DCR Through The IF Rack

Figure 4: Log-Log plot of DCR counts versus various source's fluxes in Kelvins. The red line is determined from the DCR results of W3(OH) and 3C 84 + $1^\circ$ Dec for the Optical Driver 2 data. Note that the red line is not linear in power out versus power in since the power law slope is not one. The blue line is a ``$\chi$-by-eye'' fit to the same data.

Our first test was to see if the DCR exhibited a non-linear response when observing sources of varying strength. We took data at the positions listed in Table 1. The results are shown in Figure 4. As can be seen from Figure 4 the slope of the DCR counts vs. input power does not have a slope of one as would be expected for a constant gain. This indicates that the gain is indeed non-linear even for changes of a few Kelvin at the system temperature of even our best receivers ($\sim 20$ K). This also indicates that the non-linearity does not arise in the receivers since it is seen from both the X-band and the C-band receivers.

Table 1: Source Fluxes. The Off source fluxes were determined from the DCR Tool which does not take into account any non-linearity. An artificial error of 1 K is thus assumed for these values. A system temperature of 20 K was assumed for the on source $T_{ant}$ values.

Source Name	5.0 GHz Flux (Jy)	$T_{ant}=T_{src}+T_{sys}$ (K)
M1 (Crab Nebula)	$680 \pm 34 Jy$	$1360 \pm 69$
M1 + $1^\circ$ Dec	-	$20.4 \pm 1$
3C 123	$16.32 \pm 0.816$	$52.6 \pm 2.7$
3C 123 + $1^\circ$ Dec	-	$18.9 \pm 1$
3C 84	$18.0 \pm 0.9$	$56.0 \pm 2.8$
3C 84 + $1^\circ$ Dec	-	$19.8 \pm 1$
W3(OH)	$1.1 \pm 0.8$	$22.2 \pm 2.7$
W3(OH) + $1^\circ$ Dec	-	$20.6 \pm 1$
Orion A	$457 \pm 10$	$934 \pm 21$
3C 274 (Virgo A)	$72.1 \pm 3.6$	$164.2 \pm 8.2$
3C 274 + $1^\circ$ Dec	-	$27.4 \pm 1$

Table 2: Attenuation settings for optical drivers during TGBT01A_004_08 tests with the DCR signals coming from the IF Rack.

	IF Rack Attenuation
Scan	OD 2	OD 4	OD 6	OD 8
19	13	12	10	15
20	10	9	7	12
21	7	6	4	9
22	4	3	1	6
23	1	0	0	3

Another test that we did was to observe one source continually while we varied the attenuation in the IF Rack. This was done to see if the DCR responded linearly with the change in power coming through the IF Rack attenuators. We tracked 3C 274 (Virgo A) and balanced the IF Rack to 1 Volt in each of the RF power samplers for Optical Driver paths 2, 4, 6 and 8. We then added 3 dB to all IF Rack attenuators (bringing each RF power sampler down to about 0.5 Volts). We then fired the hi cal ($\sim 32$ K) and took data with the DCR being input from the IF Rack at several different attenuator settings which are listed in Table 2.

Figure 5: Gain ratios versus relative input power for the DCR signals coming from the IF Rack. $3\sigma$ errors are shown. The black line is the expected result for a linear system response.

The results are plotted in Figure 5. We have plotted the gain ratio from Equation 5 versus the relative change in input power (i.e. $dB(scan n) - dB(scan 19)$). As can be seen in Figure 5 the gain ratios are not one - indicating that the DCR response is non-linear with the change in attenuation setting. This suggests that the observed non-linearity in the system comes after the IF Rack attenuators.

We can also see from Figure 5 that we have sent the DCR into strong gain compression for the last sets of values. Further investigations are warranted to determine exactly when the DCR goes into gain suppression. This is critical when observing near very bright continuum sources.