Linearity of Torun BBCs


This report indicates that Tr BBC gain generally follow specifications of NRAO, though there are differences between individual BBCs. These undesirable differences are not alarmingly large, however. More troublesome are instabilities caused by RF interference as well as originating in our hardware.

Torun station (Tr), being a member of the EVN, regularly participates in VLBI observations. Earlier, upon general request of the EVN TOG, we have performed measurements of the linearity of our Base Band Converters (BBC) with the BBC gain fixed at certain value, i.e. without Automatic Gain Control (AGC). They were communicated during the Onsala TOG meeting (see this report or its copy here). Normally during observations, however, the AGC is switched on. In this report we present similar measurements but with active AGC. Since recent introduction of new calibration procedures has led to persistent problems of unknown origin that in principle could be related to our specific terminal (VLBA terminal manufactured by Penny+Giles), all checks of BBC properties are useful also in this respect.

The measurements were performed with the C-band receiver as the source of noise, the telscope looking in an arbitrary direction. This noise signal, prior to being fed to BBCs, inputs has been attenuated gradually using an external attenuator covering nominally the range from 0 to 23 dB with step of 1 dB. The BBC bandwidths (4 and 8 MHz, USB and LSB) and frequencies (601 to 781 MHz) were kept constant during the measurements. These measurements were performed in the beginning of July 2006. 1728 = 24×24×3 readouts (each consisting of the 18 channels) obtained with the FS tpgain command were collected during about 3-hour session. The results follow.



IF signal level as seen by the BBC at the 'c' and 'a' channels (RCP an LCP, respectively). All the 1728 measurements included. In most of the cases one irregular spot or grouping of 24 points corresponds to one value of the external attenuator. Note the many overflowed values (saturation of BBCs, value being set arbitrarily to 66666) in the c-channel seen at the figure top whereas the a-channel overflowed only during the very first step of attenuation (at 0 dB). As seen, one complete series of measurements has been repeated three times. During the second and third run some data are spoiled by RFI or instabilities in our system.



Part of the previous figure data presented so as to better expose the two clear discontinuities in the attenuation stepping after each 8 steps (nominal values of 7 and 8 dB as well as 15 and 16 dB result in nearly the same attenuation). Note that this is a property of our external attenuators, and not of the BBC attenuators.



Individual channel readings (response of the tpgain command). The legend to the right lists the BBC number and sideband (u and l stand for upper and lower sideband, respectively). This figure consists of 24×24×3×16 = 27648 points plotted. 


      real*8 function BBCdB(icount)

c Converts gain number, icount, to BBC gain in decibels, BBCdB

      real*8 dB(0:255), BBCdB
c Table of dB gain setting against gain number for BBCs, taken
c from NRAO code subroutine "module/bblvcode.c" (256 levels)
      data dB/
     &-99.99,-36.16,-30.14,-26.62,-24.12,-22.18,-20.60,-19.26,
     &-18.10,-17.08,-16.16,-15.34,-14.58,-13.89,-13.24,-12.64,
     &-12.08,-11.56,-11.06,-10.59,-10.14, -9.72, -9.32, -8.93,
     & -8.56, -8.21, -7.86, -7.54, -7.22, -6.92, -6.62, -6.34,
     & -6.06, -5.79, -5.53, -5.28, -5.04, -4.80, -4.57, -4.34,
     & -4.12, -3.91, -3.70, -3.49, -3.30, -3.10, -2.91, -2.72,
     & -2.54, -2.36, -2.18, -2.01, -1.84, -1.68, -1.52, -1.36,
     & -1.20, -1.05, -0.90, -0.75, -0.60, -0.46, -0.32, -0.18,
     & -0.04,  0.09,  0.23,  0.36,  0.49,  0.61,  0.74,  0.86,
     &  0.98,  1.10,  1.22,  1.34,  1.45,  1.57,  1.68,  1.79,
     &  1.90,  2.01,  2.11,  2.22,  2.32,  2.42,  2.53,  2.63,
     &  2.73,  2.82,  2.92,  3.02,  3.11,  3.21,  3.30,  3.39,
     &  3.48,  3.57,  3.66,  3.75,  3.84,  3.92,  4.01,  4.09,
     &  4.18,  4.26,  4.34,  4.42,  4.50,  4.58,  4.66,  4.74,
     &  4.82,  4.90,  4.97,  5.05,  5.12,  5.20,  5.27,  5.35,
     &  5.42,  5.49,  5.56,  5.63,  5.70,  5.77,  5.84,  5.91,
     &  5.98,  6.05,  6.11,  6.18,  6.25,  6.31,  6.38,  6.44,
     &  6.51,  6.57,  6.63,  6.70,  6.76,  6.82,  6.88,  6.94,
     &  7.00,  7.06,  7.12,  7.18,  7.24,  7.30,  7.36,  7.42,
     &  7.47,  7.53,  7.59,  7.64,  7.70,  7.75,  7.81,  7.86,
     &  7.92,  7.97,  8.03,  8.08,  8.13,  8.19,  8.24,  8.29,
     &  8.34,  8.39,  8.44,  8.50,  8.55,  8.60,  8.65,  8.70,
     &  8.75,  8.80,  8.84,  8.89,  8.94,  8.99,  9.04,  9.08,
     &  9.13,  9.18,  9.23,  9.27,  9.32,  9.37,  9.41,  9.46,
     &  9.50,  9.55,  9.59,  9.64,  9.68,  9.73,  9.77,  9.81,
     &  9.86,  9.90,  9.94,  9.99, 10.03, 10.07, 10.11, 10.16,
     & 10.20, 10.24, 10.28, 10.32, 10.36, 10.40, 10.44, 10.48,
     & 10.52, 10.56, 10.60, 10.64, 10.68, 10.72, 10.76, 10.80,
     & 10.84, 10.88, 10.92, 10.96, 10.99, 11.03, 11.07, 11.11,
     & 11.15, 11.18, 11.22, 11.26, 11.29, 11.33, 11.37, 11.40,
     & 11.44, 11.48, 11.51, 11.55, 11.58, 11.62, 11.65, 11.69,
     & 11.72, 11.76, 11.79, 11.83, 11.86, 11.90, 11.93, 11.97/
      icoun=max0(0,min0(255,icount))
      BBCdB=dB(icoun)
      end

FORTRAN code used to convert the FS readings of gain number
into BBC gain expressed in decibels.


Graphical presentation of the function implemented in the above BBCdB subroutine



Gain in all the 16 channels during the first of the three series' of measurements, that apparently was least affected by interference. Parallelism and approximate straightness (by sections) of the lines corresponding to different channels is a satisfactory evidence that our BBCs behaviour is close to the NRAO specifications. The BBC8 is a surprising exception. Here the discontinuity seen in the IF channels at 7/8 dB and 15/16 dB attenuation is somehow entirely compensated for! This behaviour of BBC8 is equally clearly evident also in data of the second series, and only partly in the third series, where strong interference badly spoil the picture (see the following figure).


Same as in previous figure but for BBC8 channels only and data from
the series 2 and 3 (points marked as BBC 8l/u 2 and 3, respectively).


— Kaz Borkowski & Greg Hrynek
Posted: August 31, 2006; last modif.: Sept. 6th