EXPERIMENT 1.

NOISE FIGURE MEASUREMENTS

MIXER CHARACTERISTICS

(1.1) Noise Figure Measurements: In the first part of this experiment we study the noise performance of microwave devices. The importance of noise stems from the fact that microwave devices internally generate noise and signal power to be handled by the system is usually weak. If noise is not controlled it causes undesirable performance of the communication system. Noise figure is a quality factor which describes the above property of the device and it is an important parameter of interest for selecting the device for a particular application. In this experiment we measure noise figure of the given components and verify some fundamental formulas concerning Noise Figure.

Figure 1:

1.1-A) Given a calibrated Noise Figure meter (HP-8970A). Measure the gain and Noise Figure of the given amplifier as a function of frequency from 10-1500 MHz.

1.1-B) Measure the Gain and and Noise Figure of the given Attenuator at 900 MHz.

Noise Figure(F1)=-------dB., Gain(G1)=--------dB.

1.1-C) Measure the Gain and and Noise Figure of the given Amplifier (MiniCircuits, ZFL 2000) at 900 MHz after biasing the amplifier according to specifications.

Noise Figure(F2)=-------dB., Gain(G2)=--------dB.

Figure 2:

1.1-D) Connect the attenuator and amplifier of 1.1-A and 1.1-B in cascade as shown in Fig.2 and measure the Noise Figure and Gain of the complete system.

Noise Figure($F_A$)=--------dB, Gain($G_A$)=--------dB.

Figure 3: Waveguide connected with load and source.

1.1-E) Interchange the attenuator and amplifier of 1.1-D as shown in Fig.3 and repeat the measurements.

Noise Figure($F_B$)=--------dB, Gain($G_B$)=--------dB.

Answer the following questions

a) Find the theoretical Gain and Noise Figure of 1.1-D and 1.1-E) using the formula for Noise figure of cascaded systems and results of 1.1-B and 1.1-C.



b) Write your conclusion from 1.1-D and 1.1-E.



c) Choose from the following the circuit for best performance. Write the reason for your choice.

i) Antenna-Amplifier-CoaxialCable-TV.
ii) Antenna-CoaxialCable-Amplifier-TV.


d) What are the causes of noise in a microwave component?



(1.2) Mixer Characteristics:

Mixer is a three terminal device used for translating (converting) a high frequency signal(RF) to a low frequency signal(IF) using a locally generated signal(LO). In this part of the experiment we will measure some important parameters namely conversion loss, RF leakage and LO leakage of a commercially available mixer (Minicircuits-ZMX7GR).

Figure 4:

Conversion loss is a measure of the efficiency of the mixer in providing frequency translation between the input RF signal and the output IF signal. It is defined as follows,

Conversion loss(dB)= $-10log(\frac{P_{IF}}{P_{RF}})$ (1)

Where $P_{IF}$ and $P_{RF}$ are the power of IF signal and RF signal respectively.

When the LO power level deviates from the recommended level, the conversion loss will change, finally resulting in nonlinear behavior.

Before starting the experiment try to familiarize the various terms frequently used in microwave lab by answering the following questions,

a) Express 0.05mW (milli watts) in dBm.

Ans=---------dBm.

b) Express 1W in dBm.

Ans=-------dBm.

c) An amplifier has a power gain of 14dB, What is the output power in mW and dBm when the input power of 0.5mW.

Ans=--------mW =----------dBm.

d) When X-dBm power is fed into a coaxial cable the output measured is Y-dBm, Express the losses in the cable in dB.

Ans=--------dB.

1.2-A) Measure the conversion loss of the given Mixer (Minicircuits-ZMX7GR) for different LO power.

For doing this experiment, let us use a network analyser (Wiltron 360B) in continues wave mode for RF, a synthesiser for LO and spectrum analyzer for measuring the power levels.
Since the coaxial cables are highly lossy we have to consider the losses. For measuring the losses, fix synthesiser power level to 0dBm. Use coaxial cable to feed this power to a spectrum analyzer.

Measured power level at the output of coaxial cable , $P_m$------dBm.

Losses in cable, $L_c$=---------dB.


Fix the RF power level of NWA (Network Analyzer) in continues wave mode (5GHz) at 5dBm. Measure the RF power level at the out put of the cable ($P_{RF}$) using the spectrum analyzer.

$P_{RF}$=-------dBm

Take the power level $P_{RF}$ as the RF power to mixer, Change the LO power (at 5.5GHz) at synthesiser ($P_{LOs}$) from -15dBm and slowly vary it to saturation,each time noting the measured IF power level, $P_{IFs}$ at spectrum analyzer. From the cable losses we can calculate the actual LO power level, ($P_{LO}$) and actual IF power level, ($P_{IF}$). Finally we can find from the formula given in (1) the conversion loss in dB. Tabulate the readings in Fig.5. Draw a graph.

Figure 5:

1.2-B) RF and LO leakage into IF can be a serious problem for mixers, Measure and calculate the actual leakage power after considering the cable losses.


$Leakage_{RF}$=---------dBm.

$Leakage_{LO}$=---------dBm.