Frequency method

Frequency method of determining the range based on the use of frequency modulation emitted continuous fluctuations; the delay time is determined by measuring the difference between the frequencies of oscillations emitted and reflected signal. Functional diagram of the frequency modulation is shown in Figure 1.1

Figure 1.1 Functional diagram of the range meter with frequency modulation.

High-frequency generator, which are controlled by modulator, generates oscillations with a frequency that varies according to the periodic law (Fig.2.4, solid line). The frequency of the signal reflected from stationary targets, will vary according to this law, but only with a shift along the time axis to the delay time t_D

On the Fig. 2.4, (a), the frequency of the reflected vibrations shown by a dashed line. Reflected signals and vibrations generator are supplied to the mixer. Formed at the mixer output difference frequency (Fig.2.4, b) (the beat frequency) is proportional to the distance of the target. If the angular frequency radiation:

1.1

Figure 1.2 Changing of the frequency of of the emitted and received oscillations:

a) - the frequency of the emitted and received signal;

b) - the converted signal (beat);

c) – changing of the frequency of the converted signal.

The frequency of the signal reflected from stationary targets will be equal

1.2

where Δ ω _m - frequency deviation of the transmitter,

The difference frequency is allocated to the mixer output

1.3

from whence

1.4

Last 2 formulas illustrate the relationship between the target range and the difference frequency and allow us to understand the essence of the method.

To measure the difference frequencies using filters and pulse counters. When using the filter, there are two variants: when a group of filters tuned to a fixed frequency, or a filter with a variable setting. Hit the difference frequency signal in a particular filter (on which indicate the corresponding indicator, such as a neon lamp) will determine the target range.

Advantages of rangefinders with frequency modulation:

Range finders of this type allow us to define a very small range and use of transmitters with low power radiation.

Disadvantages of rangefinders with frequency modulation:

- need to use either two antennas or a complex device for dividing the emitted and received signals;

- decreasing of receiver sensitivity due to leakage in the reception path through the antenna radiation transmitter exposed to random variations;

- high demands on the linearity of the frequency change.