An amateur radio antenna receives hundreds and thousands of radio signals simultaneously. Their frequencies may vary depending on the transmission on long, medium, short, ultrashort waves and television bands. Amateur, government, commercial, maritime and other stations operate in between. The amplitudes of the signals applied to the antenna inputs of the receiver vary from less than 1 μV to many millivolts. Amateur radio contacts occur at levels on the order of a few microvolts. The purpose of an amateur receiver is twofold: to select, amplify and demodulate the desired radio signal, and filter out all the others. Receivers for radio amateurs are available both separately and as part of the transceiver.
Main components of the receiver
Ham radio receivers must be able to pick up extremely weak signals, separating them from the noise and powerful stations that are always 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 operationaldevice characteristics. These include frequency coverage and reading, demodulation or detection modes for LW, MW, HF, VHF radios, power requirements. Although 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 handle the weak signals picked up by the antenna. For a radio receiver, this functionality is primarily related to sensitivity. Most models have several stages of amplification necessary to increase signal power from microvolts to volts. Thus, the overall receiver gain can be in the order of a million to one.
It is useful for novice radio amateurs 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 arise from thermal excitation of conductor molecules and in amplifier components such as transistors and tubes. In general, electrical noise is frequency independent and increases with temperature and bandwidth.
Any interference present at the receiver's antenna terminals is amplified along with the received signal. Thus, there is a limit to the sensitivity of the receiver. Most modern models allow you to take 1 microvolt or less. Many specifications define this characteristic inmicrovolts 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 0.5 µV signal. In other words, the receiver noise level is about 0.16 μV. Any signal below this value will be covered by them and will not be heard in the speaker.
At frequencies up to 20-30 MHz, external noise (atmospheric and anthropogenic) is usually much higher than internal noise. Most receivers are sensitive enough to process signals in this frequency range.
Selectivity
This is the ability of the receiver to tune in to the desired signal and reject unwanted ones. The receivers use high quality LC filters to pass only a narrow band of frequencies. Thus, receiver bandwidth is essential to eliminate unwanted signals. The selectivity of many DV receivers is on the order of several hundred hertz. This is enough to filter out most signals close to the operating frequency. All HF and MW amateur radio receivers must have a selectivity of about 2500 Hz for amateur voice reception. Many LW/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 low-frequency component (sound) from the incoming modulated carrier signal. Demodulation circuits use transistors or tubes. The two most common types of detectors used in RFreceivers, is a diode for LW and MW and an ideal mixer for LW or HF.
Playback
The final process of receiving is converting the detected signal into sound to be fed to the speaker or headphones. Typically, a high-gain stage is used to amplify the weak detector output. The output of the audio amplifier is then fed to a speaker or headphones for playback.
Most ham radios have an internal speaker and a headphone output jack. A simple single stage audio amplifier suitable for headphone operation. The speaker usually requires a 2- or 3-stage audio amplifier.
Simple receivers
The first receivers for radio amateurs were the simplest devices that consisted of an oscillatory circuit, a crystal detector and headphones. They could only receive local radio stations. However, a crystal detector is not able to correctly demodulate LW or SW signals. In addition, the sensitivity and selectivity of such a scheme is insufficient for amateur radio work. You can increase them by adding an audio amplifier to the output of the detector.
Direct-Amplified Radio
Sensitivity and selectivity can be improved by adding one or more stages. This type of device is called a direct amplification receiver. Many commercial CB receivers from the 20s and 30s used this scheme. Some of them had 2-4 stages of amplification to getrequired sensitivity and selectivity.
Direct conversion receiver
This is a simple and popular approach for taking LW and HF. The input signal is fed 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 a beat can be obtained. For example, if the input is 7155.0 kHz and the RF oscillator is set to 7155.4 kHz, then mixing in the detector produces a 400 Hz audio signal. The latter enters the high-level amplifier through a very narrow sound filter. Selectivity in this type of receiver is achieved using oscillatory LC circuits in front of the detector and an audio filter between the detector and the audio amplifier.
Superheterodyne
Designed in the early 1930s to eliminate most of the problems faced by early types of amateur radio receivers. Today, the superheterodyne receiver is used in virtually all types of radio services, including amateur radio, commercial, AM, FM, and television. The main difference from direct amplification receivers is the conversion of the incoming RF signal to intermediate signal (IF).
