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

Description: This project arises from the need to home-build a valid tuning control for a multi band transceiver. It consists of a partial synthesis V.F.O. that fits to single conversion equipment 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 it may represent an acceptable compromise between simplicity (but not enough to be regarded as an elementary job) and 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 (especially 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 which 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) uses a BF960 MOSFET as a mixer and a 2N2222 as a crystal-driven oscillator to obtain the 36 MHz output from an 18 MHz crystal. 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 alignment can be made in the following manner: remove the crystal so the oscillator goes off, input a 41.2 MHz signal to gate 1 and tune the capacitors to obtain maximum output, insert the crystal and drive a 5 MHz signal into gate 1 tuning the 60 pF capacitor for the maximum output (0.7 to 1 Vpp). The second mixer uses an NE602 IC. This device allowed obtaining the best results concerning linearity and balance 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 to limit the spurious 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 the 2.2 pF capacitor 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 allows obtaining a substantially constant output level over the entire frequency range covered by the mixer. This is very important to ensure good working by the TTL 74393 divider.

The project involves a multi-band transceiver tuning control utilizing a partially synthesized Variable Frequency Oscillator (VFO) designed for single conversion equipment with an Intermediate Frequency (I.F.) stage around 9 MHz. The circuit operates across the entire High Frequency (HF) band from 3.5 MHz to 30 MHz, generating an output range of 12.5 MHz to 39 MHz. The design integrates Phase-Locked Loop (PLL) and crystal conversion techniques, achieving a balance between complexity and performance.

A critical aspect of this project is the alignment procedure, which necessitates the use of an RF generator, a frequency meter, and preferably an oscilloscope to facilitate troubleshooting. The VFO could be enhanced by implementing a Direct Digital Synthesis (DDS) approach, such as the Digi VFO, which offers improved frequency stability and precision.

The circuit comprises multiple printed circuit boards (PCBs). The first PCB includes two buffer stages that drive an external frequency counter and the initial mixer stage. The output level to the mixer is specified at approximately 3 Vpp, and this PCB also integrates a 7810 voltage regulator for power supply management.

The second PCB houses the first mixer and a 41 MHz filter, utilizing a BF960 MOSFET for mixing and a 2N2222 transistor as a crystal oscillator to derive a 36 MHz output from an 18 MHz crystal. The design of the inductors, such as L2 and the output filter formed by L46 and L5, is crucial for maintaining desired frequency characteristics and minimizing losses.

The alignment process for the mixer involves removing the crystal to deactivate the oscillator, applying a 41.2 MHz signal, and adjusting capacitors for optimal output. Once aligned, reinserting the crystal and driving a 5 MHz signal allows for further tuning to achieve a specified output level.

The second mixer utilizes an NE602 integrated circuit, which enhances linearity and balance across the frequency spectrum. The design incorporates a capacitive divider to lower the input VCO signal, and a broadband transformer ensures that balanced signals are fed to the mixer, limiting unwanted spurious outputs. The tuning of the bifilar inductor L3 is critical for achieving the desired performance, and adjustments may be necessary on the 2.2 pF capacitor to optimize output levels.

Finally, a buffer stage with dual 2N2222 transistors is implemented to stabilize the output across the entire frequency range, ensuring compatibility with subsequent stages, such as the TTL 74393 frequency divider. This comprehensive design approach facilitates a robust and efficient multi-band transceiver tuning control system.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