Chapter 1: General design and oscillator circuits (This posting)
Chapter 2: Receiver (coming soon)
Chapter 3: Transmitter (coming soon)
Chapter 4: Cabinet, frequency counter, antenna etc. (coming soon)
Chapter 5: Final remakrs (coming soon)
This radio also is some sort of “rebuilt”. in 2015 I developed a handheld transceiver with about 5 watts of output power capable for use in the 20-meter-band (Link). The idea was great for hiking, traveling and cycling tours. But the output power was slightly inferior particularly when using a shortened rod antenna. Also was the battery pack too small to provide sufficient energy for longer operation. These shortcomings had to be overcome because spring and summer season are approaching and I intend doing much outdoor activities in upcoming warmer weather conditions.
The design thus had to meet the following requirements:
- Output power set in the range of 10 to 20 watts pep.
- Powered by rechargeable batteries of sufficient capacity
- Size must be as compact as possible under the given technical data mentioned before
- The rig has to provide a rugged design for outdoor and field use.
- An antenna holder should be integrated into the cabinet to enable the operator to use a rod antenna in the field.
These expectations lead to the following design guidelines:
- Four stage transmitter with driver and final stage in push-pull mode.
- A set of 11 nickel-metal-hydride solderable AA cells with the possibility to charge while mounted in the radio (no need to open cabinet for recharging process).
- Size of about a vintage CB hand held transceiver of the 70s.
- Construction made of aluminum sheet metal and aluminum bars (equal leg structural angle).
For energy saving purposes I decided not to use any microcontroller or digital equipment. The radio is based completely on analog technology except from a ready made frequency counter purchased in the late 80s in the last century. The counter has been added because the main oscillator (VFO) is varactor tuned and this does not allow to use a simple frequency readout that would be possible when using (for example) a vernier reduction drive.
This time we will start with the oscillator sections. As standrd with radios that use one interfrequency we use 2 oscillator: A VFO for main tuning and an LO for providing interfrequency range signals to produce SSB signal for transmit mode and to demodulate received signals.
I am running out of vernier drives. 😉 Two most practical main solutions that remain for setting the frequency in a VFO are either a permeabiilty tuning (by inserting some ferro- or diamagnetic metal into the coil and thus changing inductance) or by making use of a varactor. Because the first method involves a lot of mechanical challenges to ensure stable operation and therefore has been considered not being achiveale the decicion was to use a varactor tuned VFO.
The VFO is Colpitts type:
- Main coil is made of 50 turns of 0.2mm enameled wire on a T-37-2 toroidal core. I used the Colpitts design this time because without a tapped coil (as required for the Hartley design) experimenting is easier when to determine the exact number of turns.
- The varactor is a MV104 type (purchased via ebay). This one is used as tuning device in AM radios or so and works fine because it provides high capacity swing with low DC voltage span.
- The “fine art” of building this oscillator is to spread the 10 turns of the helipot so exactly that the full band from 5.00 to 5.35 MHz is coveren and wasting any space (except from 1 or 2 kHz for each edge) is avoided. The possibilities you have are modifying the two resistors (2.2k and 150 ohms), changing the number of turns in the coil ans, at last, changing the capacitors (27pF) and setting the trimmer value properly at last.
- Fixed capacitors in the first stage of the VFO should be polystyrene (best choice) or NP0. Polystyrene caps, according to my findings, inherit contrary temperature coefficient compared to the material of the coil (iron powder), therefore excellent frequency stability of this oscillator is achieved. In practical the oscillator needs 1 or 2 minutes to warm up and subsequently drifts with some 10 kHz per hour.
To my consideration it is not necessary to install the VFO into a separate cabinet or shielding because it is, as mentioned before, absolutely stable concerning frequency. But, as you guys all know, it is not the best idea to place it right next to the final RF amplifier stage ;-).
The Local Oscillator
This oscillator also is a Colpitts one. Two crystals (9001.5 kHz and 8998.5kHz) are switched by a two position switch in the front panel. The oscillator is followed by 1 amplifier stage putting out about 1 Vpp.
(To be continued…)
Thanks for watching! Peter (DK7IH)