Finally the 20dB attenuator I ordered from China has arrived. I need this as even the lowest output setting of my VFO is too much drive power for the small 3W PA amplifier I have. So now I can turn it down to get the PA operating in its linear range. So I have been trying agin to send WSPR weak signals and am looking for a result on WSPRnet.org.
Here's the set up, from right to left
- Windows PC (I am a Mac man, but this seems not so bad) running HDSDR and WSTJ software, using VB to pipe audio from one to the other.
- The SDR receiver, which is an Elektor SDR Arduino shield and an Arudino Uno.
- The VFO, which has an AD9851, RTC (using a DS3231) and a buffer amp. Software is loaded to run WSPR on 40m at 7040.1kHz, the 20dB attenuator is plugged in the back of the VFO.
The small PA amplifier module, from eBay, running 2W output from a supply of 13.8V
More on SWR
In the last post I talked about an SWR meter using a normal bridge circuit. The output from the bridge FWD & REF RF was handled by two AD9307 log amplifiers and fed to the analog inputs of an Arduino Nano. This drove an OLED display showing FWD power and SWR. Fiddle as I might I could not get stable results, probably as a lot of the circuitry was on a plug-and-play breadboard. So I am having a rethink.
I have browsed the web for ages and this is what I have come up with. I will wire this up on a proper PCB to test it:
I have abandoned the AD8307s and gone back to what most people use - a couple of germanium low drop diodes. The PA amplifier this will be used with is 2-3W output, so their should be enough volts to run the thing. Calibration can be done in software.
As you can see I want to retain the measurement of RF output - just the voltage on the line, which can be shown as power when the system is matched to a 50R load. For this I am using a tried and trusted AD8307 circuit which will display 10mW to 10W. This AD8307 output will also be used to trigger the auto RX/TX switching of the PA.
Wish me luck.