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VLA LO Chain Page
This page allows specification of the details of the VLA LO chain.
A frequency can be specified as either a total sky frequency, a
frequency offset or velocity (using a line rest frequency from the
Frequency 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 LO Page will refuse to display itself if the Synthesizer setting option
on the Frequency 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
Bandwidth (Bw)
The total bandwidth in MHz for each IF is specified in the appropriate
Choice.
The bandwidth may also be specified on 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
Frequency 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
Crontrols 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.