du1vss

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Oct 25, 2016 / 0 Comments / in homebrew

This circuit is a simple one, audio signal from the telephone line is sampled by the 600:600 ohm isolation transformer. The combination of 0.1uF/200V capacitor and 500 ohm resistor suppresses the high voltage present in the line during the ring signal.
Signals appearing at the secondary side of the transformer is clamped at 1V peak to peak by the two diodes connected back to back. Audio level at this point can be easily processed by an audio mixer before going to the transmitter. ---73 de du1vss.

Oct 25, 2016 / 0 Comments / in homebrew

This is my first time to implement the 4:1 unbalance to unbalance coaxial transformer to match the input and output impedance of an rf power transistor. My prototype rf amplifier originally had a lumped LC network in both input and output sections and only have +/- 3MHz of bandwidth but after the addition of 4:1 coaxial transformer, the bandwidth has now increased to +/- 6MHz from the center tuning frequency of the amplifier.

Oct 25, 2016 / 0 Comments / in homebrew

This prototype rf amplifier uses two 2SC2640 VHF power transistor wired in push-pull configuration. The 50 ohm impedance of the input is transformed to lower impedance by the 4:1 broadband coaxial cable transformer providing a close match to the base of the two transistor. To operate in class C, the base of the transistor must be at ground potential so the secondary winding of the 4:1 coaxial transformer is grounded at the center position( null).

Oct 25, 2016 / 0 Comments / in homebrew

Home made VHF rf choke. It has a typical value of 0.3uH made by winding 15 turns of 0.2mm enamel wire over a high value resistor (27K 1/2watt) as a former. Inductor leads are soldered across the resistor leads and the instant glue secure the winding on the former.

Most of my home made vhf rf amplifiers uses this rf choke in its bias and also in the DC supply section. Commercial vhf choke generally comes in a small package suitably for smd but since I have so many enamel wires in my junk box, home brew comes in handy. ---73 de du1vss

Oct 25, 2016 / 0 Comments / in homebrew

I was inspired by the MDT40, a double side band suppressed carrier transceiver designed by VK2DOB because of its simplicity and how he simplified the use of a balanced modulator. His circuit uses a balanced modulator that serves two purpose that is to generate a double side band signal during transmit and also serves as a demodulator to detect incoming ssb signal during receive. Another advantage of the dsbsc is that it does not need a mechanical filter like the one found in a typical ssb transceiver.