LAB A

Microstrip Trainer MST532

INTRODUCTION TO THE MICROWAVE VCO SOURCE AND DETECTOR AND ACTION OF A 3–PORT CIRCULATOR CONTENT

EQUIPMENT REQUIRED

ASSIGNMENT 1

A simple microwave test system comprising the microwave voltage controlled oscillator source, a 3-port circulator and the crystal detector used as a power meter is set up. The operation of the VCO source and detector is explained and the nonreciprocal transmission properties of the circulator and its action as an isolator are investigated. Qty

Designation

Description

1

VCO

Voltage controlled oscillator, microwave source

1

CIR

3-port circulator

1

D

Crystal detector

1

MT (red spot)

50 ohm coaxial termination

1

PPC

SMA plug-to-plug coaxial connector

1

SC (white spot)

Coaxial short-circuit termination

1

–

Power Supply for VCO source

1

–

Digital voltmeter for diode detector, 1mV to 1.4V

Note:

The power supply should not be used in ‘tracking’ mode.

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Microstrip Trainer MST532

INTRODUCTION TO THE MICROWAVE VCO SOURCE AND DETECTOR AND ACTION OF A 3–PORT CIRCULATOR OBJECTIVES

KNOWLEDGE LEVEL

ASSIGNMENT 1

When you have completed this assignment you will: •

Be able to use the VCO oscillator and set its frequency to a given value within its tuning range

•

Be able to use the crystal detector for the measurement of microwave power

•

Know the basic properties of a circulator and its applications in microwave systems

No prior microwave knowledge is required to carry out the assignment. For those interested in knowing how the VCO source was calibrated in terms of frequency and how the diode detector was calibrated for use as a power meter, see sections 4.7 and 4.5 respectively in Chapter 4: 'Introduction to Microwave Measurements'.

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Microstrip Trainer MST532

Assignment 1

INTRODUCTION The VCO Source, Diode Detector and Three-Port Circulator Components

The microwave generator supplied in the MST532 and used as the microwave source in all assignments is voltage tuneable allowing the oscillator frequency to be set to any desired value within its range. It incorporates a silicon transistor operated as a negative resistance oscillator element which may be tuned by a varactor diode acting as a voltage controlled capacitor in a thin film microwave resonator circuit. The approximate specification of the voltage controlled oscillator, the VCO, is as follows: frequency range: 2.4 to 3.7GHz power output into 50 ohms: 10mW, minimum power variation: ±1.5dB, maximum tuning voltage limits: low frequency (2.4GHz), 2V approx. high frequency (3.7GHz), 30V approx. dc supply: 15V, maximum Tuning voltage - microwave frequency output data for the VCO in the MST532 Microstrip Trainer are provided. +15V dc supply

0V ground

Tuning voltage (2-30V)

Light emitting diode (LED)

Microwave power output

Black Red White 2mm sockets Continuous - CW rf output Flashing - 1kHz modulated rf output

SMA coaxial connector

Modulator On-OFF switch

Fig 5.1.1 External connections to VCO Microwave Source

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Microstrip Trainer MST532

Assignment 1 Fig 5.1.1 gives the external connections to the VCO unit. Three 2mm sockets mounted on the side casing are available for the connection of the dc supply and the VCO tuning voltages. The Connections are: Black Red White

ground power supply = +15V tuning voltage +2.0V to 30V

The output from this oscillator unit may be either constant wave (CW) or switched-keyed (on-off type) modulation at 1kHz. Switching between CW and modulated output is achieved by operating the modulator switch. A light-emitting diode, LED, indicator is used to indicate in which mode (CW or modulated) the oscillator is operating. In the CW mode the LED indicator remains on. When switched to 1kHz modulation the LED flashes at a rate of approximately once every 2 seconds. The diode crystal detector, see fig 5.1.2, is used in the MST532 Microstrip Trainer to detect (rectify) microwave signals and measure microwave power. The crystal detector is designed to effect an excellent match to 50 ohm lines and for CW inputs produces a dc voltage output which may be accurately measured by a digital voltmeter and converted to power using the calibration curves provided. The detector sensitivity is better than 0.5mV per microwatt at low power levels and is used to measure power levels over a wide dynamic range, typically 1µW to 30mW plus. Note

The connection of the diode in the crystal detector used is such as to produce a negative rather than a positive voltage output. Thus a negative voltage will be measured on the digital voltmeter. The negative sign is of no consequence in using the calibration curves and may be discarded.

