SM6LKM's OPTIMIST 80 - Building Instructions (DOS text version) IF you are a QRP enthusiast, IF you have heard about a strange tool called "soldering iron", IF you are getting tired of CW-only operation, IF you think commercial QRP kits are too expensive, IF you think small is beautiful... This may be the rig you are looking for! Optimist 80 is a small single board DSB transceiver designed for the 80 meter band. Almost all components are available from the Swedish supplier ELFA AB. There are no esoteric components in this rig. The only exception I can think of is the small variable capacitor, CV1, but it can probably be found in numerous junk boxes. Short description: - Optimist 80 is a QRPp transceiver for 80 meter DSB. - Direct conversion receiver. - Variable input attenuator. - Narrow preselector. - Drives a loudspeaker directly. - Varicap tuned VFO, 3600-3800 kHz (3500-3800 kHz is possible, but not recommended). - Low LO leakage radiation means less hum problems when powered from a mains supply. - Linear class A power amplifier. - 1 watt P.E.P. output power, QRPp class. - Overload indicator with LED = poor mans ALC... - A joy to operate. Functional description, receive: Preselection is handled by the resonant circuit formed by CV1, C1, C2, C3 and L1. The signal from the antenna, attenuated by RV1 to a proper level, is capacitively coupled into the bottom of the resonant circuit. The J-FET Q1 exhibits a very high input impedance. Together with the relatively loose antenna coupling, this results in a high Q. The circuit is so narrow that even small excursions with the VFO will necessitate re-adjustment of the preselector. Besides beeing an impedance converter, Q1 also acts as a phase inverter providing a symmetrical input signal to the mixer, IC1. Q1 has no voltage gain at all in this circuit, rather a few dB loss, but, the high Q of the preselector circuit results in a considerable gain anyway. The mixer circuit, IC1, contains both a doubly balanced mixer and a local oscillator. The oscillator tank consist of C11-C16 and L2. Tuning is realized with the varicap diodes D2 and D3. The 10-turn potentiometer, RV2, is the tuning control. RT2 and RT3 are used to adjust the band edges. The symmetrical audio signal coming from IC1 pins 4 and 5, goes straight through T1, passes through the audio filter R21, R22 and C21-C26 and finally gets amplified to loudspeaker level by IC2. Functional description, transmit: During transmit, the relay K1 routes supply voltage to the +12TX line. A small current flows through the diode D1 which loads the preselector tank, thereby shifting its resonant frequency and lowering its Q. This is very important because even the slightest coupling between the LO and the preselector tank would otherwise excite the preselector tank to prohibitive levels. If too much LO finds its way into the mixer input, it will upset the mixer balance resulting in degraded carrier suppression. The +12TX line is also supplying power to IC3, the microphone amplifier. The amplified speech signal from IC3 pin 6, is routed via L1 to Q1. At audio frequencies, L1 can be considered a dead short. Q1 acts as a phase inverter during transmit also. The mixer balance is adjusted with RT1. It should be adjusted for minimum residual carrier. When the speech signal is mixed with the LO in IC1, a double sideband signal will result. This DSB signal, coming from IC1 pins 4 and 5, goes to the broadly tuned transformer T1. At radio frequencies, the center taps of T1 are practically shorted together by the capacitors C21 and C22 in the audio filter. T1 and C18-C20 forms a broad resonant cicuit which picks out the mixing products that belong to the 80 meter band. The secondary winding of T1 has an output impedance very close to 50 ohms. The power level can reach about -10 dBm (0.1 mW) P.E.P here without excessive distortion. The DSB signal is routed from T1 to the linear amplifier consisting of Q2-Q4 and their surrounding components. All three stages are operating in class A. The available gain in this amplifier chain is around 55 dB. Because of this, A -10 dB attenuator, R33-R35, has been insterted before the amplifier in order to minimize residual carrier and noise from the DSB generator. The amplifier chain is designed so that the final stage has the lowest power margin i.e. the final stage will be the first to suffer from flat-topping when the amplifier is overdriven. For obvious reasons... The amplifier output goes through the T/R relay, K1, it is cleaned in a 5-pole lowpass filter and finally it reaches the antenna terminal. The choke RFC4 shorts any 50/60 Hz hum present at the antenna terminal and, also, it provides a DC path to ground that prevents static build-up on the antenna. The 5-pole lowpass filter may seem