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HF-VHF-UHF IQ Mixer with a Single SPDT Switch
Rick Campbell
Portland State University, Portland OR 97201
This Talk:
Technical details of the mixer
Our Applications
Experimental Platform
Simulation Platform
What we’ve learned so far
....Next Steps
Application: Off-Grid Wireless...seriously
Wind Power, Environmentally Conscious
Starved Current
Student experiments run on 9v battery
Short path from sketch through prototype circuitry, then deploy measurement instruments
Low Cost
Attracts an interesting community of students from diversebackgrounds and all genders.
“What voltage zener makes a better tuning diode?”
Overheard:
Frequency Conversion 101
High Frequency Low Frequency
Multiply by 1 or 0
If Low Frequency V is plus or minus dc, then RF comes out the� High� Frequency� port,� at� potentially� very� high� efficiency� depending on reactive loading. Stored Energy...
Our basic mixer unit cell V
HF inHF out
Redraw:
Use a pair of unit cells in an IQ mixer with SPDT switch:
HighFrequency RF
LowFrequency I
Note: bidirectional, no attempt at optimum drive or load at switch.� � Could� benefit� from� some� waveform� engineering...
basic conceptual I Q cell
LowFrequency Q
First Pass Waveform Engineered version
Design network components using classic Smith Chart techniques to optimize switch impedance environment
RF I
Q
Examples: Higher Z to reduce effect of On resistance, lower Z to minimize switch varactor effects, etc.
Next, Simulations and Lab Experiments
Simulation� first:� ideal� switch� exists� in� the� simulator.
RF I
Q
a
b
Note: component values in IMS paper
a
b
Simulation Schematic
all passive baseband signal processor
I
Q
Experimental Platform:
Switch candidates go on the back, alternately connecting points a and b to ground
RF
Note:� 7� MHz� low� frequency� test� fixture� allows� connecting oscilloscope to observe waveforms
I
Q
a
b
LO
a
b
a
bLO
Two different SPDT switches built on the back of the experimental I Q mixer board
VN10 MOSFETs with gate bias
Agilent 5082-2835 diode ring andhybrid coil
bias
a
b
a bLO LO
bias
Two different SPDT switch schematics
Two MOSFET SPDT switch
Hybrid coil and diode ring SPDT switch
Summary of Results: Note I Q stability and 2nd Order
MOSFET mixer typically has several mV of dc offset at I and Q ports.Variable with LO drive and RF signal level. Various 2nd order tests.
� � 3rd� order� products� with� ideal� switch� are� above� the� simulation� noise� floor
Measured opposite sideband suppression typically degraded by 10 dB for a few dB change in LO drive level--several degree phase shift.*
*
SSB Loss dB
IIP3 dBm
LO dBm
2nd Order
I Q stability vs LO drive
Ideal Switch VN10 MOSFET Hybrid/Diode
Simulated vs Measured Performance
4.1 5.3 5.9
+22.8 +21.2 +6.9
N/A +10 +4
in simulation noise floor
depends on FET model
>40 dB LSB suppression
6.2
+16.5
+17
several mV of dc offset
poor
6.4
+6.2
+4
no change for LO0dBm to 10dBm
sim simmeasured measured
*
>40 dB LSB suppression
>40 dB LSB suppression
in simulation noise floor
in measurement noise floor
*
sim
Measured MOSFET 2nd order performance is highly variable and always poor--may be related to gate capacitor varactor
I Q stability vs LO drive also likely due to gate cap varactor
Simplified� MOSFET� model inserted into simulator� with� fixed� gate capacitor and on resistance adjustedto match measuredconversion loss
With� simplified� model, 2nd order performance and IQ stability are ideal
Note that Off pair of diodes in the ring is clamped to precisely the forward voltage of the On diodes:
diode� reverse� capacitance� clamped� to� fixed� value--good� for� 2nd order balance and IQ stability.
Two� significant� implications:
Off diodes have relatively small reverse bias--limits 3rd order IM performace: 350 mV reverse voltage, 100 mV signal...
Conversion Loss and Sideband Suppression Measurements
LO level
passive basebandsignalprocessor
Measurements are basic and calibrated by substitution
Higher� Frequency� tests.� � Start� with� a� modified� ADE-1� diode ring mixer:
Interesting manual work under the microscope. Lead pitch is 2.5� mm.� � This� required� tweezers� and� a� fine� soldering� iron� tip.
144 MHz same schematic, scaled reactances, physically smaller parts
SMC� trifilar� transformerair-core hybrid transformer
surface mount parts and diode pairs
VHF surface mount versions have no surprises. A family of examples compared with conventional I Q diode ring pairs.
VHF #2
HF reference
VHF #1
Compared with reference: more modest IM; very good I Q stability; comparable loss; lower 0 dBm LO drive.
Summary
A simple and different I Q mixer topology has been explored using a platform that facilitates experiments with different switches.
Competitive conversion loss of 6 dB and modest IM performance at 0 dBm LO drive are attractive for simple battery power applications at HF and VHF.
The only active component is a single SPDT switch, which facilitates frequency scaling.
The mixer specs for frequency conversion are not the most important specs for I Q applications.
Next Steps:
Clearly already too much work to squeeze into a conference� paper.� � If� there� is� sufficient� interest� a� more complete Transactions paper will be prepared.
Unlike� broadband� mixer� topologies,� band-specific� mixers present opportunities for optimization using waveform engineering methods.
We have already begun work on very low power I Q receivers using 0 dBm LO power and the modest IM performance offered by this topology.
THz and milliHertz versions are possible with an appropriate SPDT switch.
Thank you!
Note: bow is low. Minutes after photo, discovered that prior generation electronics was soaked in salt water.
hence these experiments...
2013 photo