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Keying the Crystal Oscillator |
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Keying the Crystal Oscillator And Some Observations on Blocked-Grid Amplifier KeyingBy Byron Goodman* WIJPE * Assistant Technical Editor, QST. The subject of crystal oscillator keying
is complicated somewhat by the differences in various crystals,
tubes and circuits. All crystals do not key alike, some circuits
are better than others, and different types of tubes in the same
circuit behave differently. For these reasons, it is wellnigh impossible
to set down any hard and fast rules about crystal keying that will
apply in every case. However, work in the laboratory with the more
common tubes and circuits has resulted in some general principles
that can be applied to all crystal oscillators that are being adjusted
for keying.
Fig. 1 - Some
oscillograms of the keying of a grid. plate oscillator (see Fig.
3). The oscillator is keyed in the negative lead, with no key filter,
so that the pure characteristic can be seen. A, B and C show a 6V6
grid. plate oscillator tuned for optimum keying, the high-frequency
side and the low-frequency side of optimum, respectively. D shows
a 6L6 substituted for the 6V6 and tuned for optimum. "Adjusting an oscillator for keying" is nothing new to the experienced amateur who uses several different crystals and has worked with the problem, but it may come as a shock to those who work on the premise that a crystal oscillator adjusted for maximum output need only be turned on and off rapidly with a key to affect good keying. As with self-excited oscillator keying, the best procedure for adjustment of a crystal oscillator seems to be first to adjust it so that it follows the key closely at quite high speeds, and then to introduce some filter to reduce the clicks to the degree necessary only for good communication at amateur code speeds. The better crystal oscillator circuits are all capable of keying speeds up to well over 100 w.p.m., but a keying circuit capable of handling this speed cleanly results in more key clicks than are necessary for the more normal speeds of from 20 to 35 w.p.m., and so some lag should be added.1 One slight disadvantage of crystal oscillator keying is that, when several crystals are used (for different parts of the bands), the total current to the oscillator is not the same in every case. This means that a key filter adjusted for one particular voltage-current combination may introduce too little or too much lag on "make" and too much or too little lag on "break" when a different crystal (with different total oscillator current) is used. This is likely to be the case, since all crystals do not key best with the same tuning adjustment. It is, however, a fine point that is mentioned only to explain the apparent discrepancies some operators encounter. As is the case with self-excited oscillators, cathode keying of a crystal oscillator seems to be more difficult to filter than power-supply keying (in the negative or positive lead). The time constant of the oscillator grid circuit has an effect on the keying, and simply adding a lag circuit at the key is not as effective as might be thought. The photographs in Figs. 5B and 5C show a comparison between the effectiveness of key filters in the cathode and negative leads of a crystal oscillator. Cathode keying has won popularity because, for the same oscillator, the sparking at the key and the voltage across the key is less than with power-supply keying. The obvious answer is, of course, to key a low-power circuit where these factors become unimportant. Combination oscillators such as the Tri-tet (Fig. 2) and the grid-plate oscillator (Fig. 3) that use the screen grid as the grounded anode in the oscillating circuit can, when used with wellscreened tubes, be keyed satisfactorily in the screen circuit when doubling. If the plate voltage is too high or if the screening is poor, the crystal will oscillate weakly all of the time and discourage break-in work on one's own frequency but the circuit has the advantage that the screen dropping resistor, R2, and the screen by-pass condenser, C3, serve as a filter that helps to reduce clicks. When adjusting for minimum clicks, the values of C3 and another condenser across the key should be varied until the results are satisfactory. Well-screened tubes like the 6SK7, 6AG7 and the 10-watt pentodes are satisfactory in this application, but results with the more common beam tubes (6V6, 6L6) will be discouraging, since the crystal will oscillate continuously. 1 Goodman, "Some Thoughts on Keying," QST, April, 1941.
