|
|
The Background.
This project came about from building the DB6NT Transverter and Amplifiers. These excellent designs form the basis of equipment used by many 24Ghz activists. The progression of construction from transverter through three stage low noise amplifier producing tens of milliwatts using the inexpensive NEC NE32584 GaAsFet device is straight forward. Skill with surface mount construction being the main requirement to produce the expected results. The DB6NT Power Amplifier design uses the Mitsubishi device MGF1303 and whilst it was possible eventually to achieve 100mw of output power many hours were spent by the author adjusting the foil matching tabs to achieve the full output. It seems from corresponding with locally active 24Ghz stations that I was not alone in finding it difficult to achieve the full output power from this amplifier, some others achieving 60mw from a pair of MGF1903 (Birkett short lead version of the MGF1303) devices.
As a constructor I wanted to experiment further
for a little more power. Some Rogers Duriod PCB material type 5870
was acquired and using the Laser jet printing method to produce
layouts some experiments began. The output circuit of the DB6NT
amplifier was copied and a pair of NE325 devices which have a high
IDSS were used
as an experiment to see what might be achieved. The NE325 is a lower
dissipation and lower voltage device than the MGF1303 the literature
for the NE325 stating a maximum of 4 volts at the drain. Since the
DB6NT circuits already constructed used a 5 volt supply and no loss
of devices had been suffered, 5 volts was initially tried for the +ve
supply voltage. Negative bias is needed to keep the drain current and
dissipation in check. This is an advantage in so far that the
recommendation is that devices should be biased at ½ IDSS for linear amplifier application. The results were
immediately promising and the drain voltage was pushed beyond 5 and
it was found by experiment that output power peaked at 5.2 volts and
then fell away again beyond this figure. This seems to be fairly
consistent across a number of devices. An output power of 100mw was
obtained from this circuit driven directly from the 3 stage low noise
preamplifier, the measured gain being 6db, the dissipation limits
being exceeded somewhat for the devices but no GaAsFet smoke being
visible unless the supply voltage was extended towards 7 volts. I had
ideas to print multiple 3db coupler connected output stages for more
power but I had been corresponding with Neil, G4BRK during these
experiments and he sent me an article from Microwave Journal. This
article then formed the basis for a new design phase, three prototype
amplifiers producing 125mw, 250mw and 500mw output.
The 125mw Amplifier Design
The Microwave Journal article discusses the use of
Wilkinson combiners for the distribution of power to a phased antenna
array, Fig 1. I could see that this arrangement might lead to the
possibility of using a number of GaAsFet devices in parallel to
achieve more power from easily available devices at a realistic
price.
|
|
One of the difficulties at such a high frequency
is achieving consistent component positioning within the divider
network and therefore repeatable results, Fig2. This problem is
addressed by using a ¾ wave network with ½ wave arms connecting to
the terminating resistor, Fig3.
|
|
|
A penalty of this arrangement is that the
operating bandwidth is reduced. An prototype PCB layout was drawn
using a PC Paint Package and the layout printed onto the PCB
substrate, again using the Laser jet printing method, Fig4
|
|
The PCB track dimensions for the combiner and dc feed lines are dictated by the wavelength and reference to the parameters for the substrate. The answer to the dimension for the lines connecting to the devices was found in a Siemens application note, No. 022. A ½ wave line is recommended with move able matching tab. The resultant layout provides good physical separation of the devices and is open and easy to work on.
For simplicity it was decided to arrange for the bias voltage to be applied equally to both devices and for the drain current to be applied without individual current monitoring. The bias circuitry was also not included on the PCB simply because it could easily be included as part of the PSU board.
Resistor positions are included to stop the gates floating when the board is being handled and for the drain voltage adjustment which provides some supply lead rf isolation.
It was decided that a milled box using integral wave guide connections below the PCB would afford the best results to properly access the performance. The input and output from the board is via 0.085" rigid coaxial probes into the wave guide. This arrangement, although not the best due to the discontinuity in the transmission line enables the board to be easily removed from the box.
Construction
The construction is along the same lines as for the DB6NT PA Amplifier. Constructional experience is necessary but since preceding amplifiers are needed to provide enough RF drive for this amplifier it is assumed that this will have given experience in the techniques needed. So far the amplifier has been constructed with wave guide terminations only which eliminates the need for dc isolating capacitors in the input and output rf connections.
