Measuring radar receiver dynamic range & frequency with a data logger

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.

Dynamic range-frequency measurements: prerequisites

Presentation of the Dynamic Level Range Linearity Measurements of the Radar Receivers, RSM970 BDQ003, using the Pico ADC-11 converter and Windows laptop

Note: the PicoLog ADC-11 has now been superseded by the PicoLog 1000 Series data loggers.

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.

• Three, equal length, coax cables (50 ohm) terminated using BNC fittings to connect to Sigma, Delta, Omega, outputs of the BDQ003.
• A Mini-Circuits ZA3PD-1.5 Power Splitter to connect to the RF coupler at rear of IR cabinet, at the Dummy load Couplers.
• Three, short length, RF coax cables, terminated with N Type fittings to fit to the couplers at rear of IR cabinet.
• RF Signal generator with RF Output Level Sweep facility in range of -100 dB to 0 dB in 90 seconds.
• RF Signal generator with RF Frequency Sweep facility in range of -1000 MHz to 1100 MHz sweep time in 100-150 seconds, or 1 second to 1 MHZ.
• Pico ADC-11 converter with BNC terminations, marked Sigma, Omega, and Delta.
• PC or Laptop with Windows 95/98 operating system.
• PicoScope proprietary software to interface with ADC-11.
• Colour or B/W Printer.
• RS232, 2 port Data Switch box.

When all these components are assembled, the measurement can proceed.

Dynamic range measurements procedure

• Connect Mini-Circuits ZA3PD-1.5 Power Splitter to the coupling ports of Sigma, Omega and Delta, at read to IR cabinet, using the short N type coax cables.
• Use any long coax N type feeder from the input of the ZA3PD-1.5 to the Gigatronics Signal generator.
• Set the Signal Generator frequency to 1090 MHz and the output level to -100 dB level, the incremental steps to 0.1 dB resolution.
• Connect the three coax cables (BNC), between the BDQ 003 Receiver outputs, and the ADC-11 terminations, marked Sigma, Omega, and Delta. (See Fig. No.1).
• Connect Output from ADC-11 to the Switch Box on the LPT1 (printer) port of the PC / Laptop and select connection to computer.
• Run Picotech Scope proprietary software for ADC-11, selecting Time base to 10 secs/div, Trigger to Auto, this will give over 90 seconds display time on screen, or load a previous file.
• Set Notes to reflect the Title of Test, and Serial No. of receiver under test, and conditions of test as appropriate. dB levels
• Select Channels 1, 2, 3, to A, B, C and set to Red, Blue, Green, respectively. Select Volts and channel gain to “multiply by (1)”. This shows the tree outputs in individual colours respectfully.
• Get release of the Radar Head and set transfer Unit to manual, to prevent high levels of RF in the coupler, leaking into active receiver to the antenna, causing Receiver Alarms, and any subsequent equipment switching.
• Press the Space Bar on the PC to start the plotting sweep, observing receiver noise on monitor. Approx. 250 mV base line level (See diag. No.1)
• When the displayed sweep reaches the 10 seconds point, initiate the Signal Generator Sweep at a constant speed, from -100 dB up to no more than 0 dB in less than 90 seconds.
• Observe the computer display as the Red, Blue and Green plots rise diagonally to the top right-hand corner of screen, near the 90 second point, switch off the RF from the signal generator and press the space bar to save the displays contents.
• You can now save the measurement under a New filename that reflects the source of the test, (i.e. H22FEB023, Head 2, IR 2, Month, Receiver No.). The display is time stamped.
• Turn the Switch Box over to Printer and make a copy of the measurement to confirm the details of the notes and date, etc.
• If the other receiver is to be tested, select the second receiver using the transfer unit, return to step 6 and follow the sequence through.
• Return the station to normal by clearing the receiver alarm, disconnecting all the coax cables, and putting the transfer unit to Auto.

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.

