Figure 1: Double sided copper clad PCB makes good shielding compartments.
Continued from page 2 …
So, maybe the reader wonders where all the pieces and parts, used in this project, came from. As mentioned on page two, most of the project parts are in the form of kits purchased from QRP-Labs. They have a PA (amplifier) kit that uses an Atmega MCU for shaping – utilizing the MCU for a raised cosine output form.
They have BPF and LPF filters that are packaged on very small boards, and a receiver module kit that can take LO (local oscillator) input from their “Progrock” Si5351A based synthesizer. The Progrock can then serve the dual purpose of controlling RX frequency, and supply transmitter drive as well. The progrock supplies about 25 milliwatts to the BSP-170 homebrew driver, which subsequently drives the PA with about 250 millwatts. I used all of the above referenced pieces, in addition to a homebrew TTL/RS232 converter that will allow the TX/RX combo to be controlled with one of my “homemade” tablets. 🙂
Figure 2: Maiden voyage – and RF OUT! Yae!
So, I mounted a DB9 female connector onto the RX box, fixed up some BNC cables to connect Rx box RF output to the Tx box RF input, put the Progrock, BPF, and TTL/RS232 converter into the RX box, zero biased the PA in the TX box (with no drive, adjusting the bias such that PA current was very close to zero), and then used the Picocom serial program to set register #5 in the Progrock to a good 40 meter CW frequency. I then applied drive to the PA by keying the SCK line.
The result? Output! The old Micronta is not a very good device to measure power accurately, but its needle is kicking up pretty good for a QRP rig. The IRF-510 PA device’s current was at 800mA, key down, with drive. So, that’s some watts.
The BS-170 driver, which was the suggested QRP-Labs design, but “homebrewed” onto a piece of proto board, seemed to be happy with the bias set at about the midpoint of the 4.7 k bias control pot.
The reader may wonder why there’s a piece of paper under the TX box cover. Well – there was a “too long” screw that costs me two 2.5A fuses. It’s OK now.
So, now that we have some output, the next step is to check it for spurious (unwanted output) – harmonics or otherwise. Finally, it remains to build the RX and put it into the RX box.
Running the TX into a dummy load, and listening to the signal on a regular receiver (not the one I’m building into the purple RX box) – the signal is very nice sounding. It seems clean and crisp. As for harmonic suppression, the LPF in the TX box is supplemented with an LPF filter between the Progrock and the BS-170 driver. Before I connect an antenna to this setup, the proper effect of the PA output and Progrock transmit filters needs to be verified. I figure, “why should I amplify the harmonics going into the amp?” No reason that I can think of, so the filter is mandatory. I’m using T50-6 cores, simply because I have a lot of them. Between the two filters, we should be able to clamp the prodigious harmonics of the Progrock pretty well.