This page descibes a new SMT LO for use in the WDG 23cm and 13cm transverters and other applications in the 1GHz region. Also, progress towards a replacement for the G4DDK004 2.5GHz LO is described at the bottom of the page.
G3WDG016 SMT LO Chain
Photo of the prototype SMT LO chain
Draft Beta Test Documentation
A Surface Mount Local Oscillator Chain
Description
A new local oscillator chain has been developed, mainly for use with the G3WDG 1.3 and 2.3GHz transverters. It is based on the highly successful G4DDK001 design, but was designed to use SMT technology in keeping with availability of components. It does not currently have provision for fm, but this could be added later if there is a demand to make it compatible with the G4DDK001B. Reference - Microwave Manual volume 3 p. ???.
The circuit diagram is shown in Fig 1. Current circuit shown is approximate only for general guidance - board layout in Fig 2 is correct below.
Construction
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. This is shown in Fig 3.
4. Scribe a line 10mm from the top edge of the box, as centreline for the SMA centre connector pin. Mark out position of connector pins, to align with relevant pcb tracks.
5. Drill holes 4mm diameter for SMA connector centre pin.
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 connector to box with M2 screws and nuts.
10. Solder connector 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 pin to output track. Remove and refit the connector if alignment is poor. Remove pcb. Cut connector pin to extend approx 2-3mm 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
Fit supplied pins into holes in PCB with head of pins on the circuit pattern side. It may be necessary to use considerable force and gentle hammering is OK (support the board from the groundplane side). Alternatively, pressing them in with a hot soldering iron requires much less force. 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 output track is 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 (refer to Fig 2)
1. Fit resistors, capacitors (not the trimmers) and transistors as described in Appendix 1, except the 2.2pF capacitor which is mounted on end.
2. Make and fit L2-L6. Note L5 goes from the top of the 2.2pF capacitor to the output line of the 600MHz filter, opposite the point where the collector of the transistor taps in to the other line.
3. Any grounded veropins have not been soldered to during earlier assembly stages should now be soldered. Take care that none have been missed!
4. Fit the trimmer capacitors, theoscillator coil and the crystal.
5. Fit the wire link going over the top of the grounded end of the 600MHz filter lines.
Alignment
An absorption wavemeter covering 96, 288 and 576MHz and multimeter are the minimum items needed for alignment. A 50 Ohm load (a 0-25W non-inductive resistor mounted, for instance, in the body of a bnc plug) and a simple power meter, such as that described in the RSGB Microwave Manual are also desirable.
The initial tuning settings with a 96MHz crystal for 1152MHz operation are:
L1 core nearly level with top of the former
C7 75% meshed (assumes 10pF trimmer)
C8 50% meshed (assumes 10pF trimmer)
C13, 14 50% meshed (assumes 5pF trimmer)
C18, 19, 20 10-20% meshed (assumes 5pF trimmer)
This photo shows a trimmer 0% meshed, ie at minimum capacitance. Note that the rotor inside the trimmer has two small indentations. The trimmer is at minimum capacitance when these indentations are nearest to the flat edge of the trimmer. 100% meshed is 180deg from this.
These positions depend mainly on the type of trimmers used, and to a lesser extent on the dielectric constant of the board.
Connect the multimeter in the supply lead to the unit and check that the current taken from a 12V source does not exceed about 180mA (it should be in the region of 20mA). If it does, switch off immediately and check for short circuits or incorrectly placed components. When satisfied all is well, align the crystal oscillator stage. This is done by tuning the wavemeter to 96MHz and placing it close to L1. The core of L1 is tuned until a response is observed on the wavemeter. Peak the response by turning the core. Check that the oscillator restarts after switching off and then on. If it does not, turn the core slightly and repeat until it does. An fm radio can also be used to check the oscillator is running.
Retune the wavemeter to 288MHz and place it close to L2 and L3. Peak the reading by adjusting the trimmers C7 and 8. Connect the multimeter between ground and the emitter of TR3. The meter should be on a range no higher than 2V fsd. It is best to use a moving coil meter rather than a digital meter, since changes in the reading can be more easily seen. Peak the reading by adjustment of the two trimmers. The wavemeter should be used to confirm the circuits are tuned to 288MHz.
Transfer the meter to the emitter of TR4 and tune C13 and 14 for a maximum reading on the meter. Again use the wavemeter to confirm these circuits are tuned to 576MHz. Go back to C7 and 8 and repeak the reading at the emitter of TR4.
