An amateur radio antenna receives hundreds and thousands of radio signals simultaneously. Their frequencies can vary depending on the transmission on long, medium, short, ultrashort waves and television ranges. In between, amateur, government, commercial, maritime and other stations operate. The amplitudes of the signals supplied to the antenna inputs of the receiver range from less than 1 μV to many millivolts. Amateur radio contacts occur at the level of the order of several microvolts. The purpose of the amateur receiver is twofold: the selection, amplification and demodulation of the desired radio signal, and the screening of all the others. Receivers for ham radio are available both separately and as part of the transceiver.
The main components of the receiver
Amateur radio receivers should be able to receive extremely weak signals, separate them from noise and powerful stations that are always present on the air. At the same time, sufficient stability is necessary for their retention and demodulation. In general, the performance (and price) of a radio receiver depends on its sensitivity, selectivity, and stability. There are other factors related to the performance of the device. These include frequency coverage and reading, demodulation or detection modes of LW, CB, HF, VHF radios, power requirements. Although the receivers vary in complexity and performance, they all support 4 basic functions: reception, selectivity, demodulation, and playback. Some also include amplifiers to boost the signal to acceptable levels.
Reception
This is the receiver's ability to process weak signals collected by the antenna. For a radio receiver, this functionality is primarily related to sensitivity. Most models have several stages of amplification necessary to increase the power of signals from microvolts to volts. Thus, the total gain of the receiver can reach about one million to one.
It is useful for novice hams to know that the sensitivity of the receiver is affected by electrical noise generated in the antenna circuits and the device itself, especially in the input and RF modules. They occur during the thermal excitation of conductor molecules and in amplifier components, such as transistors and tubes. In general, electrical noise does not depend on frequency and increases with temperature and bandwidth.
Any interference present in the antenna terminals of the receiver is amplified along with the received signal. Thus, there is a limit to the sensitivity of the receiver. Most modern models accept 1 µV or less. Many specifications define this characteristic in microvolts for 10 dB. For example, a sensitivity of 0.5 μV for 10 dB means that the amplitude of the noise generated in the receiver is about 10 dB lower than the signal of 0.5 μV. In other words, the receiver interference level is about 0.16 μV. Any signal below this value will be blocked by them and will not be heard in the speaker.
At frequencies up to 20-30 MHz, external noise (atmospheric and man-made) is usually significantly higher than internal noise. Most receivers have sufficient sensitivity to process signals in this frequency range.
Selectivity
This is the ability of the receiver to tune to the desired signal and reject unwanted ones. The receivers use high-quality LC filters to pass only a narrow frequency band. Therefore, the receiver bandwidth is important for eliminating unwanted signals. The selectivity of many DV receivers is of the order of several hundred hertz. This is enough to filter out most signals close to the operating frequency. All amateur radio receivers of HF and CB ranges should have a selectivity of about 2500 Hz for amateur voice reception. Many DV / HF receivers and transceivers use switchable filters to ensure optimal reception of any type of signal.
Demodulation or detection
This is the process of separating the LF component (sound) from the incoming modulated carrier signal. Demodulation circuits use transistors or lamps. The two most common types of detectors used in RF receivers are the diode for LW and CB and the ideal mixer for LW or HF.
Play
The final process of reception is the conversion of the detected signal into an audio signal for supply to the speaker or headphones. Typically, a high coefficient cascade is used to amplify a weak output from the detector. The audio amplifier output is then fed to a speaker or headphones for playback.
Most ham receivers have an internal speaker and an output jack for headphones. A simple one-stage audio amplifier suitable for working with headphones. A speaker usually requires a 2- or 3-stage audio amplifier.
Simple receivers
The first receivers for hams were the simplest devices, which consisted of an oscillatory circuit, a crystal detector and headphones. They could only receive local radio stations. However, the crystal detector is not able to correctly demodulate the signals of the DW or HF. In addition, the sensitivity and selectivity of such a scheme are insufficient for amateur radio work. You can increase them by adding an audio amplifier to the detector output.
Direct gain radio
Sensitivity and selectivity can be improved by adding one or more cascades. This type of device is called a direct gain receiver. Many commercial SV receivers of the 20s and 30s. used such a scheme. Some of them had 2–4 gain levels to obtain the required sensitivity and selectivity.
Direct conversion receiver
This is a simple and popular approach for receiving DV and HF. The input signal is supplied to the detector along with the RF from the generator. The frequency of the latter is slightly higher (or lower) than the former, so that you can get a heartbeat. For example, if the input is 7155.0 kHz, and the RF generator is tuned to 7155.4 kHz, then a 400 Hz sound signal will be generated by mixing in the detector. The latter enters the high-level amplifier through a very narrow sound filter. Selectivity in this type of receiver is achieved by means of oscillatory LC circuits in front of the detector and a sound filter between the detector and the audio amplifier.
Superheterodyne
It was developed in the early 1930s with the aim of eliminating most of the problems that the early types of amateur radio receivers faced. Today the superheterodyne receiver is used in almost all types of radio communication services, including amateur, commercial, as well as for amplitude and frequency modulation and television. The main difference from direct amplification receivers is the conversion of the incoming RF signal to an intermediate (IF) signal.
