PDM Generator help

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PDM Generator help

Postby K9ACT » Tue Jul 20, 2010 6:52 pm

I just finished assembling my PDM Generator and need some help understanding what I have. I am not sure I even know about this to know how to tell if it's working.

With a 1kc nominal 1v sig in, I get a square wave out in one of the two positions of the output switch. Changing the amplitude of the input changes the duration of the output. So PDM it is but.....

The output is only about 50mv p-p and I assume it should be either 5v or 12v depending on the position of the switch.

In the other position of the switch, all I get is a much bigger triangle wave of constant amplitude.

So, I assume something is not right here.

Another question I have is the "feed forward" and 47k to the power supply. What does this mean and what power supply.

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Re: PDM Generator help

Postby kf1z » Tue Jul 20, 2010 8:56 pm

Jack,

Maybe something there...
What revision board are you using?

Make sure the 2n3904 (?) transistors are in the right way around...
Also can't remember what those outputs act like if they are not terminated...

If you want, send me a good high res picture of the board, I'll see if anything jumps out...

The feed-forward :
You connect a 47k resistor to your HV DC (125 to 130vdc) supply, to the point on the board..
This allows you to "null out" the PWM power supply ripple... if there is any.


BTW: I used your wooden telescope mirror cell design from your website for my "sauna-tube" 8inch F6 scope..
first ever build... didn't grind the mirrors or any of that... but it is fun to play with!


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Re: PDM Generator help

Postby K9ACT » Wed Jul 21, 2010 10:40 am

Board is Rev D, Output is terminated in 50 ohms

Pic of board is at http://schmidling.com/pdmboard.jpg

Pic of 5v output is at http://schmidling.com/pdm5v.jpg

Scope is set at .05v per div and .2 ms

I could not take a pic of the 12v output for two reasons:

1. There was a sync problem with the camera and I could only get a little tail at one end.

2. I started to smell something and quit. No smell in 5v position.

What I see in the 12v position is a much larger and slower triangle wave.

Roger on the mirror cell. That was another (of many) lives.

Jack
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Re: PDM Generator help

Postby kf1z » Wed Jul 21, 2010 12:30 pm

I guess I'd try clipping the scope onto pin 4 of the generator IC, see what's there... in both posistions..

Not sure what could be going on with the buffer circuit... assuming the correct parts are installed properly...

Chances are, you'll use the 5 volt setting anyway...

Maybe the smell was the resistor (50ohm...) ?

Does this do the same thing with no audio generator input (I assume so)


I was just curious... I see three wires to the line-input... are you using balanced audio?
(nothing to do with this, just curious..)
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Re: PDM Generator help

Postby kf1z » Wed Jul 21, 2010 1:35 pm

Jack,
I just realized something.....

Looking at your board, I bet you haven't done this........because I see no solder, or signs of a temporary connection.

You need to put +5 vdc at the "overload low" connection in order to turn the pwm on.


I'm not sure what reading you'd get in the case you haven't....

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Re: PDM Generator help

Postby steve_qix » Wed Jul 21, 2010 5:16 pm

The overload line should "float" and allow the generator to produce output in the absence of an overload low signal... More later !

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Re: PDM Generator help

Postby steve_qix » Wed Jul 21, 2010 6:30 pm

It appears as if the transistors are in backwards. Looking at the transistor, flat side facing you, the emitter is to the far left. The tab on the silk screen layer denotes the emitter. So, it does indeed appear from the picture that the transistors are backwards. Check this out before removing them. Hopefully, no damage to the transistors took place :?

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Re: PDM Generator help

Postby steve_qix » Wed Jul 21, 2010 6:43 pm

The feed forward takes a sample of the power supply [ripple] and feeds it "forward" into the PWM generator out of phase with the actual power supply ripple. So, in essence, you put power supply ripple onto the pulse train, cancelling some of the power supply ripple which would otherwise appear in the resultant output (after the filter).

