BLF188XR HF Amplfier Project

       I started this project sometime mid 2014. After trying many different amplifier kits from the EB104 to the MRF429. The EB104 kit worked ok but needed many modifications to make it work ok. Although even after modifying the original board i found output to be unsatisfactory on the higher bands.
The project that was using the MRF429 transistor was a complete failure as the transistor would blow up for no apparent reason. They did not like to be run at anywhere close to 50v even though they are rated at this voltage.
       
     So after the release of the new LDMOS devices which seemed to be more robust i decided to start another project with the BLF188XR ldmos. So two complete boards were purchased from a russian seller on eBay. The workmanship on these boards seemed quite good.

The heatsink was sourced from a seller in the US. The copper spreader is 12mm thick and was sourced locally. They weigh close to 9 KGs in total and should be more than capable of cooling the BLF188xr devices.



Here is another photo of the heatsink drilled and tapped using M4 bolts.



The copper was drilled and tapped using 6-32 screws ready for the mounting of the ldmos.
I have since modified the output matching transformers with different types as the ones seen in the
original circuit seemed to overheat.



The BLF188 was soldered onto the copper spreader using a hotplate. This is supposed to increase the thermal conductivity of the ldmos.

CONTROL CIRCUIT

      The amplifier is controlled by a pic controller which monitors all aspects from voltage, current, power,swr to temperature. It will trip the amplifier protection circuit and set off an alarm if any of these are out of range. The software for the controller was written by Dan MW0UZO. He was kind enough to let me copy his original design and was very helpful with all the issues i came across whilst building these circuits.

      The PCB for the controller was designed with Eagle CAD software and i had the printed by Fusion PCB which has very competitive pricing. The pcbs arrived after a few weeks and then i started populating it with the smd components.








The code was uploaded to the pic and it fired up with no issues.
So far so good.



This board consists of TD62783 Darlington drivers.
They drive the LPF and attenuator circuit relays.
It also allows me to select band switching from the amplifier or
from an external source. I use the outputs from my Hermes board to drive the relays in the amplifier.



VOLTAGE AND CURRENT SENSE BOARD

The voltage sensing was made with a simple voltage divider.
The current sensing is done by a ACS758 current sense module.
This board also has the bias transistor on it.



DIPLEXER FILTERS

The PCB's for the diplexer filter were ordered from WB6DHW website. The Iron powder toriods were also sourced from the internet. The Toroids are quite large for the application but i thought better to over engineer that have things fails. The relays are Omron G1R-1-E. They have 16 Amp contacts.







Here's a picture of the 80m Diplexer completed. The toroids were wound with two parallel 1.2mm enamelled wires. Several ceramic capacitors were installed in parallel in order to handle the RF current without overheating.



Finally a photo of the completed Filters. It covers:

• 160m
• 80m
• 40m
• 20m
• 15m
• 10m

The boards were joined together using m3 threaded rod and nuts.

The Filters were tested using my VNA and all seems good.

CHASSIS 

     I needed a large chassis to fit all the stuff so i ordered a 4RU rack mount chassis from RS Electronics. Initially i wanted to include the Hp blade supply in the same enclosure. Although it quickly became apparent that it wasnt going to fit. So i had to settle for having it in a separate enclosure.

Rear panel drilled and sprayed with matt black paint ready to install.

In this you can see mounted to the heatsink the Output power combiner, SWR Bridge and Output select relays.

Here you can see the gain balancing circuit that is driver by the lpf circuit, the input power splitter and swr bridge.

This is the input attenuator circuit. It is selected via the menu.

You are able to select 10db, 6db, 3db or 0 db.

Also on this board is the input select 1 and 2 and another SWR Bridge.

On the far right of the board is a P-channel Mosfet Swtich for the 50v supply.

Attenuator board all wired in. Almost ready for testing.

Rear panel installed.

   Amplifier mounted in rack ready for some on air testing. Months of work finished time to see some results. The amplifier seems to perform as expected. After some initial issues with some rf getting into the controller circuitry. It has now been running for a week with no problems.
I still have to order the second BLF188 module. All my tests so far has been with one module running.

 It does more that 750w on cw on the power bands. I have tested imd @ 600w and it is about -30 dBc. I still need to do some modifications to the transmission line transformers on the boards.

Will post more pics and info when i do these.