Dynamic frequency measurements

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

Dynamic frequency measurements procedure

• Connect Mini-Circuits ZA3PD-1.5 Power Splitter to the coupling ports of Sigma, Omega and Delta, at read to IR cabinet, using the short N type coax cables.
• Use any long coax N type feeder from the input of the ZA3PD-1.5 to the Gigatronics Signal generator.
• Set the Signal Generator frequency to 1000 MHz and the output level to -60 dB level, the incremental frequency steps to 1MHz per second, giving a sweep of 1000 MHz to 1100 MHz in 100 seconds.
• Connect the three coax cables (BNC), between the BDQ 003 Receiver outputs, and the ADC-11 terminations, marked Sigma, Omega, and Delta. (See Fig. No.1).
• Connect Output from ADC-11 to the Switch Box on the LPT1 (Printer) port of the PC / Laptop and select connection to Pico . This feeds the ADC info to the Parallel port on the computer.
• Run PicoScope proprietary software for ADC-11, selecting Time base to 10 secs/div, Trigger to Auto, this will give over 90 seconds display time on screen
• Set Notes to reflect the Title of Test, and Serial No. of receiver under test, and conditions of test as appropriate. dB levels
• Select Channels 1, 2, 3, to A, B, C and set to Red, Blue, Green, respectively. Select Volts and channel gain to "multiply by (1)". This shows the three outputs in individual colours respectfully.
• Get release of the Radar Head and set transfer Unit to manual, to prevent high levels of RF in the coupler, leaking into active receiver to the antenna, causing Receiver Alarms, and any subsequent equipment switching.
• Press the space bar on the PC to start the plotting sweep, observing receiver noise on VDU monitor. Approx. 250 mV base line level ( See diag. No.1)
• When the displayed sweep reaches the 10 seconds point, initiate the Signal Generator Sweep at a constant speed, from -1000 MHz up to 1100 MHz, sweep of 100 MHz in 100 seconds.
• Observe the computer display as the Red, Blue and Green plots rise and fall to the base line inscribing a Bell curve. This represents the Band Pass frequencies of all three receivers.
• You can now save the measurement under a New filename and folder that reflects the source of the test, (i.e. H22FEB023BW, Head 2, IR 2, Month, Receiver No.). Picotech display is time stamped.
• Turn the Switch Box over to Printer and make a copy of the measurement to confirm the details of the notes and date, etc.
• If the other receiver is to be tested, select the second receiver using the transfer unit, return to step 6 and follow the sequence through.
• Return the station to normal by clearing the receiver alarm, disconnecting all the coax cables, and putting the transfer unit to Auto.

Figure 1. set up of test circuit

Horizontal polar diagram procedure

• Connect Gigatronics Signal generator to the Eternal Hard-line co-ax to the Test Beam on the Fire Services pole.
• Set the Signal Generator frequency to 1090 MHz and the output level to -30 dB level CW. This may not cause a RF alarm the R to the antenna.
• Connect the three coax cables (BNC), between the BDQ 003 Receiver outputs, and the ADC-11 terminations, marked Sigma, Omega, and Delta. (See Fig. No.1).
• Connect Output from ADC-11 to the Switch Box on the LPT1 (printer) port of the PC / laptop and select connection to computer.
• Run Picotech Scope proprietary software for ADC-11, selecting Time base to 10mSecs/Div, Trigger to CH 8, this will give a trigger on NorthMark. Depending on Head 1 or 2, the trig delay can be set to 300 mSecs for Head 2, looking NE at the Test Beam, or 2840 mSecs for Head 1 looking SW.
• You can always load a previously saved file, which will have all the settings adjusted already.
• Set Notes to reflect the Title of Test, and Serial No. of receiver under test, and conditions of test as appropriate. dB levels
• Select Channels 1, 2, 3, to A, B, C and set to Red, Blue, Green, respectively. Select Volts and channel gain to "multiply by (1)". This shows the three outputs in individual colours respectfully.
• Get release of the Radar Head and set transfer Unit to manual, to prevent, the blocking of the Receiver, causing Receiver Alarms, and any subsequent equipment switching.
• Press the space bar on the PC to start the plotting sweep, observing receiver noise on monitor. Approx. 250 mV base line level ( See diag. No.1)
• When the displayed sweep reaches the trig point, The Sigma, Delta and Omega, received strengths will be displayed, in the characteristic shapes.
• Observe the computer display as the Red, Blue and Green plots are repeated after each NorthMark, and adjust the trig point to place the waveforms in the centre of the screen.
• You can now save the measurement under a New filename that reflects the source of the test, (i.e. H22FEB023, Head 2, IR 2, Month, Receiver No.). PicoScope display is time stamped.
• Turn the Switch Box over to Printer and make a copy of the measurement to confirm the details of the notes and date, etc.
• If the other receiver is to be tested, select the second receiver using the transfer unit, return to step 5 and follow the sequence through.
• Return the station to normal by clearing the receiver alarm, disconnecting all the coax cables, and putting the transfer unit to Auto.

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.