Frequency synthesizer for the range of 137 kHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Frequency synthesizer for the range of 137 kHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The range 135,7 ... 137,8 kHz, relatively recently allocated for amateur communications, has recently attracted more and more attention from radio amateurs. The new range also requires new equipment. This article describes a frequency synthesizer at 137 kHz, which is based on a frequency synthesizer chip from a CB radio station.

In the 137 kHz range, high requirements are placed on the stability of the transmitter frequency, so the conventional VFO is of little use here. It is necessary to use frequency synthesizers that have higher stability. The proposed synthesizer has the following technical characteristics:

Frequency range, kHz ..... 135,7 ... 137,7
Frequency step, Hz.....50
Measured frequency instability, Hz.....0,1
Supply voltage, V.....9...15
Consumed current, mA, not more than ..... 150
Output waveform ..... square wave
Output voltage amplitude, V ..... 2...2,5

The scheme of the device is shown in fig. 1. The choice of a frequency synthesizer chip from a radio station of the civil (CB) range (DD1) is not accidental. In 40-channel radio stations, the transmission frequency is approximately 27 ... 27,4 MHz, and the synthesizer generates a signal with a frequency of 13,5 ... 13,7 MHz, respectively. Using such a synthesizer and dividing this frequency by 100, we get a frequency just within the range of 137 kHz. The HG1 indicator does not show the frequency, but the channel number, as in a CB radio station. It is not difficult to establish a correspondence between the frequency and the indicator readings.

(click to enlarge)

The voltage controlled oscillator (VCO) circuitry is also similar to that used in these radios, only slightly simpler because the VCO does not need to be used in both receive and transmit modes. The VCO is made on the transistor VT1. The RF voltage generated by the VCO is supplied to the DD1 chip from the emitter of the transistor VT1 through the capacitor C8- In the DD1 chip, this frequency is compared with the reference one and a voltage is generated that is proportional to the magnitude and sign of the error. This voltage is supplied to the VCO to the VD2 varicap, which changes its own capacitance and, thus, changes the frequency in the right direction.

The control voltage for the VD2 varicap is supplied through a T-shaped filter R4C7R5. At the input of the pulse shaper, the voltage is taken directly from the circuit through the capacitor C18.

The pulse shaper is designed to amplify and limit the VCO signal. It is made on transistors VT5 and VT6.

From the output of the shaper, the pulses go to the frequency divider, which divides the input frequency by 100. The collector of the transistor VT6 is connected to the counting input of the binary-decimal counter DD2, which divides the frequency by 10. The second counter (DD3) also has a division factor of 10. The peculiarity of the counters is that first the input sequence is divided by 5, and then by 2. Thus, the output is a voltage close in shape to a meander. Such a signal can be applied to the mixer of the direct conversion receiver or through a filter to the input of the transmitter's power amplifier.

Using the SB1 and SB2 buttons, you can select one of 40 frequencies in 50 Hz steps. The SA1 toggle switch must be closed during normal operation of the synthesizer and opened at the time of the frequency change. At this time, the synthesizer should not be connected to transmitting devices, since the highest possible frequency is generated.

The device is mounted on a single-sided printed circuit board (Fig. 2). Almost all parts are installed on the board, with the exception of the indicator, channel selection buttons, SA1 toggle switch and capacitor C1.

Frequency synthesizer for the range of 137 kHz

The voltage regulator DA1 must be mounted on a small heat sink, for example, a duralumin plate. After adjustment, the board is placed in a shielded case.

The synthesizer chip and the indicator are used from CB radio stations START-1, GOLT-359, CONTACT-3. It is quite possible to use microcircuits from other radio stations, since most of them are built according to a similar scheme. The ZQ1 quartz resonator can also be used from a CB radio station, i.e., at a frequency of 10240 kHz, but in this case the frequency range will shift and will be approximately 135 ... 137 kHz. Changing the frequency of the crystal by 10 kHz will change the output frequency by about 100 Hz.

Coil L1 is wound with PEV-2 wire with a diameter of 0,63 mm on a frame with a diameter of 5 mm, contains 9 turns of continuous winding. The coil is placed in the screen. After pre-tuning the VCO, it must be impregnated with varnish.

Transistors VT1, VT5, VT6 can be of the KT312, KT315 series. Transistors VT2, VT3, VT4 - any low-frequency low-power pnp structures. Counters DD2 and DD3 can be replaced by K155IE2. Ceramic capacitors - KM-5.

Now let's move on to tweaking. Before switching on, check the correct installation. Unsolder resistor R4 from pin 14 of the DDI chip. Connect a variable resistor with a resistance of 22 ... 100 kOhm with one output to the common wire, the other - to a +5 V source (after the voltage stabilizer). Connect the variable resistor engine to the soldered terminal of resistor R4. Set the engine to the middle position. Connect the output of the synthesizer (pin 12 DD3) to the oscilloscope and frequency counter. Apply voltage. With serviceable parts and correctly performed installation, rectangular pulses with an amplitude of 2 ... 2,5 V will be observed on the oscilloscope screen.

Let the structure warm up for 10-15 minutes. By rotating the L1 coil trimmer, set the oscillation frequency within 136,5 ... 137 kHz. Rotating the variable resistor "from lock to lock", measure the frequency in the extreme positions of the engine. It should be within 130 ... 142 kHz, and in the middle position of the engine - approximately 136 ... 137 kHz. The minimum frequency limit is 134...139 kHz, the maximum is 125...150 kHz. If the frequency range is wider than necessary, you can use a capacitor C11 with a smaller capacity, and C17 with a larger one.

Remove voltage from the device, unsolder the variable resistor and solder the resistor R4 in place. Apply power to the synthesizer, check its operation on different channels and, if necessary, adjust the frequency. This can be done within a small range by selecting capacitor C13. This correction changes the frequency on all channels at once.

Check the indicator glow and, if necessary, select resistors R21 and R23.

When using a synthesizer in conjunction with a transmitter, it is necessary to use good filters to suppress higher harmonics. When used with a direct conversion receiver, it is sufficient to apply a T- or U-shaped single-stage RC filter.

A variant of the device without a synthesizer chip is possible. If you collect only the VCO, the shaper and the divider on the board, you get a regular VFO. In order for it to turn out stable, you need to take some measures.

Set the frequency to 136,7 kHz (with a variable resistor as described above). Having directed a stream of hot air to the circuit elements, note in which direction and how much the oscillation frequency changes. Let the structure cool. Now, carefully heating the individual elements of the circuit C12, C11, C17, C18 and VD2 with a soldering iron tip, determine the one that gives the greatest frequency drift at the same degree of heating. Do not hurry! Heating one element, wait until it cools down and only then check the next one.

If the varicap causes the greatest frequency drift, select a capacitor C11 with such a TKE that their simultaneous heating does not cause a significant frequency drift. If one of the capacitors C12, C11, C17 or C19 causes frequency drift, replace it with another with the same rating, but with a different TKE. The ultimate goal is to achieve a minimum change in frequency as the parts of the circuit heat up and cool down.

Do not forget to allow the circuit elements to cool after each soldering. This process is the most time-consuming, but with careful adjustment, you can get very high frequency stability. The better you make thermal compensation, the more stable the design will work, the more success you can achieve in the future.

When debugging the synthesizer, I easily managed to get my own GPA frequency stability no worse than 3 Hz after ten minutes of warming up. If you need more stability, for example, for a lighthouse, you can install a quartz resonator at a frequency of 1 ... 13570 kHz instead of the inductor L13780.

Author: N.Filenko (UA9XBI), Inta, Komi Republic

See other articles Section Civil radio communications.

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