Radio Amateur Receiver: Specifications

The antenna of the amateur radio receiver receives hundreds and thousands of radio signals simultaneously. Their frequencies can vary depending on the transmission on long, medium, short, ultrashort waves and television bands. In between, there are amateur, government, commercial, marine and other stations. The amplitudes of the signals applied to the antenna inputs of the receiver range from less than 1 μV to many millivolts. Amateur radio contacts occur at the level of several microvolts. The purpose of the amateur receiver is twofold: selecting, amplifying and demodulating the desired radio signal, and screening out all the others. Receivers for radio amateurs 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, they require sufficient stability for their containment and demodulation. In general, the performance (and price) of the radio depends on its sensitivity, selectivity and stability. There are other factors related to the performance characteristics of the device. These include frequency coverage and reading, demodulation or detection modes for DV, CB, HF, VHF radios, power requirements. Although the receivers vary in complexity and performance, they all support four basic functions: reception, selectivity, demodulation and playback. Some also include amplifiers to increase the signal level to acceptable values.

Reception

This is the ability of the receiver to process weak signals collected by the antenna. For a radio receiver, this functionality is primarily related to sensitivity. Most models have several cascades of amplification necessary to increase the power of signals from microvolt to volts. Thus, the overall gain of the receiver can reach about a million to one.

It is useful for beginners to know that the sensitivity of the receiver is affected by the electrical noise generated in the antenna circuits and the device itself, especially in the input and radio frequency modules. They arise when thermal excitation of the conductor molecules and in the components of the amplifier, such as transistors and tubes. In general, the electric noise is independent of 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 sensitivity limit of the receiver. Most modern models allow for 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 approximately 10 dB below the 0.5 μV signal. In other words, the receiver interference level is about 0.16 μV. Any signal below this value will be overlapped by them and will not be heard in the dynamics.

At frequencies up to 20-30 MHz, external noise (atmospheric and anthropogenic) is usually much higher than internal noise. Most receivers have sufficient sensitivity for signal processing in this frequency range.

Selectivity

This is the ability of the receiver to tune in to the desired signal and reject unwanted signals. The receivers use high-quality LC-filters to pass only a narrow frequency band. Thus, 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 sufficient to filter out the majority of signals close to the operating frequency. All amateur radio receivers for HF and SW bands should have a selectivity of about 2500 Hz for amateur voice reception. Many receivers and transceivers DV / KV use switch filters to ensure the optimal reception of any type of signal.

Demodulation, or detection

This is the process of splitting the LF component (sound) from the incoming modulated carrier signal. In the circuits of demodulation, transistors or lamps are used. The two most common types of detectors used in HF receivers are a diode for DV and CB and an ideal mixer for DV or HF.

Playback

The final reception process is the conversion of the detected signal into an audio signal for feeding to a speaker or headphones. Usually a cascade with a high coefficient is used to amplify the weak output from the detector. The audio amplifier output is then fed to the speaker or headphones for playback.

Most radio amateur receivers have an internal speaker and an output jack for headphones. A simple single-stage audio amplifier is suitable for working with headphones. A speaker usually requires a 2 or 3-step audio amplifier.

Simple receivers

The first receivers for radio amateurs were the simplest devices, which consisted of an oscillating circuit, a crystal detector, and headphones. They could only receive local radio stations. However, the crystal detector is not able to properly demodulate the signals of the DV or KV. In addition, the sensitivity and selectivity of such a scheme are not sufficient for amateur radio work. You can increase them by adding an audio amplifier to the output of the detector.

Radio receiver direct amplification

Sensitivity and selectivity can be improved by adding one or more cascades. This type of device is called a direct gain receiver. Many commercial CB-receivers of the 20's and 30's. Used such a scheme. Some of them had 2-4 stages of amplification to obtain the required sensitivity and selectivity.

Direct conversion receiver

This is a simple and popular approach for receiving DV and KV. The input signal is fed to the detector together with the RF from the generator. The frequency of the latter is slightly higher (or lower) than the first, so that one can get a beat. 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 is created 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 oscillating LC-circuits in front of the detector and a sound filter between the detector and the audio amplifier.

