ELEC 518: RF/Microwave Circuit Design and Measurement
Lab5: Mixer
1.0 Overview
This lab involves the design and simulation of a mixer, following the design
procedures discussed in class. The goal is to understand how to use harmonic balance
simulation techniques to simulate the conversion loss and IM3 of a mixer.
2.0 Design
Your assignments will be given during the laboratory.
Group#
Specifications for mixer
900MHz, 90-degree
2GHz, 90-degree
3GHz, 90-degree
5GHz, 90-degree
2.1 Description (An example of a 3 GHz microwave mixer)
The 3-GHz microwave mixer shown in Figure 1 consists of two tuned circuits, a
90°-hybrid coupler, two diode-matching networks, two zero-bias schottky diodes
MSS20, a 3.05-GHz ac short, and a IF-matching network. The two tuned circuits
suppress the harmonic components of the RF and LO signals from signal
generators and prevent any harmonics generated by the diodes from escaping the
mixer. The 90°-hybrid coupler directs both the RF and LO signals to the diodes
with 90°-phase shift for mixing. Before the zero-bias schottky diodes, two openstub matching networks, which matches the diodes to 50 Ω, are adopted to
improve the isolation between the RF and LO ports. A 3.05-GHz ac short is
inserted at the end so that all RF and LO voltages drop across the diodes for
effective mixing. At the end, an IF-matching network is adopted to maximize
conversion gain since the input impedance of a spectrum analyzer is 50 Ω.
Diode-Matching
Figure 1. Architecture of the 3-GHz microwave mixer.
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2.2 Diode model
Figure 2. Circuit model of zero-bias schottky diode.
The first part of the design is to create a model of the metelics diodes which is
same as the diode model used in Lab #4.
2.3 90˚-Hybrid Coupler
A 90°-hybrid coupler is a 3-dB directional coupler with a 90° phase difference in
the outputs of the through and coupled arms. With all ports matched, power
entering port 1 is evenly divided between ports 2 and 3, with a 90°-phase shift
between these outputs. No power is coupled to port 4 (the isolation). The Sparameter matrix has the following form:
⎡0 j 1 0⎤
⎢
⎥
−1 ⎢ j 0 0 1 ⎥
[S ] =
2 ⎢1 0 0 j ⎥
⎢
⎥
⎣0 1 j 0⎦
(1)
The circuit implementation of the 90-hybrid coupler is shown in Figure 3. It
consists of 4 quarter-wavelength (λ/4) microstrip lines with characteristic
impedance of Z0 and Z0/√2. To reduce the discontinuities between the coupler and
50-Ω microstrip lines, 4 tapers are used to convert the line width.
Simulations involved:
Plot: S21, S31 – 900 and 1800 phase shift respectively
S11, S44, S14, S41 – isolation
Tune: vertical & horizontal lengths of the coupler
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Figure 3. Example of circuit implementation of the 90-hybrid coupler
2.4 RF & LO Tuned Circuits
As shown in Figure 1, there are two tuned circuits which appear to be open at the
fundamental frequency but to be relatively short at other harmonic frequencies.
Such tuned circuits filter out the harmonics from the RF and LO signal generators.
Moreover, they also confines all harmonics generated by the diodes from escaping
the mixer. This is very important in improving the conversion gain and spurious
performance of the mixer.
Figure 4a shows the circuit implementation of the tuned circuit. It consists of a
quarter-wavelength 70°-radial open stub and a quarter-wavelength microstrip line.
The radial-open stub which is adopted for wide-band consideration converts the
open end into a 3-GHz ac short at point A. After the quarter-wavelength
microstrip line, the tuned circuit becomes open at 3 GHz at point B. Similar to the
bias-stub design of the LNA in the Lab #3, a narrow microstrip line is adopted to
maximize the input impedance. Figure 4b shows the admittance at point A and the
impedance at point B, it shows the input impedance is high at the fundamental
frequency and is low at harmonic frequencies.
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Figure 4. (a) Circuit implementation, and (b) point-A admittance and point-B
impedance of the tuned circuit.