The board is first drilled in two positions for the coax probes and then between the GaAsfet positions for the M2 screw fixing to aid heat sinking. These dimensions are then marked through to the box housing. Next, slots in four positions are made for the GaAsFet source lead earthing foils. These foils are 2 mm wide, 0.1 mm. thick copper. These are also used to provide as much thermal conduction through to the back plane of the board as possible. It is vital that the thermal gradient across the foils is as low as possible to ensure in operation the source leads are held close to ambient temperature. The foils are soldered on both sides of the board. Solder wick was used to mop up surplus solder on the top side of the board but the solder was puddled on the ground plane side right around all the foils and around the fixing hole to afford the best flat heat transfer surface area to contact with the box.
The wave guide probes are the most difficult part
of the board assembly. The wall thickness from the box to the
integral guide was made between 1 and 2 mm thick in the milled box
housing and therefore the probe outer coax must be cut to this
length. This is achieved by rolling a short length of 0.085 rigid
coax on a cutting board under a sharp knife. This will cut through
the copper sheath far enough for it to be broken away allowing the
inner PTFE to also be trimmed away. This probe construction is an
important element in obtaining good output power and eliminates the
need to fix the board into the box with silver epoxy, so that the PCB
can be assembled and easily removed again if required. The ideal
length of the copper sheath is a fraction longer than the hole
thickness in the box. The trimming of the ground plane around the
probe feed through to the PCB track also calls for care as the board
is only 0.010" thick. Carefully use a small drill to remove the
copper ground plane leaving a countersink hole which goes almost to
the track foil on the other side. The diameter of the removed copper
needs to match the inner of the coax as closely as possible. The coax
probe inner is first soldered in place to the PCB track, this then
holds the probe whilst the outer copper sheath is soldered to the
board. The excess inner can be cut away on the track side of the
board and the assembly checked for short circuits between inner and
outer of the probes. Finally any excess inner protruding above the
PCB track can be trimmed down carefully with a small file and excess
solder removed with solder wick. At this stage the board can be
tested for assembly into the box. If the probe outer finishes flush
with the wave guide inner surface all is well. This is unlikely
however, so if it has not protruded enough either countersink the
probe hole in the bottom of the box to allow the solder around the
probe to seat better and or remove a little excess solder from around
the probe. If the probe outer protrudes into the wave guide as long
as it is not excessive it can be ignored at this stage. Remove the
board and fit the surface mount resistors and chip capacitors. The
GaAsFet are fitted before the board is placed in the box. The heat
sinking will make the source leads very difficult to solder to the
foils once the board is in the box if it is done well. If you are
confident that you have your static discharges under control then
both GaAsFets can be fitted. If not then connect dc supply leads to
the board and fit the first of the two devices. Apply about –1.0
volts negative bias to the gate and then around 3 volts to the drain.
Measure the drain current and change the gate voltage to prove you
have an operational device. Note the current, switch off and fit the
second device. Switch on again and note you have an increase in
current proving the second device is good. The board can now be
inserted into the box. Further work shows the power supply leads are
best connected by running up through the box metal work, through the
underside of the PCB to the track. This way any variation of wire
position above the PCB surface in the rf field which may affect
performance is eliminated. Feed the supply wires through their
respective holes Apply heat sink compound to the soldered area around
the foils on the board underside and push home into the box. Secure
the board with an M2 screw between the devices. Turn the assembly
over and cut the probe inner to 2.2 mm. Adjust the probe outer to be
flush with the inner surface of the guide and using a cocktail stick
with a fine point dribble silver loaded paint (car windscreen heater
repair paint) between the outer of the probe and the hole through the
guide. This forms the rf earthing between the amplifier board
assembly and the wave guide. The board ends may well be flexed upward
slightly within the box housing due to the probe final adjustment.
The paint around the probes will be strong enough whilst the corners
of the board are pushed down and secured with a blob of instant glue.
Both the paint and the glue will break away if the board needs to be
removed again. The small variable space between the underside of the
PCB and the box seems to have no rf consequence. It is unlikely the
board will be perfectly flat after construction anyway, but this does
not seem to affect performance to any extent.
Alignment and Performance
Check that the PSU board is providing –1.0 volts for the gate supply before connecting the +ve supply to the drain dc connection. A bench PSU is helpful for the positive supply as it allows the +ve drain voltage to be set low during initial tune up with gradual increases until 5.2v at the drain is reached. Use say 4 volts initially. Do not exceed a supply voltage of 6 volts.
