Lab Three for Advanced Analogue Electronics

1

Vin

Rin

1?

L1

40µH

L2

40µH

Rout

1?

L3

22µH

C

10k?

Riso

100M?

+

–

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 1?

Vin

L1

40µH

Rout1 1?

L2

40µH

L3

40µH

Rout2 1?

VLO

D1

D2

L4

11µH

L5

11µH

C

288pF

RL

10k?

Riso

100M?

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.