2-7 OPERATING THE 540A ABOVE 5 KMC - INCREASING SENSITIVITY
The maximum sensitivity of the Transfer Oscillator and the highest
frequency that can be measured depends largely upon the strength
of the harmonics that can be generated in the crystal diode mixer.
The mixer in the Transfer Oscillator is of a broadband design and
operates from low frequencies to more than 5 kmc. As higher
and higher frequencies are measured, however, the strength of the
harmonics generated by the crystal decreases with an accompany-
ing loss in sensitivity (see Figure 13). Therefore, any means of
strengthening the harmonics would increase sensitivity and extend
the upper frequency measurement range.
When frequencies above approximately 5 kmc are to be measured,
it is desirable to use an @ 440A Tunable Detector externally,
to generate and tune the harmonics for increased amplitude. Fig-
ure 14a and b show how one or two 440A Tunable Detectors can be
used toextend the range of frequency measurement to 12,400 mega-
cycles with increased sensitivity. Although only one detector mount
is needed for measurements to 12 kmc, better sensitivity and lower
noise is obtained if separate detectors are used, one for harmonic
generation and the second for mixing, as shown in Figure 14b. The
graph in Figure 13 shows how measurement sensitivity decreases
with increasing frequency, due to the decreasing strength of the
harmonics that must be used. The right hand curve shows the in-
crease in sensitivity and frequency-measurement range that is ob-
tained by using two 440A Tunable Detector Mounts shown in Figure
14b. Due to the fact that microwave crystals vary considerably
frcm sample to sample in their harmonic generating ability, a varia-
tion of 10 db is quite possible in the useful sensitivity obtainable at
the higher frequencies. If signal levels are low, it may be possible
to improve the system sensitivity considerably by finding a crystal
which generates stronger harmonics in the frequency region being
me as ur e d.
2 -8 THE FREQUENCY CONTROL JACK ON THE REAR PANEL
The FREQUENCY CONTROL jack on the rear panel of the 540A is
to increase the flexibility of application. Electrically, this jack pro-
vides a means for modulating the fundamental frequency generated in
the 540A, or a means of adjusting the fundamental frequency by small
increments. Variation of the oscillator frequency is accomplished by
applying a steady or varying voltage, as desired, to the jack or by con-
necting a variable resistance across the jack. In either case, the ef-
fect of the applied voltage or resistance is to alter the plate-to-ground
capacity in the oscillator circuit to produce a slightly different fre-
quency of oscillation. The degree of frequency shift which results when
a 25,000-ohm potentiometer is connected across the jack is approximate-
ly 80 kilocycles at 200 megacycles (0. 04oJ0) and about 10 kilocycles at
100 megacycles (0. 005oJo). A few uses for this feature are electrical
fine frequency adjustment, spectral analysis, automatic frequency
control or extension of frequency measurement range.
2-15 |
ACEC
