This application note was submitted by Andrew MacMahon of the Irish Aviation Authority. If you have any interesting applications using Pico products that you would like to share with others, please get in touch.
The information presented in this application note is believed to be accurate. However, the Irish Aviation Authority makes no guarantees as to the accuracy or correctness of any information included within.
It is necessary to observe the linearity of each receiver output, as it effects the accuracy of the OBA calculations throughout the Extractor ERM 870 Bd1, Bd2, doubt bit settings, and the Post Processing Reply to Reply correlation, in a Garble process of the TPR1000.
It is possible to test and inspect all three receivers simultaneously, with the Pico ADC-11, and have a relative comparison view summed up on the computer VDU or printer.
There are a number of prerequisites to be put in place before the dynamic level range linearity can be examined for any departure from the standard required, for proper operation of the receivers.
When all these components are assembled, the measurement can proceed.
Diagram 1. Sample of Head 1 Dynamic Level Range Survey, showing good linearity of receivers.
For the purposes of calibration in dB scaling, the selection of "Settings - Custom Ranges" allows the scaling of the number of mV contained in a dB scale.
I have selected 250mV = -100 dB and 2250 mV = -30 dB as a scale to represent a linear dB/milli-Volts scale. This will give a close approximation of the dB value that can be related to the output of the signal generator.
Taking into account the coupler of -20 dB attenuation, followed by the Mimi Circuits ZA power splitter of -3 dB per leg, and the cable losses .3 dB to 2 dB, gives an overall loss to each receiver of -29 dB approx.
For a value of -84 dB to register on the plotter, then a level of -55 dB has to be outputted from the signal generator.
The Step approach can also be used by selecting a larger incremental resolution of 10dB per increment. This will readily show the level produced by a successive 10dB increase in the signal generator output on the plotter.
Any departure from the correct slope or step can be identified immediately and a more concentrated examination of that receiver can be embarked on.
The Dynamic Frequency Measurement is essentially the same set up as the Dynamic Range measurement, in that all the connections are the same, and the signal generator is used in a similar way.
However, in this inspection, it is the frequency that is varied with time, and the output level of the generator stays constant. This graph will display the extent of the bandwidth or the RSM 90 receivers, including their diplexors, dummy load resistors, and co-ax switch contacts.
The display below has characteristic Gigatronics bursts of level changes due to the nature of the frequency changing operation of the generator. This can be smoothed out using a filter in the settings of the PicoScope menu.
Diagram 2: Sample of Head 1 Frequency Bandwidth Survey
Figure 1. set up of test circuit
Diagram 3: Sample of a Head 1 HPD survey looking SW at the Test Beam antenna
An assessment can be made of the quality of the antenna system, by examining the crossover points between the Sigma and Delta waveforms.
They are required to conform to a beamwidth of 2.5 degrees, and a level difference of 3 dB from the peak of the Sigma waveform.
It is possible to zoom in on the very spot of the beam and make a millivolt measurement of the crossover.
The X axis can be calibrated in dB's by carefully selecting a Scale value of 30 millivolts per dB, from a range of 250 millivolts up to 2250 millivolts, being the max RF level injected into the Test Beam antenna.