A while back I decided to build a transceiver based on the "Beach 40" double side band rig by Peter Parker (VK3YE). This would be my first transceiver home brew project. The "Beach 40" is a 40 meter double side band direct conversion transceiver with a (originally) minimum of parts. It has some frequency agility and should put out a few watts.
I wanted to used mostly parts I already had on hand. I started off with the VXO which is a ceramic resonator variable oscillator. As you can see below I built it into an Altoids tin. These seem to be ideal for RF, it is shielded and actually very solderable. The chassis of the can was used as the ground plane, ugly type construction was used. Leads were kept short as best I could.
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| Start of the oscillator for the DSB transceiver. -AC9LF |
I decided on using varactor type tuning instead of going with a variable capacitor. I did have some proper varactor diodes but I did not get much tuning range out of them. I think I ended up using a couple of parallel 1N4001 type diodes as the varactors with good results. From the image below you can see the varactor diodes, a paralleled ceramic resonator and a small variable capacitor. Paralleling two ceramic resonators extended the tuning range of the oscillator. The variable capacitor was used to set the area of the band I wanted to tune in. In this case around 7.200MHz. I used a pot from a dead function generator for the tuning pot. I also used a voltage regulator for a stable tuning voltage across the tuning diodes.
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Testing oscillator -AC9LF
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| Oscillator output with associated FFT. -AC9LF |
I added a balanced mixer in the Altoids tin, winding the single transformer on a binocular core ferrite that I picked up from Science and Surplus for around 40 cents. The blue potentiometer is used to null out the carrier leaving only the upper and lower sideband signals.
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| Mixer added. -AC9LF |
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| Output of mixer with an audio input test signal. -AC9LF |
I built the rest of the rig on a piece of copper clad board starting with the low pass filter. I used toroids pulled from a dead PC for the inductors. I wound and measured the inductance before soldering them in and securing them with hot glue.
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| Low Pass Filter -AC9LF |
I built the transmitter section starting with the mic amplifier. I then proceeded RF amplifier stage by RF amplifier stage. I made use of the binocular core ferrite as needed. I used the microswitch below to enable the transmitter for taking measurements on the oscilloscope. The center relay was put on dead bug style to switch between the transmit and receive circuits.
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| Beginning of the transmitter. -AC9LF |
I built the audio output amplifier with an LM386, a jumper is available to switch between a fixed 20x gain or 200x gain. The screw terminal block goes to an 8 ohm speaker. The input the the LM386 has a potentiometer for volume control or "AF gain". I did use some perf board for the IC and construction of the audio amplifier. I have a heat sink on both the final transistor and the driving transistor. The driving transistor seemed to dissipate more heat then the actual final. The RF choke on the final transistor is just another scrap toroid that I wound a few turns of wire around. The output varies a bit with the number of turns on this core. For initial testing the mic and BNC were soldered directly to the PCB to be transferred the a makeshift case later.
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| Running out of space! -AC9LF |
During some initial testing the rig seems to put out about 1 watt. I will need to see if I can squeeze any more out of it. There also seems to be a little bit of self oscillation initially at around 1MHz if I recall.
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| DSB RF output into 50 ohm dummy load. -AC9LF |
I stuffed everything into a scrap enclosure I found. It seemed to be just big enough to fit. I still have to play around with the hardware a little more to get some more output power on it and see what it can do on the air. In the mean time I have added the schematic I based this on below. It was found on the soldersmoke blog. I did add a small filter to the front end receiver to try and remove some AM broadcast overload I was getting. I should be able to post more about this later. It has been sitting on the shelf for several months now collecting dust. I will get back to it soon...
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| http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html |
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| http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html |
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