Fig. 2 - The Tri-tet oscillator. The value of C3 will introduce some lag and thus reduce clicks if a wellscreened tube is used. Negative high-voltage keying is done at "X", and screen-grid keying at "Y." General Considerations One sometimes sees crystal oscillator circuits with no r.f. choke in series with the grid leak across the crystal, but the slight saving in expense hardly justifies the improvement in performance that can be obtained by using the choke. Several circuits that gave mediocre keying with no choke showed a marked improvement when the choke was added. The same sort of improvement is obtained when the value of the grid leak is increased to 0.25 megohm or so, but this value of grid leak cuts down the output of the oscillator to a point where it is of little value. The use of the r.f. choke (and also a large value of grid leak) removes some loading from the crystal and leaves it freer to start oscillating. As "musts" for most crystal oscillator circuits that are keyed, it is recommended that the grid r.f. choke be included and the value of grid leak be made as high as possible consistent with adequate output to drive the following stage or, in the case of a single-stage transmitter, to give sufficient output without a compromise with good keying. The straight tunedplate triode oscillator is an exception, and it is best operated with cathode bias only. Frankly, we are at a loss to explain why the cathode-biased triode works better than one with leak bias while all of the other circuits are better with leak bias but such seems to be the case, as Figs. 4A and 4B show. Simply using an r.f. choke and a high value of grid leak is not enough to give good keying, of course. A suitable choke and condenser filter circuit must be used at the key, and the key should be used in the negative (or positive, if it's hard to get at the negative) lead, as described in the keying article last month.1 The same principles apply to adjustment - more choke is used to remove the click on "make" and more condenser is used to remove the click on "break." It appears to be slightly more difficult to smooth out the keying of a crystal oscillator than of a self-excited oscillator, possibly because one is dealing with a partially mechanical oscillator instead of a purely electronic one, but in general it will respond to the same treatment.
Fig. 3 - The grid-plate oscillator. The remarks about F.g.2 also apply to this type of oscillator. C2 should be 50 μμfd. and C5 will range from 50 to 250 μμfd., depending upon the tube. Some versions of this circuit use only the input capacity of the tube for C2, but the addition of the small condenser is worth trying. The oscillator should be capable of oscillating with only 3 or 4 volts on the plate and an excellent test is to connect several dry cells in series for the plate supply to check this point. An oscillator that won't oscillate at a low plate voltage will drop in and out of oscillation with a "plop" as the voltage is increased from zero and decreased back again, and hence is not as susceptible to key filtering as one that will work at a low voltage. Straight pentode oscillators and some triode oscillators will require additional feedback to make them oscillate at less than 10 or 15 volts. Under critical adjustment, the Tri-tet will oscillate with no apparent plate voltage when the circuit is closed (as is well known), but this is caused by the contact potential of the tube and the drop through the cathode circuit. The grid-plate oscillator (Fig. 3) will oscillate at very low voltages with proper proportioning of the cathode condenser, C5. Last month we presented a story pointing out some of the factors influencing the keying of amplifiers and self-excited oscillators. This follow-up article treats some of the considerations in crystaloscillator keying and the blocked-grid keying of amplifiers (and keyer tubes) and, although it may not offer the cureall for your particular problem, it may start you in the right direction towards clearing up your keying troubles.
Fig. 4 - Oscillograms
of a keyed 6C5 crystal oscillator. A shows the triode adjusted for
optimum keying with a 10,000-ohm grid leak, B shows the triode with
cathode bias and tuned for optimum keying, and C shows what happens
when the cathode- (or grid-) leak biased triode oscillator is tuned
too much to the lowfrequency side of the optimum keying adjustment.
The dots in C become light and the keying is somewhat erratic.