Connect a load and output monitor. Apply some rf drive, a three stage DB6NT low noise amplifier is a good source, if it is supplying 20mw or so. Typically at this initial stage only the same amount or maybe even less rf will come out from the amplifier.
Place a small tuning tab on the input line and adjust for more output. The tab should be the same width as the line ideally. A wooden cocktail stick is useful for moving the tab without having any rf effect itself. The starting position is opposite the dc feed line. An increase in output should be observed. Move to the output line and adjust the tuning tab again starting opposite the dc feed line. . Finally tabs can be added to the gates and then to the drains. These may make a small improvement otherwise leave them out. It is a good idea to tin the back of the tuning tabs prior to fitting to aid soldering down using the minimum of solder.
The supply voltage, drive, and matching tabs are all interactive so it is necessary to go around several times and readjust until maximum output is achieved. It is important to make final tuning adjustments at full output power as the device matching characteristics will almost certainly be changing at increased power levels . The lid will enhance the output further when adjusted to the optimum height feeding radiated power back in phase to the output. If the lid is fitted too close to the board eddy current losses will increase in the metal cover. Between 12mm and 15mm lid height gave good results on the 125mw prototype. The input and output line matching tabs appear to compensate well for the guide match. No need for tuning screws in the guide has been found if a matched load is available during alignment. If a matched load is not available then matching screws can be added to the guide for comfort, ensuring full power transfer can be achieved .
The GaAsFets are run beyond the manufactures recommended ratings leading to elevated channel temperature, hence the importance of the heat sinking. The prototype was finally set to the following dc conditions with no input signal. Drain current, 40-45mA per GaAsFet, set with the gate bias voltage at a drain voltage of 5.2 volts. Output levels of 100mw with 1db compression and 135mw saturated power were obtained. As saturation commences gate current will flow and the drain current will increase accordingly.
Gain was measured at 7db for power levels just below compression on the authors prototype. A Beta amplifier constructed by Arie, PA0EZ produced performance shown in the graph below. Thanks to Arie for permission to reproduce his results graph.
|
Power Supply
It is not intended to deal with the PSU
construction in detail since this will be well within the capability
of the 24Ghz constructor. Some suggestions however. Since the high dc
input power is only required in transmit mode a suitable PSU can be
arranged where the receive mode applied voltage is reduced with the
Tx/Rx logic. This will reduce the dissipation within the GaAsFets to
a more acceptable level. A power supply due to F1GHB can be easily
modified for this, The basic circuit is shown in Fig5. The
negative bias for the gates, circuit not shown here, should be
derived from a low impedance source such that the voltage is not
modified substantially by the flow of gate current. This is
particularly important for the higher power amplifiers.
|
|
A simple +ve supply using a 7806 regulator is in
use by the author. The regulator is housed separately so that its
dissipation is not conducted to the GaAsfets.
|
Fig 6. Interior View of the 125mw amplifier |
Since this picture was taken the resistors in the Wilkinson divider network have been changed for 0603 types. The supply leads are also routed away by going down below the board via through holes in the box.
The 250mw Amplifier
Once the basic module described above was proven, the artwork was copied and pasted using Paintshop Pro to produce an amplifier using four GaAsfets. The Wilkinson divider was also copied and pasted to split and combine the pairs of GaAsFets. Curved transmission feed lines were chosen to avoid any right angle bends which might cause additional radiation loss. The artwork is shown in Figure 7.
|
|
|
|
|
The 500mw Amplifier
With good results being obtained from four
GaAsFets the PCB artwork for eight devices was created again by
copying and pasting the layout using Paintshop Pro. Additional input
and output dividers were added with associated connecting
transmission lines to complete the circuit. The PCB layout is shown
in Figure 10.
|
|
|
I would like particularly like to record my thanks for help and ideas to the following 24Ghz Golfists. Bob, G3GNR . Neil , G4BRK. & Chris, G8BKE.
References:
Microwave Newsletter May 1998 Issue, Making Microwave PCBs.
Microwave Journal November 1995 Issue, Modified Wilkinson power Dividers, Pages 98-104
Siemens Application Note No.022
NEC Data Sheet for NE32584C
Rogers Duroid Web site
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig 7.
Fig 8.
Fig. 9
Fig.10
Fig.11