SSB

Description: To resolve Single Sideband (SSB) or Continuous Wave (CW), a Beat Frequency Oscillator (BFO) signal may be coupled into the intermediate frequency amplifier. In these situations, the BFO must operate near the intermediate frequency of the receiver for all frequency transmissions. With a BFO of this type, signals can be received at any frequency throughout the ranges provided for the receiver. Some shortwave receivers have an intermediate frequency of approximately 1.6 MHz, and the BFO must be designed accordingly. In most general-purpose or domestic receivers, the intermediate frequency is around 455-470 kHz, and the BFO must be configured with this in mind. Aside from the requirement for the correct frequency, BFO units for both 1.6 MHz and 455-470 kHz are fundamentally the same, utilizing the circuit depicted in Figure 28. A general-purpose RF FET is employed as the oscillator, with its frequency determined by variable capacitor VC1, along with the associated winding and parallel capacitor. The smaller winding serves for drain feedback. A stabilized supply featuring a 6.2V, 400mW Zener diode allows the unit to be powered from the receiver if desired. For frequencies around 1.6 MHz, a Denco (Clacton) IFT17 intermediate frequency transformer is used, while for frequencies in the 455-470 kHz range, the IFT14 is fitted. Each transformer includes a parallel capacitor located inside the screening can, with pin connections illustrated in Figure 28. Components can be assembled on a small insulated board, which may be accommodated within the receiver. An on/off switch is integrated into the positive supply lead, as the BFO should not be active during standard AM reception. A control knob is necessary for VC1, as it must be adjustable during reception. If it is impractical to incorporate this control into the receiver, the BFO can be constructed as a separate unit. The optimal degree of coupling from the BFO coil to the receiver's intermediate frequency circuits should be determined through trial and error. This can be achieved by running an insulated lead from the BFO to the vicinity of the first transistor in the intermediate frequency amplifier. Alternatively, a capacitor of 8.2 pF can be included, as indicated, allowing the output lead to connect to the receiver diode at the final intermediate frequency transformer. If coupling is too loose, only relatively weak CW or SSB stations will be resolved, while stronger SSB signals may sound distorted or over-modulated. However, excessive coupling is also undesirable, as it may drown out weaker transmissions. The operation resembles that of the signal frequency resolver, with the distinction that all signals are converted to the intermediate frequency within the receiver, necessitating only minor adjustments to VC1. For CW, VC1 will have two positions (above and below the station frequency). In contrast, for SSB, a single suitable frequency setting for the BFO is required; placing it incorrectly relative to the SSB signal will result in inverted or unintelligible speech. Initially, tune into an AM signal and set VC1 to approximately half of its open position. Then, rotate the core of the oscillator coil until a heterodyne is audible and can be adjusted to zero. Rotating VC1 in either direction should produce an audio tone that increases in pitch. It is essential to verify that this occurs with signals at any frequency to ensure that harmonics of the BFO in the aerial circuit are not the cause of any issues.

The BFO circuit design is critical for effective SSB and CW reception, particularly due to the varying intermediate frequencies used in different receivers. The choice of components, such as the RF FET and the intermediate frequency transformers, is essential for achieving the desired performance. The BFO's output coupling to the receiver's intermediate frequency amplifier must be carefully adjusted to ensure optimal signal clarity without distortion. The inclusion of an adjustable capacitor (VC1) allows for fine-tuning of the BFO frequency, which is particularly important for SSB reception, where precise frequency alignment is crucial for intelligible audio output. The design should also consider the physical layout within the receiver, ensuring that components are mounted securely and that the BFO can be easily accessed for adjustments. Proper shielding and grounding techniques should be employed to minimize interference and maintain signal integrity. Overall, the BFO circuit must be robust, versatile, and capable of handling a wide range of frequencies to accommodate various reception scenarios effectively.To resolve SSB or CW, a BFO signal may be coupled into the intermediate frequency amplifier. In these circumstances, the BFO has to operate near the intermediate frequency of the receiver, for transmissions of all frequencies. So with a BFO of this type, signals may be received at any frequency throughout the ranges which are provided for the rece

iver. Some short wave receivers have an IF of about 1. 6MHz. The BFO must then be constructed for this. With most general purpose or ordinary domestic receivers, the IF is near 455/470k Hz. If so, the BFO has to be arranged with this in view. Except for this point of providing the correct frequency, both 1. 6MHz and 455-470kHz BFO units are the same, and employ the circuit in Figure 28. A general purpose RF FET is used as oscillator, Its frequency is determined by VC1, with the associated winding and parallel capacitor. The smaller winding is for drain feedback. A stabilised supply with 6. 2v 400mW Zener diode allows the unit to be run from the receiver, if wished. For frequencies around 1. 6MHz, a Denco (Clacton) IFT17 intermediate frequency transformer is used. For frequencies in the 455-470kHz range, the IFT14 is fitted. Each has the parallel capacitor present inside the screening can, in the usual way, and pin connections are shown in Figure 28.

Components may be assembled on a small insulated board, and it may be possible to fit this somewhere inside the receiver. An on/off switch is included in the positive supply lead, as the BFO must not be in use for ordinary AM reception.

A control knob is necessary on VC1, as this must be adjustable during reception. If it is not practical to include this control on the receiver, then the BFO can be constructed as a separate unit. The best degree of coupling from 2 of the BFO coil to the receiver IF circuits needs to be found by trial.

It should be, sufficient to run an insulated lead from 2, to the vicinity of the first transistor in the IF amplifier. Alternatively, a capacitor of 8. 2pF can be included here, as shown, and the output lead can be connected to the receiver diode, at the final IF transformer.

If coupling is too loose, only relatively weak CW or SSB stations will be resolved, and stronger SSB will sound like badly distorted and over modulated AM. However, very tight coupling is not wanted and will tend to swamp weaker transmissions. Operation is similar to that described for the signal frequency resolver, except that as all signals are converted to the intermediate frequency in the receiver, it is only necessary to make trifling adjustments to VC1.

For CW, VC1 will have two positions (above and below the station frequency). But for SSB, there is only one suitable frequency setting for the BFO, as if this is placed the wrong side the SSB signal, speech will be inverted, or unintelligible. Initially, tune in an AM signal, and set VC1 to about half open position. Then rotate the core of the oscillator coil until a heterodyne is heard, and can be set to zero. Then rotating VC1 either way should cause an audio tone which rises in pitch. Check that this happens with signals on any frequency, to make sure harmonics of the BFO in the aerial circuit are not responsible.