VLBI System Documentation

Mark IV Field System

Mark III Hardware Setup

N. R. Vandenberg and B. E. Corey

NVI, Inc./GSFC and Haystack Observatory

Operations Manual

NASA/Goddard Space Flight Center Version 8.2

Space Geodesy Project September 1, 1993

• 1.0 Introduction
• 2.0 I/O Connections
• 3.0 Powering Up the Racks
• 4.0 Setting the Formatter Clock
• 5.0 Computer Communications
• 5.1 Reset the MATs
• 5.2 Set Field System Time
• 6.0 I.F. Distributor
• 7.0 Video Converters
• 8.0 Formatter
• 9.0 Tape Drive
• 9.1 Loading a Tape
• 9.2 Remote Operation
• 9.3 Manual Operation
• 9.4 Reproduce Capability
• 9.5 High Density Head Calibration
• 9.6 Head Location Chart
• 9.7 Recording Check
• 10.0 Decoder
• 11.0 Data Buffer

This manual describes briefly how to set up the Mark III VLBI system. Further details of the performance specifications, theory of operation, wiring of the individual modules, etc. may be found in the Mark III VLBI System documentation, Volume 1 (the "Blue Books").

Refer to the companion manuals which cover system setup and tests: the VLBA Hardware Setup manual and the System Setup and Tests manual.

Portions of this manual were originally written by Brian Corey.

This section lists the I/O connections to be made between the front end, Mark III rack, tape recorder, and control computer.

Most input connections to the Mark III are made on the I/O panel at the bottom rear of the acquisition rack. The external connections listed in the following table must be made there.

I/O panel connection	Notes
IF1	100-500 MHz IF from receiver, normally X-band
IF2	100-500 MHz IF from receiver, normally S-band
5 MHz input	From maser, power level between +12 and +14 dBm (+13 dBm = 1 V rms into 50 ohms)
1 pps	terminated with 50 ohms in formatter (see note below)
5 MHz output	5 MHz to receiver
5 MHz output	5 MHz to recorder controller. The input BNC jack is on the rear edge of the recorder controller board.
TTYS (slave) output	ribbon cable to tape drive
Receiver communications	ribbon cable to receiver MAT
Met sensor comunications	ribbon cable to met sensor box

The 1 pps input signal should be a fast (less than 30 nanosecond rise time), positive voltage pulse with amplitude between +2 and +5 volts. (Signals with different characteristics, e.g., larger amplitude or negative voltage, may be accommodated by changing the connections and resistor values on a carrier in the formatter.) If the pulse rise time is greater than about 30 nanoseconds, it may be necessary to sync on the leading edge of the pulse to guarantee noise immunity. (See section on formatter.)

Connections other than via the I/O panel are also made to the points in the Mark III system listed in the table below.

Signal	Location	Notes
5 MHz	front of 5 MHz distributor	2 buffered outputs available
Cable calibration	back of delay calibrator	Type N connector for the antenna unit of the cable calibration system
Data from formatter	J11, J12	2 ribbon cables to I/O board on tape drive
Alternate IF	rear of IF distributor	alternate inputs for IF1 and IF2 available

Before powering up the racks, first make sure you have connected the station 5 MHz and 1 pps on the I/O panel at the bottom rear of the acquisition rack and that the 5 MHz has been connected to the tape drive. Turn on power in the following sequence:

1. Turn the STBY POWER switch on power module 1 to normal and turn on the +15 STBY circuit breaker. The 5 MHz distributor and formatter should start running on battery power.

2. If it is not connected, plug in the electronics rack power cord and turn on the LINE POWER switch on power module 1.

3. Turn on both power supplies and observe green LEDs as an indication that each supply is working. Also check voltages with a digital voltmeter. They should fall in the voltage ranges given below:

Voltage Tolerance

+24 24.00.3 v

+15 15.00.2 v

+15 STBY 15.00.2 v

+5.2 5.00.1 v

+5 5.00.1 v

-5.2 -5.20.1 v

-12 -12.00.2 v

4. Turn on AC power to the tape drive. There are two power switches: a master circuit breaker at the bottom front of the drive, and a pushbutton at the bottom of control panel. If all the lights on the tape drive control panel light up and stay lit after the power is turned on, it's likely that the drive is not getting 5 MHz. Turning the power off and then back on within a couple of seconds may also cause all the lights to come on and the recorder controller logic to get hung up. If that happens, just power down, wait approximately 5 seconds, and then power up. Footage counter should be 0000.