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Microstrip Trainer MST532

Assignment 1

Fig 5.1.2 As well as introducing and using the VCO and detector, this first assignment serves to investigate the characteristics of a 3-port circulator and in particular its isolation properties. A circulator is an important microwave component and is extensively used in microwave systems. It depends on its operation on the non-reciprocal properties of ferrites — nonconducting magnetic materials with high permeability and permittivity. Fig 5.1.3 shows a simplified diagram of a 3-port circulator. The ferrite, placed at the centre of the junction produces non-reciprocal effects on the transmission of energy between junctions when correctly magnetised. Magnetisation is usually produced by a permanent magnet, not shown in the diagram; however, for switching applications current carrying magnetisation coils are also used. The effect of the magnetised ferrite on transmission is as follows: Microwave energy entering at Port 1 leaves at Port 2 with ideally zero energy reaching Port 3. Energy entering at Port 2 leaves at Port 3 and energy at Port 3 emerges at Port 1. 56

Microstrip Trainer MST532

Assignment 1

P2 Port 2

Port 1

Ferrite

1

2

P1 3

Port 3 P3

Zero

Fig 5.1.3 Action of a 3-port circulator Typically, the transmission loss between coupled ports is only about –0.5dB whilst the isolation at the decoupled port is of the order of –20 to –30dB, i.e with reference to fig 5.1.3 with power P1 incident at Port 1: P2 transmission loss, 1 to 2 = 10log10 ≈ −0. 5dB P1

isolation, 1 to 3 = 10 log 10

P3 P1

< −20dB

most of the incident power emerges at port 2 and less than 1% at port 3. The input vswr is typically less than 1.2 and the bandwidth ±15% of the centre frequency. Circulators are widely used in microwave systems and some major applications are illustrated in fig 5.1.4.

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Microstrip Trainer MST532

Assignment 1

Fig 5.1.4 Some applications of circulators

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Microstrip Trainer MST532

Assignment 1

NOTES

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Microstrip Trainer MST532

Assignment 1

Fig 5.1.5 Connections for basic test system

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Microstrip Trainer MST532

Assignment 1 In Practical 1.1 described below a basic test system is set up and the operation of the VCO microwave source and detector to measure microwave power is explained. The transmission power from the source via the low loss transmission path, port 1 to port 2, of the circulator is measured at a number of different frequencies in the S-band range centred on 3GHz. In Practical 1.2 the power to the decoupled or isolated port of the circulator is measured. In Practical 1.3 port 2 is terminated in a short circuit and the power to port 3 remeasured. Finally in Practical 1.4 the circulator is set up so power is incident from the source at port 2 and the power transmitted to port 1 is measured. From these measurements the non-reciprocal transmission properties can be quantified for a practical circulator and its applications as an isolator, diplexer, channel separator reinforced. 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 (distinguished by a red spot on its outer casing).

2

The DC power supply has a dual 0 to 30 volt output. Using one set of these terminals and the leads provided connect the positive (+) terminal of the supply to the red 2mm socket terminal of the VCO unit, connect the negative (–) terminal to the black 2mm socket terminal of the VCO. Strap the negative terminals of the dual supply together. Connect the positive terminal of the second pair of the power supply to the white terminal, the tuning voltage terminal, of the VCO.

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Microstrip Trainer MST532

Assignment 1 3

Connect the crystal detector output by means of the coaxial cable provided to the digital voltmeter. The BNC female 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 VΩ. Set the rotary switch to DC volts. When switched on, the voltmeter is in the auto-ranging mode and this should not be changed. The DC output voltage from the crystal detector is 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 2.5GHz. 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 measurments for the frequencies given in Table 5.1.1. Record the results in a copy of Table 5.1.1, reproduced at the end of this assignment.

