stability with the new Pulsecal generator at Pie Town D.S.Bagri 1992Sep22

Transcription

1 VLBA TEST MEM N Phasecal phase 39 stability with the new Pulsecal generatr at Pie Twn D.S.Bagri 1992Sep22 The bserving setup fr the phase stability tests (n 1992SEP01) is shwn in the schemetic diagram (figure 1). The bservatins were made at frequency bands S/X with fexfer in split mde and the antenna was stwed till atleast 1500 UT. The RF frequencies f the 10 khz tnes in varius baseband cnverters, and their phases (in terms f phase f the injected rf signal, first lcal scillatr, and BBC L) are als given in figure 1. Ntice the dependence f the BBC utput phases n the RF signal, first L and the BBC L signals fr varius BBCs. Fr BBCs 1,2,3 it is rf phase minus first L phase minus BBC L phase; fr BBCs 4,5,6 it is rf phase minus first L phase plus BBC L phase; fr BBCs 7,8 it is minus rf phase plus L phase minus BBC L phase. ur bjective was t find ut magnitude f phase variatins f different signals and causes f their variatins by cmparing dependence n the RF, first L, and BBC L signals and their frequencies. We assume that t first rder all rf sdgnals are effected by same delay, and similarly all three 2-16 GHz synthesizers, and 5 MHz reference fr all BBCs are affected same amunt. DATA: 1 Figure 2 shws the length variatins with time f the 500 MHz reference cable between the L Transmitter in C-rack and the L Receiver in the B-rack. 2. Figures 3 and 4 shw temperature with time mnitred by varius temperature mnitrs in the antenna and the statin building. Unfrtunately the ambient temperature mnitr (Address #411B) was nt wrking during this perid. 3. Figure 5 shws phasecal phase f the 10 khz tnes in all the BBCs. Frequency f the RF signal, first L and the BBC L fr each 10 khz BBC signal are marked n the phasecal phases in the figure. DISCUSSINS: Building temperature plts in figure 3 shw peridic temperature variatins f abut ne hur peridicity with p-p temperature variatins f abut 6-7 deg C fr the Cntemp (air cnditiner unit) air utput. These variatins are damped ut t abut 3-4 deg C in C-rack, ~2 deg C in DAR, and shw <2 deg C variatins in the electrnics and cntrl rm temperatures. The cable length variatins (cablecal) in figure 2 dnt shw any nticable ripple with peridicity f abut 1 hur. This indicates that the length changes f the.141 semirigid cable in the C-rack plus the Heliax cable under the flr level due t the temperature cycling are nt significant (at a few psec), and the assumptin f the temperature cefficient f the standard.141 semirigid cable in the VLBA Electrnics Mem. #134 are t high. The BBC utput phases in figure 5 shw a lt f features. There are a number f ccasins like epch tl,t2 etc. marked in the figure when the phases shw jumps r rapid variatins. Als there is abut ne hur peridicity in the phases, which has larger p-p variatins with higher BBC L frequency and ppsite sign fr BBCs 1,2,3,7,and 8 than BBCs 4,5 and 6. This indicates that it is due t the BBC L and is prbably due t variatins in the 5 MHz reference. BBC7 phase has a lt mre jumps than any ther BBC phase. This is prbably due t input cnnectins t the BBC7. Phase jumps at epchs tl, t4, and tl seem t be prprtinal t rf/first L frequency, and are nt seen in the cablecal and therefre are (prbably) due t phasecal rf signal. Phase jump arund t8 seems t be first L dependent. By summing r taking difference f the utput phases frm different BBCs we can see phase stability dependence n RF signal, first L and BBC L frequencies. This gives an idea abut the phase stability f the rf (injected) pulsecal, first L, and BBC L signals. In turn we can estimate stability f the 500 MHz reference, the 5 MHz BBC L reference. We have cnsidered the fllwing cmbinatins f phases:

2 Figure Trace 6 1 BBC3-BBC1 2 BBC6-BBC4 3 BBC8-BBC7 4 BBC4-BBC1 6 BBC5-BBC2 5 BBC4+BBC1 7 BBC5+BBC2 l 8 Phase cmbinatin Dependence <p9f2$v ~ fa I**** (p#f " 17 (ptf " 4>U The phase dependence n the RF frequency difference/ first L frequency difference, and BBC L frequency differences are marked n the traces in figures 6-8. Cmpare traces 1,2, and 3 in figure 6. We wuld have expected that trace 1 phase variatin will be prprtinal t 480 MHz f rf phasecal generatr minus prprtinal t 480 MHz f 5 MHz BBC L reference. Similarly we wuld have expected the phase variatins f trace 2 t be just thse f trace 1 with sign reversal, and the trace 3 wuld have variatins f abut.8 cmpared t that f the trace 1. Hwever amplitude f 1 hur ripple in trace 2 is almst 1.5 times that in trace 1, and the amplitude f the ripple in trace 3 is less than half f that in the trace 1. This shw that all the BBC phases dnt vary in the same way. We reach at the same cnclusin by seeing ne hur peridicity ripple in traces 5 and 7 in figure 8. Frm a cmparisin f varius traces in figures 6,7 and 8 we can mdel delay variatins f 500 MHz, and 5 MHz reference signals. A crude (mdel) estimate gives variatins f abut 25 psec (p-p) fr 5 MHz reference, and abut 15 psec (p-p) with ppsite sign fr the 500 MHz reference signal. In figure 8 there is a large phase drp f mre than 35 degrees arund 11.5 UT in traces 5 and 7. The slpes fr the drp in the tw traces are nt excatly same. It is nt clear what is causing this, thugh it is likely t be due t mre than ne reasn. It seems t be RF r first L dependent, and smething in the vertex rm is likely cause. CNCLUSIN: It seems that phase variatins are nt smth and there are a number f phase glitches f upt abut 10 degrees. Als utput f all BBCs dnt get effected by same amunt (f phase/dely). The phasecal system is ding its intended jb f measuring phase stability f the receiving system, and very rughly there seems t be abut 15 psec p-p delay variatins f the 500 MHz reference. It is likely t be due t 100 MHz reference frm MASER and the L Transmitter. This needs t be minimised. There is als sme indicatin that phase f either first L r pulsecal rf signal (in vertex rm) changes, but this needs t be varified and its extent determined.-

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