To synchronize and set the formatter clock follow these steps:

1. Set the SYNC switch on the rear of the formatter to + or -, depending on which edge of the 1 pps pulse is to serve as trigger.

2. Move the SET-RUN switch on the formatter front panel to SET.

3. Hit the 1 pps SYNC button on the front panel. Decimal points should appear at all positions on the LCD clock readout. When the clock achieves sync with the 1 pps tick, the decimal points should disappear.

4. Set the last two digits of the year, day number, and UT time on the clock by holding down the appropriate buttons and waiting for the digits to cycle through to the desired values.

5. During the second of the time set on the clock, move the SET-RUN switch to RUN.

The SYNC TEST LEDs indicate whether the formatter clock is synchronized with the station 1 pps tick. First make sure the formatter REM/LCL switch is set to LCL. The synch test is enabled manually by pressing the SYNC TEST pushbutton until the OFF LED goes out (either the PASS or FAIL LED should then be lit). A rotary switch on the rear of the formatter sets the time interval within which the formatter and station 1 pps ticks must occur for the sync test to pass. Disconnecting the station 1 pps signal from the Mark III rack has no effect on the operation of the system other than to guarantee failure of the sync test.

After the racks have been powered up, you will need to initialize communications with the computer via the Field System. Refer to the Operators' Reference manual for instructions on how to start the computer and the Field System.

5.1 Reset the MATs

If you are starting the system "cold", i.e. you have just powered on the racks, then the MATs are in a reset condition waiting for some bits. The yellow "pend" lights should all be on. First check that all of the modules are set to REMOTE. To make sure of the reset condition, press down on the spring-loaded RESET switch on the TTY distributor. This forces hardware reset for the MATs if the modules are in remote. Then type the command:

reset=9600

This will sync up the MATs, and the PENDing lights should go out. If they do not, repeat the hard reset and try again. If the PENDing lights still do not go out, then there is no MAT communication. Check to see if the cables are connected correctly (section 2.0). Also check that the MAT device port as specified in the dev.ctl file (see Control Files manual) corresponds to the cable that is connected to the MATs.

5.2 Set Field System Time

If the MATs were synced in the last step, and the formatter clock has been set to the correct time (see section 4.0 of this manual), the Field System time can now be set using program setcl. The program first reads the formatter clock to obtain the correct time. The computer clock is not actually changed, rather the Field System maintains an offset between the formatter clock and the computer clock. The time that is logged for commands and responses and monit's time reflect the Field System's clock as it was last synchronized with the formatter clock.

After setting the formatter time, you must set the Field System time by issuing the SNAP command sy=setcl offset. Refer to the manual on setcl and the description of the time.ctl control file in the Control Files manual for a detailed explanation.

The IF distributor provides two totally independent channels (IF1 and IF2) for distribution of IF signals to the video converters. Each channel includes a switch for selecting one of two IF inputs, a step attenuator, a power divider for splitting the IF into two different frequency bands (upper and lower subchannels), bandpass filters, and eight-way power dividers to send the IF signals to the converters. The 1 dB ranges of the subchannels are 90-224 MHz for the lower subchannel and 216-504 MHz for the upper.

The front-panel switches labelled 1 and 2 are used to select the input IF signals for channels IF1 and IF2. With switch 1 in NORMAL position, IF1 comes from the connector labelled IF1 on the bottom rear I/O panel. With switch 1 in ALTERNATE position, IF1 comes from the SMA connector labelled ALT. in the upper left corner of the IF distributor rear panel. The NORMAL and ALTERNATE IF2 inputs are the connector labelled IF2 on the I/O panel and the SMA connector in the lower left corner of the IF distributor rear panel, respectively.

