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1. Introduction

A VLA antenna primary surface points with two axes of movement, azimuth and elevation. The antenna uses position encoders to convert the angle of each axis into digital position information. Both of these encoders must be functional and accurate for the antenna to be usable.

The encoders being used now are the original encoders built during the construction of the VLA ^[1]. They are based on rotary transformers known as synchros. Each encoder contains two synchros, coarse and fine.

The coarse synchro is a standard size eight synchro transmitter. The fine synchro is a multi speed resolver manufactured by Farrand Controls and is called an Inductosyn. This synchro outputs a signal that repeats 256 times per revolution. See Figure 1.

Figure 1 - Original Encoder Layout

Each of the two synchros supply some of the bits for the digital position. The existing data converter overlaps the most significant bit of the fine synchro with the least significant bit of the coarse synchro using a combination algorithm. This system yields 20 bits of position information which is about 1.2 arcseconds of resolution. See Table 1.

Table 1 - VLA Position Bits

bit	degrees	Min	Sec	c	f
20	180.0000			X
19	90.0000			X
18	45.0000			X
17	22.5000			X
16	11.2500			X
15	5.6250			X
14	2.8125			X
13	1.4062			X
12	0.7031	42.2		X	X
11	0.3516	21.1			X
10	0.1758	10.5			X
9	0.0879	5.3			X
8	0.0439	2.6			X
7	0.0220	1.3			X
6	0.0110	0.7	39.6		X
5	0.0055	0.3	19.8		X
4	0.0027	0.2	9.9		X
3	0.0014	0.1	4.9		X
2	0.0007	0.0	2.5		X
1	0.0003	0.0	1.2		X

Dusty Clark of the MMTO ^[2] used a monolithic integrated circuit to perform the conversion. Ron Weimer successfully adapted the new circuit and demonstrated that it could be used at the VLA. The new design is summarized in Figure 2.

Figure 2 - New Encoder Layout

The synchro design is inherently reliable and insensitive to environmental conditions such as temperature, dirt, and vibration. However, the conversion electronics which read the synchros are now at the end of their service life. They have become unreliable and they are expensive and time-consuming to fix.

We replaced the electronics and used the original encoder housing and synchros to reduce the cost to a small fraction of that of entirely new encoders.

We have successfully retrofitted two prototype encoders and have installed them on a VLA antenna. We hope to finalize the design and retrofit all VLA antennas with the new encoders. These encoders are also suitable for use on the VLBA antennas.

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