wires are twisted together neatly. Iden- the two capacitors are fixed using adhesive calibrated knobs.
brated knobs. You can then calibrate
tify the start and finish of each winding (hot melt glue is preferred). Take care not the settings of the three controls so that
with a buzzer and battery or an ohm- to let any tags from the capacitors touch they can rapidly be reset when you change
meter. The finish of the first winding is the box as both sides of both capacitors frequency bands.
joined to the start of the second, as shown are not earthed.
in Fig 19.67. An appropriate RF socket, such as a Operation
The two capacitors are twin 200-pF SO-239, BNC or phono socket may be The best indication of optimum match-
polyvaricon capacitors with both gangs used—the choice is yours and should suit ing can be achieved using an SWR bridge;
connected in parallel to give 400 pF max. your existing equipment. 2-4 mm sockets the ATU controls are adjusted sequentially
You have to drill holes in the box for the may be used for the balanced output. See and several times for minimum SWR. If
control shafts of the capacitors and the Fig 19.68 for layout. It is a good idea to used for receive only, the best antenna-to-
switch and the RF sockets. The switch is make a graduated dial for each of the three receiver match can be achieved by adjusting
fixed using a nut on the control shaft and control knobs. An alternative is to use the controls for maximum signal.
A SWITCHED ATTENUATOR
How many times has a signal been too sion is not essential. section exactly 4 dB, the resistor values
strong for the experiment you wish to The most useful attenuator is a switched would have to be 220.97 and 23.85 Ω. The
carry out? It could be from an oscilla- unit covering 0 to 60 dB (or more) in 1-dB values used are 220 and 24 Ω, giving an
tor on the bench or from signals from an steps. This is not as difficult as it first seems attenuation value of 4.02 dB. The switches
antenna overpowering a mixer. This at- because, by summing different attenuators, must have low capacitance between the
tenuator will solve those problems and is we can obtain the value we need. It takes contacts, and simple slide switches are the
presented from Practical Projects, courtesy only seven switches to cover 65 dB. The best selection. J1 and J2 must be coaxial
of the RSGB. seven values of attenuation are 1, 2, 4, 8 jacks (builder’s choice).
and 10 dB, and two at 20 dB; these can be The attenuator is housed in a box made
ATTENUATORS switched in or out at will. As an example, from epoxy PCB material. The top and
When designing the attenuators, we for 47 dB, switch on the two 20-dB pads sides are cut to size and soldered into a
must take into account the possibility of plus the 4, 2 and 1-dB pads. box. It is easier to cut the switch holes
poor shielding. There is hardly any point prior to making the box. After the box
in designing a 20-dB attenuator when the Construction has been constructed, screens made from
leakage around the circuit is approaching The resistor values shown in Fig 19.69 de- thin brass shim should be cut and soldered
this value. It is also important to decide termine the attenuator’s accuracy at around between the switch holes to shield each
on the accuracy required. If it is intended 5%. This is done for practical reasons, section. Next, the switches are fitted and
to do very accurate measurements, the to make use of available 1⁄4 W, 1% resis- the unit wired up. When this is done, the
construction has to be impeccable, but for tor values. For example, if we wanted to unit is checked and a back cover, securely
comparisons between signals such preci- make the attenuation value of the 4-dB earthed to the box, is fitted.
Fig 19.69 — The attenuator consists of seven pi network sections, so-called because each pad (eg, R1, R3 and R2) resembles the
Greek letter pi (π). Input and output impedances are 50 Ω.
19.44 Chapter 19
