Skip to main content

March 16, 2024. Testing the small find. (Video now working!)

Our small find is tested on transmit.


 

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.






Small Find Transmit Test.





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

Popular posts from this blog

January 26, 2024. A simple CW Transceiver/Transmitter

Cruise through the lower part of the ham bands bands and what do you hear? Well, FT-8 and CW. Often you will not hear any SSB stations yet go to the lower part of the bands, and it is a cacophony (I love that word) of bad sounding signals and some high-speed keying. Fast is not so much of the issue as is bad, run together and jerky keying. But none the less our hobby started there.    So, you could crank down your ICOM 7300 and watch the waterfall on CW or you could homebrew a radio. Actually, to do CW right you need more thought up front than you do with a SSB transceiver. Often, I will state that a CW Transceiver is much more difficult to build than a simple SSB rig. I published two articles in QRP Quarterly on CW transceivers and all I got was a yawn so maybe history will repeat itself.  Yawn!   30M CW Transceiver with RIT!   Of interest is that the LO is a Varactor tuned LC oscillator using a NE602. Look closely at the RIT circuit which is only activated on ...
Read more

March 31, 2024. Happy Easter to those who celebrate this day.

What a great day to Binge on Chocolate and experience the pain of that filling that has been leaking.  I would be in that category with the leaking filling(s) had I not just spent an amount equivalent to one of the fancy new uptown appliance box transceivers on two filling repairs. Well at least I can binge on the Chocolate bunnies without fear of pain. Regrettably everything appears to have jumped in price including the price of parts. Well not so much the parts as the shipping costs.  That notably is seen in the eBay treasures. I spotted a nice heathkit DX-20 for about $50 and the shipping was $65. Likely it is a twofer with part being a way to in effect charge a higher price by inflating the shipping and in part by increased shipping costs. Shipping with insurance across the US was about $150 for this jewel and that was three years ago. 6AM on the Left Coast ~ 20M Easter Sunday! My only hope is the cost of Chocolate Bunnies remains steady although a pound of See's Candies f...
Read more

August 30, 2024. A PNP 20M SSB Transceiver

Shown below is the Block Diagram for the 20M PNP SSB Transceiver steered in the  Transmit Mode . The components shown in the dotted block are relay steered so that the block module is single pass and amplifies in a single direction. The Block diagram show steered in Transmit.  Essentially the steering process works so that the IF Module input follows the Balanced Modulator on Transmit and then the input side follows the Receive Mixer on Receive. All done with some relays and a bit of RG174U coax. For those who count things in detail, this block diagram is not unlike what was used for the PSSST Transceiver which can be found on my website . Yes, a warmed over P3ST only using PNP devices. TYGNYBNT. 73's Pete N6QW
Read more