DDS based VFO

Another weekend project is this variable frequency oscillator based from dds chip. I found this project useful in my home brew dsb transceiver and testing several filters and rf amplifiers.

I bought the dds chip from Lazada, the cloned arduino is from Alexan, and the lcd module is from E-Gizmo. The dds is able to generate an rf signal from 1MHz to 30MHz with adjustable tuning steps from 10Hz, 50Hz, 100Hz, 500Hz, 1KHz, 2.5KHz, 5KHz, 10KHz, 100KHz and 1Mhz.

A broadband rf amplifier is also included to boost the amplitude of the dds signal. Around 3V peak to peak was measured at 7Mhz and decrease gradually on higher frequencies.

Credit is given to AD7C in which the schematic and the arduino code was copied. The broadband rf amplifier is not shown however.

A beautiful 7MHz sinusoidal wave  can be seen in my Tektonix oscilloscope. One thing I observed however, the 125MHz reference clock is heating specially when the dds module is supplied with 5V but this did not affect the quality of the signal output generated by the module. One option is to use the 3.3V supply found in arduino uno but the signal level from the module is reduced. EMI and other interference could also stop the module from generating rf signal.    ---73 de du1vss

Home Brew Static Wick

By theory, this device bleeds excess voltage potential of your tower and thus minimize the risk of direct lightning strike. This device is totally different from the lightning arrester and though it does not guarantee 100% protection but there is no harm in trying.

I made my wick from fine stainless wire cut 12 inches long. I prepared 15 pieces of these wires then I folded it in half and crimped them together in a 3/8" stainless tubing. The more pointed tips, the better it perform so it is your option. The wick can be attached to the highest portion of your tower and must be provided with grounding below.   73 de du1vss

Diode Ring Mixer

40m band is poor today and I want to listen to the AM broadcast band. However, my IC-M700TY HF transceiver can only go down to 2MHz.  Upon checking  my junk box, I found the right components to assemble an up converter so that I can still hear AM broadcasts on my transceiver.

The illustration above depicts a diode ring mixer, In my prototype, I am utilizing four closely matched 1N4148 diodes based on their forward voltage and a pair of  FT37-43 toroids as a core for my broadband transformer. The transformer is wound with four trifilar turns of #22 AWG insulated wire.

After completing the diode ring mixer, I constructed a 10MHz crystal oscillator using the circuit mentioned earlier. The oscillator is producing 2.8V peak-to-peak of 10MHz signal which I believe is well within the requirements for the diode ring mixer. The 10MHz signal will be fed into the VFO port of the ring mixer.

I've enclosed the finished prototype in a plastic container and I've installed a pair of connectors on the side for easy connection of the aerial antenna and the patch cable leading to the antenna connector of my IC-M700TY. The aerial antenna will be connected to the RF port while the baseband output signal can be extracted from the IF port of the ring mixer.

To make this set-up operational, the AM broadcast frequencies will be present at both (10MHz + AM band) and at (10MHz - AM band). For instance, if DZMM is broadcasting at 630KHz, it can be received either at 10.630MHz or at 9.370MHz   ---73 de du1vss

180W Pallet Amplifier Using MRF9180

This is my first time to play with the 180W pallet amplifier kit from China. The kit comes with only a printed schematic but there was no details for the input transformer and the coaxial balun. The bias for the LDMOS is not also indicated so I have to download the datasheet of the MRF9180 from the internet.

The input transformer has a 9:1 impedance ratio so it has a 3 turns in the input while a single turn in the output. Connected to the drains are the 25 ohm teflon cables. This coaxial transformer is responsible for stepping up the low impedance drain to approximately 25 ohms. Next is a 50 ohm cable used to combine the 25 ohm balance signal into 50 ohm unbalance at the output. I choose to cut this cable to 15cm so that pallet operation would be optimal at the fm broadcast band.

The datasheet says that for MRF9180, gate voltage bias may be set anywhere between 2 to 4V but in my amplifier the VGS was set to 2.2V. At full output power, the pallet amplifier is consuming around 8 to 10 amps at 24V dc supply.

After the test, I was getting 150W at 144MHz (2W) drive and 200W at 100MHz (1.5W) drive and this pallet amplifier is remarkably broadband. The test was conducted without the LPF at the output.
--- 73 de du1vss.

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