⚡ MINIPA70 Test Thread! 🧵
The board is populated and does not put out smoke when powered up. So what's next?
As I see it, the power amplifier has three main parameters that I need to find an optimal value for:
For each point in this three-dimensional space, I get the following resulting values:
I have some reasonable starting points for the three parameters I need to tweak. A blog post by F5NPV has been invaluable for me, and in it he suggests good values for all the parameters.
https://f5npv.wordpress.com/irf530-mosfet-amplifier/
[#]AmateurRadio #HamRadio #electronics
=> View attached media | View attached media
=> More informations about this toot | More toots from raek@tilde.zone
The kit states "13.8 V" on its schematic (the "standard" amateur radio voltage), so I'll use about that voltage for testing.
The power supplies I own are a bit limited. My HP E3631A is adjustable, but can only put out 1 A.
I have a fixed 13.2 V (so not quite 13.8 V) power supply that can deliver 3.5 A continuously / 4.5 A intermittently. It's what I've used so far.
During the tests, I managed to pull about 6 A pretty early on, so a new power supply is needed. I ordered a new one from AliExpress called "PS30SWVI" (Power Supply, 30 A, Switching, sixth generation). It even have double Anderson powerpoles on the front! It should arrive in about 2 weeks from now...
With the new power supply I should be able to adjust the voltage and have plenty of current available. My only worry is that it doesn't have any settable current limit, but I guess that I could ad a fuse in series.
=> View attached media | View attached media
=> More informations about this toot | More toots from raek@tilde.zone
My options for generating the input signal are basically by BITX40 and my QMX. The BITX40 has a trimpot for its PA driver. But instead I went with the QMX and used its "SWR meter" mode.
The QMX lets you adjust the SWR test output power as a percentage of the voltage used for full power. So "100%" is full power, but "50%" is a quarter of full power (due do P∝V²). I measured the power for various settings using my oscilloscope (12.0 V, 7.100 MHz):
I also looked at the QMX output on the TinySA Ultra spectrum analyzer. There weren't any visible harmonics at all, so I trust that this signal is suitable as a test signal.
=> More informations about this toot | More toots from raek@tilde.zone
I hooked up a current meter in series with the power supply and started to tweak the gate bias. The board draws 22 mA with the bias completely off. I turned the potentiometer until the current reached 42 mA, which should mean that each FET get about 10 mA of current.
I tested with the PA connected to the 13.2 V power supply and a few different power levels:
The harmonic content was pretty terrible, as expected. I haven't added any low pass filter on the output yet. I've also heard that lower gate bias means more crossover distortion and non-linearity, so that makes sense.
=> More informations about this toot | More toots from raek@tilde.zone
I let the amplifier run with constant 13.2 V power supply, fixed gate bias voltage, and constant 0.2 W input power. The transistor tabs reached 70 °C in about a minute. The current consumption kept increasing.
This is in line with what F5NPV wrote about on his blog and @vk3kri pointed out here on mastodon. The bias circuit doesn't have any temperature compensation. So this is to be expected!
Links to YouTube and MakerTube videos:
https://youtu.be/kbxbTp3CwPo
https://makertube.net/w/5Nx4uXhbry3zhF468iNwru
[#]AmateurRadio #HamRadio #electronics
=> More informations about this toot | More toots from raek@tilde.zone
I wanted to know how the input power and gate bias voltage together affected the output power. So I ran some tests for two bias levels. The first series of tests were made with 38 mA total quiescent current, so about 8 mA Id for each FET. The board draws 22 mA with zero drain current. The second series of tests were made with 64 mA total quiescent current, so about 21 mA drain current per FET.
The results can be seen in the table in the picture. I might edit this post later and type it to text. See the ALT text for definitions of the columns.
Since the current (and output power) changed over time, I read the meters some seconds (maybe 5) after I turned on the input signal. The values varied a lot, so these measurements are not super reliable.
Also, the power supply seems to current limit about 6 A. It's a 4.5 A power supply anyway, so the results at these levels are doubly unreliable.
=> More informations about this toot | More toots from raek@tilde.zone
@raek @vk3kri What kind of MOSFETs are being used, LDMOS or ordinary vertical MOSFETs? LDMOS should have a negative temperature coefficient limiting the quiescent current. With vertical you need to drive them very hard to make it negative:
https://www.onsemi.com/pub/collateral/and90187-d.pdf
=> More informations about this toot | More toots from profdc9@mastodon.social
@profdc9 This kit uses two IRF530 MOSFETs made for power switching (and not RF amplification). Do you know what kind they are? I don't think they're LDMOSes...
=> More informations about this toot | More toots from raek@tilde.zone
@raek They are vertical MOSFETs. You can see the quiescent drain current rise rapidly for low Vgs (Vgs=4.5 V). A diode placed on the heatsink is used as a temperature sensor to drop the gate voltage as temperature rises to keep the quiescent drain current roughly constant.
https://www.vishay.com/docs/91019/irf530.pdf
=> More informations about this toot | More toots from profdc9@mastodon.social
@raek @vk3kri If you do not have a diode compensation in your circuit, you can try turning on the device, letting it warm up, once it is getting hot (like a drain current of 4 A), turn the gate voltage until the drain current gets to a desired quiescent value (like 100 mA). This will decrease the gain somewhat but can help prevent thermal runaway.
=> More informations about this toot | More toots from profdc9@mastodon.social
"My only worry is ..."
🤔
In case you are in short supply of worries to worry about: The power supply could generate HF noise that ruins your reception.
I wish you luck it doesn't!
@raek
=> More informations about this toot | More toots from dj3ei@mastodon.radio This content has been proxied by September (3851b).Proxy Information
text/gemini