VLBA Electronics Memo No.^n

National Radio Astronomy Observatory

VLB Array Program

VLB Array Electronics Memo No. l̂ fO

VLBA Standard Interface Board Retrofit

Ron Weimer March 1992

Ref: Tech Report No. 12 VLBA STANDARD INTERFACE MANUAL, & EDTN 159

Problem:

Some time ago I wrote Electronics Division Technical Note No. 159 describing a couple of problems with the Standard Interface Board (SIB). One problem described was analog address selection problems. This can be avoided by careful design of the interface controlling the SIB. The second problem described was one of the loading of analog monitor points by the Burr Brown INA 101 instrumentation op amp on the SIB. The loading occurred when the A/D was reading a positive voltage right after reading a negative voltage. During the slew from negative to positive, the input impedance of the INA 101 dropped from 10 gigaohms to around 1.6 K ohm. This low impedance could drain charge off of filter caps in monitor circuits. Depending on the source impedance of the monitor circuit, its voltage might not be recharged completely by the time the A/D converter sampled the voltage (around 200 ps).

Problem Fix:

Since most of the VLBA modules are already designed and in the field it did not seem practical to try to correct the SIB problem by redes igning the monitor circuits. We looked 'at a number of op amp suppliers and could not find a pin compatible replacement amp. Most new op amps do not come in the 10 pin metal can package that was used in the SIB. The Analog Device/PMI model AMP-02-EP looked like it would be electrically compatible.

Testing:

Some tests were run to determine if the AMP-02 would reduce the loading problem. The results are in Table 1. The tests were run using the weather station since it was easy to modify. Test 1 used the existing ±15 volt monitor points on the weather station. This gave a -7.5v reading followed by a +7.5v reading. The voltage divider gave a source impedance of 5K ohm. The voltage test points went through a MUX 08 and then to channel 1

of the SIB. Capacitors were added to the voltage dividers to simulate different monitor point time constants. A value of O.Olvif gave the same T(T = 50 microseconds) as the L104 test points that were first noticed with the loading problem. Test 2 was similar except that voltage dividers were connected directly to channels 3 and 4 of the SIB. This was done to see the effect of the MUX 08 in test 1. There was no significant difference in the results. Finally voltage dividers with a source impedance of 10 K ohms were connected to channel 3 and 4 and used in test 3. This simulated the L104 conditions even closer. A cap of 0.05 yif gave the T = 50 microseconds condition. Figures 1 through 6 show some scope traces taken during the above tests. A probe was connected across the cap and the traces show the discharge/charge voltages.

Test Results:

From the results shown in table 1 it is concluded that the AMP-02 is superior to the INA 101. Each code step of the A/D converter is 5 mv. The voltage offsets for the AMP-02 were always 3 code steps or less. With the L104 conditions the offset was typically 0 or 1 code steps. This compares to 100 to 780 mv offsets with the INA 101 op amp in use.

Implementation:

Since the AMP-02 is not pin compatible it was decided to design a circuit board which would plug into the 10 pin socket on the SIB and hold an AMP-02 in another 8 pin DIP socket. W. Zamora and W. Koski came up with the design and parts to implement this. Circuit board drawing is number B55002Q003. Pins to plug into the SIB socket were Augat part No. 8128-94P3. SIBs will be outfitted with the new amps, test and adjusted, and then sent in blocks to VLBA stations for retrofit into the system. Returns from the stations will then be retrofit etc.

Voltage Readi ng On Readblock Screen |

BB-INA 101 AMP AD/PMI-AMP-02

Voltage Difference Voltage Difference Conditions |

Volts Millivolts Volts Millivolts

Test 1 7.529 ref 7.529 ref 5K-no cap |

7.465 64 7.529 0 5K-.01 cap

7.099 430 7.514 15 5K-.047 cap

Test 2 7.524 ref 7.524 ref 5K-no cap

7.519 5 no read no read 5K-.005 cap

7.451 73 7.519 5 5K-.01 cap

7.016 508 7.514 10 5K-.047 cap I

Test 3 7.504 ref 7.509 ref lOK-no cap I

7.397 107 7.504 5 10K-.005 cap I

6.962 542 7.500 9 10K-.01 cap I 6.723 781 7.500 9 10K-.047 cap

1

Table 1

H* 00 c M (t»

iek Stopped: 13 Acquisitions i

R1

R3

—;—i—i \ 1 1 1 1 1 1 1 S —i—:—i—:— —i—i—i—i— —i—i—i—i— —i—i—i—i— —i—i—i—i— 1 1 1 1 1 1 1 1

•

-

- / iiii i 1 1 1 1 1 1 ! 1 f\ 1 1 I I I I i i y i I I I I I I I ! 1 I 1 \— \ ! | i i t i I I I I I I I I 1 f\ I 1 I I I I 1 1 jA I l l l l I I I I I I I I

- -

- -

1 1 1 L_ L—J 1 L....J

A —/ i—i—i— —I—i—i i —i i J i —i—i—i—J— —i i i i 1 1 l,:, 1 1. 1 1 ! U..L. 1— 1 ...