Superheterodyne

Developed in the early 1930s with the goal of eliminating most of the problems encountered by early types of radio amateur receivers. Today the superheterodyne receiver is used in almost all types of radio communication services, including radio 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 into an intermediate RF signal.

RF amplifier

Contain LC-circuits that provide some selectivity and limited gain at the required frequency. The RF amplifier also provides two additional advantages in the superheterodyne receiver. First, it isolates the cascades of the mixer and the local oscillator from the antenna circuit. For a radio receiver, the advantage is that unwanted signals are attenuated, 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 oscillator generates oscillations whose frequency can be either higher or lower than the carrier. This choice is determined by the bandwidth and the requirements for tuning the RF. Most of these nodes in the CB 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 IF amplifier. The mixer outputs 4 main output signals from 2 input signals: f ₁ , f ₂ , f ₁ + f ₂ , f ₁ -f ₂ . In the superheterodyne receiver, only either their sum or the difference is used. The others may cause interference if proper measures are not taken.

IF amplifier

The characteristics of the IF amplifier in the superheterodyne receiver are best described from the point of view of the gain factor and selectivity. Generally speaking, these parameters are determined by the IF amplifier. The selectivity of the IF amplifier should 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 cut off. This results in a loss of clarity when playing back the sound with a 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. The selectivity of 400-500 Hz is sufficient for the operation of DV. This narrow band helps to reject any nearby frequency signal that can interfere with reception. In amateur radio receivers, the price of which is higher, 2 or more IF amplification stages are used with the previous highly selective crystalline or mechanical filter. With this arrangement, LC circuits and IF converters are used between the units.

The selection of the intermediate frequency is determined by several factors, which include: amplification, selectivity and signal suppression. For low-frequency bands (80 and 40 m), the IF used in many modern radio amateur receivers is 455 kHz. The IF amplifiers can provide an excellent gain and selectivity of 400-2500 Hz.

Detectors and beat generators

Detection, or demodulation, is defined as the process of separating the audio components from the modulated carrier signal. Detectors in superheterodyne receivers are also called secondary, and the primary is the mixer assembly.

Automatic gain control

The purpose of the AGC node is to maintain a constant level of the output signal, despite the changes in the input signal. Radio waves propagating through the ionosphere are weakened, then amplified due to a phenomenon known as fading. This leads to a change in the reception level at the antenna inputs over a wide range of values. Since the voltage of the rectified signal in the detector is proportional to the amplitude of the received signal, some of it can be used to control the gain. For receivers using tube or npn transistors at the nodes preceding the detector, a negative voltage is applied to reduce the CW. Amplifiers and mixers using pnp-transistors require positive voltage.

Some radio amateur receivers, especially the best transistors, have an amplifier with AGC for greater control over the characteristics of the device. Automatic adjustment can have different time constants for signals of different types. The time constant specifies the duration of the control after the broadcast is terminated. For example, during the intervals between the phrases, the HF receiver immediately resumes the full gain, which will cause an annoying burst of noise.

Measurement of signal strength

Some receivers and transceivers have an indicator indicating the relative strength of the broadcast. Usually, 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 approximately 6-dB change in the received signal power. The average reading or S-9 serves 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 received signal strength is 60 dB above 50 μV and is equal to 50 mV.

The indicator is rarely accurate, since many factors influence its operation. 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 serves to display the status of device functions, such as the final current of the RF amplifier and RF output power.

Interference and limitations

It is useful for beginners to know that any receiver may experience reception difficulties due to three factors: external and internal noise and interfering signals. External interference on HF, especially below 20 MHz, is much higher than internal noise. Only at higher frequencies the receiver nodes are a threat to extremely weak signals. Most of the noise is generated in the first block, both in the radio-frequency amplifier and in the mixer cascade. To reduce the internal interference of the receiver to a minimum level, a lot of effort has been exerted. As a result, low-noise circuits and components appeared.

External interference can cause problems when receiving weak signals for two reasons. First, the interference trapped by the antenna can mask the broadcast. If the latter is near or below the level of incoming noise, reception is almost impossible. Some experienced operators can receive broadcasts on DV even with large interference, but the voice and other amateur signals in these conditions are incomprehensible.