September 1956 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Grid dip meters, aka grid dip oscillators (GDO in this article),
are extremely useful for determining when a circuit is functioning
at resonance. In the words of Sherlock Holmes, "It is simplicity
in itself" the way a GDO operates. The grid dip oscillator emits
power at a calibrated frequency which is absorbed (or not) to
a degree depending on how close to resonance the external circuit
under test is to that frequency. The nice thing about this type
of instrument is that it does not need a wired or other type
of physical connection to the circuit. Of course modern day
GDOs do not use vacuum tubes with grids that register relative
current levels as an indication of resonance, but the nomenclature
has persisted. Besides, there are still many classic grid dip
meters both in use and available for purchase at Ham swap meets
and on venues like eBay. In fact, the exact
Heathkit model GD-1A featured here is currently listed.
How to Use a Grid Dipper
By B. Van Sutphin
Within the past ten years, the grip-dip oscillator - or GDO,
as it is sometimes called - has enjoyed a new surge of popularity.
Numerous experimenters now consider the GDO as necessary in
the modern ham shack or workshop as the VOM or VTVM. Grid-dip
oscillators have been used in electronics laboratories for many
years, but recently wide-range units in kit form became available
at prices to fit the budget of the average ham or experimenter.
With a grid-dip oscillator, you can check
circuits, frequency, and realign your receiver
The grid-dip oscillator is really a calibrated, wide-range,
low-power r.f. oscillator coupled to an indicating device. An
example of a commonly available GDO unit is shown in the photo
above. Note the use of plug-in coils so that the wide frequency
range - generally from about 2.0 mc. to 250 mc. - can be obtained
with minimum r.f. losses, and so that the GDO can be easily
coupled to the circuit under test.
When the oscillator grid circuit is coupled to an external
resonant circuit and the oscillator is tuned to the frequency
of the external circuit, power will be absorbed from the oscillator
and the grid current will decrease. The resonant frequency of
the external circuit can then be read from the calibrated dial
of the grid-dip oscillator. The "sensitivity" control is used
to set the initial grid current reading to approximately half-scale
on the meter - so that the dip will be sharp and easily recognized,
and. to allow for differences in oscillator activity on the
various ranges. When the instrument is used as an indicating
wavemeter, this control limits the current through the meter.
Photo illustrates method of coupling the
GDO to a coil when making tests. This type of coupling is best
both when the instrument is used as a grid-dip oscillator and
when it is used as indicating wavemeter.
Testing Circuits. There are many uses for
a GDO. Suppose that you have just finished winding the coils
for a receiver or converter described in Pop'tronics and want
to be sure they cover the desired frequency range. Merely couple
the GDO to each of the coils in turn and look for the dips in
the meter reading. By tuning each external circuit over its
range and checking for resonance at each end, you can quickly
determine whether the coils have the desired tuning range.
Almost any resonant circuit can be checked just as easily.
One important point in connection with this type of testing
is that it is not necessary to apply power to the circuit under
test. The only power required is for operating the GDO.
Method of coupling the GDO to an AM receiver
loop during alignment is shown at the left. The individual r.f.
and i.f. trimmers should be adjusted for maximum reading. On
the opposite page is an alignment table for a typical a.c.-
d.c. receiver, adapted for using GDO as a signal generator.
Here is another example of the kind of tests you can make with
the GDO. In a ham transmitter having frequency multiplier stages,
you must be sure that the individual stages are tuned to the
proper frequencies. Consider the four-stage 2-meter transmitter
shown in Fig. 1. The plate circuit of the oscillator must be
tuned to 16 mc., the plate circuit of the first tripler to 48
mc., the plate circuit of the second tripler to 144 mc., and
the plate circuit of the final stage to 144 mc. By setting the
GDO to each of these frequencies in turn and trying to tune
the particular transmitter circuit so that a dip is obtained,
you can tell whether the circuits can be tuned to resonance
at the proper frequencies. This type of testing is particularly
useful in preventing damage to expensive transmitting tubes
in case one of the plate circuits cannot be adjusted to resonance.
