23cm VLNA

Very Low Noise Amplifier for 1.3GHz

                                                Sam Jewell, G4DDK

Introduction

One of the most effective 1.3GHz low noise pre-amplifier designs of recent years was designed by Tommy Henderson, WD5AGO. Tommy's design uses an NE32584/86 HEMT followed by an ATF10135 MESFET with ‘air spaced’ input ‘T’ matching to minimize parasitic losses from the substrate that the pre-amplifier is built on. The same technique was also used by W5LUA in his successful 2.3GHz low noise pre-amplifier design [2]. The WD5AGO 23cm pre-amplifier is used extensively by 23cm EME operators around the world.

Last year it became apparent how difficult it was becoming to obtain ATF10135 MESFETS. They had been discontinued for some years prior to this and supplies have been quickly drying up. This caused me to look at an alternative second-stage device. I chose to use the ATF54143 GaAsFET from Avago. As this is a very different device to the 10135, it was found necessary to change the bias arrangement and matching between stages.

WD5AGO's 23cm pre-amplifier achieves between 0.3 and 0.33dB noise figure with about 30dB gain, according to the article [1] and also verified by numerous measurements.

The new 23cm VLNA prototype measured 0.26dB NF with 35dB gain using the ‘Martlesham’ HP8790A and HP346A noise head. The same pre-amplifier measured 0.25dB/34dB at RAL RT 2007 and 0.26dB/34dB by WW2R. I took one to the CSVHF 2007 meeting where Tommy measured it at 0.25dB NF and 35dB gain. G3LQR regularly measures 0.5dB more sun noise on his 23cm EME system with this pre-amplifier than with his original WD5AGO pre-amp. Early in 2008 DL4MEA measured my 'example' pre-amps in Munich at 0.25dB NF and 35.5dB gain using an Agilent N8973A NF meter.

The pre-amplifier is well suited to 23cm EME use,with its high gain and low noise figure.  The 23cm VLNA should prove useful across 1 - 2GHz and especially at 1420MHz (Hydrogen line). The ATF36077 is particularly good at 1420MHz, with a slightly lower NF here than at 1.3GHz. The 2.3 and 3.4GHz versions should be useful for both EME and terrestrial use, although the  achievable noise figure is a little higher than with the 1.3GHz VLNA.

Commercial, emersion silvered, plated-through-hole (PTH), silk screened and solder resist coated PCBs and kits for the 23VLNA, 13LNA and 9LNA are available from the author.

Circuit description

The circuit schematic is shown below.

Schematic for the 23cm VLNA. Table 1 shows the full component list.
The same schematic is used for 2.3 and 3.4GHz, except for some component value changes as shown in table 2

The input circuit consists of a ‘T’ match with suitable low loss capacitors and inductors. Fig 2 shows the input arrangement. Low noise matching is achieved by slowly adjusting the spacing of the turns of L1.  Careful adjustment is critical to achieving lowest NF. Lowest NF will not coincide with maximum gain. Maximum gain will occur at about 1200MHz when the NF is lowest at 1296MHz. The strange  positioning of the turns of L1 are important and the adjustment is critical.

Input impedance match is improved by the use of source series inductance. This is already designed into the PCB, so you don’t need to worry about tuning this parameter.

Except where indicated, 0603 size surface mount components are used on the board in order to minimize component parasitics. This has proven most successful and it is a genuinely good reason to move towards 0603 or even 0402 size parts in all designs above 1GHz.

Negative bias for the NE325 is provided by an  7660 DC-DC inverter IC. R14 allows a range of adjustment, from approximately -0.13 to -1.0V. Tr1 drain voltage (to ground) should be adjusted to approximately +2.0 volts by adjusting R14.  The drain current will be around 9 to 11mA. Do not get too hung-up about this value at this stage. After adjusting L1 to lowest noise figure it will be necessary to go back and adjust R14 to give the lowest noise figure. This will be in the range 9 to  11mA. The drain voltage  will be  2.0v at 9mA. 

Active bias was chosen for Tr2 as the drain current is set quite high, at 65mA, to achieve a good dynamic range. At this elevated current I felt that active bias would help to maintain circuit performance. This is provided by Tr3, a BC807 PNP transistor .

The whole unit runs from a 5 volt, 500mA regulator IC that uses a surface mount (D-Pak) 78M05 regulator soldered to the PCB ground plane as the heat sink. A TO92 packaged 78L05 will not supply enough current without over-dissipating.

D1 is there to ensure that an accidental reversal of the supply doesn’t destroy the pre-amplifier. The Trucap tantalum capacitors, especially C17, seem to be very sensitive to even small reverse voltages. If you do accidently connect up the supply with reversed polarity, the preamp should survive, although C17 may need to be changed to ensure longer term reliability. This is probably true for many tantalum capacitor manufacturers.

The RF absorber material IS PART OF THE DESIGN and must be used if the full performance of the pre-amplifier is to be achieved.

Construction  - new-

The PCB is designed to fit into a popular 74 x 37 x 30mm tin plate box .

Do construct exactly as shown. L1 is particularly critical.



Build Order
File the small cut outs in the two corners of the PCB. These must be as shown.

Do NOT solder the box together at this stage.

Offer up the PCB to the box and ensure a tight fit.

Clean the board with IPA or similar.
Solder down all parts except TR1, L1 and 2 and C1. Start with the 10uF Tants first, then the regulator, D1, variable resistor.
Solder down all Cs and then Rs.
Then Ls except L1 and 2
Solder the ICL7660, TR2 and 3.


Box
Mark the box 10mm down from the rim, making sure you mark from the correct rim!