Monitor jacks for IF1 and IF2 are on the distributor rear panel. The two IF's may be made the same by running jumper cables from the monitor ports to the IF alternate inputs and then selecting the NORMAL or ALTERNATE inputs with the front-panel switches. For example, if the IF1 monitor output is connected to the IF2 alternate input, and if switches 1 and 2 are set to NORMAL and ALTERNATE, respectively, then channels IF1 and IF2 will both be connected to the input IF1 on the I/O panel. With an 18 dB attenuator included in the jumper cable and the IF2 front-panel attenuators all turned off, then the power in the two IF channels will be the same.

The IF signals are routed to the video converters via the patch panel in the rear of the IF distributor. The SMA connectors in the top row provide the IF1 outputs, the connectors in the bottom row give the IF2 outputs, and the connectors in the middle row are the inputs to the fourteen video converters. Note that each IF output is split into the two different subchannels. The two subchannels are referred to as "low" and "high", with the split at approximately 220 MHz.

The IF attenuators should be set so that the video power (USB or LSB) reading of the video converter front-panel meter is approximately 1 volt (see section on video converters). The 10 dB attenuators in the converters provide gain adjustment to compensate for slope in the IF bandpass. The 10 dB attenuators should normally be on. Only if the video power reading falls below 0.1 volt should an attenuator be taken out. If the slope across the IF passband is so steep that the 10 dB attenuators do not provide sufficient gain adjustment to get roughly equal power in each video, then an external cable compensation filter must be used to flatten the IF passband.

It is probably easiest to get the levels set using manual control and then set up the appropriate parameters using the IFD command. For example:

IFD=8,32,NOR,ALT

would set IF1 up with 8 dB attenuation and NORmal input, IF2 with 32 dB and ALTernate input. Refer to the SNAP Commands manual for more details on this command.

Note that the A/D which reads the IF distributor levels saturates at about half scale on the analog meter. For the TSYS command to work properly for IF1 and IF2 you must keep the meter below half scale with the cal on.

For wide-band operations, an IF3 module is required. The standard wide-band configuration has four video converters (5 through 8) patched to IF3; these are the four highest frequencies.

Refer to the System Setup manual for patching for the standard frequency sequences.

There are normally fifteen video converters in the acquisition terminal. The fifteenth converter is used as a spare and test signal generator. Each converter's address (as used by the formatter in assigning video signals to record channels) is hard-wired into the connector at the back of the VLA bin. That is, the address depends on the location of the converter in the rack. As viewed from the front, the converters are numbered in the following manner:

01	02	03	04	05	06
07	08	09	10	11	12
			15	13	14

For computer control of the modules, use the SNAP command vcn, e.g.

vc01=123.45

will set the first video converter to a frequency of 123.45 MHz. The other settings are defaulted to 2 MHz bandwidth and attenuators set to 10 dB. Whenever the REMOTE/LOCAL switch is changed from REMOTE to LOCAL, the settings are reset to values of zero frequency and external filter. Whenever the switch is changed from LOCAL to REMOTE, the previous frequency which was set remotely is restored.

In manual operation, the LO frequency and bandwidth are set in each video converter by means of the front-panel pushbuttons. Turning the front-panel rotary switch to LO FREQ allows one to set the LO frequency (as read on the panel meter in units of MHz) with the top row of five pushbuttons. Permissible LO frequencies are between 99.99 and 499.99 MHz. A front-panel BNC jack provides a monitor of the LO signal.

All the video converters contain 2 MHz lowpass filters in the USB and LSB video channels. In addition, some video converters contain 0.125, 0.25, 0.5, and 1.0 MHz lowpass filters in the upper sideband channels, which may be selected via the front-panel pushbuttons. These units are indicated by a small black dot near the top of the front panel. Provision is made for using external lowpass video filters through the SMA jacks on the rear panels of the converters.

The USB and LSB video signals may be viewed on a scope with the BNC monitor jacks on the front panel. The level is 10 dB below that of the signal going to the formatter.

