Before starting

As the parts used in this project are quite small, some form of optical aid is recommended unless your eyesight is exceptional! A bench magnifier (some have inbuilt extra lighting) can be useful, although we prefer to use a headband binocular magnifier (eg RS part number 606-989). A good pair of tweezers is also essential. Chip components are hard to find on the average shack floor and can fly an amazing distance if they jump out of the tweezers! A soldering iron with a fairly small tip is also highly desirable, and the use of the correct type of solder is recommended for the SMT parts. The solder supplied with the kit should work well for this.

Box and RF PCB preparation

1. Assemble the tinplate box, by placing the side sections in the lids. If necessary, bend the sidewalls to give a good fit with the lids. With the box still all together, solder the outside seams as far as can be done (without soldering to the lids!).

2. Remove the lids and solder up all box seams inside and out.

3. Trim pcb to fit box. Generally pcbs are generally supplied slightly oversize and need a small amount of filing to fit the box. The board should be made a reasonably tight fit in the box, but not so tight that there is any risk of damaging it while putting it into the box. Remove any rough edges from the board which could interfere with later soldering. Identify the TOP side of the box, by reftting the pcb in the box and noting the side of the box from which the circuit pattern can be seen. This is defined as the TOP of the box.

4. Scribe a line 10mm from the top edge of the box, as centreline for SMA centre connector pins. Mark out position of connector pins, to align with relevant pcb tracks.

5. Drill holes 4mm diameter for all SMA connector centre pins.

6. Flange mount SMA connectors may be soldered directly to the box by first locating them carefully in position, clamping them with two crocodile clips (one either side of the flange) and then soldering. Alternatively, follow steps 7-10 below.

7. Mark out positions of connector mounting holes using connector as a marking-out jig.

8. Drill connector mounting holes 2mm dia. Deburr using a drill on both sides. It is difficult to reach the inside of the holes with small boxes but it can be done. Filing is not recommended, as areas where the tin plating is inevitably scratched off will be difficult to solder to.

9. Assemble connectors to box with M2 screws and nuts.

10. Solder connectors to box all round using sufficient heat so that the solder flows well and when cool remove fixings. The use of two soldering irons can help! Note: Stainless steel SMA connectors cannot be soldered, and are therefore not recommended.

11 Reassemble pcb into box and check alignment of connector pins to tracks. Remove and refit any connectors where alignment is poor. Remove pcb. Cut connector pins to extend approx 2mm into box.

12. Drill and deburr holes in the box for feedthrough capacitors for power connections. If desired, drill a 1mm hole nearby, to allow a pin to be fitted for the ground connection.

RF PCB Pre-assembly

1. Fit supplied pins into holes in PCB where grounding is required with head of pins on the circuit pattern side (locations are shown on the layout with solid circles on the pads). Solder pins to groundplane side. This is best done by applying solder and soldering iron to the pin and flowing the solder down to the groundplane. Do NOT solder to the heads of the pins on the circuit pattern side at this stage. Trim the pins with sidecutters after soldering.

Assembling the PCB into the box

1. Reassemble the pcb into the box from the bottom side, and push it up against the RF connector pins. Try to get the board "square" in the box, not tilted to any great extent. Tack solder the groundplane in the four corners and recheck the RF input/output tracks are still touching the connector pins.

2. Solder all round the groundplane to the box walls. Two soldering irons may be helpful here also. Make especially sure there is a good joint where the connector are located. This is necessary to ensure a proper low inductance ground path from connector to pcb. It is best not to solder the track side groundplane areas around the preamp to the box wall. The veropins provide adequate grounding on their own.

3. Fit and solder feedthrough capacitor(s) for power connections, and box grounding pin (if used).

Fitting components to PCB

CAUTION

It is possible to damage some  components by overheating them if the soldering iron tip is too hot.   If the iron is too hot, the metallisation on the ends of the chip components may dissolve,  or the parts may crack.  For soldering chips and semiconductors, a maximum tip temperature of 350 - 400C seems to work OK.  

1. Fit chip components as described in Appendix 1, except the MAR6s and HEMT.

2. Fit any wire ended components required (on RF side and/or on groundplane side as specified).

3. Make and fit the inductors. Before fitting L1, cut the supplied wire to length using calipers if possible to check. Length is fairly critical. Then bend to the required "U" shape. and using pliers bend down the ends for 0.5mm length by about 45 degress. When L1 is soldered into position, try to keep the wire flush to the board, if necessary by pushing it flat.

4. Fit the the HEMT and MAR6  (see Appendix 2 and 3 below).

5. Any grounded veropins have not been soldered to during earlier assembly stages should now be soldered. Take care that none have been missed!

