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LDMOS for dummies , could it be this easy ? HAPPY NEW YEAR PROJECT !!
- Thread starter WINKDOGG
- Start date
Unfortunately this only holds true for the output side of LDMOS devices and the only real benefit this offers is the device can hold up to a near infinite VSWR. This is the side of the amplifier that almost all operators get right by providing a decent VSWR from the antenna.
The input gate of a 1500 watt LDMOS is many times more sensitive than the base of an older 100 watt 2SC2879. One watt of extra drive on some LDMOS devices is equivalent to 100% overdrive and virtually guarantees instant failure. Ten watts of extra drive on a 2SC2879 (and we rarely have just one) will push it into saturation but it will almost always survive this if properly cooled.
When you have one large device that has both the dissipation combined with more than enough gain to produce the same power as 10 or more smaller devices, you are playing with fire if you don't have lighting fast protection monitoring the input drive conditions. If that is too much work and you're determined to experiment, buy a 6db attenuator before you wreck the transistor.
Don't rely on your radios carrier or power control to hold it at one watt. They rely on your radios ALC or AMC to hold the output and that reduced level and are often not fast enough to prevent noticeable excursions above that level on sudden audio peaks. You are in a much safer position to let the radio run where it operates reliably and comfortably while attenuating that drive back down to a safe level for the LDMOS device.
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Note the hundreds of small holes punched in this professional PA board. Each one is a conductive feed through bushing to the groundplane surface on the back of the board. This provides the lowest inductance paths to ground and offers the high gain amplifier the best chances of running completely stable.
Contrast that to the other board pictured that has none of these. What you will see is copper foil soldered around the edges of the board near critical areas. My guess is that the thing will oscillate without that foil and probably is not 100% stable even with it.
A resistor divider network and a couple of Zener Diodes can effectively clamp the input to safe levels.
Correct on all counts.
Anecdotal:
Off the record conversations with several of the NXP design and build/fabricate team revealed that at higher frequencies and full output power "waving a few pf (capacitance)" at the output transformer, (note the ground plane under the output transformer) resulted in additional stability of the amplifier. Sometimes
.Omitting that ground pad also caused some "wonky" behavior around 13.56MHz (ISM Freq) in "some" examples.
The point of conversation was , as much as these people know about RF there are still some unexplained phenomena. I was impressed by there willingness to say " We don't know..."
I've only had one conversation with a representative from NXP that resulted in a less than enlightening and usually humorous discussion. I believe he is the the exception and needs to work on his people skills.
NXP eventually sent me a non disclosure agreement to sign. I sent it back unsigned with " If my word isn't worth a damn my signature sure isn't " . They continue to send me information, samples, and bad jokes. It's been a sometimes fun at other times taxing path. You see I'm a hack, a good one but still a hack. Damn the math and theory , full speed ahead! Cut , fit , and try , it's the English way !
I'll continue to post what I can . Hope it helps .
Honesty is the best policy lol Even when it hurts a bit
is Baruch a builder or ebay board seller ? Thanks for the warning ! One thing I love about the forums is honesty without the shit talking or dissing ! I don't know of Baruch but I will steer clear just bc noone had anything to say about your post ! That tells me the great minds here approve with you or they would have jumped in with pics and facts to prove you wrong lol 73's Winkdogg 4-2-ZERO NJ
Nomad is one of the great minds out here and passing his approval or not, is usually good enough. Having electronic knowledge is one thing but simultaneously having the skills to explain the theory behind that knowledge in a way that the average person can understand, is another skill set altogether.Honesty is the best policy lol Even when it hurts a bit
73's Winkdogg 4-2-ZERO NJ
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Here is a little teaser.
This guy is selling kits.
73
Jeff
This guy is selling kits.
73
Jeff
Thanks Shock ! I had that feeling about Nomad lol
Again , I love these forums!!
73's Gents 4-2-ZERO NJ
Two things I noticed are there do not appear to be any RF bypass caps on the center taps of either broadband transformer and the two caps feeding the low impedance gates are high inductance ceramic types rather than surface mount chip. He should also know that transistor is only designed for 375 watts of AM carrier, not 800!
This guy, http://www.w6pql.com , seems to have some good designs including protection circuitry, some even into UHF territory.
73s
73s
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