Connect a power meter or 50 ohm diode probe to the output connector. Adjust the trimmers for maximum indicated power. This should be at least 10mW. Repeak all tuned circuits, making sure not to retune them to some other harmonic of the drive frequency.
Exact frequency setting is best done by measuring the oscillator frequency with an accurate frequency counter. If this is done at 96MHz remember that the error is multiplied by 12, hence a 1kHz error becomes 12kHz at 1,152MHz. This would be regarded as too much by most operators and a final offset of no more than 5kHz would be appropriate.
Component Information
Coil Details
L1: Toko coil with ferrite core
L2: 2 turns 4mm id 0.6mm dia tinned copper wire about 3mm above pcb
L3: as L2
L4: 4 turns 1.6mm id 0.25mm dia tinned copper wire
L5: approx 4mm length of 0.25mm dia wire from top of 2.2p cap to printed line
(opposite to collector tapping point on other line)
L6: as L4
Component Identification
Part Marking
BFS17 E1P
BFR106 B7p
0.1u u1 (solid line is +ve end)
1.0u 1u (solid line is +ve end)
10p trim white
5p trim blue
resistors 1st no = 1st no, 2nd no=2nd no, 3rd no =number of zeros eg 223=22k, 100=10R
other caps sort by number in kit or value on small bag
Use the de-kitting sheet provided below to sort out the components before assembly.
SMT LO De-kitting sheet
1K 3 |
18R 1 |
390R 1 |
560R 1 |
2k2 2 |
15R 1 |
osc coil 1 |
470R 1 |
22K 2 |
15p 1 |
10R 3 |
820R 1 |
78L09 1 |
1U 1 |
0.1U 1 |
12p 1 |
22p 2 |
1n 5 |
10p trimmer 2 |
5p trimmer 5 |
BFG97 1 |
100p 11 |
27p 1 |
BFR106 1 |
BFS17 2 |
2.2pF 1 |
wire for coils (2) solder for chips (1) veropins (24) |
De-kit sheet available as a word document for easy printing
Appendix 1 - Fitting chip components
To fit chip components across two circuit tracks or pads, the following procedure should be adopted. Refer to Fig. 4 .
1. Lightly tin one of the tracks or pads. Silver loaded solder is recommended, and is supplied in the kit.
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.
Observe static handling precautions when fitting the transistors. They are not especially sensitive, but it pays to be careful.
Spectrum of SMT LO chain from 10-2700MHz (1296MHz output)
Boxing the unit
One of the beta test boards has been successfully boxed up in a 7754 box. It was found necessary to add a small screen over the 600MHz tuned lines to prevent large tuning changes when the lid was put on (and some evidence of instability without the lid). The screen was made from copper sheet and drilled to allow the trimmers to be adjusted. Refer to photo below for an idea of what he screen looks like (left hand one). Output power was over 15dBm at 1088MHz (BFG97 in output stage).
Other observations
Experiments with the 2.5GHz version of the LO have led to a few pointers which may be useful for the SMT LO.
1. BFS17A should not be used in the oscillator - tendency for instability. BFS17 is recommmended.
2. Coupling cap TR2 to TR3 may need changing for different output frequencies (eg 5.6pF for 2592 o/p, 22pF 1088, 1152).
3. Change TR4 to BFG97 for more o/p. Experiment with decoupling on upper emitter (100pF for 1088, ~1.5pF for 2592).
4. Try adding 10R in series with TR2 collector if instability is noted.
Progress towards G4DDK004 replacement
The G4DDK004 design is a very reproducible LO chain with an output frequency of around 2.5GHz. However it is getting quite difficult to source some of the components, and work is underway to add a stage to the SMT LO described above to double the output frequency. It is hoped that this new board will be able to take the place of the G4DDK004 sometime later this year.
It is hoped to replace the output filter with a pretuned one, made on 31mil duroid, similar to the filter used in the G3WDG010 13cm transverter. This will obviate the need for tuning the filter (perhaps the most difficult operation in setting up the G4DDK004) and removing the need for the 5pF SKY trimmers, which are becoming increasingly expensive and hard to get.
The responses of the original filter and the proposed replacement are very similar and the next stage is to prototype a multiplier using the new filter.
The first prototype of the DDK004 replacement (WDG021) is now working. A power output of 18mW is being achieved at 2592MHz from the bare board and 14.5mW when boxed. At this stage, it was decided to use the 3 x sky trimmer output circuit. A fixed tuned fiter remains a possible option if sky's become difficult to source. Photos of the first prototype are shown below.
Bare pcb
Boxed up