# lab 3 mixers

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Lab Three for Advanced Analogue Electronics 1 Vin Rin 1Ω L1 40µH L2 40µH Rout 1Ω L3 22µH C 10kΩ Riso 100MΩ + – vout 288pF RL VLO D LAB THREE Mixers The mixer is an important component in RF system. In this assignment you will review and demonstrate the basic mixer theory by means of PSPICE simulation so that you can have a further understanding for the characteristics of mixer. This assignment consists of two parts: (1) simple diode mixer; (2) balanced diode mixer. Part One: Simple diode mixer Figure 1 A simple diode mixer is shown in Fig.1 (The nodal numbers have been shown in this figure). The input signal source, Vin, is coupled with the nonlinear part of the mixer by means of the transformer L1, L2 having a coefficient of coupling of 0.99. Resistors Rin and Rout are used to model losses in the transformers. The signal of the local oscillator is produced by the voltage source VLO. The parallel LC circuit has a resonant frequency corresponding to the frequency of the output signal, fout, which is the difference of the input frequency and the local oscillation frequency. The load resistor, RL, is also the resonant resistance of the tuned circuit. Resistor Riso is used to model the resistance of the isolation in coils because PSPICE cannot simulate a circuit completely isolated from ground. Firstly, you should write the PSPICE input file for the circuit shown in Fig.1, then run it to find the resonant frequency of the tuned circuit and its Q-factor using AC analysis. Then, you can calculate the frequency of the local oscillator, fLO, assuming the input frequency, fin(fRF), is equal to 10MHz and larger than fLO. Secondly, you will do a transient analysis to find the spectral components of the circuit in Fig.1. You should choose a suitable time interval (long enough) so that you Lab Three for Advanced Analogue Electronics 2 are able to reach the steady state. From your simulation results you will observe the diode voltage, the diode current and the output voltage, as well as their spectra. You will find the original spectral components of the current and assess the spectral purity of the output signal. You will know the output spectrum contains a noticeable spurious component Vout(fLO) at the frequency of the local oscillator. Thirdly, you will calculate the conversion loss out in IF RF P P P P CL =10log =10log where Pin is the power delivered by the input signal source, and ( ) L out out R V f P 2 ( ) 2 = is the useful output power. You can find Pin by plotting the instantaneous power V(4)*I(Vin) and its spectrum. The DC component of the spectrum is equal to the average power Pin. Finally, the efficiency of the mixer depends on the amplitude of the local oscillator amplitude, VLO. You will calculate the conversion loss and assess the spectral purity of the output signal for two different amplitudes: VLO=0.5V and VLO=0.6V. Experimental steps: 1. Write the PSPICE input file for the circuit in Fig.1. The statements for the transformer L1, L2 are as follows K12 L1 L2 0.99 L1 3 0 40U L2 7 5 40U The diode model is as follows .MODEL D1N914 D(Is=168.1E-21 N=1 Rs=0.1 Ikf=0 Xti=3 Eg=1.11 +Cjo=4p M=0.3333 Vj=0.75 Fc=0.5 Isr=100p Nr=2 Bv=100 Ibv=100u +Tt=11.54n) The input signal amplitude Vin=1V and perform the small-signal AC analysis of the circuit. The AC analysis command is as follows .AC DEC 100 100K 30MEG Find the resonant frequency fr, the the bandwidth BW, and the Q-factor of the tuned circuit. Calculate the local oscillator frequency fLO (Here it is smaller than fin (fRF)). Lab Three for Advanced Analogue Electronics 3 Rin 1Ω Vin L1 40µH Rout1 1Ω L2 40µH L3 40µH Rout2 1Ω VLO D1 D2 L4 11µH L5 11µH C 288pF RL 10kΩ Riso 100MΩ 2. Modify the file to enable the transient analysis. Take the input signal amplitude Vin=20mV and the local oscillator signal amplitude VLO=0.5V (They are sinusoidal signals). Perform the transient analysis using the following command: .TRAN 1.0E-20 10E-4 Observe the voltage across the diode, V(1,2) and its spectrum. Determine the frequency of the main spectral components. You should use a log scale along the y-axis. 3. Observe the diode current I(D1) and its spectrum. Identify spectral components corresponding to the input signal, the local oscillator signal and one or two intermodulation products. You should use a log scale along the y-axis too. 4. Observe the output voltage V(2,6) and its spectrum. Measure the amplitude of the useful spectral component Vout(fout) and calculate the ratio ( ) ( ) out out out LO V f V f which is the spectral purity of the output signal. Calculate Pout, measure Pin and calculate the conversion loss CL as it is described previously. 5. Change the amplitude of the local oscillator from 0.5V to 0.6V. Perform the transient analysis and repeat step 3 and 4. What has happened to the diode current spectrum and what is the reason for this change? Analyse the change in the output signal spectral purity, the output amplitude and the conversion loss. Part Two: Balanced diode mixer Figure 2 A balanced diode mixer is shown in Fig.2 (The nodal numbers have been shown in this figure). Its tuned circuit has the same resonant frequency as that in Fig.1. However, the secondary winding of the transformer has a tap and the coil of the tuned circuit also has a tap. The local oscillator is connected to the circuit in such a way that the current spectral components at the frequency fLO flow through the tuned circuit (L4 and L5) in opposite directions. Since the circuit is practically Lab Three for Advanced Analogue Electronics 4 symmetrical, their amplitudes are almost the same and, as a result, the output spectrum has a very small component at the local oscillator frequency. Experimental steps: 1. Modify the file in Part One to describe the circuit in Fig.2. The statements for the transformer L1, L2 and L3 are as follows K1 L1 L2 L3 0.99 L1 3 0 40U L2 7 5 40U L3 5 8 40U Take VLO=0.6V and Vin=20mV. 2. (1) Observe the output voltage V(2,10) and its spectrum. Give your conclusion. (2) Calculate the spectral purity of the output signal and Pout. Measure Pin and calculate CL.

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