Message-ID: <1085595147.40b4de0b1a742@webmail.telus.net>
Date: Wed, 26 May 2004 11:12:27 -0700
From: "Scott Tilley, VE7TIL" <sthed475@telus.net>
To: rsgb_lf_group@blacksheep.org
References: <53.cd0d8a2.2de5e9f7@aol.com>
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Subject: Re: LF: G0MRF-TX-ouput circuit troubles
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David

I have been examiming your drive cct from the FET driver to the gate of the 
FETs.  In this cct you have a cap., a diode with the anode to common this node 
connects to a 10ohm resistor in series with the gate that has the gate side 
tacked to common via a 4.7k resistor.  Am I correct in assuming that the 
purpose of the cap is to prevent DC voltages from getting to the gates of the 
FETs in the case of an exciter failure?  Is the 4.7K just there to keep undue 
charge off the gate capacitance? Also, what is the design intent of the 'fast' 
diode you have installed.  I am also curious as to what the waveform will look 
like out of this driver circuit.  Is it a 12V p-p from common to 12V or is it 
something else.

Thanks for any insight you can offer.

Regards,

Scott, VE7TIL
 

Quoting G0MRF@aol.com:

> In a message dated 25/05/2004 22:01:42 Central Europe Standard Time,  
> knaap159@zonnet.nl writes:
> 
> 
> Maybe somebody has something to say about  this......
> 
> 
> I think you're spending too much time measuring things at frequencies that 
> 
> are not LF. It's a great analysis of the detail, but I can't help thinking
> you  
> are overlooking an obvious fault. 
> BTW.  What is the output power? The supply Voltage? and the  efficiency?
> Just concentrate on the basics. The circuit works, but if you start  removing
> 
> diodes adding resistors etc, then there's not much anyone can do to  help as
> 
> you are now developing your own circuit and not trying to duplicate  mine.
> Build the circuit as close to the original as you can.
> Check you have the correct waveforms at the gates. (keep wire lengths  short.
> 
> This maybe 136kHz, but the waveform rise and fall time neeed to be fast.  
> Around the HEF4013, FET driver and FETs, the construction should be
> consistent  
> with good VHF practice ) 
> Measure the 136kHz output power, the supply voltage and calculate 
> efficiency.
> Then you'll have a benchmark of performance. If you have less than 200W  
> output from a 36V supply or an efficiency less than 60% then you have a fault
> in  
> a component or in the construction that you need to find.
> After you have a reasonable benchmark, change one thing at a time and  
> compare. If you have an improvement, leave it in circuit. If you don't, then
> put  it 
> back as before and try something else. Jim's work on the transformer is  
> excellent and I'm now comparing my original transformer with a version of
> Jim's  
> built on a 3C90 42mm core as supplied in the kit.
> I've built versions of this circuit from 400W at 73k to 700W (4 FETs) at  
> 136kHz. There's even a version running in the Medium waveband around 
> 1200kHz.
> Good luck
> David   G0MRF
>