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.