---

Intro:

• azimuth: The encoder rack gear runs around the inside of the platform ring girder. There are two azimuth encoders (azenc1, azenc2) that are mounted on the top of the azimuth arm and run in  the encoder rack gear. The encoders are fixed relative to the azimuth arm.  Azenc1 is mounted on the dome side of the azimuth arm, azenc2 is mounted on the carriage house side of the azimuth arm (next to the vertex shelter). The azimuth pointing uses azenc1. Azenc2  measures the bending in the azimuth arm.
• Gregorian: The elevation rack gear sits at the bottom of the azimuth arm. The gregorian encoder is mounted on the  gregorian trolleys and rides on the elevation rack gear. It is fixed relative to the dome.
• Carriage house: Same rack gear as the gregorian (but on the other side of the azimuth arm). Encoder mounted on the carriage house trolley.

---

History:

oct05:  az swings 327:

azimuth swings were done to look at the 327 Mhz birdies. About 7 birdies had the same  az dependence. At the time we did not know that these birdies were coming from the az,za encoders. This was determined later when the comb frequencies were measured with the new encoder. The 7 birdies from oct05 all sat at one of the comb frequencies.

02aug06: new azimuth encoder

The old encoders were getting water in them and failing from corrosion. A new waterproof encoder was purchased (same company). It had  a continuous metal case (that we thought would be better for rfi shielding).

sep06: 327 pulsar observations saw really bad interference.

03oct06: data was taken with the 327 Mhz receiver. The comb was seen with a spacing of 614 Khz.

04oct06: Using 327 Mhz, az spins and za motion showed that the birdie was not coming from inside the dome (since it had and az and za dependence).

04oct06: The 614 khz comb in seen in the 430 receiver while new encoder is installed. A2125 did 430 mapping (with the dome) on 04oct06 when the new encoder was installed and again on 05oct06 (night) when the old encoder was back in. The 614 Khz come was seen when the new encoder was in. It may or may not have been seen with the old encoder back in (some comb elements were there).

05oct06: We found that the comb was coming from the az2 encoder. We replaced the "new" az2 encoder with the "Old" encoder around 1pm.

05oct06: after replacing the new encoder with the old encoder, data was taken to see how strong the 614 Khz comb is with just the old encoders. Most of the comb elements disappeared. The ones that had been seen back in oct05 remained.

05oct06: In the lab

We saw that the new encoder had a 614 Khz comb. We opened the encoder casing and saw the comb go to > 1000 Mhz. At 327 Mhz the strongest comb element was at 326.269 Mhz. At 430 Mhz the strongest comb element was at 432.539 Mhz. When we closed the casing the 327 Mhz comb was strong (> 10 db). It was probably coming out of the cable.

We opened the case of the old encoder and saw  a few elements of the comb were present. The strongest was at 324.415.  When we closed the encoder, we could still see the strong birdies (though not as strong as the new encoder.

The new encoder has all the elements of the comb while the old encoder only has a few of these. There is an offset of about 4 khz between the birdies from the old encoder and the new encoder..

---

10oct06 strengths of birdies from old encoder measured in the lab

    The amplitudes have not been corrected for the response of the small loop antenna used. When searching for possible birdies caused by the encoders, start at the strongest values  in the table.

---

Summary:

• The 614 Khz comb is coming from the az,za encoders.
• The comb from the new encoder is about 1 to 3 khz wide.
• The comb in the new encoder (installed 02aug06) is a lot stronger than the comb in the old encoders. We can only see a few elements of the comb in the the older encoders.
• Back in oct 05 elements of the encoder comb were seen (but we didn't know they came from the encoders). We should verify that these are really coming from the old encoders by monitoring these frequencies while turning off all of the encoders.
• The new encoders comb is shifted about +5 Khz from the old encoder comb (as seen in the lab).
• In the lab, the strongest birdie from the new encoder is at 326.269 (327 band)  and at 432.539 (430 band). Using a log periodic antenna on the platform, the birdie was strongest at 324.408 (this is one of the strongest comb elements seen with the old encoders back in oct05).
• In the lab, the strongest birdies from the old encoder are at 324.415 (327 band). This is probably the same birdie as the 324.408. In the 430 band the strongest birdie is at 427.640. This was seen in the "oldEnd,newEnc" data taken at 430 Mhz.
• With the new encoders we've seen the comb at 327 and 430 Mhz using the telescope. With the old encoders we've seen it at 327 and maybe 430. In the lab the comb goes to > 1000 Mhz.
• The Az dependence of the various comb elements is repeatable at that frequency. Some comb elements show different azimuth dependence.
• The comb gets at least 10 db   stronger when the dome is above 17 degrees.
• at az=270 a peak is at 17.295 (ch at stow)
• the strength of the comb also changes when the dome sits still and the carriage house is moved from za=2 to za=20 degrees.  With the dome at za=17.295 the comb peaks at ch=stow and ch=10degree. This probably the scattering path from the ch encoder into the dome (or it could be the platform tilting and the scattering path from azenc2 into the dome changing).
• We have concentrated on the azimuth encoders, but there is probably birdies coming from the dome and ch encoders (although they are the "old" type).

home_~phil