Balanced Modulator circuits available for trial are not just two or three; a variety of circuits are available, some using valves and some either transistors, Diodes or ICs. I think it is diode ring/IC based balanced modulator circuits that are more popular in India. Diode ring is the simplest of all and it gives the necessary 40 db carrier suppression too. Needed here are four diodes that are best in high reverse resistance, low forward resistance, maximum temperature stability and least noise generation. While choosing them measure and confirm that they have equal forward resistance. Minor differences are solved by the tuning circuits in balanced modulator. germanium diodes are preferred because it has comparatively less forward resistance when compared with the silicon makes. Temperature stability is considerably low and noise possibility is high in both silicon and germanium. Hot carrier diodes are better than germanium diodes here. Hot carrier diodes are noted for their low forward voltage drop and fast switching potential.
The next two important factors that improve the quality of a any Balanced modulator are the stability of the Oscillator signal and the higher relationship proportion between the Modulating and Carrier signals. Crystal Oscillators might give better stability but not always the necessary strength. In an ideal situation, the carrier wave is ten times stronger than the modulating signal. When the balanced modulator is tuned for zero carrier, better to change the frequencies also accordingly so that it gets the best mixing.
Because any change in the internal resistance of a diode causes carrier leak, there would naturally be a variable resistance circuit intended to compensate this. Instead of using presets that give big differences at slight adjustments, it is better to use trimpots in which the change is comparatively very very slow, perhaps in single digit ohms. However, the variation range of any Trimpot is adjustable, if the value of resistors used in series or parallel to it is changed.
Extreme care should be taken in the quality of capacitors also that are used n a Balanced Modulator. This does not mean that the quality of the printed board and design specialities are unimportant. Confirming the frequency using a standard Frequency counter and tuning the wave shape using a Oscilloscope also helps to increase the performance of the Balanced Modulator. A Balanced Modulator requires these settings only once. Even a minute difference in the modulator performance will adversely hit the following stages in an SSB transmitter. A home brewer will certainly check the Balanced Modulator on a working SSB Transmitter before proceeding to the following stages.
In C-18/1 we have given the full circuit of a Balanced Modulator and in Fig. C-18/2 the print details of the circuit board is given (print side view - 4" X 2 "). A given print layout may vary at situations in which the parts available are different. Always check the layout with the circuit and size and proposed assembling mode of the components collected.
As per C-17/1A whatever be the transmitting SSB signal, LSB or USB, the oscillator frequency in the receiver will be the same. But in C-17/1B we use two different carriers for LSB and USB and this difference will be reflected in the receiver also.
When a VFO and a Mixer is used to convert the signal coming out of SSB exciter to transmission frequency, much care is required not to exchange the input signal between USB and LSB.
As told before, to convert an external frequency of 9 MHz into 7 MHz it should be mixed with either 16 MHz or 2 MHz and the VFO should belong to that frequency range. Since the stability at higher frequencies are proportionally low with any VFO, what we opt will be a 2 MHz VFO. In that case it will be the difference between the frequencies mixed that we take out. Here, if it is LSB that we give in the input what we get int the output also will be LSB. Same is the case with USB too. But if a 9 MHz signal from the exciter is mixed with a 5 MHz VFO signal to generate a 14 MHz output, the case will be different. If it is LSB that we give in the input what we get in the output will be USB and vice versa. This is because it is the sum of both the signals that is taken out. This working principle helps anyone to transmit LSB or USB using the same Crystal.
Briefing this conversion philosophy, if we are using a 9.0015 MHz Crystal for Oscillator, what we get in the output on meter bands 40 and below will be LSB and at frequencies above 40 meters it will be USB. It is perceived that this earlier working arrangement is being continued even today. In this case, if the same Gang condenser is being used to tune at both meter bands of 40 and 20, if high frequency is tuned towards right at 40 Meters, it will be low frequency side that gets tuned at 20 Meters. Technically, at 40 Meters, as capacitance increases the output frequency also increases.
Briefing this conversion philosophy, if we are using a 9.0015 MHz Crystal for Oscillator, what we get in the output on meter bands 40 and below will be LSB and at frequencies above 40 meters it will be USB. It is perceived that this earlier working arrangement is being continued even today. In this case, if the same Gang condenser is being used to tune at both meter bands of 40 and 20, if high frequency is tuned towards right at 40 Meters, it will be low frequency side that gets tuned at 20 Meters. Technically, at 40 Meters, as capacitance increases the output frequency also increases.
In the filter stage of an SSB exciter, even though it is always a Crystal that best fits, other inexpensive techniques also are used. If the carrier frequency is 1000 KHz or below, a coil-capacitor net work is enough to design a bandpass filter of the intended bandwidth. Another method is building a ladder filter. The details of such a filter using six no.s of 4.433618 MHz a bandwidth of 2.26 KHz. was shown in C-9/5A (Chapter 9). What we require is not Crystals with the same rating but Crystals that oscillate exactly at the same frequency. Sometimes it happens that way also. There is an easy method to identify the defective. Connect a Crystal of the same rating, in series to a working Crystal. If the oscillator Crystal continues to be oscillating as before, it can be presumed that both Crystals are of the same frequency in performance. Since Crystals of suitable ratings are available in India, building an SSB is not a hazardous task for an average home brewer. But it is always advisable to reserve an experienced expert at reach. Even grinding a Crystal into desired frequency is also not a difficult thing.
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