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Author Topic: Stretchyman 40 W TX Reliability Modifications  (Read 9517 times)

Offline Pigmeat

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #30 on: October 06, 2022, 1422 UTC »
I see you topped the "Feudin' ,fussin', and a fightin' ", thread, Albert. Welcome to the board and I hope you get an answer before the action starts. 

Offline Charlie_Dont_Surf

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #31 on: October 08, 2022, 0634 UTC »
I got really annoyed trying to get true frequency accuracy and stability for Medium Wave from the three or four DDS modules I tried....  I gave up, and built a PLL that's locked to the 198kHz BBC Radio 4 transmitter.  The 198 kHz rig is run from the National Physical Laboratory's frequency standard source, so the Medium Wave boxen I built were rather more accurate than I could show on my frequency counter!

My question is: how do you prevent the (obviously very proximate) HF transmission from interfering/blocking the reception of 198 KHz?

Incidentally, I didn't mind adding proper gate driver ICs, rather than trying to cane a logic IC for the drive, and generated twice the frequency I wanted, then used a final bistable to guarantee an absolutely 1:1 M/S ratio.

I'm not sure what or whom this is aimed at, if anyone or anything.

The advice given on Steve Cloutier's "ClassEradio" website about component ratings is pretty useful, and has helped me avoid the fireworks displays that others have had! 

Steve is a well-known ham who operates primarily (or solely) AM in the New England area of the US. My problems with some of the things he does (currently) are these:

1) Posts on AMFone and his own website suggest that, even though he builds solid state stuff these days, he still relies on (what I call) "boat-anchor construction techniques", that are appropriate for vacuum tube/valve rigs but largely ridiculous in an environment where the FET gate driver better be within less than 2 cm of the transistor it is driving and minimizing parasitic L and C is key. http://www.classeradio.com/24_fet.htm  (Holy shit, dude. If you use some more modern transistors and construction techniques, maybe you wouldn't have to use 24 freaking FETs.)

2) Much of the things on his website use obsolete or ridiculously old ICs and transistors.

3) His main design is essentially a half-bridge Class-E with the upper half being the PWM modulator and lower half being the RF stage. Unfortunately, when you do this sort of thing, you create a scenario where you have great "pull-up" capability owing to the upper half of the half-bridge but there is weak "push-down" capability because the upper half can't push down well, and so I think it is very difficult to not create a distorted (mis-shapen) modulation waveform, as a result of the mismatched capability of the two halves.

Suffice to say, I'm not a huge fan of his work.

Now.... back to designing a little (80W PEP) PWM rig for as little cost as possible.....

Curious to see what you have so far.
« Last Edit: December 17, 2022, 0541 UTC by Charlie_Dont_Surf »
I don't STRETCH the truth.

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Every minute Charlie squats in the bush, his signal gets stronger."

Offline Stretchyman

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #32 on: October 08, 2022, 0840 UTC »
I'd actually agree with Mr Surf here. Not using a PCB these days is just plain dumb, so cheap to get made and not exactly difficult to design. Those crappy old FETs they harp on about when these SiC and GaN around ffs.

Most folks on there are afraid of change however one ham sent me a design he'd being working on and the O/P devices are from GaN Systems. Very easy to drive and a 100W design would fit in the palm of your hand.

Yes Albert please post up any details you would like to share.

I bringing out a 60W all in one design soon.
'It's better to give than receive' so why Rx when you can Tx!

                                              ;)

Offline Stretchyman

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'It's better to give than receive' so why Rx when you can Tx!

                                              ;)

Offline Charlie_Dont_Surf

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #34 on: October 15, 2022, 2300 UTC »
I have been using commercially-made encapsulated power inductors lately and so far they have worked well. They generally take up less board space than an equivalent homemade toroidal inductor and each one I buy is one less inductor to wind.

https://www.pulseelectronics.com/molded-inductors/

They should not be used for an RF tuning network because the losses are far too high, but for PWM filters and RF chokes they seem OK. In any RF choke application, high loss at RF is desirable characteristic anyway. In a PWM, as long as the PWM isn't operating at high frequency, losses aren't an issue. (Some manufacturers don't publish the SRF for the power inductors because it's not a consideration for most of their market.)


I don't STRETCH the truth.

"Every minute I spend in this room, my signal gets weaker.
Every minute Charlie squats in the bush, his signal gets stronger."

Offline Albert H

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #35 on: October 18, 2022, 1831 UTC »
CharlieDS : Thanks for the moulded inductor suggestion..  It's really speeded up construction, and goes some way to saving my arthritic fingers!  You're right about them being nicely lossy, and they've made my PWM modulator filter both smaller and better.  My PWM circuit goes at 75kHz (it was a convenient frequency for several reasons), and the filter is now trivially simple.

I don't entirely agree about the ClassE site info - granted he's still using 25-year-old FETs, and his resistance to PCBs is rather silly, but some of the info about voltage rating specifications, and the provided equations are useful.

I'm wedded to the idea of using proper gate driver ICs for the FET drive, and have experimented with opto-isolation of the drive, which has proved to be useful in keeping the sections of the transmitter well isolated.  I am still using a very simple CMOS PLL for frequency generation, despite the availability of the pre-programmed oscillator modules.  Frequency agility (over a few hundred kilohertz) is important to the people who want to use these rigs.  I also want to keep them as "low cost" as is feasible - I don't want the loss of a rig to the "Authorities" to be financially significant.

I've also been looking at "low level" modulation - gate mod in the case of the FET experiments.  Obviously the audio quality suffers somewhat, so I've been playing with methods of "Envelope Feedbck" (like I did on my screen-grid modulated medium wave rigs back in the 70s!). The results are encouraging.

Another approach I've been experimenting with is the Cheirix "Outphasing" approach, using very high efficiency PAs, and a semi-digital approach to generation of the phase-modulated carriers.  Getting the PLL VCOs to swing linearly enough in frequency/phase has proved to be tricky, and I don't like the way that the antenna loading can affect the modulation, but it looks like a way of gererating a hell of a lot of modulated power at relatively little cost.  Having experimented extensively with "Ampliphase" rigs back in the 70s, this looks like an approach that could be made to work....

It's great to be retired from the Rodent Race these days - plenty of time to get to the bench and experiment!

Offline Azimuth Coordinator

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Re: Stretchyman 40 W TX Reliability Modifications
« Reply #36 on: October 31, 2022, 2035 UTC »
Ampliphase you Say....   Good Answer... I'll be watching you..
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