D limit Diode and C limit capacitor

[Gilly]

Hi everyone...

Have replaced my 80M station with another....
The circuit is much the same as on Steves WEB site....
VFO / PWM kit and TX using 4 Fets ( but without the overload circuit SORRY )....

It all works fine ... except there is what would be best described as a crackle that seems to be related to the audio peaks......or Modulation level.....

I am not using the D limit c limit components.....

I use a carrier voltage of 30Volts and 126Volts under load Head Room.....the carrier alone is quiet....

Up to appox 50% Modulation level all seems OK ... but over that particularly 100% the pops / crackle are too noticeable....

I have built 4 PWM and never had this problem at all.....they all worked from word go really ...

I wonder if anyone has had this problem before ?


Wayne
VK3ALK

[Gilly]

I installed the Diode and capacitor....

Conditions now have changed for the better...
At our legal limit of 120Watts Carrier ... modulating at 100% and over NO PROBLEMS....

But if the power is reduced to 50Watts the crackle appears again.....modulating at 100% plus a little bit....

Sometimes I run at lower power due to TVI etc: so will need to look at little closer I suppose .....


Wayne

When you reduce power, are you only reducing the current, or are you reducing the input voltage to the power supply?

[kf1z]

Wayne,

To lower your power with PWM , you MUST lower the voltage AND the current.
You want to keep the impedance at the design of the PWM filter.

If you lower the carrier voltage, you will need to re-tune the transmitter slightly as well, to keep the current/voltage ratio the same.

Bruce
kf1z

[Gilly]

Hi Bruce

Yes OK on your comments.
When I do reduce power I'm a bit slack I suppose and just change the load Capacitor.
I don't do it all the time and cannot be bothered retuning etc:

But when I do this on my other TX's there is no sign of crackles and pops etc:

Just this new system......

However I did not say this on my other 2 posts but when I run it in the low Voltage setting ( 60Volts ) and tune it correctly for the Filter the noise is not there.....only in the High Voltage 125Volts.....

At the end of the day it does not matter I suppose but something is not quite right going on....

But the D Diode and C Cap have made a difference though ! !


Wayne

[steve_qix]

Usually, there are two possible causes:

1) There is a parasitic in the RF amplifier. Parasitics usually show up at high voltages and light loading. An oscilloscope hooked to the drain busses will show this.

2) There is coupling between the PWM output and the PWM generator or input to the PWM output. Usually caused by poor or missing grounding. Put an oscilloscope on the PWM output to see this. If the pulse chain changes significantly when the crackling takes place, you probably have coupling output to input. Also check the input to the PWM generator and look for the same thing.

Not using an overload system is not a good idea, as there is nothing to save the modulator or RF amplifier components in the event of an overload Lots of people start out without an overload system, and after replacing modulator MOSFETs and other components a couple of times, will then add some type of protection.

Let us know what you find!

Regards,

Steve

[Gilly]

Thanks for all the advise.....

I'm going to improve the 12Volt supply to the drivers of the TX....

Be back soon......



Wayne