Modulating a 60 watt Mosfet Transmitter

[N4LTA]

I have a single ended mosfet class C transmitter that is about 60 watts input and outputs about 43 watts.

The modulating impedance would be about 10.5 ohms with a  drain voltage of 25 volts.


I would like to modulate it with a solid state amp. Any easy way to do this?

I assume I could use a tube type amp of 30-40 watts and use the output transformer as the modulation transformer but I want to use this as a portable.


Pat
N4LTA

[Steve - WB3HUZ]

Heising modulation?

[N4LTA]

I was trying to find the equations for heising modulation earlier.

I assume that the choke would need to be in the range of 10.5 ohms at the low frequency.  That is just an assumption and I have not been able to find the heising formulas.

If the above is close, then a mh sized inductor would be required. That would allow a small value high current inductor. These are available from Hammond

[Steve - WB3HUZ]

Good assumptions. Pick the lowest freq audio you wish to pass and do some math to back out the inductance value. I'm pretty sure you should be able to just place the inductor in the Vcc line and couple a solid state amp to the high side of the inductor via an appropriately sized blocking cap. You may want to think about if there would be enough inductive kick back when shutting off the  Vcc to damage the audio amp.

[N4LTA]

I think I am going to give it a try. I have a couple of Hammond inductors that were sent to me incorrectly a few years ago. They are rated 50mh at 2 amps. The inductive kick back can probably be handled with a solid state type surge suppressor (transorb) sized to quench anything larger than the amps max output  voltage x 1.5 or so. I am glad you mentioned it - I had not thought about it.

[Rob K2CU]

Suppose you had a solid state audio amp with 30 W output, totem pole output stage, and for simplicity, +/- 25 V power supplies. You would need that much just to drive the RF amps supply (25V) for 100% modulation. You could isolate the audio amp and tie it's neutral (GND) to the + 25V DC supply of the RF amp. Then you could drop the need for a non polarized coupling cap. At zero audio, the audio output would sit at 25V (virtual ground). A PWM type supply would be much better, but this would do if you didn't mind the extra heat. You would still need the Heising inductor.

[N4LTA]

That is actually what one of my my driver amps look like - It has a +- 25 volt supply and is rated at about 40 watts max.

It is one on the National type Chip Amps.

Looks like rainy weather this weekend. Maybe I can destroy a few FETs.

I also have built a pwm driver board and tested it but have not built the mosfet switching part or the filter yet.

Anyone ever done a lower power pwm supply ?

Pat
N4LTA

[W3RSW]

The 35 watt RS model MPA-45 transistor PA amp. that I have actually has an output transformer to get the 4, 8, 16 ohm and 70 volt line 'impedances.'  Shouldn't take to much to lift the ground off one side of the output and see if that would work. Use the common and 8 or 16 ohm tap and see what happens. 

I don't think any feedback circuit is attached to the output of the transformer, just 'in' and 'out.' 

When I modified mine for a 811's modulator (feeding a backwards tube output xfor to 811 grids), I grounded the PA mike inputs directly to the chassis, put in an RF bypas cap in the mike inputs and grounded the output xfor. common TO ground.  The common was already floating when new. 

[WA1GFZ]

You need to be able to drive the drain voltage greater than 50 volts and down to zero. Might be easier to get a higher voltage supply and build a series modulator. You will need a heat sink that will dissipate at least 60 watts.
It will be very clean.

[steve_qix]

Series modulation is the best... however, if you don't want to build a series modulator, there may be other answers.

First of all, if you are only modulating 25 volts, in theory, an audio amplifier with a plus-minus 35 or 40 volt supply will modulate the transmitter using a modified Heising approach.

You simply use a choke, connecting the B+ power supply to the RF amplifier modulated DC, and then run a capacitor over to the audio amplifier output (the "ground" of the output goes to the negative side of the RF amplifier).  This should give you an instant modulator !

Values: Looks like 100mH will do it for the choke.  It has to be a good choke to take the current.  You might be able to wind such a thing using a hi flux core.  Capacitor: Looks like 450 to 1000uF will work.

Someone should verify my figures to make sure I didn't make a math error, but I think they are OK.

Regards,

Steve