. GENERAL RADIO COMPANY
!rom
the
desired
frequency,
or
at
16.631 -
0.018 = 16.613
Me
to
obtain
the
desired
frequency.
Similarly,
if
the
heterodyne reading
were high,
the
heterodyne must
be
set
high
from
~
desired
frequency
by
the
amount
or
the
correction
.
USE
OF
HARMONICS
The
principal
use
for
harmonic methods
lies
in
extending
the
range
of
the
heterodyne
freQuency meter
to
higher
or
lower freQUen·
cies.
In
t:tns equipment every
effort
has
been
made
to
make
this
harmonic
extension
as
simple and
reliable
as
possible
.
EXTENSION
TO
HIGHER
FREQUENCIES
In Figure
4.
are
shown,
on
a
logar-
ithmic·
scale,
the
frequency ranges covered by each harmonic
of
the
heterodyne frequency meter
from
1
to
20
. Each range
is
5hown
as
a
horizon-
tal
line,
the
length5
of
the
lines
being
consoant,
sinca
the percentage frequency
coverage
of
each range
is
the
same.
On
each
line
are
marked
10
intervals
corresponding
to
the
10
coil
ranges
of
the
instrument.
The
interpretation
of
these
coil
range marks
will
agree
with
the
en~
graving
of
the
coil
switch
if
the
first
is
called
10,
the
second 11, and so
on
up
to
19.
The
last
mark
is
then 20, which
repre-
sents
the
highest
frequency which can
be
reached
on
Coil
19.
For any frequency, simply
move
upward
to
the
point
where
the
desired
frequency
line
crosses
the
harmonic range
line.
At
the
intersection
read
off
the
number
of
the
harmonic (which
is
the number
of
the
line),
the
coil
range and an approximate
indication
of
the
condenser
scale
reading
.
For example,
let
the
desired
frequency
be
25
Me.
Entering
at
25
on
the
frequency
scale-
move
upward meeting
line
2
at
Coil
12, condenser approximately
1/2
.
The
de-
sired
frequency
is
then
obtained
when
the
heterodyne
is
set
to
Coil
12,
with
the
con
-
denser
at
about
half
-
scale,
~sing
the
sec-
ond
harmonic.
The
harmonic
number
being
known
(in
this
case
2),
the
exact
setting
of
the
heterodyne
is
25/2 = 12.50
Me
.
In
extending
the
range
to
higher
fre
-
quencies,
it
should be noted from Figure 4
that
the
gain
toward
higher
frequencies
1s
obtained
only
at
the
high frequency ends
of·the
harmonic ranges. Consequently,
to
use
the
lowest harmonic
in
a given high
frequency measurement, always use
as
high
a fundamental frequency
as
possible
. In
searching
for
an
unknown
high
frequency
always
start
with
the
heterodyne
set
on
Coil
19
with
the
condenser
at
1.0
(20
Me)
and
progress
toward lower
frequencies
un-
til
zero
beat
with
the
unknown
frequency
is
picked
up
. For example, suppose a
fre-
quency
near
60
Me
is
to
be measured.
En-
terin~
Figure 4
at
60
on
the
frequency
scale,
an
intersection
on
line
3,
Coil
19,
condenser
maximum
is
found.
Progressing
further
upward, an
intersection
on
line
4,
Coil
15, condenser zero
is
obtained.
Simi-
larly,
line
5,
Coil 12, condenser
zero,
line
6, Coil 10, condenser 0.
Any
one
of
these
settings
gives a harmonic
at
60
Me,
being,
respectively,
the
3rd,
4th,
5th
and
6th.
The
lowest harmonic would
in
this
case be the
third.
The
unknown
frequency
is
then 3 times
the
frequency of
the
het-
erodyne.
(In
this
case 3 x (19+1.0) -60.
If
the
harmonic
beat
against
the
unknown
fell
at
19.78
Me
(instead
of
just
20
Me)
the
harmonic frequency would
be
3 x 19.78
= 59.34
Me.
If
no
idea
is
had
of
the value
of
an
unknown
frequency,
the
procedure
is
to
start
at
the
high frequency end
of
the
het-
erodyne range and note
the
successive
set-
tings
of
harmonic
beats
as
the
frequency
of
the
heterodyne
is
progressively
reduced.
Then
with
the
coil
and condenser
settings
of
the
highest
frequency
point,
enter
a
harmonic
line,
and
search
for
the
next low-
er
settings
on
the
line
immediacely above
the
one
entered
.
If
agreement
is
obtained,
the
proper
line
was
entered
;
if
not
,
move
up
or
down
a
line
and
try
again.
If
more
than
two
settings
of
the
heterodyne were
obtained
, agreement should
be
obtained
for
every
line
crossed.
This
is
simply
another
way
of
stating
thao the
s~ccessive
funda-
mental
frequencies
of
the heterodyne, each
multiplied
by
the
correct
harmonic
number
must
give
the
same
answer
for
the
unknown
frequency. A quick
test
on
the
figure
will
generally
fix
the
correct
harmonic
num-
bers
much
more
quickly
than
numerical
tri
-
als.
·
EXTENSION
TO
LOWER
FREQUENCIES
When
measuring
.fre-
quencies below the
fundamental range
of
the heterodyne frequency meter, the
actual
measurement
is
made
at
a harmonic of the
unknown
frequency.
If
a
sufficiently
strong
signal
of
the
unknown
frequency
is
applied
to
the
COUPLING
terminals,
harmon-
ics
will
be
generated
in
the
detector
itself
. With
weak
signals,
however,
an
external
means
of
harmonic
generation
must
be
provided. In Figure 5
are
shown
the
sub-harmonic frequency ranges
of
the
het-
erodyne frequency meter.
The
general
de-
scription
of
the
chart
follows
that
al-
ready given
for
Figure 4
F'rom
this
chart
the
appropriate
submultiple
number
(sub-
harmonic number),
the
coil
switch
setting
and
approximate condenser
setting
may
be
found
for
frequencies
below the range
of
the heterodyne frequency meter. For
exam
-
ple,
suppose
the
settings
of the
hetero-
dyne
are
desired
for
a frequency
of
4800
-3-