2.5 AC short
To ensure all input microwave voltages drop across the diodes for effective
mixing, an ac short is designed at the end of the schottky diodes. Figure 5a shows
the circuit implementation of the output stage which consists of a radial stub, two
bends and a cross junction. The quarter-wavelength radial stub is used to create
the ac short. To separate the radial stub and the bends, the angle of the stub is
limited to be 30°. To utilize the effect of the ac short, two schottky diodes are
located very close to the radial stub. As the separation between ports 2 and 3 of
the coupler is different from that of the length of the two diodes, two bends are
adopted to connect the coupler and the schottky diodes. Figure 5b shows the
admittance of the radial stub.
Figure 5. (a) Circuit implementation of the output stage and (b) admittance of the
ac short of the microwave mixer.
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2.5 Diode Matching Network
For a mixer implemented by a 90°-hybrid coupler, input matching is easy to be
achieved as the waves reflected from output loads are cancelled out. Therefore,
matching networks are implemented in order to match the schottky diodes to be
50 Ω.
The diode-matching network is implemented by an open-stub matching network.
Due to the limitations from the physical size, the total length of the two openstubs can not exceeds the inner length of the coupler if located “inwards”.
2.6 IF Matching Network
As the IF output of the mixer is measured by a spectrum analyzer which is
terminated with 50 Ω, an IF-matching network is required to maximize the
conversion gain. As IF is low frequency, you can use chip components.
3.0 ADS Simulation
3.1 RF, LO and IF Matching
The reflection coefficients of the RF, LO and IF ports can be simulated by Sparameters simulation in ADS.
3.2 Conversion Gain G, 1-dB Compression Point P-1dB and 3rd-Order Intercept
Point IIP3
3.2.1 Determine the optimize input LO power
a. To determine the optimal LO_power such that the power loss of the
mixer is minimized.
b. Tools need to be used:
i. HB simulator: [SimulationÆ HB]
Define the following parameters:
“Freq”: 1. LO_freq
2. RF_freq
“Sweep”: Power_LO from –14dBmÆ 12dBm
ii. Power Sources:
[SourcesÆ Freq Domain]: P_1 Tone
Define the following parameters:
“Power”: dBmtow (RF_Power)
“Freq”: RF_freq
* Same things for LO
iii. Define node at the RF, LO inputs & IF output to get the power
in/out at that port.
iv. Define measurement equation:
P_IF=dBm(mix(VIF, {-1(-LO), 1(+RF)}))
PL=P_IF-RF_power
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3.2.2 Determine the 1-dB compression point by plotting RF_out vs RF_power
a. Same setup as above, except LO_power is fixed at optimal value
and sweep RF_power from –30 to 20 dBm.
c. Plot PL=P_IF-RF_power
3.2.3 Two-tone test: determine the intermodulation of the mixer
To simulate the IIP3 of the mixer, a two-tone test is performed.
a. Setup:
P_n Tone is used instead of P_1 Tone at RF_input
Freq[1]=RF_freq+frequency/2
Freq[2]=RF_freq-frequency/2
P[1]=RF_power
P[2]=RF_power
b. HB simulator
Freq: Fundamental
1. RF_freq+frequency/2
2. RF_freq-frequency/2
Sweep: delete all terms
b. Var*: tone=[{1,0},{0,1},{2,-1},{-1,2}]
Plot [dBm (mix (VIF, tone))] *choose Trace_option to “Spectral”
3.3 RF-to-LO and LO-to-RF leakage
The RF-to-LO and LO-to-RF leakage can be simulated in Harmonic Balance
simulation. The RF and LO power are fixed to -30dBm and optimal LO value
respectively.
3.4 Noise Figure
By fixed the RF power to -30dBm, the noise figure (double side band) of the
mixer can be simulated by Harmonic Balance simulation with “non-linear noise”
option selected.
4.0 Layout
Prepare the final layout of the mixer in ADS.
5.0 Measurements
Notes will be given during the second session.
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