RFI System Calibration

phil perillat
01 june 1995

Introduction

The system temperature for the RFI system was measured using noise diodes injected at the input to the RFI box. The noise figure meter was also used to measure the gain and noise figure for the entire RFI receiver box and for individual components of the system.

System layout.

This layout for the RFI system is shown in figure 1.

RFI system layout.

Gain/noise figure for system components.

The RFI system consists of an antenna, RFI box containing the receiver, a fiber optic link connecting the upstairs to the control room, an amp following the fiber, and an HP spectrum analyzer. The gain and noise figure for various elements in the system were measured with the HP noise figure meter.

Amplifier gain and noise figure.

There are two amplifiers in the RFI receiver box (preamp and postamp) and an amplifier downstairs after the fiber optic receiver. The gain and noise figure for these amplifiers is show for different frequencies in Table 1. The measurements were made with the amps in the RFI receiver box with the lid open (which lowers the temperature a little). The preamps are labeled 1A-1C and the postamps are labeled 2A-2B.

RFI receiver gain and noise figure.

The noise figure and gain for the entire RFI receiver box was also measured for the 725-1400 frequency bands with the postamp installed. The noise figure meter was plugged into the RFI box input, and a 30db attenuator was added to the output to keep the gain within the range of the noise figure meter (about 50 db). The results are shown in Table 2. The total gain is listed along with the gain from the individual amps. The ``other'' gain is the totalGain - ampGain. This loss is do to the filter, switches, and cables within the RFI receiver box. Before the first amplifier there are 11 cable connections. At .2db per connection this is a 2.2 db loss which matches what we see here.

Fiber optic link gain and noise figure.

The ortel fiber optic link has a bandwidth of 4 gigahertz. In the lab it has a gain of -32db and a temperature of about 4 million degrees. Installed on the hill it has a temperature of 60 million degrees and a gain of -46db. The descrepancy is do to lousy splices.

Tsys computed from system component gain/noise figure.

The system temperature was computed using the gain and noise figure for the elements in the RFI receiver system. For each frequency Tsys was computed with:

no postAmp
with postAmp
with postAmp and using RFI receiver box as a single unit.

(2) and (3) differ since (3) takes into account the losses in the connections within the RFI receiver box.

Tables 5-14 hold the gain, noise figure, cumulative gain, and system temperature contribution for each element in the receiver chain. Table 15 has a summary of the system temperature for each frequency.

The shaded regions are the configurations used when the system is used for RFI monitoring.

* the measurement of the RFI box as a whole probably had the wrong 1700Mhz filter switched in.

Tsys computed using the cals.

Cal measurements.

The system temperature for the entire system (RFI receiver box, fiber optic link, downstairs amp, and spectrum analyzer ..but excluding the antenna and sky) was measured with 3 different cals that were injected at the front of the RFI box. The system was switched between the cal and a load at 290 degrees.

The cal values were:

High cal (H) 35db excess (917060 degrees K)
Low cal (L) 25db excess (91706 degrees K).
Lower cal (LL) 19db excess (23035 degrees K).

The difference between the cal on and the load was measured on the spectrum analyzer downstairs in the control room. The spectrum analyzer was set to 100Khz rbw and integrate for 100 sweeps. The RFI receiver was configured for each of the available frequencies and the temperature was measured with all cals that could be used (saturation ruled out using the high cal in some configurations)

The results are shown in table 1. The RFI box has 3 separate pre-amps and 2 different post amps. The frequencies 70-330 uses the first preamp (1A), 550-955 uses the second preamp (1B), and 1075-1400 uses the third preamp (1C). In the table , measurements with the same preamp are grouped within the double lined rulings. The shaded regions are the configurations that are used when observing with the RFI system. The table has the frequency used, the results for measurements without the postamp, the measuremets with the postamp, and the measured cal rations. For each measurement the following is displayed:

The cal used (H,L,LL) and the measured cal step (cal on / load) in db.
The system temperature measured via the cals and the system temperature computed via the gain and noise figure of the individual components.The ratio of the measured to computed system temperatures is also listed.
For those measurements that used a H and L cal, or the L and LL cal, the ratio of the two cals is also displayed.

The definition of Tsys is:

(Tcal + Tsys)/(Tsys + Tload)=measuredRatio

Since the cals are injected at the RFI receiver box input, the sky and the loss do to the cable from antenna to RFI box is not included in this Tsys. To get the temperature that includes the sky and the cable:

TsysAll = Tsys / cableLoss + Tsky

The computed temperatures for the 70-330 Mhz frequencies are based on the individual components and assume that there is a 1 db loss in the filters and no loss in the interconnecting cables. For the 550-1400 Mhz frequencies the noise figure meter measured the gain/noise figure for the RFI box as a single element. The temperatures from the fiber optic system, downstairs amp, and spectrum analyzer where then added on.

The measured and computed temperatures for the lower frequency bands are within a factor of two. The ratio of H cal to L cal is also about 10db which shows that we were not saturating. The measured temperatures for the 725 to 1325 Mhz L cal do not correspond to the computed values. The L to LL cal ratio of 3db (it should have been 6db) means that we were probably saturating with the L cal for these frequencies. The 1400 Mhz measurement was within a factor of 2, but it had the smallest bandwidth of the group.

Power levels in RFI receiver box with High Cal.

The total power at the output of the filters and at the output of the RFI receiver box with the High cal was computed (not measured!) using the cal temperature, the filter bandwidth/loss, and the amp gains. The power is show at the filter output, the box output with no postAmp, and the box output with the postAmp. The gain of the amps is also listed. These values are 2-4 db high because they do not include the losses withing the RFI receiver box do to the connections. The power output from the filters assumed a rectangular filter shape out to the 3db points.

The power at the fiber optic link input will have 2db more loss from the cable and the power limiter.

Power level fiber input from the sky.

The power level at the input to the fiber optic link was measured for the configurations used for monitoring. The values at the output of the postamp were about 1db higher than the values in the table do to the loss in a cable. The limiter (1db loss) than goes between the postamp and the fiber was not installed.

Summary

Tsys computed and Tsys from cals.

Table 16 holds has the system temperature for all the frequencies. Tsys here includes the sky temperature and the antenna cable. There are 4 separate values for each frequency:

Tsys computed from the system elements- no postAmp.
Tsys computed from the system elements - with postAmp.
Tsys computed using RFI box as single element - with postAmp
Tsys measured using cals- configuration is that used for monitoring.f

(2) and (3) will differ since (3) takes into account the losses through the connectors/switches in the receiver box. The shaded regions are the configurations used when monitoring with the RFI system.

Things that need to be looked into.

For the higher frequencies we are losing 2-3db because of the switches/cables in the RFI receiver box.

The 725-1325 Mhz bands saturated with the L cal when they shouldn't have. The power levels should not have saturated. There must be power that is being added to the system from somewhere : poor switch isolation, some component oscillating out of band.

When on load, you still get some signal from the sky. The 60db isolation of the first switch is probably the culprit. The sky signals are more than 60db about the load temperature.

The filters for 330Mhz does not cut out the 212 Mhz tv station enough. The power is dominated by power outside the filter bandpass.