TB 9-4931-539-35
11. Capabilities
a. VLF Tracking Receiver. The VLF tracking receiver is capable of calibrating to
1 part in 1011 on a noon-to-noon basis when propagation conditions are good. Sources
having a 1 MHz or 100 kHz output, minimum stability of 1 part in 106 , and a voltage
level of 0.5 to 5.0 V into 1000Ω may be used with the receiver. The receiver is a phase
tracking system and is not directly comparable to the more commonly used, super
heterodyne receivers. The VLF receiver compares the phase of the received signal to
the phase of local 100 kHz or 1 MHz frequency source and produces an error signal. The
error is recorded on a strip chart. The receiver will not operate without a local 100 kHz
or 1 MHz source.
parts in 1010 per day. Output frequencies are 100 kHz, 1 MHz, and 5 MHz. Front and
back outputs may be used simultaneously.
c. Frequency Difference Meter (FDM). The FDM can be used to compare stable
100 kHz and 1, 2.5, 5 and 10 MHz sources to the quartz oscillator or other suitable crystal
(or atomic) sources. The FDM compares an unknown frequency standard with a
reference frequency standard and indicates the difference. The FDM indication is a
relative reading. Therefore, the accuracy and stability of the reference must be
considered in using the FDM. The comparison capability of the FDM is from 10 parts
in 107 through 10 parts in 1011 (direct reading). By making observations over a period
of time the relative stability of an unknown standard can be calculated. Addition of the
stability of the reference standard (which is available from VLF measurements) to the
relative stability of the unknown frequency standard will yield the stability of the
unknown standard.
d. Omega Gating Unit (OGU). The OGU enables the VLF receiver to track omega
transmissions. Omega stations broadcast during assigned time segment of a 10 s period
on several frequencies including 10.2 kHz and 13.6 kHz. The OGU is basically an
electronic switch that enables reception only during a selected time segment. The OGU
shall be installed with the VLF tracking receiver and used as propagation conditions
permit.
e. VLF/MSK Converter. The VLF/MSK converter enables the VLF receiver to
track a form of code known as MSK (minimum-shift-keying). This is a type of MSK in
which the bit frequency is equal to twice the difference frequency. In consequence, if a
bit is transmitted at frequency No. 1, then one bit at frequency No. 2, and a third at
frequency No. 1 again, the phase of frequency No. 1 during the third bit will differ by 180
degrees from what it was during the first bit. Therefore, there is no coherent "carrier"
at either of the two frequencies. By a process of frequency doubling, those transmissions
at zero phase and those at 180 degrees are restored to equal phase at the second
harmonic frequency. Phase tracking of the second harmonic with a VLF tracking
receiver is then possible.
