The Marmitotron 500g HF Receiver

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Gary Andrews M0CWY is back with a Direct Conversion Receiver in a Marmite jar. You either love it or hate it!

 

 

Gary Andrews M0CWY is back with a Direct Conversion Receiver in a Marmite jar. You either love it or hate it!

 

 

Marmite, you either love it or you hate it. I love it; I always have done. The jar, with its boldly coloured label and yellow lid, is very attractive and some time ago I decided to build a radio inside one. After all, if you can have a ship in a bottle, why not an HF receiver in a Marmite jar?

There’s not a lot of room inside the jar, not even in the largest 500g size, so things are a bit tight, especially if a battery must fit inside as well. The only place that the controls and connectors can be fitted is on the lid, which has an inside diameter of about 55mm. The circuit diagram of the Marmitotron is shown in Fig. 1, and there are about 50 components, a few of which are quite large. To fit everything in requires a compact construction technique, so I decided to use ugly-construction.

Ugly-construction is well named, as you can see by the photograph of the finished board in Fig. 2, but it works well and uses space efficiently. I managed to fit the whole circuit on to a board that measures 75 by 50mm. The assembled board squeezes through the mouth of the jar without too much of a struggle. On the lid there is just about enough room to mount an on-off switch, a filter-select switch, a phono connector for the antenna, a 3.5mm stereo jack socket, a tuning control and a small volume control.

The Marmitotron is a direct-conversion (DC) receiver: It converts the radio-frequency (RF) signal from the antenna directly to audio-frequency (AF) by mixing the RF signal with a local-oscillator (LO) signal whose frequency is separated from the RF signal by only a few hundred Hertz.

80m band signals from the antenna are selected by a tuned circuit consisting of C1, C2, TC1 and L1, and are coupled to a singly-balanced diode-switching mixer. The other input of the mixer is driven by a Vackar variable frequency oscillator (VFO). The Vackar oscillator is known for its frequency stability that it achieves by swamping the voltage-sensitive transistor output capacitance with a large resonator capacitance and by its loose coupling of the resonator to the transistor input. In the Marmitotron, the VFO output is also loosely coupled to the diode mixer so that impedance changes at the mixer have very little effect on the oscillator. The VFO tunes over a range of about 70kHz, which covers a good part of the CW segment of the 80m band. The frequency spectrum at the output of the mixer contains not only the audio (AF) signal, that is the difference between the radio (RF) and local oscillator (LO) frequencies but the sum of those frequencies as well. The mixer is followed by a diplexor that routes the unwanted sum frequencies to a 51Ω dummy load resistor and the wanted AF signals to a lowpass filter, which provides the receiver’s selectivity. The lowpass filter is a fifth-order Butterworth filter with a corner frequency of about 600Hz. The filter’s attenuation reaches about 60dB at a frequency of about 2400Hz. The filtered AF signals are then amplified by about 60dB by a three-stage transistor amplifier. The first amplifier stage has an input impedance of 50Ω that matches the lowpass filter. The second stage has moderately high input and output impedances that match the preceding and following stages, and the third stage has a low output impedance to drive a pair of low-impedance stereo headphones.

 

Construction

The first job is to wind inductors L1, L2, L3, L4 and L5 and transformer T1. Inductor L1 is wound on a T50-6 iron-powder toroidal core using 0.315mm diameter enamelled copper wire. Each time the wire passes through the toroid counts as one turn. Wind three turns, then leaving a large loop of wire (about 50mm), continue winding another 21 turns. Tightly twist the loop of wire together leaving a small loop at the end. This is the coil tap. Trim the ends and the tap to length, remove the enamel using a small needle file or a piece of abrasive paper and tin the wires.

Inductors L2 and L3 are identical and are wound on Epcos B64290L44X830 toroidal cores. The cores are available from Farnell Element 14. These cores are intended for use in switched-mode power supplies but because of their high magnetic permeability they are useful for making large inductances for audio filters. The size of the toroid or more importantly, the size of the hole, allows thick wire to be used, thereby giving low resistance windings and consequently low filter insertion loss. To avoid having to count lots of turns, simply cut 1.75m of 0.315mm diameter enamelled copper wire and wind it onto the core leaving about 50mm of wire spare at each end of the winding and then prepare the ends as before.