6. Fit remaining veropins and solder to track side.

7. Complete wiring

Appendix 1 - Fitting chip components

To fit chip components across two circuit tracks or pads, the following procedure should be adopted. Refer to Fig. 7 .

1. Lightly tin one of the tracks or pads. Silver loaded solder is recommended, and is supplied in some of the kits.

2. Locate the chip component and reflow the solder on the tinned track to make a preliminary joint.

3. Solder the other end of the chip to the track to make a good fillet.

4. Resolder the other end of the chip using a little fresh solder.

Note that the standard chip capacitors are mounted flat on the board as shown in the figure. Mounting them at 90 degrees will cause extra series inductance and may result in reduced performance, instability etc. It is recommended to mount resistors with the code letters visible, should later debugging prove necessary!

Note, where wire ended components are mounted surface mount style, aim for minimum lead length.

Appendix 2 - fitting MODAMPS

Assembly of the modamps on to the RF PCB is a two stage operation. Firstly, it is necessary to trim the ground leads so that when located in position, about half the heads of the grounding pins are visible. The reason for cutting the leads so short is to help prevent dry joints here which can have a bad effect on the gain and noise figure of the device. When cutting the leads, please observe the static precautions decribed below.

To solder a MODAMP in position, lightly tin one of the grounding pins, and making sure the device is the right was round, tack the device in position by reflowing the solder on the grounding pin. Aim to have the device mounted FLAT on the grounding pins not sitting up on a bump of solder, to equalise and minimise the lengths of input and output leads which go down to the tracks.

For the reflowing operation, it is best to apply the soldering iron to the side of the pin head and not to the lead of the device, to minimise the possibility of a dry joint. Use the same technique to solder the other source lead, applying solder to the point where the cut ground lead touches the top of the grounding pin, not to the soldering iron or top of the lead. Next, remake the original joint in the same way, using a little fresh solder. Finally bend the other leads leads down to the board as close as possible to the body of the device as possible. The inductance of this short lead is built in to the design, and longer (diagonal style) connections will cause detuning. Another way of describing this is to ensure that the soldered joint is made at the end of the track, not some distance away from the device.

Appendix 3 - Fitting the HEMT

Before this device is handled, some preparations need to be made to avoid potential damage to the device(s) from static discharge and/or soldering iron leakage. In the case of normal GaAs FETs, it is often possible to get away with no precautions at all and not experience any problems. With HEMTs this philosophy needs to be reexamined. HEMTs are much more susceptible to damage and this is usually not apparent in its DC characteristics, often a disappointingly high noise figure is the only observable consequence. Where devices are supplied as part of kits, the use of these devices is entirely at the constructor's risk. The Microwave Committee Components Service cannot replace damaged devices free-of-charge!!

Damage from electrostatic discharge (ESD) is avoided by never allowing the possibility of the sensitive device having a different potential to any object it touches, including yourself. A simple static-free workstation should be made, such as a sheet of metal to which a wire is attached. The free end of the wire is joined to either a proprietary anti-static wristband worn at all times, or to some form of homemade body contact (such as a wire wrapped round a ring). For safety reasons it is recommended NOT to earth the workstation, and to use a high value resistor eg 100k ohms in series with the wire if using a homemade body contact. The device may than be safely unwrapped and placed on the worksurface.

Source leads are cut short next using sharp sidecutters., to fit between the source pins so that the device is flat on the board.

The last stage in the construction of all units is the installation of the HEMT. The module should be otherwise complete and tested for correct voltages at the ends of the lines where the device is to be connected. With no power applied to the module, place it on the metal worksurface and connect a wire between the worksurface and the module box. Next. arrange some form of connection between the soldering iron bit and the worksurface, separate from the mains earth. Check that even when hot, there is a low resistance path from the soldering iron tip and the worksurface. It is also worth checking that there is no leakage in the iron by measuring resistance between the tip and the heater connections of the iron, with the iron cold and hot. The device may then be placed in position and its source leads soldered. At the same time, make sure the solder also flows to the grounding pins It is important to avoid touching the gate and drain leads with the iron during this operation. Before soldering the gate and drain the iron should be completely unplugged (retaining the tip-worksurface connection of course).

One assembled into the circuit, devices should be safe. However if for any reason soldering operations are required in the future, be sure to repeat the precautions. The author has damaged devices in the course of developing modules when soldering in tuning stubs and forgetting to connect the iron-worksurface link, and/or unplugging the iron. Some irons claim to have an ESD connection - we would not trust this!