HF Wideband Active Circulator

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2 Νοε 2013 (πριν από 4 χρόνια και 2 μήνες)

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HF Wideband Active Circulator

Team Laplace Sauce: Jake Smith & Andrew
Wajda

Sponsors: Dr. Jeff Young & Dr. Ken
Noren

A circulator is a device that directs a signal
to a particular port depending on the
direction of signal flow, allowing a single
antenna to both transmit and receive.
Team
Laplace Sauce has been challenged to
develop a high frequency circulator that
pushes the envelope in terms of power
and bandwidth.

Ultimately, a 30
-
88 MHz
circulator that can deliver 50 W of power is
desired
.


Specifications:


Frequency > 30 MHz

10W < Power < 50W

Insertion Loss <
-
.5dB

Isolation >
-
15db




Three Stage Transistor Design

From S. Tanaka IEEE article: Active Circulators


“The Realization of Circulators using Transistors” 1965

Tanaka showed that devices with certain
two
-
port system parameters can be turned
into circulators.

Two NPN circulators were realized.

Results and Lessons Learned:


2N3904


NPN General Purpose Amplifier

MPSH10

NPN RF Transistor

Purpose: To test theory at low
frequencies.


Results: Very good circulator
behavior for 1
-
10MHz.



Conclusion: The 3904 is an
excellent benchmark to test
new methods on that in the
future we may want to employ
on other transistors.

Purpose: To construct a circulator
for the frequencies 30
-
80 MHz


Results: Weak circulation at
frequencies 30
-
50MHz due to
large insertion loss.


Conclusion: Much more detail
and concern must be taken at
these frequencies. Stability, noise,
ringing, and unwanted coupling
are all issues that need to be
addressed.

-25.00
-20.00
-15.00
-10.00
-5.00
0.00
5.00
1.0
10.0
Voltage Gain (dB)

Frequency MHz

Circulator Properties of a 2N3904 Prototype Design

Insertion Loss
Signal Generator Loss
Isolation
Op
-
Amp Approach to

Active Circulators



For the circulator topology
shown to the left, the
performance relies mainly
on the Op
-
Amp. To meet
the required specifications,
choosing the correct Op
-
Amp is essential.

Op
-
Amp Considerations

Slew Rate
-

This is the rate of change of the op amp's voltage output over
time when its gain is set to unity (Gain =1).


Maximum peak output voltage swing
-

The maximum peak
-
to
-
peak
output voltage that can be obtained without clipping when the op amp is
operated from a bipolar supply.


Gain
-
Bandwidth Product
-

This is the product of the op amp's open
-
loop
voltage gain and the frequency at which it was measured.


Current vs. Voltage Feedback?


Final Design

Op
-
Amp:
Texas
Instruments THS3061

Cost: $6.79 each (3
used)


Trim Pot:
Vishay
43P501 500

, 15 turns

Cost: $1.43 each (15
used)


Connector:
AMPHENOL

BNC R/A Jack

Cost: $2.94 each (3
used)


Total Cost: $50.64

Components Used

Testing & Results

Two methods of testing were used to determine the
capabilities of the Op
-
Amp circulator:

Method 1:
A signal was sent through the circulator using a function
generator and then the signal was observed at each output port using
an oscilloscope. Isolation was then determined from the gain seen
between the two output ports. Frequency was then increased until
either signal distortion occurred or the isolation specification was
violated.



Results:

Maximum frequency was unable to determined. The
function generator in the senior design lab was maxed out at 15MHz
at 1
Vpp
.


Method 2:
Used an Agilent N5250C Network Analyzer to determine the
S
-
Parameters. The network analyzer was programmed to sweep
frequencies from 10
-
100MHz.



Results:
Maximum frequency was found at 44.38
MHz.

The limiting
s
-
parameters were the reflection coefficients, S11 & S22.


Port A to B

Port B to C

Port C to A

Conclusion


While we were able to meet the frequency specification for our
circulator, we were unable to achieve the power aspect.



With current available Op
-
Amp technology, circulating a signal in the MHz
range with 50W of power is unachievable.



However, readily available RF transistor technology can bring us well into
both frequency and power specifications. The S
-
Parameters of such
devices are often recorded on their data sheets, and as such they can be
used to design a circulator using the Tanaka’s equations.