velocity Option on the Frequency Page is set to
Radio Velocity or Optical Velocity, then the value of this field is
+/-99,999.999 km/sec. The on-line system will use this velocity in
the Rest Frame defined on the Frequency Page to determine the apparent
frequency of the Line Rest Frequency (also on the Frequency Page); this
will be the frequency of observation. At present, the on-line system
is not smart enough for this to be sufficient. Correct settings for
the Front End L.O. and the 2-4 GHz L.O. must also be supplied.
The button under the Text Field is depressed and labeled "set" if the
corresponding value is fixed. Otherwise, the value is computed and may
change if other fields are modified.
IF Filter
Controls the "Front End" filters.
There are three options:
- 50 MHz centered at 1025 MHz
- 25 MHz centered at 1027 MHz
- 12.5 MHz centered at 1027 MHz
These filters are useful at 4 and P band. The default is 50 MHz.
Don't mess with this field unless you know what you're doing.
Frequency
This field contains the Sky Frequency. A frequency can be entered
directly in this field (preferred) or can be computed from Local
Oscillator settings (only if you know what you're doing).
For line observers, we calculate the sky frequency based on your rest
frame reference setting on the Frequency Page and the velocity from
this page. If necessary, LO setting may also be specified.
The button under the Text Field is depressed and labeled "set" if the
corresponding value is fixed. Otherwise, the value is computed and may
change if other fields are modified.