Though the concept of SSB transceivers scare a few hoe brewers there are lot of Hams doing good business with SSB Transceivers. It should be remembered that AM equipments also have problems. Ham equipments, whatever it be, requires much care and preciseness. The purpose of all these illustrations are helping Amateurs to avoid unnecessary losses and choose the best option for each. AM transmitters demand more bandwidth and are much behind in efficiency when compared to SSB sets. The number of Hams have increased much and AM transmitters might be compelled to keep away at least during peak traffic hours. If I share my experience, just a 1W SSB transmitter was enough for me to contact all of South India, some stations in Russia, Taiwan, Maldives and Sree Lanka. This is why elaborate explanations are given on SSB technologies.
Even if there is only one common Crystal Oscillator in the SSB transmitter, if two different exciters are used for LSB and USB, transmission is possible on LSB and USM at the same time, using the same Antenna - and that at two different modulations. Another requirement in SSB is that speech amplifier (mic amplifier) circuit should be modified to fit to the audio frequency range of the regular user. Even the slightest difference in the strength of the audio signal fed to the balanced modulator makes big differences in the out put signal strength.
This facility can be used as power control itself. Another arrangement to control the out put power is regulating the strength of the 9 MHz sideband carrier that is mixed with the VFO signal. Whenever we use a bandpass filter, it is compulsory that we should keep both the input and output signals well isolated and the input output impedances matched perfectly. A crystal filter is best at doing both these functions. Other factors that disturb the signal quality are power supply interferences and random signal strength at the input. For best performance, there always is an optimum signal strength rating, which varies according to every situation. To satisfy this signal strength requirement, when a signal from bandpass filter is fed into the mixer, perhaps we may need a post filter amplifier itself. However, this should be decided based upon the peculiarities of the circuit and the efficiency of the active components used.
This facility can be used as power control itself. Another arrangement to control the out put power is regulating the strength of the 9 MHz sideband carrier that is mixed with the VFO signal. Whenever we use a bandpass filter, it is compulsory that we should keep both the input and output signals well isolated and the input output impedances matched perfectly. A crystal filter is best at doing both these functions. Other factors that disturb the signal quality are power supply interferences and random signal strength at the input. For best performance, there always is an optimum signal strength rating, which varies according to every situation. To satisfy this signal strength requirement, when a signal from bandpass filter is fed into the mixer, perhaps we may need a post filter amplifier itself. However, this should be decided based upon the peculiarities of the circuit and the efficiency of the active components used.
Mixer is perhaps the most important part of a SSB transmitter. It is here that the transmission frequency is decided. With a single ended transistor mixer a lot of signals at different frequencies are possible in the output. Even best class filters may not be powerful enough to harm signals close to transmission frequency. In valve amplifiers, those tuned circuits used in each stages are enough to check unwanted signals. This is not the case with solid state active components. In other words, if the final amplifier is solid state, the input signal should strictly be a signal at the transmission frequency. Sometime we may require traps here.
This needs a brief explanation. Everybody knows that at the output of a SSB mixer in a 7 MHz transmitter, in which a 9 MHz signal is mixed with a 2 MHz signal the circuit used need to be tuned to 7 MHz. A little 9 MHz also will pass through this filter.
To eliminate this 9 MHz, we can add a trap. Fig.C-21/2 is the circuit of a 7 MHz Mixer. In this if T2 is tuned to 9 MHz it will get grounded through the T2. Substituting 47 pf capacitors in this area with 22 PF ones will help us to avoid T2. This method is practical in any type of mixers. Double Balanced Mixer circuits using ICs like CA3028, MC1496 (Motorola) and NE 602 are found to to be giving best performance. At the out put of Double Balanced Mixers in which 9 MHz is mixed with 2 mHz, the out put signals will be limited to just 7 MHz and 11 MHz. Here, neither a low pass nor a high pass filter is required in the circuit.
Another condition to be observed while assembling a SSB transmitter is the material used for coil cores in it. Everywhere, the core should be competent to handle the intended frequency. The 'Neosite' cores used in TV IFTs and Philips Shortwave Antenna coils efficiently handle HF signals. Be it in mixer amplifier or linear amplifier the transistor used should not only be working in that particular frequency range but also should not be noise generating. BC 109 transistor is found efficient in early stages. Carbon resistors also require a check. Carbon resistors used at emitters are found to be causing self-oscillations.
W can avoid this by using either CC (composition carbon) or adding a choke in place by winding a few turns (number of turns depending upon the frequency) on a balun core. Another general instruction is that in SSBs it is always better to keep all stages in modules, well shielded in separate boxes. This will eliminate a permanent disturbance - interstage interference. This makes the Machine repair friendly too. In C-21/1 the circuit of side band filter using 9 MHz Crystal Filter is given. A ladder filter can be used instead of this stage. In a SSB transceiver this circuit is enough to filter a 9 MHz receiver signal (signal that is converted into 9 MHz through the front end mixer).
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