Mark 1 2.695GHz > 151MHz Down Converter

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These are my construction and progress notes for the Mark 1 2.695GHz to 151MHz Downconverter. It should be noted that this is not the official BAA-RAG 2.695GHz down converter which is developing under the guidance of Terry Ashton.

This unit comprises of two modules:

Local Oscillator

Converter Module

Local Oscillator

The local oscillator has been built from a G3WDG016 kit produced by Charles Sucking, G3WDG. It is available for purchase from the G3WDG website. I have used a 106MHz crystal sourced from Quartslab. The Oscillator is a low microwave source and comprises of a x12 multiplier to give an output at 1272MHz. With careful tuning, 10dBm can be achieved from this source.

A couple of things to watch for when building this unit. Order the tin box at the same time and follow Charles' construction notes carefully. I made two minor changes to the layout, I mounted my Crystal under the board - this means that two holes will be required plus "lands" clearing on the ground plane. I also routed all the DC supplies under the board rather than looping on top as suggested by Charles.

A couple of points that are not too clear in the original instructions

L5 Detail

Note that the 2p2 capacitor stands up from the board. This is actually quite easy to do, simply tin the location on the pad, bring the capacitor to the correct location, apply heat from the soldering iron, but without touching the capacitor and as the solder melts, capillary action will pull the capacitor up vertically. L5 simply connects from the top of the capacitor to the filter line.

BFG97 Detail

Good RF grounding is essential at Microwave Frequencies. The BFG97 is a 4 pin device, with the outer two pins of the case being emitters. 3 x 100pF capacitors in parallel are used to provide good RF decoupling. The connection to the lower emitter lead (on the photo) takes two of the three 100pF chip capacitors and the 10 ohm resistor. The 3rd 100pf capacitor connects the other (upper) emitter lead to ground. But there is no track, so it is advisable to solder this final capacitor in place after the transistor has been located.

The Down Converter

This unit is based on the G3WDG040 'S' band receive converter originally designed for the ill fated Oscar 40 Amateur Satellite.This unit uses the same active devices, but has a pipecap filter instead of a stripline bandpass filter as used on the original design. The converter uses a sub harmonic mixer. The benefit of this type of mixer is that the local oscillator freqency is doubled at the mixer. The original A040 design used an onboard oscillator. I have chosen to locate this as a separate unit - see notes above. This allows me to keep to my intention of a fully modular approach.

The converter is built on an epoxy ptfe pcb material 0.8mm thick with an Er of 2.6. Although having a slighly higher Er than equivalent materials such as Duroid from Rogers, I would not expect there to be any problem reproducing this if necessary

The completed converter on the bench

The board measured 70mm x 118mm and was designed to fit into a tinplate box. However, the filter would protrude below the box so a decision was made to make a box out of brass sheet. Big mistake. It takes a lot of heat to get the box to solder up neatly and I wished that I had stayed with tinplate as originally planned. The mixer diodes are HP5082-2835. Buy 10 off and select a "matched" pair by forward biasing the diode with about 1mA of current and then measuring the voltage drop across the diode. I used a 10V source with a 10kohm resistor in series with the diode. Choose two with similar characteristics.

The PCB and circuit layout are available if you would like to build a version.

Construction Notes

The underside of the board is copper and used as the groundplane

Drill all holes first (1mm diameter)

Solder the cavity to the board then the board into the box. To align the cavity, mark the centre of the cavity and scribe a circle of 15.5mm radius. Place the cavity in the centre of the circle and heat gently with a blowtorch. Test the temperature at the joint using a small piece of solder. As soon as the solder starts to melt, remove the heat and then run the solder around the joint. Try to avoid using too much solder, as solder inside the cavity is known to be very lossy.

All components (except the filter) are mounted on the track side of the board

The black dots on the layout show where the board should be drilled and grounding veropins inserted. The heads of the pins should be on the track side and the pins pushed through. Solder the pin on both sides and then cut to length. The pins around the actice devices should be lightly soldered until the active devices are installed.

The blue dots show where the veropins for the filter are inserted. The copper on the ground plane around the hole should be gently cut away with a 3mm drill to prevent the pin shorting against the ground plane.

The red dot shows where the 13.8V supply should be brought in. Connect to a veropin pushed through from the track side. Again, the copper on the ground plane should be cut away to prevent a short circuit. The supply to the converter box should be brought in via a feedthrough capacitor of about 1nF

The polarised capacitors are 16V tantalum and can be SMD or discreet radial devices

The input inductance is a loop of (preferably) silver plated wire of 0.5mm diameter. Cut to length and bend the ends down very slightly so the wire is 0.5mm off the surface of the board.

Performance data to follow.