MODULATION DIODE

[WA1GFZ]

your simulation doesn't prove squat

[K1JJ]

Glad you added a NPL, actually Friday night I was thinking of a way to add a NPL to SS the driver

Frank,

Where were you thinking of adding the NPL in the SS MOSFET driver?

T

[KA2DZT]

Brian,

Sorry that some of the posters are breaking ur chops .  Ur doing good with the computer modeling.

As I mentioned here and on the air many times, I don't agree with the whole concept of using three diodes to effect a keep alive circuit.  For at least five years I've been using a single diode setup and it works perfect.

Like many of us hams I'm from the old school.  We have been designing and building circuits and xmtrs long before anyone ever heard of a PC.

There is no need for D2 or D3 as noted in ur circuit.  The fact that u have a 60K resistor in series with the bias supply effectively disconnects the so-called 6K load resistor from the mod iron.  Look at ur own circuit and tell me how the 6K resistor along with D1 is doing anything.  Where is it across the sec of the mod iron with a 60K resistor in series with the bias supply?  The 6K load is completed from ground thru the internal resistance of the bias supply thru the 6K resistor thru the sec back to ground thru the filter caps in the plate supply.  If u add 60K in series with this path the so-called load is now in excess of 66K.

This should somewhat prove that there no need to use this separate load resistor,  therefore no need for using the steering diode D2 and following this D3 can be eliminated.

This leaves one diode, the only one u need.  A single diode string in series with the bias supply.  The addition of added resistance in this path gives u the high impedance supply u need so the bias voltage drags down when the PA pulls current from the supply on neg peaks.

The overall impedance of the bias supply and added series resistance creates this so-called load on the sec of the mod iron.  I use three 3K resistors with progressively smaller caps to ground to create a time constant in the bias voltage.  This creates the needed soft clip.

There is no need for D2 or D3  since any current flowing from the bias supply is going only one way and that is to the PA.  Any current flowing out of the bias supply in not going to flow backwards thru the sec of the mod iron when there is a 2KV voltage on the other end of the winding.

Nodel analysis tells us that all the current that flows into a junction will flow out of the junction.  Current flows into the junction of the mod iron sec and the line going to the PA.  With a 200V bias supply,  I see a 2KV voltage on the other end of the sec.  This tell me that the current is going only one way, to the PA.

Conclusion,  there is no need for D2 or D3.  And to repeat,  I see no need at all for D4 or R4.

Hope some of this helps.

Fred

[WA1GFZ]

Fred,
The blocking diode isolates the mod transformer when it swings below zero. A stronger bias in your case allosw one to delete this diode. Keeping with the proven 3 diode design that has been around since dirt the only thing you can do is soften the slope of the modulation waveform as it transitions to the clamp voltage. You do the same thing in your case. Adding all kinds of useless parts just makes the circuit bigger.

[Steve - WB3HUZ]

Fred, stop your smear tactics. We must all completely agree with Brian, no matter what he says. Don't you know he is all knowing and much smarter than any of us. His big signal over the years proves this.

Brian,

Sorry that some of the posters are breaking ur chops .  Ur doing good with the computer modeling.

As I mentioned here and on the air many times, I don't agree with the whole concept of using three diodes to effect a keep alive circuit.  For at least five years I've been using a single diode setup and it works perfect.

Like many of us hams I'm from the old school.  We have been designing and building circuits and xmtrs long before anyone ever heard of a PC.

There is no need for D2 or D3 as noted in ur circuit.  The fact that u have a 60K resistor in series with the bias supply effectively disconnects the so-called 6K load resistor from the mod iron.  Look at ur own circuit and tell me how the 6K resistor along with D1 is doing anything.  Where is it across the sec of the mod iron with a 60K resistor in series with the bias supply?  The 6K load is completed from ground thru the internal resistance of the bias supply thru the 6K resistor thru the sec back to ground thru the filter caps in the plate supply.  If u add 60K in series with this path the so-called load is now in excess of 66K.

This should somewhat prove that there no need to use this separate load resistor,  therefore no need for using the steering diode D2 and following this D3 can be eliminated.

This leaves one diode, the only one u need.  A single diode string in series with the bias supply.  The addition of added resistance in this path gives u the high impedance supply u need so the bias voltage drags down when the PA pulls current from the supply on neg peaks.

The overall impedance of the bias supply and added series resistance creates this so-called load on the sec of the mod iron.  I use three 3K resistors with progressively smaller caps to ground to create a time constant in the bias voltage.  This creates the needed soft clip.

There is no need for D2 or D3  since any current flowing from the bias supply is going only one way and that is to the PA.  Any current flowing out of the bias supply in not going to flow backwards thru the sec of the mod iron when there is a 2KV voltage on the other end of the winding.