Fig. 5 - A 6AG7 leak-biased Tri-tet oscillator with the plate tuned to the fundamental frequency. A shows the oscillator tuned for optimum keying, and B shows the same with optimum key filter. Both A and B are keyed in the negative lead; C is keyed in the cathode with the same filter as B. Adding capacity to C did not extend the tail on "break" enough to reduce the click to a good value. Another important factor in the adjustment of a crystal oscillator for best keying is that it be keyed while tuned. Electronic bug owners will find this a simple matter, while the straight key or mechanical bug owners will have to content themselves with sending a series of dots while tuning the oscillator. It is relatively easy to hit the best tuning adjustment by listening to the signal in the receiver, but one can end up with some rather horrible keying if he just tunes the oscillator for maximum output and then keys it. This is assuming, of course, that a proper key filter has already been installed and that the switch to a different crystal has just been made. The key filter constants can be determined in the same manner as described last month for self-excited oscillators. Be sure to listen with the r.f. gain of the receiver well reduced, else the receiver is likely to give too pessimistic a picture of the clicks. If the oscillator circuit is one using a screen grid tube, the screen circuit should not be overlooked when adjusting for minimum clicks. The size of the dropping resistor is usually fixed by the screen operating voltage, but the size of the by-pass condenser can increase or decrease the clicks, depending upon the type of circuit. It is suggested that a 0.001 by-pass condenser be used right at the socket, for a short r.f. path, and then different values of shunting condensers can be tried at some more accessible point. Here again testing is most readily done by sending a steady string of dots and listening to the signal (with the b.f.o. turned off) while different values of screen by-pass condensers are tried. The screen adjustment is best made before the key filter is adjusted. The additional capacity should be added on the tube side of the dropping resistor, of course. Loading has an effect on the keying of oscillators where the feedback is obtained from the plate circuit, as in the case of the straight tetrode or triode oscillators, but it doesn't seem to be very important in circuits like those shown in Figs. 2 and 3. A conclusion from the work described in this article is that the regenerative type of crystal oscillator (Tri-tet and grid-plate) keys better than the straight triode, tetrode and pentode oscillators. Not only do they seem to work more uniformly with different crystals, but their optimum keying is more likely to occur at the maximum output point. It may very well be possible to make a triode or multi-element tube oscillator show similar results by adding additional feedback from plate to grid, but the Tri-tet and gridplate oscillators are easier to control. Grid-Block Keying The use of a blocking voltage on the control (or suppressor) grid of a tube to cut off its output until the blocking voltage is removed by the shorting of the key, as shown in Fig. 6, is an excellent method of keying an amplifier. The resistor R1 is the normal grid leak and R2 is a resistor used to prevent the blocking-voltage supply from shorting when the key is down. The capacity C1 is the normal r.f, by-pass plus any additional capacity necessary for a good keying characteristic. A nice feature of grid-block keying is that it requires no inductance to give a lag on "make," the lag coming from the time constant of C1 discharging through R1. On "break," the constant is determined by C1 charging through R1 plus R2. Since the grid leak, R1, is determined by the tube .that is being keyed, adjustment of a grid-block keying system consists of adding enough capacity across C1 until the" make" is as soft as desired and then, if the "break" still shows some click, raising the value of R2 until desirable keying is obtained. The same rule as set forth in the previous article applies - it is preferable to have a harder "make" than "break" for good copy at high speeds, and this is obtained automatically with grid-block keying. The same adjustment procedure applies to tube keyers (that use a blocking voltage) and to suppressor-grid keying. Grid-block keying is most convenient in amplifier stages using high-μ tubes that aren't being driven too hard, since such stages will require a lower voltage for cut-off. Unfortunately, grid-block keying does not work any too well with oscillators. It can be used, of course, but it isn't possible to get a soft "make" characteristic because the bias must be brought down to a value that gives a high enough mutual conductance before the tube will oscillate and it then plunges into oscillation in the usual manner. Further, a soft "tail" is not added to the oscillator when grid-block keying is used as is added to an amplifier keyed this way. The closest approach is suppressor-grid keying of a Tri-tet or grid-plate oscillator, and these both require that the oscillator run constantly, prohibiting break-in on one's own frequency without elaborate shielding and neutralization.
Fig. 6 - Grid-block
keying circuit. R1, is the normal grid leak and C1,
is the r.f. by-pass condenser plus enough capacity to give a good
keying characteristic. R2 is included to prevent a short
circuit of the blocking-voltage supply when the key is closed. Increasing
the size of C1, will make the keying "softer" on both
"make" and "break;" making R2 larger will soften "break."
Summary In addition to the keying checks listed last month, the following applies specifically to keyed crystal oscillators. 11. Holding the key down and tuning the crystal oscillator for maximum output does not always give the optimum keying adjustment. Send a string of dots and tune the oscillator for best keying. 2. A crystal oscillator should be capable of oscillating with only 3 or 4 volts on the plate if it is to key well. 3. In adjusting the lag filter at the key, don't overlook the effect of the value of screen bypass condenser if the oscillator is one that depends upon the screen for feedback (Tri-tet or grid-plate oscillator doubling or with wellscreened tube). 4. Use an r.f. choke in series with the grid leak and as high a value of leak as is consistent with adequate output. 5. Don't be surprised if some crystals key better than others in the same circuit. Posted 8/9/2011 |
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