Practical 1

Microstrip Trai PRACTICAL

1.1

Initial setting-up and measurement of transmission power 1. Connect up the microwave source (VCO), circulator (CIR) and crystal detector (D) as shown in fig 5.1.5. The VCO and circulator at its port 1 input are interconnected using an SMA plug-to-plug connector, 8 of which are supplied. The crystal detector input is an SMA plug connector and mates directly with the circulator at its output port 2. These connectors can be easily tightened by hand and finally by the spanner. Do not over-tighten. Terminate port 3 of the circulator in a 50 ohm coaxial termination. 2. The DC power supply (unit PS3000) has a dual 0 to 20 volt variable and +15V fix supply output. By using three pin circular connector to DB9 connector for connecting PS3000 to VCO units. By this VCO units is automatically biased with Vcc=+15V and Vt= 0-20V. Tuning voltage Vt can be adjusted through VOLTAGE knob mounted on PS3000. 3. Connect the crystal detector output by means of the coaxial cable provided to the digital voltmeter. The BNC male connector on the cable connects directly to the BNC jack output of the detector. At the voltmeter end connect the black lead to the terminal marked COM and the red lead to the terminal marked VΏ. Set the rotor

Microstrip Trainer MS3000

switch to DC volts, the position denoted by V… . When switched on, the voltmeter is in the auto-ranging mode and this should not be changed. The DC output voltage form the crystal detectors displayed directly. The display also shows the measurement units millivolts (mV) or volts (V) and the polarity, in this case negative, since the detector produces a negative output.

4. If not already done so, switch on the digital voltmeter and switch on the DC power supply. Set the VCO supply voltage to + 15 volts. Set the VCO tuning voltage to 10 volts or so. The digital voltmeter will display the crystal detector voltage corresponding to the power output at port 2 of the circulator. Ensure the VCO is in its CW mode. I.e. LED indicator is on continuously. 5. Now complete the following. Using the tuning voltage frequency data supplied for the VCO set the oscillator frequency at 2.7GHz. Measure the transmission power P2 at port 2, i.e. record the digital voltmeter reading and use the detector voltage – power calibration curves to convert the reading to microwave power. Use this procedure to obtain similar measurements for the frequencies give in Table 5.1.1. Record the results in a copy of Table 5.1.1, reproduced at the end of this assignment.

Fig 5.1.5 Connections for basic test system

Microstrip Trainer MS3000

PRACTICAL 1.2 Measurement of Power Isolated Port 3

transmitted to Decoup led /

Interchange the crystal detector D and 50 Ω coaxial termination as shown in fig 5.1.6 and using the VCO voltage settings found in Practical 1.1, measure the detector voltage and hence the power P3 transmitted to port 3 at the five frequencies, 2.5, 2.6, 2.7, 2.8 and 2.9 GHz. Record the results in a copy of Table 5.1.2, reproduced at the end of this assignment. Calculate the ratios P3/P2 and 10log P3/P4, where P2 is the power transmitted to port 2, measured in Practical 1.1.

Fig 5.1.6 Set-up for measuring power P3 at decoupled port 3

Microstrip Trainer MS3000

PRACTICAL 1.3 Measurement at port 3 with power reflected at port 2 Replace the 50 ohm co-axial terminations at port 2 with the coaxial short-circuit termination. Make measurements of P 3 at the five reproduced at the end of this assignment. Calculate the ratio P3 /P2

Fig 5.1.7 Set-up with short circuit at port 2

Microstrip Trainer MS3000

PRACTICAL 1.4 Measureme nt of Transm issi on Powe r P 1 with Circulator reversed

With the circulator reversed so that port 2 is now connected to the VCO output and port 1 to the detector. Terminate port 3 in the 50Ώ coaxial termination. Measure P1 and record the results in a copy of table 5.1.4 reproduced at the end of this assignment. Calculate the ratios P1/P2 where P2 is the reference power measured in Practical 1.1.

Fig 5.1.8 Set-up with circulator reversed in direction

PRACTICAL 1.5 Results summary characteristics

and

observations

on

circulator

The results obtained in Practical 1.1-4 and calculations performed enable a summary of the basic characteristics of the 3-port circulator to be made and its performance specified over the range 2.5 to 2.9GHz. Use a copy of Table 5.1.5.reproduced at the end of this assignment, to record its performance.

Exercise 1.1 Comment on the results recorded in the first and third columns as regards the directional/isolation properties of the circulator. Explain also the significance of the results in the second column.

SUMMAR Y A basic microwave test system has been set up and the non-reciprocal transmission properties of a 3-port ferrite circulator have been investigated.It is observed that with power incident at a given port the circulator directs the power with low loss in a given direction but not in the reverse sense, e.g. with power incident at port 1 low loss transmission occurs to port 2 but little power reaches port 3 when both these ports are terminated in matched (50 ohm) impedances; if power were to be reflected at port 2 it will be directed to port 3. An important application of the circulator is as an isolator – a one-way transmission device

Microstrip Trainer MS3000 which presents low-loss transmission in one direction but high loss (isolation) in the reverse.

Microstrip Trainer MS3000

In microwave measurements it is standard practice to use an isolator to protect the source. Any reflections produced in the system will be effectively absorbed in the isolator (see fig 5.1.4b) thus preventing these affecting the source output. The application of the 3-port circulator as an isolator for the VCO microwave source is used in most of the following assignments for this reason.

RESULT TABLES VCO Frequency f GHz 2.5 2.6 2.7 2.8 2.9

VCO Tuning Voltage

Detector Voltage Output at P2

Transmission Power P2

Table 5.1.1 Results

f GHz

VCO Volts

Port 3 Detector V

Port 3 Power P3

P2 Power P2 from Table 5.1.1

Isolation P2-P3 dBm

2.5 2.6 2.7 2.8 2.9 Table 5.1.2 Results

f GHz

VCO Volts

Port 3 Detector V

Port 3 Power P3

2.5 2.6 2.7 2.8 2.9 Table 5.1.3 Results

P2 Power P2 from Table 5.1.1

P2-P3 dBm

Microstrip Trainer MS3000

f GHz

VCO Volts

Port 1 Detector V

Port 1 Power P1

Power P2 from Table 5.1.1

P2 / P1 dBm

2.5 2.6 2.7 2.7 2.9 Table 5.1.4 Results

Power input at Port 1 Ports 2 and 3 Terminated in 50Ω i.e. matched f GHz P2=power measured in practical 1.1

2.5 2.6 2.7 2.8 2.9

P3=power measured in practical 1.2 10 log10 P3 / P2 dB

Port 2 short-circuited Port 3 matched

Power input at Port 2 Ports 3 and 1 Terminated in 50Ω

P2=power ref. in practical 1.1

P2=power measured in practical 1.4

P3=power measured in practical 1.3

P3=power ref. in

10 log10 P3 / P2 dB

Table 5.1.5 Results

10log10 P1

/

P2 dB

Microstrip Trainer MS3000