Nodel analysis tells us that all the current that flows into a junction will flow out of the junction.  Current flows into the junction of the mod iron sec and the line going to the PA.  With a 200V bias supply,  I see a 2KV voltage on the other end of the sec.  This tell me that the current is going only one way, to the PA.

Conclusion,  there is no need for D2 or D3.  And to repeat,  I see no need at all for D4 or R4.

Hope some of this helps.

Fred

[WD5JKO]

another good thread comes to an abrupt end. sad

[ka3zlr]

You'll have that...

73

Jack.

[ka3zlr]

Brian,

I experimented with R3, the bias resistor.  Using just 138 volts of bias, I found that the best fit was 4K.  I expanded the scope vertical to get a close-up of the negative pinch-off line. As I hit negative ~94% with voice peaks, the line was 1/4" thick. But as the modulation increased, the line grew proressively thinner down to 1/16" thick as expected. It never hit -100% as a fine line. So we are now seeing a soft negative peak limiter as hoped.

I can see that if I started with say, 225 volts of bias, I could use a larger R3 and create a larger dynamic range. However, the present soft limit has plenty of range as-is to handle my negative peaks. In fact I start to flat top at 140% positive just as the negative peaks gets close to -99%. Perfect.

So I think the additional R3 was a good add-on that will now make the 3-diode or 4-diode limiter into a soft limiter rather than a clipper.  I like that.   The side crud test on a receiver still shows nothing up 8kc when hitting the limiter hard. Without the limiter the crud is S9, NG.

Fred:  The large power resistors are used for when the diode gets back biased. There can be full HV (3KV on high tap) across certain diodes and resistors at that point. These big 200 watt wire wounds can handle substantial breakdown voltages, whereas the small 5w types would flash over. A small rig would be more forgiving as long at the small resistor voltage ratings didn't get exceeded.  I've been changing things around so much that I used all 100 or 200 watters just to be sure.

Jack: The reason for a high level limiter is because plate modulated rigs using a mod transformer have phase shift thru the audio system. This makes maintaining voice asymmetry very difficult. Thus, a low level limiter can have poor effect by the time it reaches the final.  With the limiter at the final, limiting and phase can be well controlled.  When using a class E rig, low level ricebox balanced modulator or SDR rig, low level limiting works fine cuz of minimal phase shift.  A plate modulated transformer rig is old technology and has its limits and problems - but is cool to ride, just like a Harley.

T

Tom,

 I would never argue with you that which you know best, but also you know I believe in the 80 to 90% rule.

And that applies with some of these problems here.

73

Jack.

[WA1GFZ]

Tom,
Put your values in the simple simulation I sent you and tell us if they match performance..

[KA2DZT]

Fred,
The blocking diode isolates the mod transformer when it swings below zero. A stronger bias in your case allosw one to delete this diode. Keeping with the proven 3 diode design that has been around since dirt the only thing you can do is soften the slope of the modulation waveform as it transitions to the clamp voltage. You do the same thing in your case. Adding all kinds of useless parts just makes the circuit bigger.

I know the diode isolates the mod iron as u state.  I'm still not convinced that that is necessary.  Remember,  so long as the bias voltage at the class C end of the winding remains positive, its not negative.  It can't be both at the same time.

The problem with Brian's circuit is that the load resistor R1 is connected to the wrong side of the 60K resistor.  R1 should be connected at the bias supply.  The value of 60K is way to high.  If the bias voltage is about 200V (about 10% of the plate voltage) it would only take about 3ma of current to drop the entire 200 volts.  That would not create much of a soft clip.  Tom's 4-1000 would pull way more current than that even with only 40-50 volts on the plate

My 6146 on standby, (no plate voltage, no drive) pulls about 5-8ma with only 25 volts on the plate coming from the bias supply.  On transmit, the bias voltage comes back up to about the 80 volts.  As I hit the neg peaks the bias voltage drops down, but it never hits zero no matter how hard I hit the neg peaks.

I guess that there is nothing wrong with staying with the 3 diode setup, as u say it has been around for a long time.  But I still think that the single diode along with a properly designed bias supply is all that is needed.

Fred

[WA1GFZ]

Fred,
I think you are right as long as the bias is strong enough to over come the negative swing of the modulation waveform. I like simple myself.  The extra diodes just isolate the transformer and loads it. This makes the bias supply smaller. I only pointed out that the wavefoem could be rounded off eliminating some of the HF emissions created by the slope going through the switch point.

[KA2DZT]

Fred,
I think you are right as long as the bias is strong enough to over come the negative swing of the modulation waveform. I like simple myself.  The extra diodes just isolate the transformer and loads it. This makes the bias supply smaller. I only pointed out that the wavefoem could be rounded off eliminating some of the HF emissions created by the slope going through the switch point.