RF amplifier
Contain LC circuits that provide some selectivity and limited gain at the desired frequency. An RF amplifier also provides two additional benefits in a superheterodyne receiver. Firstly, it isolates the cascades of the mixer and the local generator from the antenna circuit. For the radio, the advantage is that unwanted signals are weakened, the frequency of which is twice as high as required.
Generator
It is necessary to create a sinusoidal signal with a constant amplitude, the frequency of which differs from the incoming carrier by an amount equal to the IF. The generator creates oscillations, the frequency of which can be either higher or lower than the carrier. This choice is determined by the bandwidth and RF tuning requirements. Most of these nodes in the SV receivers and the lower range of amateur VHF receivers generate a frequency above the input carrier.
Mixer
The purpose of this unit is to convert the frequency of the incoming carrier signal to the frequency of the inverter amplifier. The mixer outputs 4 main output signals from 2 inputs: f 1 , f 2 , f 1 + f 2 , f 1 -f 2 . In a superheterodyne receiver, only either their sum or the difference is used. The rest may cause interference if proper measures are not taken.
Inverter amplifier
The characteristics of an IF amplifier in a superheterodyne receiver are best described in terms of gain (gain) and selectivity. Generally speaking, these parameters are determined by the IF amplifier. The selectivity of the IF amplifier must be equal to the bandwidth of the input modulated RF signal. If it is greater, then any adjacent frequency is skipped and causes interference. On the other hand, if the selectivity is too narrow, some side bands will be cut off. This results in a loss of clarity when playing sound through the speaker or headphones.
The optimal bandwidth of the short-wave receiver is 2300–2500 Hz. Although some of the higher sidebands associated with speech signals go beyond 2500 Hz, their loss does not significantly affect the sound or information transmitted by the operator. A selectivity of 400–500 Hz is sufficient for the operation of a DW. This narrow band helps reject any adjacent frequency signal that may interfere with reception. In amateur radios, the price of which is higher, 2 or more IF amplification stages are used with the previous highly selective crystal or mechanical filter. With this arrangement, LC circuits and frequency converters are used between the units.
The choice of intermediate frequency is determined by several factors, which include: gain, selectivity and signal suppression. For the low-frequency ranges (80 and 40 m), the IF used in many modern amateur radio receivers is 455 kHz. IF amplifiers can provide superior gain and selectivity of 400–2500 Hz.
Beat Detectors and Generators
Detection, or demodulation, is defined as the process of separating audio-frequency components from a modulated carrier signal. Detectors in superheterodyne receivers are also called secondary, and the mixer assembly is primary.
Automatic gain control
The purpose of the AGC unit is to maintain a constant level of the output signal, despite changes in the input. Radio waves propagating through the ionosphere are either attenuated or amplified due to a phenomenon known as fading. This leads to a change in the reception level at the antenna inputs in a wide range of values. Since the voltage of the rectified signal in the detector is proportional to the amplitude of the received, part of it can be used to control the gain. For receivers using tube or npn transistors in the nodes prior to the detector, a negative voltage is applied to reduce the gain. Amplifiers and mixers using pnp transistors require positive voltage.
Some amateur radio receivers, especially the best transistor ones, have an AGC amplifier for greater control over the characteristics of the device. Automatic adjustment may have different time constants for various types of signals. The time constant sets the duration of control after the broadcast is stopped. For example, during the intervals between phrases, the HF receiver will immediately resume full gain, which will cause an annoying burst of noise.
Signal Strength Measurement
Some receivers and transceivers have an indicator indicating the relative strength of the broadcast. Typically, part of the rectified IF signal from the detector is fed to a micro- or milliammeter. If the receiver has an AGC amplifier, then this node can also be used to control the indicator. Most meters are calibrated in S-units (1 to 9), which represent an approximately 6-dB change in received power. The average reading or S-9 is used to indicate a level of 50 μV. The upper half of the S-meter scale is calibrated in decibels above S-9, usually up to 60 dB. This means that the strength of the received signal is 60 dB higher than 50 μV and equal to 50 mV.
An indicator is rarely accurate because many factors influence its performance. However, it is very useful in determining the relative intensity of incoming signals, as well as when checking or tuning the receiver. In many transceivers, the indicator is used to display the status of device functions, such as the final current of a radio frequency amplifier and RF output power.
Interference and restrictions
It is useful for novice hams to know that any receiver may have difficulty receiving due to three factors: external and internal noise and interfering signals. External interference at RF, especially below 20 MHz, is much higher than internal. Only at higher frequencies are receiver nodes a threat to extremely weak signals. Most noise is generated in the first block, both in the radio frequency amplifier and in the mixer stage. A lot of effort has been put into reducing the receiver's internal interference to a minimum. As a result, low-noise circuits and components appeared.
External interference can cause problems when receiving weak signals for two reasons. Firstly, the interference picked up by the antenna can mask the broadcast. If the latter is near or below the incoming noise level, reception is almost impossible. Some experienced operators can receive broadcasts on the Far East even with large interference, but the voice and other amateur signals in these conditions are incomprehensible.