AMATEUR RADIO
By GARRY CHATT, VK2YBX
Designing and building attenuators
Many people are haffled by the theory behind
attenuator design hut once understood, attenuators
can he easily made for most applications at
minimal cost. This article outlines the theory
behind attenuator design and provides practical
guidelines for home construction.
Basically, an attenuator is a network of resistors intended to produce a specific loss between a
known source impedance and a
kriown load impedance. Attenuation is normally expressed as a
ratio in decibels, and is the same
regardless of the direction of
operation.
Two basic forms of symmetrical
resistive networks are available
that can be used as attenuators.
Although there are many other configurations that could be discussed
here, this article restricts itself to
easily constructed, symmetrical
networks. These two configurations
are called the "T"-section and the
"1r" -section.
Fig. l(a) shows the circuit for the
T-section attenuator, while Fig. l(b)
shows the 1r-attenuator. The value
of both types Gan be calculated as
shown. For the T-section attenuator
with 50 ohms impedance:
Attenuation in dB = 20 log (Rl +
5'0)/(Rl - 50) when R2 = (50 +
R1)/2R1
T SECTION
7r SECTION
(a)
(b)
Fig.1: circuit configurations for
T·section (a) and 1r-section (b)
attenuators.
For the 1r-section attenuator with
50 ohms impedance:
Attenuation in dB = 20 log (Rl +
50)/(Rl + 50) when R2 = 2 x
50R1/(R1 + 50)
For those not mathematically
minded, Fig.2 shows calculated
resistance values for values of attenuation between 0dB and 40dB,
for an impedance of 50 ohms.
Multiply these figures by 1.5 to obtain 75-ohm attenuators and by 12
for 600-ohm attenuators.
For fixed attenuators, intended
for the lower frequencies (ie, from
audio to 50MHz or so), the physical
layout is of no real consequence.
These " pads", as they are called,
can be used between transmitter
stages to stop interaction, to match
levels, and to calibrate S-meters.
Switched attenuators
A more useful instrument is the
switched or stepped attenuator,
which can be used to insert preset
amounts of attenuation while maintaining the correct impedance.
Such a device is useful for
evaluating the gain of antennas,
preamplifiers and power amplifiers, for preventing receiver
overload and for determining
power amplifier compression. Once
the required amount of attenuation
is determined, a fixed value pad
can be built into the equipment.
At VHF and UHF, the techniques
used in the construction of such attenuators become important. This
is because the higher frequencies
are prone to attenuation errors,
due to stray coupling between
stages. For this reason, it is better
to cascade several stages having a
lower value of attenuation, rather
than use a single stage of high attenuation, as the coupling error will
be a much smaller percentage of
each stage. In practice, 20dB is the
largest single step of attenuation
achievable.
At these higher frequencies , the
OUTPUT
INPUT
SBG;fl
68Q
! 68i
?
f
Fig.3: practical design for a 50-ohm 8-step attenuator with an attenuation range from 1dB-80dB and an upper frequency
limit of 450MHz. It should be built into a diecast metal case with shielding between stages to prevent RF leakage.
70
SILICON CHIP
resistors must be non-inductive
types such as carbon composition,
or better still, cracked carbon.
Fig.3 shows the design of a
50-ohm, 8-step attenuator having
an attenuation range from ldB to
81dB and a useable upper frequency limit of 450MHz. Such a device
can be used with a directional
antenna to locate or track a hidden
transmitter. The closer you get to
the source, the more attenuation required to maintain the same signal
level.
ATTENUATION NETWORKS
R1
I
Attenuation
dB
0
Construction
The physical construction of a
stepped attenuator for VHF and
UHF use is important, and the
following guidelines should be
noted:
(1). House the attenuator in either a
diecast metal box, or fabricate the
housing from double sided printed
circuit board. Shields between the
stages can be made from sections of
double sided PCB, and can be
soldered into place, or slid into the
internal ribbing of the diecast box.
(2). Use good quality connectors.
Normally the most convenient type
is BNC but ensure that it is the correct impedance and rated for RF
operation. Avoid cheap video BNC
connectors.
(3). Use non inductive resistors.
Remember that if you use quarterwatt types, the power handling
ability of the attenuator without
causing damage is one quarter of a
watt! Parallel combinations of halfwatt resistors will give a higher
power rating, but in all cases keep
lead lengths to an absolute
minimum.
(4). It is preferable to use full size
DPDT slide switches as they give
greater isolation than smaller
types. Subminiature toggle or slide
types are not acceptable.
(5). Ensure that your design is practical. It is extremely difficult to accurately provide more than 80 to
90dB of attenuation, as leakage
around the outside of the unit will
affect any measurements that are
made. Design for the maximum
practical attenuation you are likely
to need. Use several smaller stages
of attenuation rather than one large
stage, and never attempt to exceed
20dB in one step.
11
11
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
11 .0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
38.0
40 .0
R1
R1
IT
7r SECTION
T SECTION
50 Ohms
R1
0
0.298
0.568
0.86
1.15
1.43
1.74
2.02
2.29
2.58
2.87
4.32
5.73
7.16
8.56
9.93
11 .32
12.68
14.00
15.33
16.61
17.88
19.14
20.35
21.53
22.69
23.82
24.91
26.0
28.0
29.92
31.71
33.37
34.9
36.32
37.62
38.82
39.92
40.87
42.64
44 .07
45.22
46.17
46.92
47.55
48.04
48.42
48.76
49.01
50 Ohms
R1
R2
00
4184
2190
1455
1089
872
720
619
545
482
434
287.7
215.2
171.1
i41.9
120.7
104.1
92.42
82.3
73.9
66.99
60.9
57.1
51 .3
47.3
43.75
40.6
37 .75
35 .13
30.62
26.81
23.57
20.78
18.37
16.26
14.41
12.71
11.37
10.1
7.99
6.3
5.02
3.98
3.17
2.51
1.99
1.58
1.26
1.00
0
0.6
1.14
1.71
2.29
2.87
3.47
4.04
4.59
5.19
5.72
8.69
11.62
14.62
17.71
21 .5
23.96
27.05
30.37
33.82
37.29
41.05
44.75
48.72
52.85
57.12
61 .57
66.2
71.17
81.65
93.25
106.1
120.2
136
153.9
173.6
195.3
222.5
247.5
312.7
394.7
498.75
629.2
791.7
994.2
1250
1580
1985
2500
R2
00
8375
4782
2908
2175
1743
1436
1240
1092
966.7
870
579.2
436
349.6
292.1
251.7
220.9
197.1
178.4
163
150.6
139.9
130.7
122.9
116.1
110.2
105
100.3
96.17
89.25
83.5
78.84
74.92
71.63
68.83
66.44
64.4
62.64
61 .11
58.63
56.73
55.28
54.14
53.27
52.57
52.03
51 .61
51.27
51.01
Fig.2: calculated resistance values for 50-ohm attenuators (0-40dB attenuation).
The values are simply scaled for 75-ohm and 600-ohm attenuators.
Component Suppliers
(1). Cracked carbon resistors: Allen
Bradley Pty Ltd, 22 Parramatta Rd,
Lidcombe, NSW 2141. Telephone:
(02) 648 2652.
(2). DPDT slide switches: use 3PDT
types from Dick Smith Electronics,
Cat. No. S-1017, $2.99.
it
MAY1988
71
