Based on the above, to measure the sensitivity of the radio station receiver, we send a test signal of the nominal frequency at which the radio station is used, with a deviation of 3 kHz at a modulation frequency of 1 kHz, to the antenna input from a high-frequency signal generator.
The level of the test signal supplied to the antenna input is set equal to 0,5 mV, i.e. 500 µV, as suggested by GOST. With the sensitivity of modern communication radio stations 0,1 - 1 µV, this is a lot. It is assumed that a mistake has been made in the GOST. Therefore, we will use a level equal to 0,5 μV.
The signal-to-noise ratio is measured using the SINAD method. SINAD - Signal-to-noise and distortion ratio, which means signal-to-noise ratio plus distortion. In technical terms, this is the ratio of the total signal power to the sum of the noise and distortion powers. The formula, describing this is as follows:
If the signal level is expressed in terms of voltage, then this formula will look like:
On the other hand, the nonlinear distortion factor (FND) is determined by the formula:
If there is noise in the signal when measuring FND, then the formula will look like the reciprocal of the SINAD value:
Thus, we can measure the signal-to-noise ratio by the SINAD method through measuring the FND with subsequent calculation.
The enterprise ETC "KONTUR" (Novosibirsk) has created a radio communication service tester (tester) RST-430 for measuring, including the sensitivity of a radio station receiver. The device is multifunctional, it performs a number of tasks in the field of monitoring and measuring various parameters of communication facilities. You can find out about all the functions and technical characteristics here.
In the PCT-430 tester, the main purpose of the FND meter is to measure the signal-to-noise ratio. That is, the noise in the signal is the main information parameter. If you try to measure harmonic distortion in a noisy signal, the result will be incorrect. To measure harmonic distortion, the signal must have a minimum presence of non-fundamental harmonics.
The measured SINAD signal-to-noise ratio is displayed in decibels (dB) and with a FND measurement in percent.
SINAD signal-to-noise ratio of 12 dB corresponds to 25% when measured through FND. How to measure sensitivity, by SINAD or FND, is a matter of personal preference. A change in SINAD signal-to-noise ratio of 1 dB corresponds to a 3% change in FND. Since the noise level is not uniform, the SINAD measurement is easier, since the value on the indicator fluctuates less, however, the measurement through the FND allows you to catch the pattern of noise change.
Measuring the sensitivity of the radio station receiver using the RST-430 tester is performed as follows.
using the F2 button, activate the item for setting the level of the high-frequency signal of the set frequency and set the required value (for most receivers, it is enough to set the level to 0.5 μV, which corresponds to -113 dBm), and through the SPACE button, set the level adjustment step;
using the SHIFT button, turn on the restructuring of the high-frequency signal level with a specified step, while the cursor showing that the option is active will take the form of a double-sided arrow;
using the F3 button, activate the item for setting the deviation and set it to 3 kHz;
using the F5 button, activate the item for setting the modulation frequency and set it to 1 kHz;
use the F4 button to turn on modulation;
press the ESC button to switch to the MEASUREMENT RECEIVER mode and press the F1 button to select how we will measure the signal-to-noise ratio (using the SINAD method or with a SOI meter).
with the ESC button we switch back to the SET RECEIVER mode;
activate the item for setting the high-frequency signal level with the F2 button, the cursor should look like a double-sided arrow;
using buttons 4 and 6, increase or decrease the level of the high-frequency signal to such a value, at which the SINAD value of 12 dB or 25% by FND will be reached in the measurement result display window;
The resulting value of the high-frequency signal level will determine the sensitivity of the radio station receiver.
Do not forget to use the SHIFT button to exit the level adjustment option with a given step. This option does not allow you to set the desired level value with the number buttons, as buttons 4 and 6 are used to decrease or increase the level.