VHF frequency synthesizer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Frequency synthesizers

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At present, the requirements for the frequency stability of the local oscillators of receivers operating in the range of ultrashort radio waves have increased. Unfortunately, there are few publications about such devices. But on the other hand, a wonderful KN1015PL5 microcircuit appeared, suitable for creating a frequency synthesizer used as a VHF local oscillator. The structural diagram of the IC is shown in Fig. 1, the main parameters are in table 1.

Fig.1. Structural diagram of the IC

The block diagram of the synthesizer is shown in Fig.2. The synthesizer has a voltage-controlled oscillator (VCO), from the output of which a voltage of the desired frequency is supplied to the receiver mixer. The VCO frequency is tuned by applying a constant voltage of various magnitudes to a reactive element (RE) - usually a varicap.

Fig.2. Synthesizer block diagram

The voltage from the VCO is fed to a controlled frequency divider (UDCH), the division factor of which is set using the division factor setting register (RUCD). The state of this register (code) is changed using the tuning generator (GN). The VCO signal, after dividing it into UHF, is fed to a frequency-phase detector (FPD), where it is compared with the frequency of the reference oscillator, which is formed by dividing the frequency of the crystal oscillator (KG) in the corresponding frequency divider (DF). From the PFD output, the frequency setting error signal is fed through a low-pass filter (LPF) to the RE. Thus, the frequency-phase adjustment of the VCO frequency is carried out.

The output frequency of the VCO is maintained with the stability of the VCO. As can be seen from the circuit diagram (Fig. 3), the VCO is made on the transistor VT1. Its circuit includes a VD2 varicap. Through the emitter follower VT2, the signal is output. The VCO supply is stabilized by VD1. The controlled divider, KG, DCH, FFD are made on a DD6 chip (KN1015PL5). The division factor is set by applying "0" or "1" to the inputs 7 ... 18 DD6 with the RUKD, made on the DD3 ... DD5 microcircuits. It is a reversible counter, which is controlled by a generator on microcircuits DD1, DD2.

Fig.3. Schematic diagram of the synthesizer (click to enlarge)

The generator frequency is changed by the potentiometer R13. When its moving contact is in the middle position, the generator does not work. If you move it up (according to the scheme), generation starts on the top three elements of DD1. In this case, from the output 10 DD1.4, the signal is fed to the input 5 DD3, and the step-by-step switching of the register begins with an increase in the number stored in it, which means that the DPCD division factor begins to increase. The frequency of the VCO increases by 1 kHz with each pulse. The frequency of the GN pulses depends on how far the R13 slider is moved up, and can vary from 0,5 Hz (slow stepping) to 1000 Hz (fast tuning), i.e. the further the R13 engine is moved up, the faster the restructuring. To decrease the frequency, the R13 slider moves down. Then the generator will start working on the three lower DD1 elements according to the scheme, and the register will go to "decrease". This is how the setting is done. It's a bit unconventional, but you quickly get used to it. To fine-tune the frequency of the crystal oscillator assembled on DD6, ZQ1, C14 is used.

Table 1

Parameter	designation	Parameter value	Unit	Notes
Ranges of DPKDv division factors (step 1)	Nv	225 131071 ...	MHz	40 output
Range of division coefficients DPKDg (step 1)	Ng	3 8191 ... 100 900 ... 20 800 ...	MHz	37 output Group "A" Group "B"
Operating frequency range DPKDv	fiv	5 600 ... 20 900 ... 10 800 ...	MHz	Group "B" Group "A" Group "B"
Operating frequency range DPKDg	fig	0,1. ..80	MHz	Group "A"
Maximum input frequency FRF	Fg max	5	MHz
RF input sensitivity DPKDv	Sv	0,2 0,8 ...	В	19 output
Exhaust gas input sensitivity	Sg	0,1 ...0,15	В	Conclusion 22, fir=10 MHz
Maximum NMOS drain voltage	Umax	12	B	Conclusion 42, Ids=0,1 mA
Maximum residual drain voltage of the NMOS transistor, no more	Udsmin	0,1	B	Ids=10 mA
The steepness of the NMOS transistor, not less than	S	40	mA/V
Output impedance of PFD, no more	R0	600	Om	39 output
Input currents of low level, not less than	IiL	-5 -15	μA	Conclusions 2..18, 20, 24..36 Findings 19, 22
Input currents of a high level, no more	IiH	0,1 15	μA	Conclusions 2..18, 20, 24..36 Conclusions 19,22
Current consumption maximum (group "A") Icc max	17	mA	Ucc=5,5V; fi=900MHz; Ng=400; fg=10 MHz; Nv=225
Current consumption typical	Icc	5	mA	Ucc=3,5 V; fi=500MHz; Nr=400;fr=10MHz;Nv=22
Weight, no more		2,0	г	Pin pitch - 1 mm
Operating temperature range	T	-60 ... + 85	° C