Mark the position of the feedthrough.
Offer up the PCB to the box and mark CAREFULLY where the SMA holes will go. The output should be 12.3mm above the 10mm line
The input one should be 10mm down from the other rim, and almost in line with the input to TR1. Actually, make it 0.5mm further inboard towards the variable resistor.



Drill a 2.0mm hole for the feedthrough and 2 x 4mm for the two SMA connectors.

Connectors
Cut the SMA Teflon insulation back so it leaves just 0.5 - 1mm . Use a sharp scalpel for this.
Cut off the spill to a length of 3mm. File the end flat. This is particularly important for the input connector because C1 needs to be carefully soldered to the end of the spill. This will give the best, lowest resistance, contact when soldered.

Mounting into the Box
Tin the end and one edge of the PCB groundplane on the output connector side and offer up to the box side. Ensure the output connector lines up and that the groundplane side of the PCB is in line with the 10mm line you marked on the inside of the box side. i.e. the groundplane should be 10mm below the rim of the box. Tack the PCB to the side. Place the PCB and side inside the lower lid and add the other side. Make sure it all fits into the lid.

Tack the corners of the two sides together.
Ensure the PCB lines up with the groundplane marked line on the second side and tack solder in place.
Fit the other lid and now remove the first lid once you know it all fits OK.

Once you are happy it all fits, carefully solder all round the edge of the PCB groundplane, ensuring a good, tight, soldered joint to the tinplate box. i.e., no gaps.

Solder the sockets in place ensuring you press down solidly on the connector so that it makes really good, flat, contact with the box side. DO NOT solder the connector spill yet. Only after the connector flanges are soldered should you solder the connector spill to the PCB track.

Fit the feedthrough into the solder tag and solder to the box side.


Completing the build
Solder Tr1 into place, ensuring correct lead orientation. It is best to use a small insulated soldering iron to prevent static damage. Touch the soldering iron to the tin plate box before soldering the GaAs FET leads.  I suggest you tin the 6 holes by applying the small gauge solder and the iron so that the solder just runs into the holes.

Carefully wind the two coils, L1 and L2 (for 23cm version) or cut the wire to length for the 13 and 9cm versions. These must be wound to 2.5mm inside diameter. Use a 2.5mm drill shank as the coil former. Remove the former after tinning the coil ends.

Tin the end of the coils for 0.5mm only. The temperature required to 'burn off' the insulation coating is about 400C.

Solder C1 to the end of the spill. Make sure it is in line with the spill and not perpendicular to the spill or at any appreciable angle.
Solder L2 in place, soldering first the C2 end and then C1 end.

Solder L2 in place to C1/L2 with the TR1 gate end positioned close to the FET's gate. Solder to the gate. Ensure a good soldered connection. It helps to slightly bend TR1 gate lead UP before installing it onto the PCB.

Solder a short piece of insulated wire from the feedthrough to the pad at the end of D1.

This completes the build.

Fig 4 Component overlay

Fig 5 PCB mask

Initial setting up

Connect between +12v and  +18v to the feed through capacitor. Check the output of IC1 for +5V at its output.

Check that the output of IC2 is close to -5.0V

Check that the variable resistor R14 adjusts the output voltage, at the free end of L1, down to the range -0.13 to -1.0V approximately

If any of these tests fail, check for incorrect component values or bad joints.

Correct the power supply to the box and adjust R14 so that Tr1 drain voltage is 2.0v. Also check that Tr2 drain voltage is about 3.0V.
With L1 still close-wound, measure the noise figure. Now carefully bend the first two (top) turns up and away from the remaining turns. The turns should be spaced as shown in figure 2. Re-measure the noise figure. It should now be very low. Now CAREFULLY adjust the spacing of these two turns for the lowest NF. Care here will be rewarded. Now go back and adjust R14 to obtain the lowest noise figure. The drain current (as measured as the voltage drop across R2, R3 and R4 in series = 223R) should be in the range 11 to 13mA. This applies to the NE325. The ATF36077 may be slightly different.

See the 13 and 9cm LNA pages for details of setting L1 for those two bands.

RF absorbent material should be stuck to the inside of the lid of the tin plate box. If using the supplied piece of absorber material, remove the protective paper from the rear of the absorber. Stick the absorber towards the end of the lid nearest the amplifier section. Putting the lid in place should not result in any increase in noise figure or loss of gain.

The magnetic field absorber material supplied with the kit has been carefully selected to ensure stability.

Results

These should already speak for themselves. The input third order for the NE325 version is about -8dBm. Whilst this is not outstanding, the gain of the pre-amplifier will degrade the overall dynamic range of the pre-amplifier and transceiver or transverter combination. Careful attention to system gain distribution will allow you to achieve a very sensitive receiver with a useful dynamic range when used with, e.g. a TS2000X or LT23S.

Where strong out-of-band signals are a problem, the very low noise figure of the pre-amplifier will allow the use of a low-loss inter-digital filter in the antenna lead, without increasing overall system noise figure too significantly.

Caveat Emptor

Very low noise figures are notoriously difficult to measure with any accuracy. No specific noise figure is claimed for this pre-amplifier. I have quoted the numbers measured at different VHF/Microwave events and with different noise figure measuring equipments and operators. Since the pre-amplifier is offered as a kit, the noise figure and gain achieved will depend on the individual constructor’s ability with the soldering iron and patience in setting up the pre-amplifier.

The pre-amplifier can be operated without a lid and the stability will be good. However, without the high quality commercial magnetic field absorber material inside the tin plate box lid, putting this in place is guaranteed to degrade performance. Foam absorber, such as ‘CMOS’ foam, will not work very well in this application. Please use the right material. It is as much a part of the design as the FETs used!

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Further info from

Sam (at) G4DDK (dot) com.........

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References

[1] ARRL Microwave Update  1999

[2] ARRL Microwave Update 1994

 

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This page last updated 30/11/2009