This basic testing method can be used in any receiver or
transmitter containing a tuned circuit. The ability to reach
resonance of the tuning range of the circuit can be checked
very quickly. Everyone who displays any interest in radio has
had a neighbor or friend come by, at one time or another, dragging
his little table-model radio and explaining: "It just stopped
suddenly. I took the back off to reach the tubes and saw some
loose screws, so I tightened them. One tube was bad and I replaced
it - but the set doesn't play now."
Obviously, alignment is in order. Many experimenters hesitate
to undertake such a job because they do not have a commercial
signal generator. With a GDO, however, they have a signal generator
and quickly can realign the receiver, although it is still generally
best to obtain complete alignment data beforehand. This is included
with the service information which can be purchased from many
radio parts jobbers.
Fig. 1. A four-stage 2-meter transmitter.
By setting the GDO to each of the frequencies shown, in turn,
and trying to tune the particular transmitter circuit so that
a dip is obtained, you can tell whether the circuits can be
tuned to resonance at the proper frequencies.
Aligning Receivers. This table will be used
as an example in the following discussion. Incidentally, this
alignment procedure is standard for sets of this type and can
be used if it is inconvenient or impossible to buy the service
information for a particular brand of receiver.
Since grid dip oscillators do not have a provision for modulating
the output signal, some indicating device must be connected
to the receiver during alignment. A sensitive d.c. voltmeter
(20,000 ohms per volt, on VTVM) connected across the receiver
volume control is the method commonly used.
Connect the indicating device to the receiver circuit. Tune
the set to the low-frequency end of the dial. Plug the proper
coil in the GDO and set the dial to the desired i.f, The preferred
method of coupling the GDO to the receiver during alignment
is shown on the preceding page.
Adjust the i.f. trimmers for maximum reading of the indicator.
Then set the receiver dial to the 1500-kc. point and adjust
the oscillator trimmer in the receiver so that a 1500-kc. signal
from the GDO gives maximum indication. Finally, adjust the r.f.-or
mixer trimmer-on the receiver for maximum indication. (If there
is a strong local station operating between 1300 kc. and 1600
kc., you can use the signal from the station in adjusting the
oscillator and r.f. trimmers in the receiver. Simply adjust
the oscillator trimmer so that the signal comes in at the proper
point on the dial and adjust the r.f. trimmer for maximum output
from the receiver.)
For output indication, connect a sensitive d.c. voltmeter
across the second detector load resistor or receiver volume
control, the positive meter lead going the the B- side of the
Note: A signal from a station operating between 1300 kc.
and 1600 kc. can be used in adjusting the oscillator trimmer
and , the mixer trimmer.
Aligning home-made receivers and converters is just as easy.
Connect an indicating device to the equipment and loosely couple
the GDO to the input circuit. Set the GDO to the proper frequency
and go on to adjust the various stages. Most grid-dip oscillators
have a switch which will convert the instrument to a wide-range
indicating wavemeter or simple diode phone monitor with the
addition of a pair of headphones.
Checking Frequencies. Novice hams are aware
that FCC regulations require all ham stations to have some means
of checking their frequency other than the calibration of the
crystal or VFO used in the transmitter. With the GDO set for
use as an indicating wavemeter, it is ideal for frequency checking
or "band spotting."
Of course, the calibration of the GDO dial is not accurate
enough for use in calibrating a VFO, but it will indicate whether
a transmitter is operating in the band. If greater accuracy
is desired, the GDO calibration can be checked against a frequency
standard of known accuracy and a specially drawn calibration
scale can be substituted.
When the GDO is set for use as an indicating wavemeter, it
can also be employed as a phone monitor to check the modulation
quality of the output signal. Simply plug headphones in the
jack on the panel, lightly couple the GDO coil to the transmitter
output, and listen while someone else uses the mike. This should
provide an accurate check of your ham station phone signal.
Only a few of the many uses for the valuable grid-dip oscillator
have been discussed here.
You will find others described in the instruction manual
accompanying this versatile instrument.
Posted March 15, 2015