Transcript AM HD Load Z - Crawford Broadcasting

April 26, 2006
Presented By:
Ky Luu – Harris Corporation
Ronald Rackley – du Treil, Lundin & Rackley
Page 2
Power Amplifier Loading
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The complex load presented by the antenna system to
the power amplifier in an AM transmitter has a large
impact on both the analog modulation and digital radio
performance.
What is the benefit if the antenna rotation is optimized?
The advantage of output network (DAX, DX and 3DX
transmitters)
Page 3
Transmitter Output Network
To Antenna
(Filter Out)
To PA
(Filter In)

Table outlines typical output network bandwidth of Harris
AM Broadcast transmitters
Transmitter
Model
Filter
Zin
Fcarrier (kHz)
Fupper(-3dB)
Flower(-3dB)
Q
DX10
4.00
600
624.0
576.0
12.5
DX50
8.00
600
640.7
556.2
7.1
3DX50
3.15
600
647.2
552.6
6.3
DAX 5/6
10.77
600
663.6
541.3
4.9
DAX 1
10.43
600
696.3
491.7
2.9
Page 4
Optimum antenna rotation
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The impedance of the antenna load reflected to the Power
amplifier can vary greatly through the transmitter’s output
filter.
Optimum phase rotated load presented to the RF Power
Amplifier MosFET’s needs to provide for minimum impedance
variation (ie: small VSWR circle)
Using the an artificial antenna (VSWR=1.4:1 at +/-15kHz)
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Optimum antenna rotation through the Harris output network actually
reduces the antenna VSWR from 1.4:1 to 1.15:1 (+/-15kHz) at the
power amplifier devices.
If the antenna rotation is NOT optimum, then VSWR increases from
1.4:1 to 2.0:1 at +/-15kHz from carrier frequency
Page 5
50 Ohms Load at Transmitter Output
VSWR 1.5
Transmitter’s load
VSWR 1.0
Power Amplifier’s load
Page 6
Poor Antenna Rotation – Not Recommended
VSWR 2.0
Transmitter’s load
VSWR 1.4
Power Amplifier’s load
Page 7
Optimum antenna rotation
VSWR 1.15
Transmitter’s load
VSWR 1.4
Power Amplifier’s load
Page 8
Antenna Load Rotation Summary

Power Amplifier loading in detail
VSWR 2.0
VSWR 1.4
Load @ PA
Load at Tx output port
VSWR 1.15
Page 9
Load Optimization – Key Points
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The Harris transmitters have best RF mask performance
into a symmetrical load that is rotated from 11:00 to
2:00 o’clock positions presented to the RF output
connector.
Optimum rotation for DX10/15 is 45 degrees instead of
135 degrees for DX, 3DX, and DAX
PDM transmitters are more sensitive to non-optimum
antenna phase rotation.
3DX, DX transmitters are less sensitive to load change.
Harris can provide "equivalent circuit" models that
can be integrated into computer models of antenna
systems
Page 10
3DX Transmitter Spectra
Optimum PA Load
Less than Optimum
Page 11
PDM Transmitter Spectra
Optimum PA Load
Less than Optimum
Page 12
Location of Spectrum Sample Point
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The position of the sample point can affect amplitude
flatness (tilt) across the sidebands and introduce
distortion to the RF envelope shape, but it can not
introduce new spectral components as long as the
transmitter is not over-modulated
Directional coupler forward sample of combined current
on voltage samples gives more accurate results than
current only or voltage only – but still not fully
satisfactory
Far field sample for non-directional may include
propagation errors.
Far field sample for directional will also include pattern
bandwidth effects.
Page 13
Impedance-Induced Measurement Errors
+X
R
F(L)
F(C)
F(C)
F(H)
F(L)
F(H)
-X
Final Amplifier
Output
Far-Field
Page 14
Mis-adjustment of HD-Radio Exciter
Transmitter Output Current
Far-Field of ND Antenna
Page 15
Summary
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Optimum performance depends on handling impedance
related issues correctly.
Transmitter final amplifiers must operate into
symmetrical load with sufficiently low sideband VSWR
(Optimum rotation of antenna complex impedance).
Sampling errors must be considered when adjusting
exciter using spectrum analyzer.
Page 16
Questions ?
Email: Ky Luu – [email protected]
Email: Ronald Rackley – [email protected]
Page 17