I finally got a few minutes yesterday to test the RF Amp board on transmit. The results seem pretty positive. The tests were done on both 40M and 20M with more output available on 40M.
The LO versus the RF Amp board likely is responsible. With a 9 MHz Filter the LO on 40M = 16MHz and on 20M the LO = 23MHz. Typically, the output reduces from the Si5351 as you go up in frequency.
Several posts ago I showed a small RF amplifier board that costs about $7 from Jeff Bezos. Previously I tested it on Receive and yesterday it was time to test it on Transmit.
The output from my test SSB Transceiver was connected on one end to the RF Board and the other end of the RF Board to a 50 Ohm load. The Vcc was 10VDC. Typical output from the test SSB transceiver is 40mv.
On 20 Meters we saw about 1.5VPTP (5.625mw, 31.5 dB Gain) and on 40M about 1.8VPTP (8.1mw, 33dB Gain). These values are consistent with the specs. The max output from the device is 20mw.
Turning to a reverse engineering exercise, we find 20mw is about 2.828 VPTP at the 50 Ohm load. Thusly, 20mw/2.5 = 8 and the square root of 8 = 2*(sqrt(2)) = 2*1.414 = 2.828. The 2.5 factor is a conversion from PTP to RMS with a 50 Ohm load.
Continuing with the math, an anchor point is that 20*log(40) is 32 which says that log(2.828/X) = log of 40 making X = 70.7mv. Thus, to get full output from the RF Board we would have to see about 70mv into the board.
We used the voltage gain dB formula where the log is multiplied by 20. If you use the power gain the log result is multiplied by 10. You can check the math in that 70.7mv is 0.012496225 milli-watts across 50 Ohms. Divide that into 20 take the log and multiply by 10 and you get the same 32dB.
The patterns look OK, but I want to re-run the test with a BPF ahead of the RF Amp board. The additive and subtractive mix out of the TUF-1 are in the pattern and inserting a BPF will eliminate some of the extra stuff in the envelope. This has some promise as a Rx RF Amp/Tx Pre-Driver stage using relay steering.
73's
Pete N6QW