Early Amateurs were daring and challenging. They were not simply tuning discarded military equipments, they knew the art of converting Crystals into their intended frequencies. Today, all Crystals available in the market are packed and sealed in metal cases and it cannot be opened safely. The Crystals used in early communication receivers could have been opened and reassembled. One time such receivers used in ships, aircrafts and military vehicles were sold in junk/flee markets. Radio Amateurs were the target. Most of them were functional at minor touches.
In fig. C-19/1, the assembly of a Crystal is shown. In the centre there is the quartz Crystal plate and on either sides metal contact plates and close to them on the outer side connection contact plates. The remaining part of a Crystal Filter is the spring fixed in between the connection contact plate and the outer cover. This was the arrangement in first generation crystals.
They were not dust/moisture free and it was enough to block its functioning. Most of the time they had to be cleaned. All that we need to do is to dismantle carefully everything onto a tissue paper, remove the dust and pack it back - dead crystals start working. Either it is for grinding or cleaning, unless very carefully handled, the crystal breaks - spoiling everything. Hold the crystal corner to corner between thumb and the index finger. Only corners of the crystal plate shall be touched. There are various methods to clean the crystal assembly. According to a popular method, a little white tooth paste is mixed with water and is added liquidity. Cleaning also is done holding the plate between index and thumb. This detergent is applied at the surface and rubbed with the middle finger or ring finger. After removing dust and stains, the paste factor is cleaned under flowing water and the plate is dried with blotting paper. After grinding also the plate is to be cleaned like this.
For grinding what we require is a clean plane glass. Keep the glass at the centre of the table and spread some medium carborundum powder at the centre of the gllass. Further add a few drops of water (the powder should not be sunk). Then this paste is spread to a diameter of three inches. It should be at the centre of this paste that the crystal is put. Now press the crystal lightly in the centre and turn it right and left in clockwise and anti clockwise direction. Th finger position also is to be changed frequently. Grinding requires much expertise. If crystals are ready, SSB exciters are possible, this is why an illustration into the details handling a crystal was given.
Simply because we use a Crystal with a frequency rating, it need not necessarily be working exactly on that frequency. Only some minor arrangements are possible in the circuit. Balanced modulators that are better than diode ring is possible with ICs, In C-19/2 a model circuit using MC1496 G is given. The specifications of each crystal varies and use it only after reading the instructions manual.
Before connecting the oscillator to the balanced modulator make sure that the signal is clear, strong and of the intended frequency. In C-19/2 the circuit diagram of a 9 Mhz oscillator using separate crystals for LSB and USB (BEL make) is given.
The disciplines followed in other circuits with different purposes using crystals are not applicable here. It applies to switching circuits also. When a single transistor is used to switch both the crystals it is the crystal lead to the base of the transistor that is to be switched. Also this switch and crystal are to be close to the transistor too.
In diode switching one crystal could be strongly oscillating and the other mildly. When a single transistor is used for both LSB and USB, it is not practically easy to keep both signal strengths equal. According to fig. C-19/4, first activate the 8.9985 MHz crystal and all its connected common oscillator circuitry and set the balanced modulator. Further set the 9.0015 MHz crystal in tune with the settings already done.
In the filter stage that comes after the balanced modulator, the input output impedances are to kept matched. A filter not only suppresses one sideband but also weakens the other sideband considerably. Naturally we may need a post filter amplifier. It was told before that the sideband signal coming out of the filter stage is to heterodyned to convert it into the transmission frequency.
If the sideband frequency is 9 MHz, to convert it into 7 MHz and 14 MHz, two VFOs of 2 MHz and 5 MHz are required respectively. The mixing using the VFO signal is intended to generate only a single signal. That is the output of the VFO shall always be only a single frequency. In almost all oscillators circuits including that of doublers what we get in the output are fundamental frequency, and harmonics. The fundamental frequency could be the strongest and that would be the only difference. If a similar VFO output is applied to SSB exciter, products mixed with harmonics also appear in the output. Since the LC filter in the output of the linear amplifier is generally not that powerful, all these signals get proportionally amplified and appear either at various Ham bands or at various spots in the same meter band in different power. This means that a very strictly designed VFO is required in SSB transmitters. That is, a VFO assembled for SSB purpose needs both High pass and Lowpass filters in the output. Most of the circuits circulated among Ham home brewers do not keep this standard.
This filter is not a difficult thing to do - LC Networks are enough. The details of such a VFO with print circuit details follows. PCB layout for both are one and the same. Along with a Receiver Incremental Tuning (RIT) circuit (for optional use - in case it is a transceiver) also is given. It is advised that the signal from IC filter (after mixer) is tested with a radio kept slightly away before feeding the signal into the linear amplifier.
The block diagram in fig. C-19/5 shows the arrangement in the receiver part when used in a transceiver. If you do not mean to use the same 9 MHz signal as IF 9 MHz the signal from the 9 MHz crystal can be mixed with 455 KHz (9MHZ + 455KHz) and generate a 455 KHz (for IF) in the out put. Here the BFO used should exactly be oscillating at 455 KHz. A front end that fits this concept is given in Chapter 14 and pictures of 9 MHz IF AF stages with its lay out are given in Chapter 12. The circuit of a model crystal oscillator is given in this chapter also.
Only some general information on SSB home brewing are given here. It is always excellent to measure the frequencies at various stages using a frequency counter, and if possible an oscilloscope to check the shape of the signal.
The circuit details of a 1.9 - 2 MHz VFO to be used for a 7 MHz SSB transmitter is given in C-19/7. Using a frequency counter the efficiency of the Lowpass filter used may be affirmed. The efficiency and clarity of the filter can also be determined along with adjusting the tuning by carefully monitoring the receiver for any beat notes anywhere elsewhere and the correct signal. It is because pretty good strength of the signal will be lost in the filter that high gain transistors like MP 102 is recommended at Q1. The core for L2 also should be one among the best in the class. A 4 X 4 glass epoxy board is enough to assemble one. Spacing between sensitive components is always a positive factor while trying circuits like this. Another thing to be done is fixing the VFO in a thick box with no unnecessary holes and box well grounded. The value of C2 will be around 22 PF for the expected bandwidth. For 14 MHz SSB transmitter also this same circuit can be used. Only the values of components change. The RIT portion requires no change at 14 MHz also.
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