Communication Concepts AN762 Application Note - Manual (Page 8)

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HIVE INF
O
RMA
TI
O
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PRODUCT TRANSFERRED T
O
M/A
–
COM
AN762
8 RF Application Reports
FREQUENCY (MHz)
OUTPUT
HARMONIC CONTENTS (ODD ORDER
)
- 10
1.5 2.0 3.0 5.0 10 30
- 20
- 30
- 40
180 W
140 W
100 W
- 50
- 60
7.0 15 20
a
c
b
a
c
b
BELOW CW CARRIER LEVEL
(dB)
V
CC
= 13.6 V
3RD
ORDER
5TH
ORDER
Figure 9. Output Harmonic Contents (Odd Order)
versus Frequency
The stability and load mismatch susceptibility were tested
at 15 and 30 MHz employing an LC network (2) to simulate
high and low reactive loads at different phase angles. The
maximum degree of load mismatch was controlled by placing
high power 50-Ohm attenuators between the amplfiier output
and variable LC network. A 2 dB attenuator limits the output
VSWR to 4.5 :1, 3 dB to 3.0:1, 6 dB to 1.8:1 etc., assuming
that the simulator is capable of infinite VSWR at some phase
angle. The attenuators for – 1.0 dB or less were constructed
of a length of RG-58A co-axial cable, which at 30 MHz has
an attenuation of 3.0 dB/100 ft. and at 15 MHz 2.0 dB/100
ft. Combinations of the cable and the resistive attenuators
can be used to give various degrees of total attenuation.
The tests indicated the 100 W and 140 W amplifiers to
be stable to 5:1 output VSWR at all phase angles, and the
180 W unit is stable to 9:1. All units passed a load mismatch
test at full rated CW power at an output load mismatch of
30:1, which they were subjected to, until the heat sink
temperature reached 60
°C. For this, the load mismatch
simulator was motor driven with a 2 second cycle period.
Output Filtering
Depending on the application, harmonic suppression of
– 40 dB to – 60 dB may be required. This is best
accomplished with low-pass filters, which (to cover the entire
range) should have cutoff frequencies e.g. 35 MHz, 25 MHz,
15 MHz, 10 MHz, 5.5 MHz and 2.5 MHz.
The theoretical aspect of low-pass filter design is well
covered in the literature (8 ).
A simple Chebyshev type constant K, 2 pole filter (Figure
10) is sufficient for 40 – 45 dB output harmonic suppression.
C1 C3 C2
50 Ω
50 Ω
L1 L2
Figure 10.
NOTE: The use of these amplifiers is illegal for Class D Citizens band
service.
The filter is actually a dual pi-network, with each pole
introducing a – 90 ° phase shift at the cutoff frequency, where
L1, L2, C1 and C3 should have a reactance of 50 Ohms,
and C2 should be 25 Ohms. If C2 is shorted, the resonances
of L1C1 and L2C3 can be checked with a grid-dip meter or
similar instrument for their resonant frequencies.
The calculated attenuation for this filter is 6.0 dB per
element/octave, or – 45 dB for the 3rd harmonic. In practice,
only – 35 to – 40 dB was measured, but this was due to
the low Q values of the inductors (approximately 50). Air core
inductors give excellent results, but toroids of magnetic
materials such as Micrometals grade 6 are also suitable at
frequencies below 10 MHz. Dipped mica capacitors can be
used throughout.
If the filters are correctly designed and the component
tolerances are 5% or better, the power loss will be less than
1.0 dB.
SUMMARY
The basic circuit layout (Figure 1) has been successfully
adopted by several equipment manufacturers. Minor
modifications may be necessary depending on the
availability of specific components. For instance, the ceramic
chip capacitors may vary in physical size between various
brands, and recent experiments show that values > 0.001
µF
can be substituted with unencapsulated polycarbonate
stacked-foil capacitors. These capacitors are available from
Siemens Corporation (type B32540) and other sources. Also
T1 and T2 can be constructed from stacks of ferrite toroids
with similar material characteristics. Toroids are normally
stock items, and are available from most ferrite suppliers.
The above is primarily intended to give an example of
the device performance in non-laboratory conditions, thus
eliminating the adjustments from unit to unit.
REFERENCES:
1. Hejhall R.: Understanding Transistor Response Parame-
ters, AN-139A Motorola Semiconductor Products Inc.
2. Granberg, H.: A Two Stage 1 kW Solid-State Linear Ampli-
fier, AN-758 Motorola Semiconductor Products, Inc.
3. Granberg, H.: Get 300 W PEP Linear Across 2 to 30 MHz
From This Push-Pull Amplifier, EB-27A Motorola Semicon-
ductor Products Inc.
4. Granberg, H.: Broadband Transformers and Power Com-
bining Techniques for RF, AN-749 Motorola Semiconduc-
tor Products Inc.
5. White, John: Thermal Design of Transistor Circuits, QST,
April 1972, pp. 30 – 34.
6. Mounting Stripline-Opposed-Emitter (SOE) Transistors,
AN-555 Motorola Semiconductor Products Inc.
7. Granberg, H.: Measuring the Intermodulation Distortion of
Linear Amplifiers, EB-38 Motorola Semiconductor Prod-
ucts Inc.
8. Reference Data for Radio Engineers, ITT, Howard & Sams
Co., Inc.