PWM Rig questions.

[Ed/KB1HYS]

Considering building a PWM rig.  Working out the Audio stuff. 

Plan on keeping it simple as possible.
Using An LM 386 as a mic-preamp, a couple of 741Opamps configured as a audio band pass filter.  Useing an XR2206 as a Function generator,to generate a 75kHz Triangle wave, into another 741 to add the triangle function to the audio and then into a comparator (yet another 741) to generate the pulses,...  BUT I was thinking the 2206 has an AM input so you can modulate the output.  Seems to me that to feed the processed audio into that AM input and run the modulated output into the voltage comparator to generate pulses would be easier than building the summing circuit-- or would that produce distortion??

for the audio bandpass I was figuring on cutting off at 3kHz for the High, but whats a good cut off for the low freq audio? 200hz? 100?

[WA1GFZ]

Ed First get rid of the 741 bipolar op amps will drive you crazy with RFI.
TL072 is a great cheap amp. You do not sum the triangle with audio.
you put audio with a DC offset (variable to set the carrier level) into one side of a comparator then the other input gets the triangle. The output is the variable pulse width. Look at qix site for ideas. There are a number of pwm chips that eliminate a lot of hardware. Steve has schematics of a good one.

[KE1GF]

Hey ed. Frank is correct also, the LM741's maximum slew rate isn't so good. Instead of using a function generator you can build a simple miller integrator and schmitt trigger to generate your triangle using op-amps . Using the two in conjunction eliminates the dc offset drift problem that you get using a pure integrator. Seeing that the integrator feeds the schmitt trigger and then the schmitt trigger feeds back into the integrator. You could build your whole PWM generator with two TL074s (mic pre-amp, anti-alias "salen and key" filter, triangle generator and comparitor) . If you need a schematic and formulas I can pull out my notes and send them to you.

-Bill 'GF

[Ed/KB1HYS]

Hey Bill. that sounds good, if you can send me the schematics and such that would be great. Sounds good, two tl074's sounds like a level of complexity that I can handle!!!

[KE1GF]

Hey Bill. that sounds good, if you can send me the schematics and such that would be great. Sounds good, two tl074's sounds like a level of complexity that I can handle!!!

Okay Ed, I'll grab my notebook and make a nice schematic and writeup etc... Tomorrow!

[KE1GF]

Ed, BTW, QTF are you going to put this in?

[Ed/KB1HYS]

Well, This is sort of evolving as I go.
I have been wanting to build a decent sounding medium power AM rig for 75meters.
My main goal is to get some fairly good audio, have a simple rig and not break the bank doing it.  PWM gets good reviews for audio quality and efficiency So I decided to go with that.
 I'm learning as I go so I figured I'd take in in small steps. Build a good audio chain first and get that working nicely. Then work out the RF end of the house (I've got this NIB Eimac 4-250 that really needs a rig to call home..)
 I did some research and I had the op amps and other stuff in the box already, so I was cobbling together what I had.

[KE1GF]

Well I can help you as far as the PWM generator, i don't have any experience switching a tube.

[W2VW]

Keep posting this stuff. It makes a great tutorial.

[K1JJ]

Ed,

Did you read Steve's paper on TUBE PDM rigs?

http://www.classeradio.com/pdm_article.html

Read it a few times and it will make sense.

OK on the 4-250 final. Contact Larry/NE1S, who has built and uses now, a 4-400 PDM rig. Just what you wanna do.

Frank/GFZ and I built up a 4X1 PDM rig about 12 years ago. I tore it down and built something else since. But it was a FB transmitter except for layout issues I had.

But, I'm in the process of designing up a new 4X1 X 4X1 PDM rig now. I will use the same PDM low level generator as the class E guys use and drive an FET in the cathode of the 4X1 PDM switching tube. I already have the PDM generator built from a previous class E rig here.  I hope to have mine done before the winter is through, probably way berfore if I get on it.

It's a great learning experience and I highly recommend you give it a shot, OM.

73,
Tom, K1JJ

[K1JJ]

BTW, Ed - you axed about audio frequency response...

Of course, the PDM rig will give you DC to 12kc or whatever you desire as a cut -off, and FLAT, if set up correctly. Design you PDM switching filter to cut off at 10kc-12kc or so.

Then tailor your audio in the low level audio stages. I think that's what you intend to do.  Most voices drop off below 70hz, so 60hz is a good place to roll off sharply, unless you want rumbles and room effects. Some people like that.  [The 3kc roll off you mentioned will be too sharp]

As for high end, 5kc is OK, some run 6kc. But you may want to have an EQ with TWO curves. Set one until you sound good [hi-fi] in your own off air monitor. It will probably have an 'S' smiley face curve. Roll it off starting at 5kc.  Then, for AM DX work, use a sharper curve from maybe 120- 4.5kc. Or whatever works best for you. Get tape recordings on the air from guys until you like what you hear.

ie, the PDM will do DC to light cleanly and you set up the EQ or other audio filtering low level to do whatever you desire on the fly.