HF amplifier
Contains LC circuits that provide some selectivity and limited gain at the required frequency. The RF amplifier also provides two additional benefits in a superheterodyne receiver. First, it isolates the mixer and local oscillator stages from the antenna loop. For a radio receiver, the advantage is that attenuatedunwanted signals twice the desired frequency.
Generator
Needed to create a constant amplitude sine wave whose frequency 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 MW receivers and low band amateur VHF receivers generate a frequency above the input carrier.
Mixer
The purpose of this block is to convert the frequency of the incoming carrier signal to the frequency of the IF amplifier. The mixer outputs 4 main outputs from 2 inputs: f1, f2, f1+f 2, f1-f2. In a superheterodyne receiver, only either their sum or difference is used. Others may cause interference if proper measures are not taken.
IF amplifier
The performance of an IF amplifier in a superheterodyne receiver is best described in terms of 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 incoming modulated RF signal. If it is larger, then any adjacent frequency is skipped and causes interference. On the other hand, if the selectivity is too narrow, some sidebands will be clipped. This results in a loss of clarity when playing sound through the speaker or headphones.
The optimal bandwidth for a shortwave receiver is 2300–2500 Hz. Although some of the higher sidebands associated with speech extend beyond 2500 Hz, their loss does not significantly affect the sound or information conveyed by the operator. The selectivity of 400–500 Hz is sufficient for the operation of the DW. This narrow bandwidth helps to reject any adjacent frequency signal that might interfere with reception. Higher priced amateur radios use 2 or more IF gain stages preceded by a highly selective crystal or mechanical filter. This layout uses LC circuits and IF converters between blocks.
The choice of intermediate frequency is determined by several factors, which include: gain, selectivity and signal suppression. For the low frequency bands (80 and 40 m), the IF used in many modern amateur radio receivers is 455 kHz. IF amplifiers can provide excellent gain and selectivity from 400-2500Hz.
Detectors and beat generators
Detection, or demodulation, is defined as the process of separating audio frequency components from a modulated carrier signal. The detectors in superheterodyne receivers are also called secondary, and the primary is the mixer assembly.
Auto Gain Control
The purpose of the AGC node is to maintain a constant output level despite changes in the input. Radio waves propagating through the ionosphereattenuate then intensify 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 signal, part of it can be used to control the gain. For receivers using tube or NPN transistors in the nodes preceding the detector, a negative voltage is applied to reduce the gain. Amplifiers and mixers using PNP transistors require a positive voltage.
Some ham radios, especially the better transistorized ones, have an AGC amplifier for more control over the performance of the device. Automatic adjustment may have different time constants for different signal types. The time constant specifies the duration of the control after the termination of the broadcast. For example, during phrase intervals, the HF receiver will immediately resume full gain, which will cause an annoying burst of noise.
Measuring signal strength
Some receivers and transceivers have an indicator that indicates the relative strength of the broadcast. Typically, a portion of the rectified IF signal from the detector is applied 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 approximately a 6-dB change in received signal strength. The middle reading or S-9 is used to indicate the level at 50 µV. Upper half scaleThe S-meter is calibrated in decibels above S-9, typically up to 60 dB. This means that the received signal strength is 60 dB higher than 50 µV and equals 50 mV.
The indicator is rarely accurate as many factors influence its performance. However, it is very useful when determining the relative intensity of incoming signals, and when checking or tuning the receiver. In many transceivers, the LED is used to show the status of device features such as RF amplifier output current and RF output power.
Interference and limitations
It's good for beginners to know that any receiver can experience reception difficulties due to three factors: external and internal noise and interfering signals. External RF interference, especially below 20 MHz, is much higher than internal interference. It is only at higher frequencies that the receiver nodes pose a threat to extremely weak signals. Most of the noise is generated in the first block, both in the RF amplifier and in the mixer stage. Much effort has been made to reduce internal receiver interference to a minimum level. The result is low-noise circuits and components.
External interference can cause problems when receiving weak signals for two reasons. First, 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 LW even with heavy interference, but the voice and other amateur signals are incomprehensible under these conditions.