0 50 h w M 1 I I Ln Ln Aji D O C fc C CO (DBS

a. a o. H-H- H« < < < I

P o

• • rt © O (D js »—

CO O t—'

O 0) (D OJ T3 <

< O l-» rt to 09 (t>

3 o o •d

<-n H « (t> K CO H rt w O H-c H I o (6 W W

H B H TJ 3 (t> > Ou (D Hi 3 O O t-n>

Ret 3 -Ch 1 200mV Ref'l 1.00 V S.OOjuis

M 50.0MS Ch i \ 7.26 V

o pa M W M

Ul 3rf P P E CA fc fc to o 01 CO C

CL hj pu a- H* o H* H> < CD < <

t-n (-3 5* (6 K 0) H a

M M 9 • -O

0 O fl> 31 JN t—>- a ̂ 1 p>

^ 1 0 ' s O 0

: (0 0 NJ fl). T) (D

Ref 1 2.00 V 10.0WS Ref 2 A s f 3 1.00 v 50 jis

lek Stopped: 109 Acquisitions

R1

R 2

H» 0Q C * fl>

1-+

; j ! , 1 1 1 1 —i—r—i—i— 1 1 1 1 —i—i—i—r— — , — | — | — | — —1—I—I—I— ! 1 1 1— 1 1 1 1 —:—i—i—r—

-

A ^ - y ^ / -

"̂Wvwk.

! I 1 1 i /f i ! 1 K* 1 l i l t 1 1 1 1 I I I ! 1 1 1 1 l i l t l i l t 1 1 1 1 1 1 1 1 S*I / i yri

/

1 1 1 1 1 1 1 1 t i l l t i l l III t 1 1 II 1 1 1 1

-

-

- -

J L_.J 1 J i i a J 1 1 L_ L 1 1 —I—i—i i 1 1 1 1 —1—I—1 1 -1 L—t 1 J 1 1 1 -L—i. 1 1 Chi 20.0mV Ref 2 10.0 mv SO jis

Ref 1 1.00 V 1.00MS

M 50.0/us Ch1 \ 7.48 V

5 n jo u H ^ (t I p rt

In Ln f? CO

CO CO **>» Q.

Ou Ou H« S3" 5? . . p o o o I— o O 0) O IS -o 0) T3 T)

W NJ O 5 S 0 (D S

H H 1 > ft> s PL hd 03 I S O o ro (0

O JO >— u>

Ln cn

CO CO a a. H« H-< <3

5 i— H 2 ® n CO

Ln f f CO CO a. a H* H< < <

CO OJ o

o o -O t—' "Vj o

O P> ftt •O

C I o CXJ (t> Cd H 3 H 13 Z <D f> Ou P3 i—. 0 o O H-> (5

lek Stopped: 17 Acquisitions

R1

R 2

R 3

H* OQ C i-t (D ON

1 +

—:—i—i—i— —I 1 1 1 —i—l—I—i— —J 1 1 1— —i—i—i—i— i ii »— —i—r—i—i— —i—i—i—i— —i—i i i i i—i—i—

pScwmi

-

-

: • I ! I 1 IF 1

/" /

1 1 I I I I i i i * l l l l i i i t I I I I 1 1 1 1 1 \ Y \ J^t 1 1 I I I ! • I I r t i l l l l l l r I II l l l l i l l l

- -

^ 1 J L H -

1, ..1 -L_. j—i—i—j— 1 1 L—1 .—j—i—i—i 1 —J—J—i i i i i —i—i—i—i— 11 t.i i— —i i i i i i II _. Chi lO.OmV

npowfUHH h W K) H O (0

CO

ft rr

Ui Ui Ui m r

CO CO CO CO C ^

hu K pi pr o a < < < < fl> 3

M M • • • 3 B >

O O O O TJ s »— O (D "Tj

vj Ui O (X I O PI P) O

O o *o p rs> P> TJ {0 O t) (D

M 50.0MS Ch1 \ 7.46 V Refl 1.00 V 1 . 0 0 M S Ref. 2 = Ref 3 50 mv 50 jus