Voice Activated Repeater Controller Using LB1403N

Here is my first attempt to construct a voice activated repeater controller using a VU driver IC LB1403N. Instead of driving a group of LEDs, the first pin is used to pull down the PTT (push to talk) of the transmitter. A small portion of the received audio is routed to the transmitter via the 100K, 0.1uF and 10K potentiometer.

The entire circuit is relatively simple and the only drawback is that it needs a high level of audio input coming from the receiver to keep the PTT close all the time during transmission. The protoype board is connected to my Icom IC-2200H as the transmitter and Icom IC-V80 as the receiver. ---73 de du1vss

Mosfet Replacement in Pallet Amp Trick

I would like to share my trick in replacing the mosfet from pallet amplifier which I learned from my experiences in pallet fabrication and repairs. Most of us would like to unsolder those mosfet legs first one by one and is the most difficult to do since too much heating on those pcb pads would accidentally delaminate the copper from the board.

My shortcut in doing this is to cut the legs of the mosfet using a cutter blade but before doing this please make sure that the mosfet is verified defective!

After freeing the mosfet, now unsolder the remaining mosfet legs from the board using a soldering iron. Cleaned the board, apply fresh thermal paste and mount the new mosfet.  73 de du1vss

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