Measuring the sensitivity of the radio station receiver.

The receiver sensitivity is determined by the level of the standard test high-frequency (HF) signal of the operating frequency receiver at the antenna input with a wave impedance of 50 Ohm at a signal-to-noise ratio at the receiver output of 12 dB using the "SINAD" method without turning off the generator signal. It is generally accepted that this is the minimum acceptable level of the signal-to-noise ratio at which speech at the output of the receiver is considered distinguishable.

Terminology:

• the standard test signal is defined as a frequency modulated signal with a carrier frequency equal to the nominal tuning frequency of the receiver and having a standard test frequency deviation at a modulation frequency of 1000 Hz; 
• standard test deviation is the frequency deviation equal to 60% of the maximum allowable deviation;
• maximum permissible deviation is the highest value of the frequency deviation of a radio frequency signal that is allowed to be used in the mobile radiocommunication service, the value of the maximum deviation should not exceed 5 kHz.

How to measure the sensitivity of a radio station receiver

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.

A device for measuring the sensitivity of a radio station receiver

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.

Measurement instruction

Measuring the sensitivity of the radio station receiver using the RST-430 tester is performed as follows.

1. We connect a radio station to the PCT-430 tester according to the scheme for measuring the parameters of the receiver.
2. Preparatory actions are performed on the RST-430 device:
• through the Tab button, go to the main menu and select the INSTALLATION RECEIVER mode by pressing the F1 button;
• in the opened window of parameter settings for testing the receiver, we sequentially perform the following operations: use the F1 button to activate the item for setting the frequency of the high-frequency signal and set the required value, and through the SPACE button we set the frequency tuning step;

  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).

3. We carry out a measurement of sensitivity, for this:
1. turn on the radio station, turn off the noise suppressor (GOST 12252 in paragraph 4.4.10 allows, in the absence of controls and noise shutdown, measurement to be carried out with a noise suppressor). A signal with a modulation frequency should be heard in the loudspeaker, and on the tester indicator screen in the VOLTAGE and LF FREQUENCY lines, the LF voltage and modulation frequency values​​at the receiver output will appear. Turning on / off modulation with the F4 button, check the presence / absence of sound in the loudspeaker for the presence of modulation. In the display window of the measurement result, the value of the measured signal-to-noise ratio is displayed in decibels (when measured by SINAD) or in percent (when measured through FND);

2. with the ESC button we switch back to the SET RECEIVER mode;

3. activate the item for setting the high-frequency signal level with the F2 button, the cursor should look like a double-sided arrow;

4. 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.