Page 7
c. Adjust the oscillator amplitude for an indication
of about 7 mV on both null meters. Set the
oscillator frequency for 1 kHz.
d. Adjust both reference supplies for null indi
cations on their respeclive null meters.
e. Change the oscillator frequency to 5 Hz.
f. Readjust the oscillator -amplitude for a null
indication on the null meter connected to the
reference thermal converter. The null meter
connected to the thermal converter under test
should read the same as the reference null
meter.
If
not, record the error. Remember,
the percent change in the thermal converter
output is equal to twice the percent change in
the input.
g. Repeat steps e and f at 50 Hz, 500 Hz, 5kHz,
50 kHz,500kHz, l MHz, 3 MHz, 6 MHz, 8 MHz,
10 MHz, 20 MHz, 30 MHz, 40 MHz, 50 MHz,
and 60 MHz. In each case, record the error.
h. Use the error data collected in the previous
steps to make corrections whenever using the
thermal converter.
If
the difference between
the reference thermal converter and the thermal
converter under test is greater than the specifi
cations in the "with reference to standard"
column in Table 1, the thermal converter is
not operating properly. Refer to repair in
structions in paragraph 25.
L
The calibration of a thermal converter should
Model l1049A
be checked against an NBS reference thermal
converter at least once every six months
during the first year, and once a year there
after.
25. REPAIRlNG THE THERMAL CONVERTER.
26. If the thermal converter is out of calibration below
10 MHz or if it has no output, the UHF thermoelement
should be replaced. Since the values of RI and R2 in
Figure 2 depend on slight variations in thermoelement
characteristics, they must be res elected when the
thermoelement is replaced.
27. SELECTION OF R2.
28. R2 adjusts the current through the thermoelement
to 5 MA with a full scale input. In the l1049A, R2 is
made up of two resistors with a total value of about
510
il,
In the 1l050A, R2 is ,about 110
il,
and in the
11051A, R2 is deleted. R2 should be a 1/2 watt,±5%
metal film resistor.
29. Figure 7 on the next page shows the test setup for
selecting R2. The power supply generates a known in
put, and the de voltmeter monitors the thermoelement
output. R2 is selected to adjust the thermoelement out
put to 6. 5 mV with 90% full scale input. Proceed as fol
lows.
a. With the power supply off, connect the power
supply, thermoelement. de voltmeter and an
initial value of H2. For 11049A, R2 should be
a 240
Q
and 270
Q
resistor connected in series.
For 11050A R2 should be a 110
n
resistor.