Help on input circuit - 6 fet 80 mtr

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Help on input circuit - 6 fet 80 mtr

Postby Brent » Tue Apr 01, 2003 8:35 am

Well...at this post, I am replacing another fet on the rf deck. I was sabotaged by the auto tuner in the ft-990....The input circuit I built has been changing impedance? I ran the unit at full power for awhile..I looked down and the swr had climbed on the ft-990? I have not been using the auto-tuner. I went into standby and loaded the input with the tuner....ran fine for about 10 minutes...after keying up in went into seek and crapped out a fet by dropping out the drive while searching....Arrgh!

I have not used the extra fet driver, but may reconsider. I may have to use an attenuater, as the ft-990 only drops to 10 watts. I know Bob K1KBW is using an input circuit at the 30 watt input level with no problems.

Well anyway....perhaps a much lower Q input circuit.....I give up

Brent W1IA
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990 auto tuner class e problem

Postby ke1gf » Tue Apr 01, 2003 4:59 pm

Brent is there anyway that you can disable the auto tuner?

I use the driver fet and run about 5-10 watts at about 1.8:1 swr and everything is fine for the past few weeks... But remeber that I battled with paracitics with my driver and had to move it to another spot on the heat-sink to solve the problem...

I would tweak your input without the auto tuner (without any voltage on the drains of the fets), put the scope on the gates of the rf-deck fets, set the output of the rig such that you get 20-24v p-t-p on the gates of the fets (put the scope probe right on the gate bus) this should be the ideal tune-up for your rig...

6 fets should take about 30-35 watts of gate drive, does your radio go up that high? You may have to adjust the internal am carrier pot to make it go past 25 watts.

Also Steve QIX mentions in his paper that as you modulate your gate voltage may drop if there is poor isolation of grounds from your input and your output... If you could post a picture of your rf-deck up close where your input tuning circuit connects to the gate bus so that I can see how it's connected.
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Brent's Input Circuit

Postby steve_qix » Tue Apr 01, 2003 10:36 pm

Hi!

I see someone reads the documentation :-( (thanks Bill :-)

Here is more or less what we did for Brent's input circuit:

The overall gate drive circuit consists of the two T3 transformers, with primaries connected together, fed directly from a tapped tuned circuit.

The tuned circuit is a parallel resonant circuit which consists of a single coil, 12 turns of #12 wire, 1.5 inches in diameter, about 4 inches long. The input [coax coming from the driver] is tapped at approximately 7 turns from the bottom [ground] end of the coil, and the output [to the T3 primaries] is tapped at approximately 5 turns from the bottom [ground] end of the coil.

The capacitor value is approximately 900 PF. I recommend using a multi-section broadcast variable for the capacitor. The capacitor is connected across the whole coil, end to end.

The capacitor is tuned for a proper match [1:1 SWR] for the external RF driver [probably a multimode transmitter operating in CW mode].

YOUR RESULSTS MAY VARY! You will most likely have to "play" around with the taps on the coil. You may need to move one or both taps up or down. You simply pick the best combination of taps, combined with the proper adjustment of the capacitor, which gives the best match for the driver consistant with a good amount of drive level.

DO NOT connect any driver to your class E transmitter without having an oscilloscope connected from the class E gates to the source bus. Start out with your driver operating at the lowest possible power level, and gradually bring it up. Be sure to adjust the input tuning capacitor and/or taps on the coil for the best possible match.

Anyway, that's what we did!

Regards,

Steve WA1QIX
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http://www.classeradio.com
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Re: Help on input circuit - 6 fet 80 mtr

Postby Glenn K2KL » Thu Apr 03, 2003 11:14 am

Hi Brent;

I plan on using the driver FET in my RF decks even though I can easily provide the 30w drive to go direct. I figured this would give me sort of an extra stage of protection before the 6 output FET's

By the way, I heard you on the air with your new rig and it sounds fantastic!!, Nice Job!!