T

[KE1GF]

Yes, speaking of QIX's PWM documentation. You are really better off going with his two chip PWM gen like I'm using in my E rig.

The design I mentioned was something that I whipped up for an EE project. It has some neat ideas in it but, it doesn't have things like a quiescent duty-cycle control for the carrier level and it doesn't have a negative peak limiter. It's really just a simple PWM to drive a boost converter. Water bubbler chat as it were.

I don't want to mislead you Ed, Steve's two chip design is hands down the best.

-Bill 'GF

[K1JJ]

Yes, speaking of QIX's PWM documentation. You are really better off going with his two chip PWM gen like I'm using in my E rig.

Yep, I was gonna mention the same thing, Bill.

Here it is:
http://www.classeradio.com/simplified_pwm_generator.gif

Simple and clean. The original generator board Franz built 12 years ago was filled with chips. What a difference today with this design using a modern 14 pin chip.

This design will be the one I use for my 4X1 PDM rig.

T

[W1DAN]

Hi Ed:

It should be a fun project. I really enjoy my SS E-rig.

Another helpful hint...the LM386 is really designed for driving a speaker. It has some bad crossover distortion. Not a good mic amp. Noisy too.

You may use another TL072 (I also like the NE5534...not a FET) as a mic amp. If ya have a mic transformer( say 250 ohms to 20k), these chips would work great. If ya wanna get high performance the SSM2019 or LT1115 are very low noise chips for mic preamp service (not really needed for ham service though).

I built Steve's multi-chip PDM generator, but had problems with it. Ground currents were hard to nail down. So, I built his single chip generator using the UCC25701 and it is really fantastic! No messing around. I also created a soft start with a cap and a switch on one of the pins. I also inverted the TTL PDM output so that when using his PDM power section, the modulated B+ turns off when I disable the PDM generator (it would normally go to full on when the pulse train went away).

Steve's circuit is here:

http://www.classeradio.com/simplified_pwm_generator.gif

The best of luck!!!

73
Dan
W1DAN
Natick, MA

[WA1GFZ]

Good Thread.
The PWM chips of today make PDM a lot easier. There are a number of them that turn the design into an op amp driving the PWM chip. Maybe an extra transistor for the negative peak limiter.
The older PWM chips have distortion problems so that is why it took a discrete triangle wave generator and fast comparator. I used an integrator and very fast window comparator with 2 stages of op amp for audio. The board was full of parts by the time all the bypass caps were added.
I prefer to switch the tube with a N CH fet in series with the cathode and Fil CT
 while Steve did grid control.
The 4-250 will make a nice final. I think Larry NE1S used an 833 as the switch.

Bill,
I got your message but got home late and going to bed send me an email of pm.

Tube PWM is a lot harder to do than solid state. be careful with high voltage and floating circuits. Remember scope probes are usually only good for 500V. I killed a nice on. The Big TEK HV probe is the one you want.  You will need 5500 to 7000
VDC to make that tube dance. 

[Ed/KB1HYS]

Thanks for that information.  I've orderd the chips from Digikey, now awaiting the postman.  I'll be running a TL072 as a preamp, a TL074 audio processor\filter into the UCC25701 Just like in the schematic.

I was pondering how to go from the PWM to the Final.  I was thinking of series modulation via a big triode. Still have to figure that bit out some, but there's time yet. Got to get the audio to PWM portion built and working 1st.


I've got a possible plate transformer, that I have to check out. I was hoping to get 6kV or so... will have to test it.

I think finding the filament tranny may be harder. though if I had to I think I could rewind one of the ones I've got holding down the floor of the shack.

Thanks for the help guys!

[K1JJ]

I think finding the filament tranny may be harder. though if I had to I think I could rewind one of the ones I've got holding down the floor of the shack.
Thanks for the help guys!

Hi Ed,

The PDM modulator fil transformer can be a conventional one, with normal insulation.

But as you know, the final's fil transformer must withstand the full HV supply.
Here's what I'm gonna do for a 4X1 fil transformer that is insulated for 10kv....

Take a variac that is big enough to normally handle the power of the filament. A 5A variac will handle it.  Pull it completely apart and get at the round core with the 120AC winding. Simply wind 5-10 turns of heavy enough wire for the current and with a good HV insulation. Adjust the number of turns to get the right fil voltage under load. Tape the primary for more insulation, use some mylar film, Kaptan? or whatever to get a good insulation between the winding layers.

Float it on plexiglass or standoffs.  If you need a grid bias or screen supply, float a TV transfomer and connect the 5.0 and 6.3 windings in series and put them across the fil transformer's output. The 600VAC [or whatever] winding will now have your 400VAC screen or grid voltages, etc.  [Frank/GFZ's idea] 

The final's grid and screen circuits all float at the HV potential and are referenced and RF bypassed to the bypassed fil xfmr C.T. of the final.

Happy Tube PDMing!

73,
T