Inductor L4 is wound on a T37-6 iron-powder toroidal core using 0.315mm diameter enamelled copper wire. Wind 26 turns and prepare the ends.

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Inductor L5 is wound on a BN43-2402 ferrite binocular core using 0.25mm diameter enamelled copper wire. One pass through both holes counts as one turn. Wind 30 turns and prepare the ends.

Transformer T1 has a trifilar winding, which simply means that the three wires are twisted together before winding them onto the core. Twisting the wires together maximises the magnetic coupling between the wires so that energy is as far as possible transferred through the transformer to the load rather than being partially reflected to the source. Cut three pieces of 0.25mm diameter enamelled copper wire about one metre long. Twist about 25mm of them together at one end and grip them in a small vice. Stretch the wires out evenly and similarly twist the other ends together and grip them in the chuck of an electric drill. A drill with speed control makes this job easier. Pull the wires tight and start the drill turning slowly. Keep the drill turning until the twists in the wire are about 2 to 3mm long. Release the wires from the drill and vice. Wind ten turns onto the BN43-2402 ferrite binocular core. Leave about 50mm of wire spare at each end of the winding. Untwist the wires at each end and trim them to length. Prepare the ends. Find three pieces of different coloured insulated hook-up wire and strip two pieces of insulation about 3mm long from each. Using a multimeter set to the resistance or continuity range, identify the ends of the three windings and colour code them by slipping the coloured insulation onto the wires. Let’s assume the colours are red, white and blue. Take one end of the red winding and twist it together with the opposite end of the white winding. The twisted-pair connects to the tap on L1. The remaining red end connects to one of the mixer diodes and the remaining end of the white winding connects to the other diode. One of the blue ends, it doesn’t matter which, connects to the VFO output and the other one connects to ground.

 

Circuit Board

The circuit board is next. Cut out a 75 by 50mm rectangle of copper-clad board and polish the copper until it is nice and bright. I built the VFO in the top left of the board, the mixer in the middle-left and the antenna tuned circuit in the bottom-left. I built the filter in the centre of the board and the audio amplifier down the right-hand side. Space is tight, so it is helpful to make a layout sketch before starting.

Assemble the board. Start by identifying a component that is connected to ground and solder the ground leads to the copper, leaving the remaining lead, or leads, pointing upwards. Then, using these grounded components as anchor points, solder other components that do not have ground connections to the leads that are pointing upwards and trim the excess. Work your way across the board top-to-bottom and left-to-right. You can get an idea of what it should look like by studying an example of an ugly-style amplifier circuit that is shown in Fig. 3.

Drill holes for the controls and connectors in the lid of the jar. WARNING! Before you start drilling, screw the lid on to the jar and mark the front: Mark the positions of the connectors and controls as you would like them to be positioned relative to the front of the jar. Use a miniature potentiometer for the volume control because there isn’t room for one of the standard 25mm diameter ones. When the holes have been drilled, mount the controls and connectors in the lid. An example of a finished lid is shown in Fig. 4.

Finally, orient the board at right angles to the lid and wire the controls, connectors and a battery clip to the circuit board. Make the wires long enough to leave about 50mm of slack between the board and the lid to give some flexibility for when the lid is screwed on.

 

Operation

Connect a 50Ω, 80m antenna, headphones and a battery and turn on. Tune across the band until you find a signal, then peak the antenna tuned circuit using TC1. If you have a calibrated signal generator, you can adjust the tuning range to cover from 3.500MHz up to about 3.570MHz by compressing or spreading out the turns on L4.

Now everything is set up, drop the board and battery into the jar and carefully screw on the lid.

The audio lowpass filter can be switched in and out of circuit using S1. With the filter bypassed, it is easy to search the band for activity and then narrow the bandwidth when you have found a signal.

The Marmitotron cannot compete with the latest big-brand rig but despite using only four transistors it is a pretty good receiver, and it looks great too!

 

This article was featured in the July 2018 issue of Practical Wireless