It will be a while before I get to building my RF decks, I'm busy working on the Power Supply/modulator.. I'm forcing myself to go slow and get it right rather than rushing to finish it up. :lol:

Brent wrote:I have not used the extra fet driver, but may reconsider. I may have to use an attenuater, as the ft-990 only drops to 10 watts. I know Bob K1KBW is using an input circuit at the 30 watt input level with no problems.

Well anyway....perhaps a much lower Q input circuit.....I give up

Brent W1IA
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Very Stable Input

Postby Brent » Thu Apr 03, 2003 6:59 pm

Thanks Glenn for the compliment....I recommend not using the driver fet for ease of construction....Steve agrees that it is simpler and requires less parts.....although the Q1 driver works fine and saves wear on the solid state rig....but I have had excellent results using the ft-990 at 30 watts of drive in AM mode.

The input circuit was an experiment that proved very stable and very easy to build....I cheated with a small 100pf air-variable and made up the rest of the 800pf with silver micas.....it is very low Q and covers the whole band.

Also the boards that Kevin (KD1J) made work FB.....I can recommend them to anyone.

It took 2 months from start to finish to build it and was a very exciting first project for me...if I can help anyone with my experiences...please ask. There are at least a dozen of these rigs now on the air in New England area alone.

73 Brent W1IA

p.s. the driver fet will not save you from problems....it will amplify parasitics if they get into that stage.....ground loops, ground loops, ground loops...I can't stress it enough...use the scope on the gates of the 6 fets....you will be saved lots of time NOT having to replace fets
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Postby Glenn K2KL » Sat Apr 05, 2003 10:36 am

Hi Brent;

I was thinking that having the driver FET would give you some protection with regards to input matching issues. If you were producing excessive gate voltage because of incorrect input transformer condition/match , you'd only blow the single driver FET instead of taking out several of the 6 finals. You could test with only power applied to the driver while you experimented with input matching with a scope on the gate... :?:

Yea, parasitics is another problem. I'd like to know where folks are positioning their driver FET's on the heat sink in relation to the main 6 to avoid problems... :roll:
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One only

Postby Brent » Sat Apr 05, 2003 2:32 pm

Glenn K2KL wrote:Hi Brent;

I was thinking that having the driver FET would give you some protection with regards to input matching issues. If you were producing excessive gate voltage because of incorrect input transformer condition/match , you'd only blow the single driver FET instead of taking out several of the 6 finals. You could test with only power applied to the driver while you experimented with input matching with a scope on the gate... :?:

Yea, parasitics is another problem. I'd like to know where folks are positioning their driver FET's on the heat sink in relation to the main 6 to avoid problems... :roll:


It seems to be only one that ever gets shorted...and not from the drive level, so much as parasitics. As far as the fet layout....close as possible....one sil-pad directly next to the other seems to be fine....the ground looping on my deck was related to gate source grounding in relation to the single turn on the input transformers....equal distance apart from each end of the gate bus. If I were to do it again I break it into thirds for spacing of the ferrites closer to center than on the ends from the first layout. I think the fet driver would be great, but one mistake on input level...wether you have a drive fet or not will kill the three legged monsters.

I can't speak for driver fet positioning from my rig, but I don't believe it would be that critical.....only the 6 are critical.

73 Brent /IA
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Input networks

Postby k3xf » Mon Apr 07, 2003 6:47 am

I drive the 6 FETs directly have used two input networks successfully;
1. Input to a 5:2 transformer and then to the two transformers driving the FETs. . . same ferrite beads as the input transformers. 25W provides plenty of drive to the FETs and the autotuner on my TS-2000 tunes it very nicely. No problems.
2. I wound a toroid and resonated it with a 2000pF mica cap around 3.88. Then tapped the assy about 8 turns from the ground end to receive drive. I then took two turns through the toroid to drive the FETs. . . that took 35W to provide the right voltage and required no tuning at all.
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