pedantic. Well, it is. You can omit C58 and C59 and replace L3 with a piece of wire. I have not tested this transceiver with a spectrum analyzer, but, even if the harmonics are only 20 dB below the fundamental frequency, that's quite acceptable when related to the 1 Watt power level... As the final amplifier is working in class A, it maintains a nearly constant collector current under normal conditions. When the final amplifier is beeing overdriven, the required RF peak current becomes greater than the available standing DC bias. Consequently, flat topping will occur. At overdrive, the DC current through the transistor will be modulated by the speech. The overload indicator uses this fact to an advantage. The voltage drop across the emitter resistors R50 and R51 is fed to a lowpass filter formed by R32 and C54. This filter is necessary to get rid of the RF that is present on the emitter. The filtered signal is then amplified by the OP-amp, IC4, to a level suitable for driving the overload LED, D5. The modulation level is just about perfect when D5 barely starts to flicker on voice peaks. The forward voltage drop of D4 and D5 results in a desirable, and distinct, threshold. Construction: The PCB is made of double-sided PCB material. The copper foil on the component side is used as a ground plane and should not be etched at all. Print the PCB layout file and make a PCB using your favourite methods. Drill all holes with a 0.8 mm drill. Then, drill the holes for Q3 and Q4 with 1.0 mm. Drill the large square terminal pads, X1-X19, with 1.2 mm, and finally, the four mounting holes and the Q4 mounting hole with 3.2 mm. Look at the ground plane view. On the ground plane, on the component side, countersink all holes that are filled with black using a 2.5 mm drill. Be careful to avoid drilling through the board. The holes marked with a thin ring on the ground plane view are ground connections to be soldered on the component side and must NOT be countersunk. The grounded component pins have octagonal solder pads. Polish the board and give it a protective layer of solder-through lacquer. Start with all the components that have at least one pin grounded. Solder the grounded pins on both sides of the board. However, some components do not need to have their ground pins soldered on the component side. They get their ground from a nearby pin that is easy to solder on the component side. Plan the placement sequence carefully. If components needing ground plane connection is "built" in between other components, it could be very difficult to reach them with the soldering iron. Try to mount horizontal components, such as resistors, about a millimeter above the ground plane. Look out for shorts between ungrounded pins and the ground plane. C3, C4, C5, C21, C22, C25, C26, C28, C29, C32, C33, C34, C35, C36, C37, C38, C40, C45, C60, C61, C64, C65, RFC4, RT1 are examples of components that do not need direct connection to the ground plane, but it doesn't hurt to connect them. The electrolytic capacitors have square pads for the positive terminal. The electrolytic's don't have to be soldered on the component side. The toroid inductors should be carefully close-wound. Wind T1 with 23 turns to begin with, leave a few centimeters in a loop and continue winding 23 more turns in the same direction. Cut the loop at the middle and you have 46 turns with a center break. The number of turns is equal to the number of times the wire passes through the hole in the toroid. It is easy to mix up the wires of the small transformer T2. Look at the phasing dots in the schematic diagram. These dots are in the same end of the twisted pair. The T50-2 toroids should be mounted horizontally a few millimeters above the board. As spacers, use pieces of perfboard without copper, or any other plastic material of suitable thickness. You may need to drill a few holes for the wires, especially those wires coming down from inside the toroids. The small toroid T2 is mounted vertically, standing on its own legs. When the rig has been tested, put some lacquer on the coils to enhance frequency stability and to prevent microphonic effects. The final transistor, Q4, has to be insulated from the heat sink using a mica washer, or equivalent, and some silicon grease. When R24, R50, R51 and D6 is soldered in place, Q4 and its heat sink can be bolted to the board. After the VFO has been tested, and covers the expected frequency range, solder a screen around the VFO tank (C10-C17, L2, D2, D3). Thin PCB material or brass will do. Make it about 20 mm high. Do not do this before the VFO has been tested. See below. Don't forget the wire link that connects one end of C39 with IC4 pin 7. Solder terminal pins to the large squar...
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