The front-panel meter may be used to monitor the USB video, LSB video, IF, and LO powers as well as the LO frequency. CAUTION: The front-panel meter monitors the quantity selected with the front-panel rotary switch. This meter is independent of the readings which can be accessed with the computer.

In normal operation the front-panel meter should read:

IF level less than 2.0

LO level between 0.50 and 2.00

USB level between 0.1 and 1.9 (adjust IF attenuators)

LSB level between 0.1 and 1.9 (adjust IF attenuators)

LO frequency between 99.99 and 499.99 MHz

Note that the USB and LSB levels should be under approximately 1.9 with the cal on so that the total power integrator does not overflow when the tsys command executes.

In normal operation the condition of the LED's should be:

2 MHz green LED on

LO UNLOCK red LED off

ALARM red LED off

10 dB attenuator green LED's on

The additional 10 dB gain provided by turning off the attenuator is normally used only when operating near the edge of the front-end receiver bandpass. The alarm light latches on whenever the LO is unlocked and the video converter is in remote; this light can then only be turned off under computer control.

The formatter clips and samples the 28 video signals and formats them with time code, parity bits, and sync words. The formatter also contains the system clock, which runs off the 5 MHz reference.

The sample rate, mode, and input data are selected via the front-panel pushbuttons.

**Note that the sample rate is in megabits/sec, NOT MHz. Thus, if the video converter bandwidth is set at 2 MHz, the formatter sample rate should be set at 4 Mbps.**

The formatter allocates the video inputs to the various record channels in four different modes. These are described in Section 4.0 of the Mark III VLBI System Description manual in the "blue books". Modes A and C are used most frequently for continuum VLBI observations, and mode D for spectroscopic work. Mode C uses 14 video converters and records 14 USB signals. Mode B uses 7 video converters and records 7 USB and 7 LSB signals. Mode A is used when all 28 tracks are being recorded simultaneously, and modes B & C are used when 14 tracks at a time are recorded (14 in forward, 14 in reverse). Note, however, that selection of the mode on the formatter is a separate operation from choosing which tracks are being recorded. The tracks to be recorded are selected via the tape drive control (see section on tape drive).

The standard procedure when using mode A is to record all 28 tracks simultaneously in a forward pass on the tape drive, and then rewind. The standard procedure in Mode B and C is to record the 14 odd-numbered tracks in the forward direction and the 14 even-numbered tracks in reverse.

In normal operation the condition of the formatter LED's should be:

4 Mbps sample rate green LED on

Mode A, B, C, or D green LED on

VIDEO or NORMAL data input green LED on

PASS green LED of sync test on

All other LED's off

There is a rotary switch on the rear of the module which affects the sync test. Turning the switch to the right (CCW towards FINE) makes the time interval for the test smaller, to the left (CW towards COARSE) makes the time interval larger. If the sync test fails, move the switch towards COARSEr settings.

Use the form command, described in the SNAP Commands Manual to set up the formatter. For example,

form=C,4

sets up mode C, 4 Mbit sample rate.

WARNING: There is NO ERASE HEAD in the tape drive. Before reusing a recorded tape, it must be degaussed. If recording in forward and reverse directions on the same tape, be sure to change the record track selection before recording in reverse, or else you will get no usable data from either the forward or the reverse pass.

9.1 Loading a Tape

Instructions for loading a tape on the tape drive are found in the Experiment Operations manual. High density tapes need to be "prepassed" as described in that manual.

The tape drive should be cleaned after every tape. Clean the capstan and idler rollers and the vacuum column windows as well as the record and reproduce heads. Use only alcohol as a cleaning fluid.

9.2 Remote Operation

Remote operation of the tape drive requires the following steps. Refer to the SNAP Commands manual for details on the commands used in this section.

