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Ah not me Allison!
I think you meant AB1WX
I could tell you a bit about 813s though :)
73s Ted
2E0THH

Allison, I'm not Ted but I think I'm the only one who brought up LDMOS in this thread.

I'm looking at ways to fit AFM907NT1 in QMX. It costs 2-3 bucks a piece depending on where and how many you buy. NXP and ST offer several other options in DFN, PLD-1.5W and SOT-89 packages. DFN and PLD have much better thermal characteristics than SOT-89, although the latter is much better than TO-92. Obviously, not everyone is comfortable with hand soldering these packages, so for kits, TO-92 is a good choice. But we can do our mods at our own risk. I have some idea of mounting AFM907NT1 dead bug style on QMX board. I'll get into detail when I have something to show.

To meet the QMX spec, AFM907NT1 is an overkill, but I'm ok with that. It's made for 7-8W output at 7.4V supply, so with a push-pull, it can be used up to 15W range... but I'll be using it very conservatively.

Now, using AFM907NT1 in HF range is an amateur job because the spec sheet says it's a V/UHF device. But usually these specs are decided based on the device's internal impedance characteristics and risk of instability; discrete devices usually work at lower freq as long as care is taken in matching and stability criteria.

Hello! I am working on soldering and setting up my QMX transceiver for the past few days. While working along the manual worked fine for most steps I've done already, I am completely confused by wiring the tapped inductor L401.

Researching the internet I've seen this step has been reworked completely due to 20m sensitivity issues, but I haven't found any unambioguous instructions on how to set L401 up. I've already done the 19 turns, then the tap with the 20th turn and than another 8 turns.

However, I am not sure which wire goes where and how to place the jumper wire as both manual and other web resources are ambiguous with regards to this.

Can someone please explain this to me in a simple and clear way and/or provide some photos of where the end of the wire and the jumper wire go?

Another straw man attack.

I did say things to the effects that, outside amateur radio world, we wouldn't see uses of general purpose switching MOSFETs as RF power amps. However, I am not rubbishing BS170 or calling Hans non-professional.

Hans himself made his engineering trade-offs clear: he is offering hobby kits not medical devices. I respect that.

Stating requirements, specifications and design criteria clearly upfront is what professional engineers do.

Looks good as long as there are no shorts and you still use the "washer and nut" to force the flat sides of the BS170s to make good thermal contact.

73 Kees K5BCQS

Yeah exactly, Nick, if we knew 15dB down at second harmonic is adequate, I could have nicer passband like Ross's for 17-10m LPF. My attitude here is that I can over-build now and slack later. Meanwhile if I get on the air with it I won't feel bad about it.

Absolutely. Professionals wouldn't use a general purpose switching transistor to squeeze RF power. They probably spend a few bucks to go straight to LDMOS. (That's what I plan to do in my QMX, by the way, so that I can avoid "stupid discussion" altogether in the future.)

I certainly WOULD call Hans a professional. He has sold more than 20,000 sets with the PA you are rubbishing.
Most manufacturers can only dream about that number.

Ted
2E0THH

On Wed, Sep 20, 2023 at 03:49 PM, Donald S Brant Jr wrote:

It's not always heat which causes failure.

Absolutely. Heat generally decreases MTTF and make the device more susceptible to other threats but heat alone is only a gradual and slow-acting cause of failure. In case of general purpose switching MOSFET, heat makes the parasitic npn transistor more likely to conduct and cause so called secondary breakdown. BS170 is supposed to be not very susceptible to that but we are also abusing it with fast and deep drive and lots of drain current so we should all keep that in mind.

A short circuit is not necessarily the worst load.

Absolutely. Another question is short circuit where? Shorting the BNC connector is not the same as RF shorting the drains on the die.

It is always tempting to over-simplify and/or trivialize complex problems but rarely the best approach.

I might add that the same applies to the brain science.

On Wed, Sep 20, 2023 at 01:28 PM, Da Amazin' man G0FTD wrote:

Only BS170's ?

Nice try, I never said that.

Not 2N2222's, BC108's, MPF102's ?

Those are fragile, too.

