Projects & Technical
Simple
test equipment to build
by Peter Parker VK3YE - first appeared in Amateur Radio, April 1997
This month we plug in our soldering irons
and put together some pieces of basic test equipment. Though inexpensive, the
projects described will prove useful in the radio shack. Any one of them can be
assembled in an afternoon. They are described in order of complexity, so that
the reader can find a project suitable for their expertise. Extensive
constructional information is not provided; refer to April 1996's Novice Notes
for advice on obtaining components, construction techniques and sources of
information.
FIELD STRENGTH METER
A field strength meter is perhaps the
simplest piece of RF test equipment that can be built. Used for checking
transmitters, antenna experimentation, and testing RF oscillators, field
strength meters provide an indication of the presence of RF energy. They are not
frequency sensitive and are useful where indication of a change in level is more
important than the actual strength of the signal indicated.
Figure One shows a schematic of an RF field
strength meter. Like a crystal set, it requires no power source. However, unlike
a crystal set, the meter has no tuned circuit. It responds to signals of any
frequency.
The meter works by converting any RF signal
present at the antenna to a DC voltage. This voltage drives a meter movement to
give an indication of relative RF. The meter includes a control to reduce its
sensitivity where required.
Because it uses few parts, a printed circuit
board is not necessary; components can simply be soldered to one another.
However, a box is desirable for operating convenience. The case and aerial from
a discarded toy walkie-talkie was used in the prototype (see photograph), though
any small plastic case will suffice. The meter movement need not be large; we
are only detecting the presence of RF, and not making precise measurements.
A meter from an old radio or tape recorder
should work fine. The diodes can be any germanium type; the actual part number
is not important. Germanium diodes can be recognised by their 6mm-long clear
glass case with two coloured bands towards the cathode end. None of the
component values shown are critical; a 50 percent variation would have little
effect on circuit operation.
To test the operation of the meter, a
transmitter is required to provide a source of RF. Placing the field strength
meter's extended antenna near a handheld VHF rig should produce an indication on
the meter, assuming that the sensitivity control has been set to maximum. No
indication means that the meter is not working. Common construction errors
include connecting the diodes or the meter wrongly and using silicon diodes in
place of the germanium diodes specified. In this case, the meter will still
work, but with reduced sensitivity. The earth wire is optional; when working
with low-powered oscillators, it is useful to clip it to ground (of the circuit
under test) to ensure a better indication on the meter.
Those without a transmitter can use an RF
signal generator or crystal oscillator (such as that described later) for
testing purposes. In this case, place the meter's antenna directly on the output
terminal to verify operation. However, only attempt this with transistorised
circuitry; component ratings and safety considerations make the meter described
here unsuitable for poking around valve equipment.
The field strength meter is a useful
instrument in its own right, but it can be made more versatile. Modifications
include adding an amplifier (for greater sensitivity), including a tuned circuit
(so it only detects signals in a particular band), or converting it into an RF
wattmeter and dummy load. Circuits for such instruments are found in the
standard handbooks.
Figure One
CRYSTAL TESTER
Figure Two shows the circuit of a simple
crystal tester. It switches on a light emitting diode (LED) if the crystal is
working.
The crystal under test is placed in an
oscillator circuit. If it is working, an RF voltage will be present at the
collector. This is rectified (converted to DC) and made to drive a transistor
switch. Applying current to the base causes current to be drawn through the
collector, thus lighting the LED.
If an indication of frequency is required,
simply use a general coverage receiver to locate the crystal oscillator's
output. Note however that when testing overtone crystals (mostly those above 20
MHz) the output will be on the crystal's fundamental frequency, and not the
frequency marked on the crystal's case. Fundamental frequencies are
approximately one-third, one-fifth or one-seventh the overtone frequency,
depending on the cut of the crystal.
The circuit may be built on a small piece of
matrix board and housed in a plastic box. Alternatively, a case made from scrap
printed circuit board material may be used. Either a selection of crystal
sockets or two leads with crocodile clips will make it easier to test many
crystals quickly. The RF choke is ten turns of very thin insulated wire (such as
from receiver IF transformers) passed through a cylindrical ferrite bead. Its
value does not seem to be particularly critical, and a commercially-available
choke could probably be substituted.
