# Lecture16 mixer

Document Sample

```					                  ELEC 483 -Microwave & RF Circuits & Systems
Mixers

SIMULATION

Part I: Transient Response
1. Start ADS, and open the schematic MIX_Diff_Spec_Iso. Push into the mixer sub-circuit
hierarchy, and look at the circuit. You should see a Gilbert cell mixer with some extra
transistors on the left that are used for biasing. Push out of the mixer and take a look at the
schematic. Take note of the frequency and power of the input signals. What are the
expected down-converted and up-converted frequencies?
Down-Convert Output Frequency:
Up-Convert Output Frequency:

1. Simulate the transient response for the circuit. If a data display window does not open,
select Window | Open Data Display, and open MIX_Diff_Spec_Iso.dds. Look at the
output signal from the mixer. You should see the up-converted signal superimposed into
the down-converted signal. If we are to use this mixer as a downconvert mixer, we need to
remove the high frequency output, using a filter. Examine the magnitudes of the harmonics
at output by adding a new rectangular plot, clicking “Advanced” in the Plot Traces and
Attributes box, then enter the expression dBm(fs(Vload)). You should see a frequency
domain plot of the output voltage Vload. Note the magnitudes of the signal at the
upconversion and downconversion frequencies.
2. Insert a series inductor between the current meter I_probe and the load TERM2 to form a
low pass filter. Use the formula R/L =ωo, where ωo is the cutoff frequency of the filter in
rad/s and R is the resistance of the TERM2 element (400 ohms), to calculate the inductance
value required to filter out the up-conversion frequency but pass the down-conversion
frequency. Simulate the circuit. Examine the output in both the frequency and time
domains. By how much did your filter reduce the high frequency output of the mixer?

1
ELEC 483 -Microwave & RF Circuits & Systems
Mixers

Part II: Mixer Characterization
In this part, we will examine the isolation, conversion gain and TOI of the Gilbert cell
mixer.
1. Remove the added inductor. Deactivate the “Transient” simulator box and activate the
“Harmonic Balance” simulator box. Simulate the circuit, and examine the spectrum at
the input (RF) port, and output (IF) port on the bottom left of the data display window.
Locate the LO, RF, and IF frequencies on the input and output spectra, and note their
amplitudes. What is the isolation between the RF and IF and the LO and IF ports (i.e.
isolation between RF and IF port is the difference in magnitude of the RF signal at the RF
port and the magnitude of the RF signal the IF port)?
Isolation between RF and IF:
Isolation between LO and IF:

2. Change the LO input power in the schematic (using the P_LO variable) to -30, -20, and -10
dBm, and notice the change in conversion gain and output spectrum. Record the
conversion gain changes for various LO powers below. Explain why this happens. Think
about how the switching core of the Gilbert cell mixer will work if the LO is not large
enough to turn the FETs on and off.
P_LO (dBm)                                 Conversion Gain (dB)
Up                     Down
-30
-20
-10
The conversion gain change is linear to the LO power change.

2
ELEC 483 -Microwave & RF Circuits & Systems
Mixers

3. Close the current schematic and open Mixer2Tone. This schematic will measure the third-
order intercept of the Gilbert cell mixer.
4. Simulate the circuit and examine the plots in the Data Display Window. Determine the
input referred third-order intercepts for each graph using the information in the table of the
data display window, and fill in the table below.
Low Side input referred TOI
High Side input referred TOI
Two toned input referred TOI

3

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 4 posted: 9/11/2012 language: Unknown pages: 3
How are you planning on using Docstoc?