Erm, ANY? semiconductor is more fragile than a lump of metal elements inserted in a vacuum tube because EVERY semiconductor relies upon a fragile atomic bond.

Semiconductors 101.

Be careful making a categorical statement like that. GaN RF transistors are very durable and many of them may be more durable than 6146, although there probably won't be a fair ground to make that comparison objectively. LDMOS is also pretty robust.

Pointing the finger only at BS170's plain daft, and only shows a lack of basic semiconductor knowledge and safe operating params, like EVERY other semiconductor on the planet.

Again, that's a straw man attack as I never made statements you criticize.

Just take a look at how Yaesu / Icom yada yada use PA parts.

They don't (ab)use general purpose switching transistors to squeeze RF power for sure. We are slightly abusing those cheap devices in a way.

They barely use them at 25-40% of their rated values to ensure that stupid users don't keep sending their rigs back for repair, have low power tune modes when you hit the auto ATU function and complicated foldback mechanisms.

That's only a part of the reason. All mode transceivers must have decent linearity to limit IMD and that requires device headroom at the expense of cost and power efficiency. RF engineering 101 that is, by the way.

These BS170 discussions really do make the amateur community look like a real bunch of amateurish amateurs, and reflects badly upon the technical training and competencies within our hobby.

Absolutely. Professionals wouldn't use a general purpose switching transistor to squeeze RF power. They probably spend a few bucks to go straight to LDMOS. (That's what I plan to do in my QMX, by the way, so that I can avoid "stupid discussion" altogether in the future.)

Time to upskill if we want to be taken seriously.

I'm probably more likely criticized being too serious, so thanks for a fresh concept.

I have been using the?Korad KD3005D for years now.

Hi Nick,

Yes, my traces look incredibly low tech. I still use those numerical design tables at the back of filter theory books and my HP calculator to design filters, so NanoVNA V2 Plus 4 is a bit new to me haha. Yes I know NanoVNA Saver, but I don't have a computer right at my bench, but maybe I should look into it. I also use TinySA Ultra (x2) and another more powerful signal generator and getting readable traces can be a bit of work...

I didn't capture the trace, but I also looked at Z1/S11 while output port terminated at 50. Those ripples correspond to where the input impedance goes a bit lower and slightly capacitive. So, it is entirely possible that the "ripple" can be partially offset by greater power pulled from the transistor, resulting in slightly lower eta but not much change in the output power at the BNC. We'll have to see that aspect of it in the actual board.

My filter is 5th order elliptic, one more zero in the transfer function than the topology in QMX's stock filters. It's barely visible in the photo but both inductors have parallel caps. A part of the lousy return loss and passband ripple is component values being slightly off, but this is actually not bad compared to a lot of wideband amplifier matching networks. The filter is also better than some used in other amateur transceiver kits. So, knowing the actual filter will probably be slightly better than this with values from E24 sequence that are closest to the theoretical calculations and also with higher quality Murata/Kyocera C0G/NP0 caps, I'm not too worried. I also implement those caps on the back of the board with SMD components so if I see a better design later, they are easy to change.

In terms of the design challenge, I think the hard part is that we have no good specification requirement as to the minimum attenuation of the second harmonic. If it's 15 dB, I could design a filter with higher cutoff and less passband ripple. It's a trade-off but I don't know what I'm trading off. So, the design shown here probably turns out wayy too conservative in terms of meeting FCC emission limits and too aggressive on the passband, but I'd rather err that way on my first version.

Incidentally, in relation to another thread of a mismatched load causing high voltage at the transistor's drains, I also looked at Z1/S11 while the output is open or shorted. Of course, the contour of Z(f) on the Smith chart looks more like a very long transmission line, basically marching right on the unit (outer) circle where the phase moves rapidly with the frequency. So, a new, easier way to find a point that maximally stresses the transistor is to short or open the antenna connector, and sweep the transmit frequency while observing the drain voltage. Why didn't I think of that while spending time on that thread.

Thanks for the good discussion, though.

These BS170 discussions really do make the amateur community look like a real bunch of amateurish
amateurs, and reflects badly upon the technical training and competencies within our hobby.