The synthesizer is made on a board with dimensions of 95x65 mm (Fig. 4). R13 and C14 are fixed on it with the help of an aluminum corner. The inductor is not critical to the parameters, and any diameter of 6 ... 8 mm can be used. It contains 3 turns of PESHO wire with a diameter of 0,3 mm. Adjustment of the VCO center frequency is performed by a brass core. The R13 potentiometer is better than the SP-1 type, as the most reliable, but a sliding one can also be used.

Fig.4. Synthesizer circuit board

Chips DD2 ... DD5 it is desirable to use the 1533 series, a little worse - 555, even worse - 155, because. the power consumption of 5 V increases from 50 to 250 mA. Conclusions 2, 3, 6, 7 microcircuits DD3 ... DD5 with pins 7 ... 18 DD6 are connected by thin insulated conductors (surface mounting) - it's easier, and it turns out quite normally. As ZQ1, you can use any quartz with a frequency of 1 ... 8 MHz, choosing the division factor of the CD (by connecting the pins 24 ... 36 DD6 accordingly), so that the frequency at pin 37 is equal to 1 kHz (depending on the desired VCO frequency and step perestroika).

The setting is carried out in the following sequence:

• check the correct installation, the absence of short circuits and breaks in the board;
• check the work of the GN. In the middle position of the R13 slider, there should be no generation at the DD1 pins. When you turn the slider to the right or left, the generation frequency at the DD1 pins should increase smoothly. This is achieved by selecting R14 and R15;
• Verify that the crystal oscillator is working properly using a high-impedance oscilloscope. Connect a 5 kΩ resistor between the 37 V bus and pin 6 DD1 and check the operation of the DC - the frequency at pin 37 should be about 1 kHz;
• check the operation of the VCO with an oscilloscope on the VT2 emitter. Between the 5 V bus and pin 40 DD6, turn on a 1 kΩ resistor. The frequency at pin 40 should be approximately 1 kHz. It is set by adjusting the L1 core, and if necessary, by selecting C8;
• measure the DC component of the voltage at the connection point R1 ... R3, C2 using an oscilloscope or a high-resistance voltmeter. It should be within 1 ... 8 V and change smoothly when configured using R13. Set the middle frequency of the range using R13 and, by rotating the brass core of the coil, set this voltage within 4 ... 5 V. The setting is completed.

The synthesizer was made by the author at a frequency of 127 ... 131 MHz. In this case, the average UHF division factor is 129000, and the DC is 3584. It is possible to make a synthesizer for a different frequency and with other quartzes, while the division factor of the DC Kd is determined as follows:

where f_square - quartz frequency; fg.cp. - the average frequency of the local oscillator.

Of course, it is possible to make a similar synthesizer for the range of 430 ... 440 MHz - KN1015PL5 allows this, but then a higher-frequency VCO is needed. The author also made a synthesizer for HF bands, similar to the one published in [1]. At the same time, the number of microcircuit cases and dimensions were halved. There, instead of DD7 ... DD12, DD14 ... DD16, KN1015PL5 is installed.

Literature

1. L. Rivanenkov. Frequency synthesizer. - Radio amateur KB and VHF, 2000, N6, p.24.
2. Mouse valcoder. - Radio, 2002, N9, p.64.

Author: L. Rivanenkov, Smolensk; Publication: radioradar.net

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