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A compact, low-cost unit using surplus crystals.

Here is a frequency standard built around the low-frequency FT-241A crystals, and using a multivibrator to obtain markers every 50 kc. throughout the communication spectrum. The oscillator circuit is one that will work with high-frequency crystals as well, and thus provide spot checks for identifying the 50 kc. harmonics.

A frequency marker providing spot frequencies at intervals of 50 kc. per second is a useful piece of measuring equipment for the radio amateur, not alone because it permits accurate determination of the amateur band limits, but because it provides means for calibrating receivers, variable-frequency oscillators and similar gear. The frequency marker described in this article provides crystal-controlled marker frequencies at 50-kc. intervals up to at least 30 Mc.

Except for the 115-volt a.c. source of power, it is entirely self-contained in a standard metal case, 3 by 4 by 5 inches in size. It uses a low-frequency crystal, such as are now readily available on the surplus market in FT-241 or FT-243 holders for two dollars or less.

Circuit

As the wiring diagram, Fig. 1, shows, the marker comprises a 6AK5 crystal-controlled oscillator, followed by a 6118 triode-pentode frequency-controlled multivibrator adjusted to provide marker frequencies at intervals of 50 kc. Output is ample for communication-type receivers up to 30 Mc.

Fig 1
Fig. 1. Circuit of the 50 kc. frequency marker.

C1,C2Electrolytic.
C3,C4Dual ceramic-mounted trimmer.
C5,C9,C10,C11,C12Mica.
C6,C7,C8,C12Ceramic or paper.
CR175 mA selenium rectifier.
S1D.p.s.t. slide.

Power to operate the unit is taken from the 60 cycle line. The frequency marker contains its own power supply, consisting of a 6.3 volt 1 ampere filament transformer, and a half-wave selenium rectifier and resistance-capacitance smoothing filter comprised of C1, C2, and R1. Protection against short circuits to ground is provided by the ground coupling capacitor, C12, in the output circuit.

The crystal oscillator uses a 6AK5 pentode in an electron-coupled Pierce oscillator circuit, with the screen of the pentode serving as the plate of an equivalent triode while output to the multivibrator is taken from the plate of the pentode. The frequency of oscillation can be adjusted over a small range by means of the variable capacitors C3 and C4. Capacitor C4 is used to raise the frequency, whereas C3 lowers the frequency slightly. Both of these capacitors are mounted on a single ceramic base as a double trimmer, and each has a capacitance range of from 10 to 170 pF. A series capacitor, C5, was used to reduce the maximum capacitance of C4 to about 60 pF. Output from the plate of the 6AK5 is fed to the grid of the triode section of the 6U8 multivibrator.

The frequency divider consists of an electron-coupled multivibrator. When free running (i.e., not controlled by the crystal oscillator) its frequency range extends from about 30 to 80 kc., depending upon the setting of the frequency-adjusting resistor, R6. When the multivibrator is frequency controlled to operate at 50 kc., R6 has a value of about 23,000 ohms. Aside from the feature of electron coupling, which virtually eliminates effect of load on operation of the frequency marker, the only unusual feature of the multivibrator circuit is the use of series resistors in the grid circuits. These are not necessary but are used as an aid in producing a reasonably good square-wave output at 50 kc.

Photo 1
This unit generates frequency marker signals at 50 kc. intervals, using surplus crystals in the 400 to 500 kc. region as the primary frequency source. In this front view the oscillator tube is at the left rear and the multivibrator tube at the right.

Photo 2
Power-supply components are at the top in this interior view of the frequency marker. Most of the oscillator and multivibrator circuit components are mounted on the Vector sockets for the two tubes.

Adjustment

The oscillator goes into oscillation easily when the crystal is plugged into its holder. The multivibrator is, perhaps, easiest adjusted by coupling its output to a communications-type receiver and varying the resistance of Rs until marker frequencies are produced at intervals of 50 kc. This adjustment is most easily done in the broadcast band, or a similar low-frequency band of a multiband receiver. Proper adjustment is that for which the note in the receiver is sharp and clean. It will probably be found that the desired condition of operation can be obtained with R6 adjustable throughout a small range of angular rotation. If adjustment is made by means of a receiver alone, the midpoint of this angular rotation is probably the best adjustment, but a check at the high-frequency end of the receiver is advisable.

If a cathode-ray oscilloscope is available, it can be used to permit the output to be adjusted more nearly to a square wave. This is done by connecting the output of the multivibrator to the input terminals of the vertical-input amplifier of the oscilloscope. The oscilloscope sweep circuit should be adjusted to provide horizontal sweep of from 10 to 50 kc., and the synchronizing adjustment should be advanced to lock the trace into a stationary pattern showing several cycles of waveform of the multivibrator. The resistor R6 may then be adjusted to that value of resistance which yields the most nearly square wave on the screen of the oscilloscope.

Operating Data

Power consumption is small (approximately 10 watts) and thus the unit may be left running continuously. Very little drift - only a few cycles per second - is observed in warming up, however, so for most frequency measurements it is not really necessary to leave the unit running. The crystal frequency can be adjusted to exact value by varying C3 or C4 until the output is in zero beat with signals received from W W V.

A score of crystals have been used with this frequency marker and in all cases oscillation occurred readily. The multivibrator has synchronized easily with crystals whose fundamental frequencies were 200, 400, and 500 kc. The oscillator also worked well with crystals having frequencies of 1000 and 5000 kc., but the multivibrator would not synchronize at all with the 5 Mc. crystal, and only with difficulty and not too satisfactorily with the 1 Mc. crystal. This is in accordance with usual good practice of not using multivibrators for dividing the frequency of the controlling oscillator by factors of more than 10. Suitable low-frequency crystals, including those operating at 450 kc., and which should be as useful as those already mentioned, are available as surplus material for $2.00 or less each.

A crystal-controlled frequency marker such as is described here can be built for from $15 to $20 if all parts, including the crystal, must be purchased new. Of course, if the receiver is provided with suitable power-supply terminals, filament and plate power could be taken from the receiver, in which case the cost can be still further reduced by omitting the filament transformer and rectifier-filter system.

The high precision and small size of this self-contained unit make it an especially useful piece of equipment for the radio amateur.

Beverley Dudley.