Experiment Mmm-1: Spectrum Analysis

Experiment Mmm-1: Spectrum Analysis

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EMF3056 Microwave Measurement Techniques

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EXPERIMENT MM-2: NETWORK ANALYSIS

OBJECTIVE

  • To demonstrate Network Analysis measurement techniques of a filter.
  • To evaluate the insertion loss, 3-dB bandwidth and spurious of a filter
  • To analyse reflection due to the impedance mismatch of a filter.

APPARATUS

1. Network Analyser (MS4630A)5. VSWR bridge (62B50)

2. Filter module (10.7 MHz)6. Coaxial cable, 20-cm, BNC: 3 units

3. Open/Short/Load calibration kit, BNC(f)7. Adapter, BNC f-f

4. Matched termination, BNC(m)8. Capacitor, 100 pF

INTRODUCTION

Network Analyser is an important equipment for testing of active and passive microwave circuits. It measures the EM powers of waves that are propagating in both forward and reverse directions. By using the appropriate inputs and terminations, all the S-parameters of 1-port, 2-port, as well as 3-port devices can be measured.

A Vector Network Analyser can measure both amplitude and phase of the RF signal. Input port matching S11 consisting of both real and imaginary parts can be obtained for impedance matching purposes. The gain or insertion loss of a 2-port device in term of S21 can also be obtained. The amplitude response together with the phase response, over the bandwidth of interest, provide useful information for prediction of signal distortion at the output.

A block diagram of a typical Network Analyser is shown in Figure 1. The RF source is tuned to sweep across the desired frequency range. A fraction of the RF power is split to act as a reference signal when the remaining signal power is transmitted to the device under test (DUT) input. The RF power reflected at the input of the DUT can be separated from the incident power using a directional coupler or a reflection bridge. This reflected signal is measured using a superheterodyne receiver (consisting of a mixer and a detector). The RF power transmitted through the DUT can be measured using the same type of receiver. The measured reflected power and transmitted power are compared to the reference signal to compute S11 and S21, respectively. By digitising the down-converted test/reference signals using an ADC, the results can be displayed in various format: magnitude/phase response, group delay, VSWR, Smith chart (for S11 and S22), polar (for S21 and S12), and impedance response.

Figure 1: Block diagram of a Network Analyser.

In order to get an accurate result, a calibration process must be carried out before conducting the measurement on DUT. The objective of the calibration is to characterise the error factors due to the equipment set up (inclusive of the directional coupler, reflection bridge, connectors, cables, etc.). For transmission measurement (S21), a simple process known as Response Calibration can be performed by a thru connection from Port 1 to Port 2. It is essentially a normalisation process which will correct for transmission tracking error. The frequency response error due to the equipment set up (not due to the DUT) will be cancelled by mean of a post processing computation carried out on the measured results to yield the actual S-parameters of the DUT. The final results will then be displayed on the screen.

For reflection measurement (S11), a standard Open-Short-Load (OSL) calibration is performed. The calibration process measures the reflected powers of 3 known standards: Open-circuit (ideal S11 = 1 + j0), Short-circuit (ideal S11 = -1 + j0), and 50-ohm matched load (S11 = 0). Due to the error factors of the equipment set up, the measured S11 for these standards deviate from the ideal values. Calibration coefficients can be calculated by the Network Analyser for use in error cancellation processing when measurement is performed on a DUT.

In the evaluation of an RF band-pass filter, the important parameters are centre frequency, 3-dB bandwidth, in-band ripple, out-of-band attenuation, insertion loss, group delay, and input VSWR. These parameters can be extracted from the measurements of S21 and S11 over the frequency range of interest. Passive filters are generally reciprocal (input and output is interchangeable), hence S12 equals S21 and S22 equals S11. To verify whether the filter satisfies the reciprocity condition, S12 and S22 can be measured to compare with S21 and S11, respectively.

EXPERIMENT & MEASUREMENT [50%]

  1. Set the Network Analyser to a known state by pressing [Preset][Yes].
  2. Select the frequency range: press [Frequency][Center Freq][10.7][MHz] [Span][2] [MHz].
  3. Select [Format][Mag & Pha].
  4. Connect Port 1 (Output B) to Port 2 (Input TA) using a Thru Connector (BNC f-f adapter). Connect the second Output B to reference input R.
  5. Press [Avg][Average No.][8][Enter].
  6. Press [Cal][Response][Cal On].
  7. Connect the Filter Module as shown in Figure 2.
  1. Sketch the displayed response.
  1. Use the marker to measure the 3-dB bandwidth (fH - fL), 40-dB bandwidth, center frequency [fo = (fH + fL)/2], pass-band Insertion Loss, and level of out-of-band spurious response. Observe the frequency response and comment.

3-dB bandwidth =…………………..MHz

40-dB bandwidth =…………………MHz

Center frequency =…………………MHz

Insertion Loss =………………………dB

Spurious response =…………..………dB

Measurement of Reflection Coefficient

  1. Connect the VSWRBridge as shown in Figure 3.
  2. Press [Cal][Setup][Cal Method][Enter].
  3. Use the arrow key to select [1-Port OSL] and then press [Enter].
  4. Connect the 50-ohm load standard to the VSWRBridge. Press [Load].
  5. Connect the Short-circuit standard to the VSWRBridge. Press [Short].
  6. Connect the Open-circuit standard to the VSWRBridge. Press [Open].
  7. Press [Cal On].
  8. Connect one port of the Filter Module to the VSWRBridge. A 50-ohm load termination shall be connected to the other port of the Filter. Sketch the displayed response and comment.

  1. Select [Format][VSWR]. Set the scale to 2 / division. Sketch the VSWR response and give the comment.
  1. Select [Format][Impd Chart]. Sketch the Impedance response and give the comment.

  1. Select [Format][Admt Chart].
  2. Connect a shunt capacitor between the input terminal of the ceramic filter chip and ground. Sketch the resultant Admittance Chart.

  1. Connect a shunt capacitor between the output terminal of the ceramic filter chip and ground. Sketch the resultant Admittance Chart.
  1. Remove the 50-ohm load termination from the Filter output. Sketch the resultant Admittance Chart.
  1. Evaluate the three resultant admittance charts and comment.

DISCUSSION [50%]

  1. The most commonly measured filter characteristics are the insertion loss and bandwidth. Why?
  1. Evaluate the relationship and comparison and among the SWR, Return Loss, and Reflection Coefficient.
  1. Explain why when working with a series-connected circuit or inserting elements in series with an existing circuit or transmission line, the "impedance" Smith chart is normally used? Similarly, when working with a parallel-connected circuit or inserting elements in parallel with an existing circuit or transmission line, the "admittance" Smith chart is normally used?

IMPORTNANT NOTE:

Report submission: Submit your report within 10 days of performing the experiment to the same laboratory. Your report should include 1-2 pages of background information on Network Analysis. Include the discussion and conclusion on the results obtained.

Evaluation Schemes (100%)

Experiment and Measurement – 50%

Discussion – 50%

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