1. Make sure the REMOTE button is depressed.

2. Use the tape command to turn on or off the low tape sensor and reset the footage counter if desired: tape=low,reset.

3. Use the enable command to enable tracks: enable=g1,g3. This would enable groups 1 and 3.

4. Use the st command to start the tape moving: st=for,120. This would start the tape moving forward at 120 ips.

5. Use the et command to stop the tape: et. This stops the tape.

6. The rw command rewinds the tape: rw.

In some cases it might be necessary to coax the tape controller to pay attention to the commands you wish to send. It apparently wants many $$$ characters to be sent to it. To do this, use the command:

>mat=#94=$$$$$$$$$$$$$ ...

for say 40 characters. The symptoms of this situation are that the tape drive communicates properly (i.e. commands are ACKnowledged), but no action is taken by the tape controller.

CAUTION: If the tape is unloaded remotely and it has run out of the vacuum columns, the controller may be left in a curious state. It may happen that any command (such as enable or repro) to the tape drive, after a fresh tape is loaded, will cause the tape to start moving. To avoid this, issue the et command to clean up the controller before issuing any other command.

9.3 Manual Operation

Manual operation of the tape drive is described in the following steps.

1. Select the tape speed. For the standard 2 MHz per track recording, the 120 ips button should be depressed (the actual tape speed is 135 ips).

2. Select the tracks to be recorded. This is done with the GAE, REC, and R/A pushbuttons at the bottom of the tape drive control panel. These buttons can be a bit hard to push.

a) The REC button turns on and off the various tape tracks. The green LED's show which tracks are turned on. When REC is pressed, those tracks for which the lefthand yellow LED's are lit will change state, i.e., tracks turned on will be turned off, and tracks turned off will be turned on.

b) The GAE and R/A buttons are used to turn on and off the yellow LED's in the lefthand column. The R/A button controls the LED's within each group of 7 tracks, and the GAE button advances control from one group to the next.

3. Press the FORWARD and the RECORD buttons simultaneously. The tape drive should start up, and the TACH light should come on after a few seconds. Data is being recorded only if the RECORD light is lit.

4. If the LOW TAPE button is depressed, the tape will stop after about 13.5 minutes. If the LOW TAPE button is not depressed, the tape will go off the end of the reel. The tape may also be stopped at any time with the STOP button.

5. The procedure for recording in reverse is the same as in steps 2 and 3, except that the REVERSE and RECORD buttons are depressed to initiate recording.

6. A tape may be rewound at 270 ips (no recording) by pressing the 240 ips button and then the REVERSE button. If fast rewind is done, the LOW TAPE button should be depressed. Otherwise the tape will fly off the end of the takeup reel too fast and damage the tape end and possibly the heads. Also, the brakes will slam on, locking the hub, while the tape will unwind all over the place. NOTE: As of late 1981, all recorders should have been modified such that the FAST button is effectively disabled and 270 ips is the fastest tape speed.

9.4 Reproduce Capability

The reproduce head is located below the record head, so the reproduce head can perform a read-after-write function only when a tape is being recorded in the forward direction. The decoder can be used to look at the signal from two tape tracks simultaneously. The two tracks are selected remotely with the repro command:

repro=byp,1,3

would set up read-after-write mode for track 1 in channel A of the decoder and track 3 in channel B. Manually, the two tracks are selected with the GAE, R/A, GBE, and B pushbuttons. Channel A of the decoder looks at the track for which the yellow LED in the lefthand column is lit, and channel B looks at the track for which the yellow LED in the righthand column is lit. Buttons GAE and R/A control the lefthand LED's, and buttons GBE and B control the righthand LED's.

9.5 High Density Head Calibration

The high density read and write heads must be calibrated for proper operation of the Mark III. Refer to the Narrow Track Calibration manual for procedures.

9.6 Head Location Chart

Refer to the chart depicting the head locations for Mark IIIA passes in the System Setup manual.

