Hi, Dick.
30mm will be OK for the zobel network. Just keep the wiring rigid and
direct, and you shouldn't have any problems.
If you want to transmit for long periods, be generous with the cooling, and
not just for FETs. The ferrite components will get hot too. Keep an "open"
layout, and allow room to add extra cooling fans to blow air onto the output
transformer, the PA feed choke (if you use the "current fed configuration),
and the toroids in the output low-pass filter(LPF).
You can use air-cored inductors for the output LPF, but don't underestimate
the gauge of the wire you need use. I found that even air-cored coils get
hot at moderate power levels (>200W)due to copper losses, as the circulating
currents in the output LPF can be surprisingly large.
The most unreliable part of my rig is the power supply. I'm on my third set
of rectifier diodes ;-)
For the divider, you can divide by nine and then two. The divider I use has
a 74LS161N LSTTL programmable divider followed by a SN74LS74 divide-by two.
You'll also need a 74LS04 inverter, plus a handfull of other bits and
pieces. Cost about 10 euro maximum. It will also generate DFCW by switching
a trimmer capacitor across the crystal.
If you (or anybody else)want the schematic, let me know and I'll mail it
direct. (700k Word document)
Good luck with the construction!
73, Dave G3WCB IO91RM
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Dick
Sent: 09 April 2006 12:25
To: [email protected]
Subject: LF: Tech questions G3YXM amp+18 divider
Hello all,
I'm building a new (and larger) 136kHz transmitter.
The output circuit is the same as the one used by G3YXM with 4 times
IRFP450.
Each FET will get his own gate input resistor (6R8) and gate-to-ground
resistor (47k)
The sources will be grounded as short as possible to ground, while the
drains of
2 FET's will be connected together.
I am now in the stage of putting the components of the output circuit on its
correct position,
and that raised some questions about where to mount what.
I am using a large heatsink 12x15x5cm.
On this heatsink I mounted a 1.5mm thick copperplate 75x150mm in order to
transport heat quick away from the FET's.
Maybe it's some 'overkill' , but I just want to set up a thing that can run
for hours
on full power; just a bit of reserve...........
The 4 FET's are mounted in a row, separation between the FET's is 1 FET
width (abt 15mm).
The resistor of the 'zobel-network' is mounted just in the center , with
wires about 30mm long connected
to the drains of the FET's.
I do not expect problems, but maybe that 30mm is too long???
I don't think I can make them shorter without changing the whole component
layout.
If anyone has experiences with this amplifierstage and the zobel-network
wire length,
I would glad to hear.
Another thing coming into my mind is to set up a fixed frequency into the
transmitter, beside the VXO.
I do have an 4.9152MHz X-tal, but I have to divide it by 18 to get the
necessary 273.066kHz.
Any simple schematic (with easy to get components) for an divide by 18
circuit?
73
Dick, PA4VHF
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