V.F.O. for H.F with PLL

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This project arises from the need to home-build a valid tuning control for a multi band transceiver. It consists of a partial syntesis V.F.O. that fits to single conversion equipments with an I.F. stage nearby 9 MHz. The circuit can cover the whole H.F. band from 3.5 to 30 Mhz (i.e. 12.5 to 39 Mhz output). This device has been developed through several experiments based on PLL and crystal conversion circuits, and I think it may represent an acceptable compromise between the simplicity ( but not enough to be regarded as an elementary job ) and the performance. Consider that some equipment is necessary for the alignment : an R.F. generator and a frequency meter are a must, but the availability of an oscilloscope makes the job easier (specially in case of troubles).The VFO circuit could be better replaced by a more sophisticated DDS unit like the Digi VFO and related Digi Brain presented in the May ‘95 and March ‘96 issues - two buffers wich drive an external frequency counter and the first mixer stage (the second PCB). The output level to the mixer should be about 3 Vpp. This PCB also contains the 7810 power supply. - the first mixer and related 41 Mhz filter (the third PCB). It uses a BF960 mosfet as a mixer and a 2N2222 as a christal driven oscillator to obtain the 36 Mhz output from a 18 Mhz christal. L2 is made by 9 turns, 0.5 mm wire on a T44-2 toroidal core (0.42 mH). A 41 Mhz output filter is obtained by L46 and L5 (9 turns, 0.5 mm wire on a T44-6 core, 0.34 mH) with a buffer stage (2N2222 transistor). L3 is made with 2 wires wound on L4. The mixer alignement can be made in the following manner : - remove the christal so as the oscillator goes off - input a 41.2 MHz signal to gate 1 and tune the capacitors to obtain maximum output - insert the christal and drive a 5 Mhz signal into gate 1 tuning the 60 pF capacitor for the maximum output (0.7 to 1 Vpp) second MIXER wich uses an NE602 IC. This device allowed to obtain the best results concerning linearity and balancement over the entire frequency range. The input VCO signal is lowered by a capacitive divider and the two balanced inputs (pins 1 and 2) are driven in opposite phase using a broadband transformer so as to limit the sporious outputs. L3 is made by 5 bifilar 0.5 mm wires into a binocular ferrite core, type 43 material 13x8x8 mm. Some tuning may be required on the value of 2.2 pF capacitor so as to obtain a level of 100-200 mV pp into pins 1 and 2 of the IC. A buffer stage equipped with two 2N2222 transistors and a compensation network on the second stage emitter allow to obtain a substantially constant output level over the entire frequency range covered by the mixer. This is very important to ensure a good working by the TTL 74393 divider visit page.

The VFO circuit could be better replaced by a more sophisticated DDS unit like the Digi VFO and related Digi Brain presented in the May ‘95 and March ‘96 issues - two buffers wich drive an external frequency counter and the first mixer stage (the second PCB). The output level to the mixer should be about 3 Vpp. This PCB also contains the 7810 power supply. - the first mixer and related 41 Mhz filter (the third PCB). It uses a BF960 mosfet as a mixer and a 2N2222 as a christal driven oscillator to obtain the 36 Mhz output from a 18 Mhz christal. L2 is made by 9 turns, 0.5 mm wire on a T44-2 toroidal core (0.42 mH). A 41 Mhz output filter is obtained by L46 and L5 (9 turns, 0.5 mm wire on a T44-6 core, 0.34 mH) with a buffer stage (2N2222 transistor). L3 is made with 2 wires wound on L4. The mixer alignement can be made in the following manner : - remove the christal so as the oscillator goes off - input a 41.2 MHz signal to gate 1 and tune the capacitors to obtain maximum output - insert the christal and drive a 5 Mhz signal into gate 1 tuning the 60 pF capacitor for the maximum output (0.7 to 1 Vpp) second MIXER wich uses an NE602 IC. This device allowed to obtain the best results concerning linearity and balancement over the entire frequency range. The input VCO signal is lowered by a capacitive divider and the two balanced inputs (pins 1 and 2) are driven in opposite phase using a broadband transformer so as to limit the sporious outputs. L3 is made by 5 bifilar 0.5 mm wires into a binocular ferrite core, type 43 material 13x8x8 mm. Some tuning may be required on the value of 2.2 pF capacitor so as to obtain a level of 100-200 mV pp into pins 1 and 2 of the IC. A buffer stage equipped with two 2N2222 transistors and a compensation network on the second stage emitter allow to obtain a substantially constant output level over the entire frequency range covered by the mixer. This is very important to ensure a good working by the TTL 74393 divider http://digilander.libero.it/ik3oil/_private/qexvfo.pdf

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