The circuit can be tested by connecting a
crystal known to work, and checking for any indcation on the LED. A shortwave
transistor radio tuned near the crystal's fundamental frequency can be used to
verify the oscillator stage's operation. Note however that this circuit may be
unreliable for crystals under 3 MHz, and some experimentation with oscillator
component values may be required.
The crystal checker also tests ceramic
resonators. Other applications include use as a marker generator for homebrew HF
receivers (use a 3.58 MHz crystal) and as a test oscillator for aligning
equipment.
Figure Two:
CAPACITANCE METER
This project is more complex than the others
described earlier. However, when finished, you will have an instrument capable
of measuring all but the largest capacitors used in radio circuits. Unlike
variable resistors, most variable capacitors are not marked with their values.
As well, the markings of capacitors from salvaged equipment often rub off. By
being able to measure these unmarked components, this project will prove useful
to the constructor, vintage radio enthusiast or antenna experimenter.
The common 555 timer IC forms the heart of
the circuit (Figure Three). Its function is to charge the unknown capacitor (Cx)
to a fixed voltage. The capacitor is then discharged into the meter circuit. The
meter measures the current being drawn through the 47 ohm resistor. The 555
repeats the process several times a second, so that the meter needle remains
steady.
The deflection on the meter is directly
proportional to the value of the unknown capacitor. This means that the scale is
linear, like the voltage and current ranges on an analogue multimeter.
The meter has five ranges, from 100pF to
1uF, selected by a five position two pole switch. In addition, there is a x10
switch for measuring higher values and a divide-by-two facility to allow a
better indication on the meter where the capacitor being measured is just above
100, 1000pF, 0.01, 0.1 or 1 uF.
Component values are critical. For best
accuracy, it is desirable that the nine resistors wired to the Range switch have
a 2% tolerance. If 0A47 diodes are not available, try OA91 or OA95 germanium
diodes instead. Construct the meter in a plastic box; one that is about the size
of your multimeter but deeper is ideal. The meter movement should as large as
your budget allows; you will be using it to indicate exact values. A round
70mm-diameter movement salvaged from a piece of electronic equipment was used in
the prototype. The meter you buy will have a scale of 0 to 50 microamps. This
scale needs to be converted to read 0 to 100 (ie 20, 40, 60, 80, 100 instead of
10, 20, 30, 40, 50). Use of white correction fluid or small pieces of paper will
help here.
The components can be mounted on a piece of
matrix board or printed circuit board. Use a socket for the IC should
replacement ever be needed. Keep wires short to minimise stray capacitance;
stray capacitance reduces accuracy.
Calibrating the completed meter can be done
in conjunction with a ready-built capacitance meter. Failing this, a selection
of capacitors of known value, as measured on a laboratory meter, could be used.
If neither of these options are available, simply buy several capacitors of the
same value and use the one which is nearest the average as your standard
reference. Use several standards to verify accuracy on all ranges.
To calibrate, disable both the x10 and
divide-by-two functions (ie both switches open). Then connect one of your
reference capacitors and switch to an appropriate range. Vary the setting of the
47k trimpot until the meter is reading the exact value of the capacitor. Then
switch in the divide-by-two function. This should change the reading on the
meter. Adjust the 10k trimpot so that the needle shows exactly twice the
original reading. For example, if you used a 0.01 uF reference, and the meter
read 10 on the 0.1 uF range, it should now read 20. Now switch out the
divide-by-two function.
If you are not doing so already, change to a
reference with a value equal to one of the ranges (eg 1000pF, 0.01uF, 0.1uF
etc). Switch to the range equal to that value (ie the meter reads full-scale
(100) when that capacitor is being measured. Switching in the x10 function
should cause the meter indication to drop significantly. Adjust the 470 ohm
trimpot so that the meter reads 10. Move down one range (eg from 0.01uF to
1000pF). The meter should read 100 again. If it does not, vary the 470 ohm
trimpot until it does. That completes the calibration of the capacitance meter.
Now try measuring other components to confirm that the measurements are
reasonable.
With care, an accuracy of five percent or
better should be possible on most ranges.
Figure Three:
REFERENCE
Hawker, P Amateur Radio Techniques, Seventh
Edition, RSGB, 1980
FURTHER READING
A Guide to Test Equipment – Novice Notes August 2000
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This page was produced by Peter Parker VK3YE parkerp@alphalink.com.au. Material may be copied for personal or non-profit use only.