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Fig. 3. The grid -dip meter. Top
portion may serve as a wavemster.
it is used as the tank circuit of an oscillator.
This provides more than adequate
range to cover the less than 2:1 frequency
range required by the new 470-
890 mc. u.h.f. band.
Inductance of the cavity is varied by
rotating a brass fin in the cavity field.
This fin acts as a shorted turn and represents
a convenient way of trimming
the inductance of the tuned circuit.
The practicability of this device lay
in the ability to manufacture the dielectric
sleeve with sufficient accuracy.
Should the plunger have lateral motion
in the dielectric once its position is set,
the unit would be susceptible to micro -
phonics and backlash, thus proving
quite unsatisfactory for use in a u.h.f.
tuning element. Production techniques
for realizing almost unheard of accuracies
have been developed exclusively for
this application, making the device commercially
feasible at a reasonable cost.
Advantages
The question now arises as to why
this particular arrangement was finally
chosen as the ideal u.h.f. tuning element
for the new band of frequencies.
Most important is (1) that the construction
is simple, and the severe close
tolerances required by u.h.f. are easily
held. Then, (2) there are no moving
electrical contacts. This results in a
long, noise -free life, with no contact
finger or plunger wiper wear. The fact
Fig. 5.
Diagram of sweep generator.
that there are no moving electrical contacts
also results in a considerably
higher Q tuning element than would
otherwise be possible.
Extremely high Q's are attainable
using this configuration (3) ; Q's of
1000 or more have been achieved. The
use of high Q circuits for the tuner element
insures low noise figure and high
gain. Also, trimming and padding of
the unit is easily achieved (4), thus assuring
three -point tracking.
Rigid construction (5) assures excellent
frequency stability. As a matter of
fact, the unit lends itself admirably to
virtual self -compensation of thermal
drift. The completely shielded construction
makes the unit independent of its
environment (6) since its field is internal;
and outside motion, such as
hand capacitance, has practically no
effect on resonant frequency. This construction
also keeps radiation from the
tuning element to a minimum.
The reasons given above were all considered
important in arriving at the
choice of a tuning element. There is,
however, another very important reason
for its choice which is (7) that the
Flq. 4. Capacitance tuning of cavity.
whole unit lends itself admirably as a
generic element for a line of u.h.f. test
equipment.
The following u.h.f. equipment is required
in a well- equipped laboratory
working to develop u.h.f. television receivers
and associated products:
1. Signal generator
2. Sweep generator
3. Wavemeter
4. Grid -dip meter
5. Composite picture, sound and sync
signal generator
These instruments have recently been
developed by Granco Products Inc. to
fill its own growing need for laboratory
and production instruments in the development
and production of u.h.f. television
products. The top unit in Fig. 3
shows the wavemeter design using the
tuning element described. A small plug -
in coupling loop is brought out at one end
of the "cavity" for use as a probe. The
signal is rectified by a 1N72 crystal and
the detected current read on a compact
meter conveniently mounted in the end
of the instrument. Provision is also
made to plug in a more sensitive meter
when required. Because of the high Q
of the cavity, this wavemeter is unusually
sensitive, and extremely weak
signals can be detected with negligible
reaction on the circuit under observation.
The instrument as presently constituted
has a 13" dial scale with accurate
individual calibration from 380 to
1000 mc. This unit is particularly useful
in the design of any oscillator both
as a wavemeter and relative power output
meter. It can be used in the latter
function by being placed in a fixed position
relative to the circuit under test,
making the meter reading a function of
the power output of the oscillator.
By converting the wavemeter into an
oscillator, capable of either external or
internal amplitude modulation, the versatile
grid -dip meter (shown in Fig. 3)
was developed. A switch is provided to
turn off the oscillator for operation similar
to the wavemeter. The addition of
the oscillator has greatly extended the
instrument's utility. Because of the accurate
frequency dial, high output and
stable operation, it may be used as a
compact u.h.f. signal source for numerous
applications - for example, as a
marker source when sweeping a television
receiver, as a probe for detection of
parasitics and in determining the self -
resonant frequency of circuit components.
Provision is also made for earphones
or other types of detection facilities.
The power cable to the oscillator has
been so designed that it will plug
directly into the power connector of
the Measurements Corporation's popular
megacycle meter, thus extending the
range of this instrument from 400 mc.
through 1000 mc.
A small, compact sweep generator
covering the u.h.f. television band, and
having a sweep width of approximately
±80 me., is shown in block diagram
form in Fig. 5. This instrument uses a
resonant cavity circuit mounted so that
the cavity rides on a smooth rod, the
position of which is controlled by the
center frequency knob. As the cavity is
moved, a plunger moves in to the cavity
to change its frequency. This plunger
(Continued on page 30)
Fig. 6. Block diagram and waveforms
for the picture signal generator.
470 MINUS VN. F SIGNAL
TO
ego MINUS VHF SIGNAL
CENTER FREQUENCY
VARIABLE FROM 470 -890 MC.
4MC.
K
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R A D I O - E L E C T R O N I C E N G I N E E R I N G APRIL 1953