It will definitely reduce power supply ripple in the output (if it is there), but generally this is not an issue at all. I don't actually use the feed forward in my own transmitters, but I have used it in the past when I had a poorly filtered HV supply.

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Re: PDM Generator help

Postby K9ACT » Wed Jul 21, 2010 6:51 pm

steve_qix wrote:It appears as if the transistors are in backwards. Looking at the transistor, flat side facing you, the emitter is to the far left. The tab on the silk screen layer denotes the emitter. So, it does indeed appear from the picture that the transistors are backwards. Check this out before removing them. Hopefully, no damage to the transistors took place :?

Regards,

Steve


Uggh! I assumed the tab was left over from a TO5 can and pushed the center lead back and lined them up with the hole pattern.

I already removed the NPN to check it out, leaned nothing and came in for dinner.

I will download the data sheets and make sure I get them right but I don't think I can salvage the one I pulled out.

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Re: PDM Generator help

Postby K9ACT » Thu Jul 22, 2010 12:00 am

I got them out and turned around but the PNP gets very hot so I must have fried it.

Will check with Radio Shack in the morning to see if the local has one.

Guess I got lucky with the other pair.

As a suggestion, a "D" profile on the board would have saved a lot of grief.

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Re: PDM Generator help

Postby K9ACT » Thu Jul 22, 2010 12:05 am

kf1z wrote:
I was just curious... I see three wires to the line-input... are you using balanced audio?
(nothing to do with this, just curious..)


I am not sure what to call a hand held, battery operated audio generator so I tried it both ways.

The regular audio chain is balanced.

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Re: PDM Generator help

Postby kf1z » Thu Jul 22, 2010 8:55 am

steve_qix wrote:The overload line should "float" and allow the generator to produce output in the absence of an overload low signal... More later !

Regards,

Steve


Hmmm...

I thought the ol board sets that pin to +5 volts during transmit?

The schematic says 0 volts for recieve...

Pretty sure Brent had to jump +5v to his to make it work while he was testing...

Oh well... I don't have one that is operational, so I couldn't test the theory.

Plus, well, you designed it, so I guess you'd know.
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Re: PDM Generator help

Postby K9ACT » Thu Jul 22, 2010 11:31 am

kf1z wrote:
steve_qix wrote:The overload line should "float" and allow the generator to produce output in the absence of an overload low signal... More later !

Regards,

Steve


Hmmm...

I thought the ol board sets that pin to +5 volts during transmit?

The schematic says 0 volts for recieve...

Pretty sure Brent had to jump +5v to his to make it work while he was testing...

Oh well... I don't have one that is operational, so I couldn't test the theory.

Plus, well, you designed it, so I guess you'd know.


The measured value is about 3v so I presume this is enough to hold it on.

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Re: PDM Generator help

Postby kf1z » Thu Jul 22, 2010 1:48 pm

The measured value is 3v ?

Huh...

You're measuring the base of a transistor with NO bias.

Seems strange...

But, as I said... I don't have one, so I can't see if I get the same readings as you...
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Re: PDM Generator help

Postby steve_qix » Thu Jul 22, 2010 2:51 pm

Well, the circuit puts a positive voltage on the emitter of PNP transistor Q301. The base of Q301 (overload low) is floating, and has a bypass cap to reduce the possibity of RF getting into the circuit. So, with the base Q301 floating (0 base current), the collector of Q301 will be pulled low (to -12V) by R309, shutting Q300 off. When the base of Q301 (overload low) is pulled low, current flows through the base-emitter junction, turning the transistor on. This pulls current through the base-emitter junction of Q300, turning the transistor on, pulling the audio line looking into the anti-aliasing filter to -12V, which appears at the output of the DC coupled filter, cutting off the pulse train.

Anyway, that's how it works. So, unless the transistor Q301 is bad and has high base-collector leakage (which is unlikely), the circuit will operator and produce PWM at the output if the overload line is left floating 8)

By the way, Jack - Q300 and Q301 appear to have been installed correctly.

Regards,

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