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I.F. Amplifier - The I.F. transformer primary has 18 turns, the secondary winding has 4 turns. The capacitors across the IFT primaries are 82pF. The input/output transformer has 12 turns, tapped at 3 turns from ground. This transformer is wound on a ferrite core. The MOSFETs are 3SK45's. The diodes in the product detector are 1N34's. I use a six pole SSB filter from a scrap CB. The centre frequency is 7.8MHz. The -6db bandwidth is about 2.5KHz. __ Designed by EI9GQ homebrew radio Radio Frequency (RF) Oscillators - Every oscillator has at least one active device (smarties don't complicate matters for me - just read on) be it a transistor or even the old valve. This active device and, for this tutorial we'll stick to the humble transistor, acts as an amplifier. There is nothing flash about that. For this first part of the discussion we will confine ourselves to LC Oscillators and I'll keep the math to an absolute minimum.__ Ian Purdie
Resonator BFO - This circuit was used to stop all the BFO drift. The circuit is extremely stable. Turn the receiver off, and then on at any time and temperature, the BFO frequency is exactly the same. __ Designed by Web David White, WN5Y
Series Regulator with Q900 - This is a series regulator with Q900 being the control element, Q901 a driver, and Q902 an error amp. ZD900 forms the emitter reference voltage source. Since the generated high voltage and other voltages are linked by means of the magnetic field of T900, any change in H. V. will be reflected back to all of the other voltages. __ Designed by Chris Candreva
Shortwave Audion - It is a Hartley-ECO Audion, in which the plate is grounded for RF. Positive feedback from cathode to grid occurs via the coil tap. The amount of feedback is determined by the 100k pot; this controls the voltage at the screen grid and thereby the transconductance of the valve (higher voltage resulting in greater transconductance). __
VFO/Buffer - It's basically a standard Hartley oscillator, followed by Roy Lewallen's buffer (page 14.20 of the 2001 Handbook). Output is +7 dBm into 50 Ohms. Don't be tempted to add a gate diode, this circuit doesn't need it, and it will degrade the phase noise performance, according to Ulrich Rohde. It should be suitable __ Designed by Leon Heller
What I had to do to stabilize a VFO… - From the basic homebrew literature (QRP Classics, DeMaw's notebooks, various ARRL Handbook projects, etc) , I could never get VFO stability that really satisfied me. __ Designed by John Seboldt, K0JD |
