Modifying the Eagle Aspen SLT 3.7 - 4.2 GHZ LNA for 3456

This type of LNA is readily available from any number of sources at low cost. Most of these are specified as having a noise figure of around 1 dB. Some are a little higher, other slightly lower. From the ones I have seen they are all specified in terms of degrees Kelvin (K) and not dB. A 74 degree rating, which is what mine had, is just ever so slightly under 1 dB NF.

They are quite easy to convert to Ham use on 3456 MHz. To do the job correctly some tools and/or patience is required. I prefer the power tool route but you may not have the tools I used available to you. What I wanted to do was to convert the input to an SMA connector and not have to use a WR-137 waveguide to coaxial adpater. As it turned out, that was fairly easy to do but I had to be very careful drilling holes. Read on to find out how I converted this from a TVRO LNA to a simple but effective 3456 LNA.

The above picture is the complete unit as I received it. It is made of aluminum and has the typical WR-137 input and a type "F" connector output. This unit is an LNA and a down converter all in one package. Some TVRO products were amplifiers only and drove a down converter through a long coaxial cable run, which was generally RG-8/U. See another of my pages for converting a straight LNA unit.

Here is the business end of the waveguide input. Notice a nice wide gold plated probe inserted into the waveguide. The diameter of it makes it very broadband to cover the entire 3.7 to 4.2 GHz TVRO band.

This picture is of the back of the unit showing the output connector which would feed the nominal 1 GHz signal to the receiver. At this timeit is best to connect a 12 VDC source to the "F" connector to see if it draws current. It should draw about 65 millimaps or so if it is working.

In this picture of the finished LNA you can see how I removed the waveguide pieces to get down to flat aluminum surface that I could mount the SMA input connector to. Cutting it down also eliminated about 50% of the total volume. I used a Portaband saw to do the work but a hacksaw will work as well. Where the SMA connector is located is the point the waveguide probe was soldered to the PC board. More on mounting the SMA later.

In this picture I have removed the rear cover to get a better look at what is inside. Pretty straight forward stuff in here. The DRO is mounted under the small metal box. The adjustment screw in it is the DRO frequency adjustment.

In this picture I have removed the DRO cover to expose the DRO itself. The incoming voltage regulator is in the top right, and the negative voltage generator for gate bias is in the lower right. The first stage is in the bottom of the picture and just left of center. You can clearly see the 4 leads of the device. Look carefully and you will see a short wire loop (used as an inductor for matching) going to a pad. That pad is the waveguide probe shown above. Also notice another small wire going to the right; that is the DC feed into the device. There are two RF amplifier stages, the one just mentioned and another to the left and slightly above the first stage.

Now look in the upper right at the voltage regulator. Just to the left of the 3 regulator leads, and right next to the center one, is a solder pad. This is the DC input and the output point for the down converted signal. Look towards the left of that pad and you will see a surface mounted transistor. That is the final IF amplifier stage. I removed that device with needle nose pliers later on. Don't start pulling things out just yet though.

The first thing I did was to remove the DRO. Just grab it with pliers and snap it off. It will come out with little effort. Don't worry about breaking it since it will not be used. If you want to be extra careful, as I did, then you can also break out the resistors and capacitors connected to the circuit board traces going to the DRO. The DRO is in the upper left corner of the picture. DO NOT remove anything outside of the area of the box you removed to get to the DRO. On the far left side, about mid way top to bottom there is a PC board trace that looks kind of like a slightly squashed egg going right to the left edge of the board. If you look carefully you can see the hole I drilled for the SMA connector directly in line with that trace. There is another IF amplifier stage connected to the top of this trace. It is the little dark blob running top to bottom. Pull that device out as well. Also remove everything else connected to that little egg shaped trace except for the blocking capacitor further down the trace. DO NOT remove the blocking capacitor. What you will be left with coming out of the second RF amplifier stage is the blocking capaitor feed that egg shaped trace and nothing else.

So this is the point you can continue hacking into the IF components. That little IF amplifier I pointed out above is now gone. Look at the open traces in what is now the lower left corner. I also removed the resistors and cut the traces that fed this device. Notice that I have also now installed an SMA connector at the point the RF is taken out of the LNA. Bolt the connector in place and then make the solder connection. The holes for the SMA should first be drilled with a number 50 drill. Redril drill the center hole with a number 20 drill (.161") and then tap the outer holes for 2:56 screws. When drilling, be very careful not to break through and damage the PC board. There is not a lot of clearance to work with. I strongly recommend using a drill press and a vise. When all that is done, replace the DRO cover with the 3 screws removed earlier. Tighten snuggly but do no over tighten.

Earlier I had mentiuoned installing the SMA connector for the 3456 input. Look back at the first RF stage, the one with what looks like a "0" on its side on it. The wire loop (RF choke) can clearly be seen going from the pad just above the device to the DC feed. The RF loop is not really visible but it is there. What you need to do is remove both leads but be very careful. Now grab the waveguide probe on the other side and put a soldering iron on that pad for a second or two and then pull the probe out. That was the easy part! Now turn the LNA over and you will find a piece of Teflon in the hole the probe just came out of. Remove that. Easier said then done, but it needs to come out in order to make room for the SMA connector Teflon. If the Teflon on the SMA you are going to use is a little long then trim it so that the flange fits snuggly against the aluminum surface and protrudes throught he pad we just took the proble out of. If the center pin of the SMA is too long, trim it off. Then solder a small 5 pf chip capacitor to the center pin of the SMA connector. Then, being very careful, solder the two leads removed from the waveguide probe.

The hole is just the right size for the Teflon of the SMA connector since this was originally a 50 ohm feedline section from the probe. Now mark the mounting flange holes for the SMA using the orientation of the SMA shown in picture 4. That puts the SMA flange running from the cut off waveguide towards the "F" connector. I strongly suggest using a two hole flange mount SMA. Now go back to the drill press and drill a pair of number 50 holes. Here you need to be extremely careful since you are drilling holes, one of which that will punch through under the PC board. The other will be in the metal of the waveguide section that is left and will not punch through under the PC board. Drill carefully until you just break through under the PC board. The other hole can be drilled deeper. Tap the deeper hole as far down as you can for 2:56. Tap the other hole until the tap just touches the PC board. Then let the mounting screw in this hole finish tapping the threads. Also selct a 2:56 screw that is just long enough to go through the hole. If it touches the PC board, that is OK; just don't push on the board or something might crack.

I lef the "F" connector there as the DC input connections. It could be removed and replaced with something else if desired, but why bother. Make sure all of the solder pieces and drilling residue have been removed. If you have an air compressor, gently blow the filings out of the LNA. Now connect a nominal 12 VDC to the "F" connector. The LNA should draw about 35 milliamps if it is working correctly. Replace the cover and you should have a working LNA ready to go for 3456.