Multi-Purpose VFO


Posted on Feb 6, 2014

The Frequency instability in VFO stage is a common problem. This is usually occurred in VFO`s due to low Q  of the tank circuit, capacitors real value, power supply regulation, stray capacitance, lead inductances and over feed back levels etc. Few months back, I had discussed about VFOs and Oscillators with one of the veteran home brewer, he is


Multi-Purpose VFO
Click here to download the full size of the above Circuit.

a senior amateur licence holder than me and he is working as JE in communication filed. In that discussion, he told me that while oscillating the bi-polar transistor based VFOs, the first transistor emitter-base voltage should shows as negative volt and this Js an indication of healthy operation. He showed me a practical demo also. When I started my 7MHz, AM/SSB/CW RX project due to heavy compulsion from HAM`S and SWL`s. I started designing work on September 2004, the aim to release the circuits and the PCB on Adoor HAM `XPO 2004 on 21 st November. I strictly started the proper designing for the benefit of new license holders and SWL`s to get practice about systematic receiver assembling. During that period, while designing the 7 Mhz RX project, I discussed with VU2ARA, Arasu, about VFOs through Kodai VHF repeater. He strictly told that the first transistor emitter-base voltage must be a positive volt. Due to this positive and negative charge a lightning stroked my brain. VU2ARA never disclosed the exact principle about a positive voltage at emitter-base while oscillating and he forced me to find out myself. I could not get any information from anywhere including ARRL or from any other amateur radio based articles. I discussed with VU2ITI, Prof. T. K. Mani and he supported the statement of VU2ARA and reminded me that the first transistor is in class A` mode. Then I realised why the positive voltage should keep at E-B...




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