7MHz VFO using 3.58MHz Ceramic Resonator

This project is a 7MHz vfo that uses a 3.58MHz ceramic resonator. The oscillator output is fed to a frequency doubler but the output of the stage contains full of harmonics. To clean the output waveform, a double tuned band pass filter is added. Each filter resonates at 7MHz and the passband is quite sharp which effectively attenuate the unwanted harmonics.

The output level is at 3volts peak to peak and has a 60Hz per hour drift after some warm-up. Using 1N4007 as varactor, i was able to tune +/- 0.200MHz from the center frequency of the oscillator.   73 de du1vss

QRP RF Voltmeter from your Junk Box

A useful tool that can be easily crafted out from your junk box is this rf voltmeter. I discovered this circuit online and created a  prototype out of it. I found this project very useful in reading RMS voltage in my HF /VHF projects. I use it to determine the output power of my rf amplifier, set the null in my balanced modulator, determining the peak voltage output of my oscillator, peaking a band pass filter, etc... 

It takes only 3 components to start with. I use 1N60 germanium diode to rectify the high-frequency AC signal into a DC voltage compatible with any digital multimeter. My prototype rf voltmeter is mounted in a small PCB assembled in surface mount fashion. Two connectors soldered on the edge of the pc board to accommodate the positive and negative probe of my digital multimeter.

Connect the alligator clip to the common ground of the circuit you are measuring. Minimize the length of the alligator clip wire to avoid introduction of errors in your measurements, particularly on higher hf bands. In my prototype the lead wire for the alligator clip is just over 6 inches.

When reading the Vrms, take the measurement across the 50 ohm dummy load. For instance, if you measure 13V using your digital tester, the computed power is around 3.38watts, calculated using the formula P = (Vmeasured)² / 50.    ---73 de du1vss

40M DSB Transceiver

The circuit described here is actually a variation of the  MDT40 project by VK2DOB,  A simple double side band suppressed carrier (DSBSC) transceiver which operates on 7MHz. I find this project simple and a good start if you are interested into home brewing your own rig so I decided to make one.

In the original circuit, it uses a ceramic resonator as a common oscillator for both receive and transmit stage but I have difficulty in finding the parts here in Manila so I decided to use a vfo whose circuit was copied from KD7REM, a rocksteady vfo. Stability is considerable during start up which usually takes less than 30minutes of wam-up and can be used with a couple of QSO without correcting the vfo for a drift. T50-6 toroid was chosen as a core of my inductor where 25 turns of enamel wire was wound. RF feedback was taken at 5 turns from the cold end. I found by experiment that by keeping the capacitance large in the L/C ratio, the oscillator is more stable than the other way around. MPF102 is also used since JFET is more stable than its BJT counterpart.

The balanced modulator and the receiver pre-amplifier stage are constructed together in one board. There are four 1N4148 used in the balance modulator and please note that these diodes are carefully matched by their forward bias voltage. A 500 ohm potentiometer is used to find the null in the balance modulator. Carrier suppression is easily done when you already have completed the transmitter stage since this can be just adjusted by the power output of the rf amplifier stage.

The rf transistors found  in the transmitter stage are uncommon so I decided to redesign completely the entire stage using common transistors that are locally available here. 2SC2078 was chosen as the final stage since it is cheap and offers a 4W to 5W ouput at the HF frequencies.

Above is the prototype of my DSBSC transceiver that I made. It is housed in a plastic casing (black) and on the top (grey plastic casing) is the additional 15watts linear amplifier constructed using a push-pull IRF510 mosfet transistor.  ---73 de du1vss