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Microstrip Trainer MST532

Assignment 1 PRACTICAL 1.2 Measurement of power transmitted to decoupled/isolated Port 3

Interchange the crystal detector D and 50 ohm 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, 2.75, 3.0, 3.25 and 3.5GHz. Record the results in a copy of Table 5.1.2, reproduced at the end of this assignment. Calculate the ratios P3/P2 and 10 log P3/P2, where P2 is the power transmitted to port 2, measured in Practical 1.1. Circulator (CIR)

P1 1

VCO Microwave Source

PPC plug-to-plug connector

Crystal Detector (D)

2 3

50Ω Coaxial Termination (MT)

P3

D

DVM

Digital Voltmeter

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

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Microstrip Trainer MST532

Assignment 1 PRACTICAL 1.3 Measurement at port 3 with power reflected at port 2 Circulator (CIR)

1

VCO Microwave Source

PPC plug-to-plug connector Crystal Detector (D)

2 3

P2

Short-circuit Termination (SC)

P3

D

DVM

Digital Voltmeter

Fig 5.1.7 Set-up with short-circuit at port 2

Replace the 50 ohm co-axial termination at port 2 with the coaxial short-circuit termination (distinguished by a white spot on its casing). Make measurements of P3 at the five frequencies recording the results in a copy of Table 5.1.3, reproduced at the end of this assignment. Calculate the ratio P3/P2

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Microstrip Trainer MST532

Assignment 1 PRACTICAL 1.4 Measurement of transmission power P1 with circulator reversed Digital Voltmeter

50Ω Coaxial Termination (MT)

DVM 3 2

VCO Microwave Source

PPC plug-to-plug connector Circulator (CIR)

1

P1 D Crystal Detector (D)

Fig 5.1.8 Set-up with circulator reversed in direction.

Finally set up the components as shown in fig 5.1.8 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 50Ω 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 and 10 log10 P1/P2 where P2 is the reference power measured in Practical 1.1.

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Microstrip Trainer MST532

Assignment 1 PRACTICAL 1.5 Results summary and observations on circulator characteristics

The results obtained in Practicals 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 3.5GHz. 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.

SUMMARY

A basic microwave test system has been set up and the nonreciprocal 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 which presents low-loss transmission in one direction but high loss (isolation) in the reverse. 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.

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Microstrip Trainer MST532

Assignment 1 NOTES

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Microstrip Trainer MST532

Assignment 1

VCO Frequency f GHz

VCO Tuning Voltage

Detector Voltage Output at Port 2

Transmission Power P2

2.5 2.75 3.0 3.25 3.5 Table 5.1.1 Results f GHz

VCO volts

Port 3 Detector V

Port 3 Power P3

Port 2 Power P2 (from Table 5.1.1)

P3/P2

10log10 P3/P2 dB

P3/P2

10log10 P3/P2 dB

P1/P2

10log10 P1/P2 dB

2.5 2.75 3.0 3.25 3.5 Table 5.1.2 Results f GHz

VCO volts

Port 3 Detector V

Port 3 Power P3

Reference Power P2 (from Table 5.1.1)

2.5 2.75 3.0 3.25 3.5 Table 5.1.3 Results f GHz

VCO volts

Port 1 Detector V

Port 1 Power P1

Power P2 (from Table 5.1.1)

2.5 2.75 3.0 3.25 3.5 Table 5.1.4 Results 68

Microstrip Trainer MST532

Assignment 1

Power input at Port 1 ports 2 and 3 terminated in 50Ω i.e matched

P2

input

f GHz

1

port 2 short-circuited port 3 matched

P2

input 1

2 3

Power input at Port 2

2 3

P3

P

S/C P2

ports 3 and 1 terminated in 50Ω 3

input 2

P1 1

P2 = power measured in Practical 1.1

P2 = power ref. in Practical 1.1

P2 = power measured in Practical 1.4

P3 = power measured in Practical 1.2

P3 = power measured in Practical 1.3

P3 = power ref. in Practical 1.1

10log10 P3/P2 dB

10log10 P3/P2 dB

10log10 P1/P2 dB

2.5 2.75 3.0 3.25 3.5 Table 5.1.5 Results

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