9.7 Recording Check

A recommended check of head motion and recording capability is to use the commands that will be invoked during a normal schedule to make sure data is being recorded on tape properly. Use the following series of commands:

sx2c1=1 (sets heads to position for pass 1)

st=for,120 (record pass 1, forward)

check2c1 (check parity errors)

sx2c2=2 (sets heads to position for pass 2)

st=rev,120 (record pass 2, reverse)

check2c2 (check parity errors)

The decoder analyzes the read-after-write output from two tape tracks. Monitor jacks are provided on the front panel for viewing various signals (data, clock, sync, etc.). An LED display shows the decoded time code, auxiliary data, sync error counts, and parity error counts in the two channels (channel A top, channel B bottom).

With the righthand rotary switch turned to TIME, the LED display shows the BCD time written in the fourth and fifth words of each frame (see drawing 6056 in the Mark III System Description manual). When the write head is positioned by the Field System, the first 12 characters in the AUX data field are automatically filled with tape format information. Characters 1 through 4 of the AUX data may be specified with the FORM command. Characters 5-7 are the pass number, with leading blanks. Character 8 is the sign for the head offset that follows. If the offset is positive, then the sign is blank. If the offset is negative, then the sign appears as a funny character that looks like an underlined "c". Characters 13-16 of the AUX data are controlled by the formatter itself and are not accessible to the Field System. With the switch turned to AUX, the display shows the contents of the two auxiliary data words at the beginning of each frame. The last two bytes in the second auxiliary data word are used. They give the sample rate selected and the hardwired system number (see drawing 6056).

The auxilliary data display shows the fields that have been written onto the tape. Refer to the description of the pass command for the details.

With the righthand rotary switch turned to ERRORS, the lefthand numbers in the LED display show the accumulated number of frame sync errors, and the righthand numbers show the parity error rate. With the P.E.R. toggle switch toward BYTEx1/1000000, the parity error number displayed is the number of parity errors per million bytes (9999 is overflow). With the P.E.R. switch toward FRAMEx1/400, the number is displayed is the number of frames out of each 400 that contain at least one parity error.

With the tape moving, the perr command will report the error rate on the requested track:

perr=track-number

perr

The number of sync errors should be, at most, a few per tape. (The display may show a non zero number after switching from one tape track to another. Spurious numbers may be cleared with the RESET button.) With a good set of tape heads and good tapes, the parity error count per million bytes should be in the range 0-20. Anything below a few hundred errors per million bytes is acceptable. Higher parity error rates, especially when several tracks show counts in the thousands, and/or many sync errors per tape are cause for worry. It may be necessary to degauss the tape heads.

The signals going from the formatter to the tape drive may be viewed directly (without going through the tape drive and recording a tape) with the decoder by using the two bypass ribbon cables on the rear of the formatter. Substituting these for the formatter-to-recorder ribbon cables in J11 and J12 sends the clock and data signals intended for track #1 directly to decoder channel A. The track #2 signals go to channel B. To set up bypass mode remotely use the command:

repro=byp,A-track,B-track

The data buffer built into the decoder is used to store up to one million data bits from the tape track going to decoder channel A. These bits may be read out at a later time for transmission via modem and phone line to a central processing station as part of a real-time fringe test. The data buffer is also used to store real time data from the tracks being recorded which is then processed by the Field System program pcalr to check for the phase cal signal.

The controls and status lights for the data buffer are on the rear panel of the decoder. To store data in the buffer manually:

1. Press the RESET button to clear the data buffer. This action should turn on the IDLE LED.

2. Data storing can begin 0, 10, 20, 30, 40, or 50 seconds after the minute. To store data, press the ARM button during the 10-second interval before data taking is to begin. The ARM LED should light.

3. While data are being taken, the TAKE LED should light. When done, the HOLD light goes on. The data will be held ready for transmission until the buffer is reset.

The INT-EXT toggle switch should always be in the INT position.

The ERROR light indicates a transmission error, e.g., undecipherable computer command to the buffer. It may normally be ignored unless it appears during computer communication.

There is no external switch to set the baud rate for the data buffer. This is done with a jumper inside the decoder module. If the data buffer appears not to be working, it could be that the Field System control file which specifies the baud rate (dev.ctl) does not correspond to the actual baud rate set in the data buffer.

To use the data buffer to check phase cal in all tracks, use the pcalr program.