I agree about the bias voltage having the current to remain positive.  I think what happens is that the negative going peak simple runs out of headroom when it reaches the lowest positive voltage the bias supply will allow.  This being the case, the two other diodes are not needed.

Keep in mind that I use extra RC filters after the basic bias supply.  Three 3K resistors in series with progressively smaller caps to ground.  Last cap being about 0.5ufd.  These caps supply sudden surges of current.  Sort of a time constant of voltage decay.

The total internal impedance of the bias supply offers the load to the mod iron while at the same time allows the bias voltage to sag on neg peaks allowing a soft clip.  It seems to work and it looks good on the scope.

I may run another test by putting a DC scope on the bias voltage and check to see if the bias voltage drops to zero or go neg on neg peaks.  Something I never did.

Fred

[Steve - WB3HUZ]

Only if you let it.

It is sad when terms like smear tactics get thrown around when someone dares to disagree on a technical point. Very sad.

another good thread comes to an abrupt end. sad

another good thread comes to an abrupt end. sad

[WA1GFZ]

The earth is flat
the earth is the center of the universe

[KA2DZT]

Fred,
I think you are right as long as the bias is strong enough to over come the negative swing of the modulation waveform. I like simple myself.  The extra diodes just isolate the transformer and loads it. This makes the bias supply smaller. I only pointed out that the wavefoem could be rounded off eliminating some of the HF emissions created by the slope going through the switch point.

I agree about the bias voltage having the current to remain positive.  I think what happens is that the negative going peak simple runs out of headroom when it reaches the lowest positive voltage the bias supply will allow.  This being the case, the two other diodes are not needed.

Keep in mind that I use extra RC filters after the basic bias supply.  Three 3K resistors in series with progressively smaller caps to ground.  Last cap being about 0.5ufd.  These caps supply sudden surges of current.  Sort of a time constant of voltage decay.

The total internal impedance of the bias supply offers the load to the mod iron while at the same time allows the bias voltage to sag on neg peaks allowing a soft clip.  It seems to work and it looks good on the scope.

I may run another test by putting a DC scope on the bias voltage and check to see if the bias voltage drops to zero or go neg on neg peaks.  Something I never did.

Fred

I ran some tests on the bias voltage with a DC scope.  On heavy overmodulation I could see where there were neg peaks pulling the bias voltage negative.  Even with this, I still feel that there no need to use a three diode circuit.

I made some changes to the bias supply.  I reduced the overall resistance of the RC filters in the bias supply.  I changed the three 3K resistors down to about 850 ohms each (just bridged 1200ohms across each).  I saw where I didn't have a cap on the last resistor, so a .68ufd to ground was added.  The first two resistors have 20ufd and 12ufd to ground.  I also added a .022ufd cap across the LED, thought that might be a good idea since it seemed that it was only higher freq. peaks that were going into the negative.  Maybe the LED isn't switching on fast enough.  Not really sure.

Anyway,  after making the changes which reduced the overall internal impedance of the bias supply,  I ran the same test again.  The results were greatly improved.  No peaks where reaching anywhere near the negative, which is good.  Even with some pretty heavy overmodulation the bias voltage was pulled down but remained in the positive.  The wave-envelope pattern looked really good with this setup.  Nice rounded neg peak modulation without hitting the baseline.

Seems that designing the bias supply with the correct internal impedance is important.  I use a single diode in series with the bias supply to the junction of the mod iron and the line running to the PA.  A proper bias supply is probably just as important when using the more common three diode circuit.  My single diode is a string of high speed diodes with 1/2meg resistors across each one (no caps).

When one considers the added problems of added two more strings of diodes for the three diode keep alive circuit, I still think that the single diode circuit is all that is needed.  Remember, the internal impedance of the bias supply is presenting the so-called load to the mod iron.  So, a separate load resistor in not needed.

Fred

[KA2DZT]

Brian,

I know there are different ways of preventing overmodulation.  I came up with my circuit about 6 years ago, long before I ever heard of the 3 diode circuit.  In fact, Timtron described it to me over the air.

True, my rig is only a 6146, but the 6550's can put out a lot of audio.  I always get good audio reports and never any distortion reports.

My first thought was that so long as the bias voltage remains positive, you can't be hitting the baseline.  But to comfirmed this, today I put a DC scope on the bias supply.  I was surprised when I saw some negative peaks dropping below the zero line on the scope.  At first I thought maybe there was something wrong with my test setup, but really couldn't see anything that would be showing me false waveforms.  That's when I decided to rework the bias supply and reduce the internal Z.  After that, the negative peaks were remaining above the zero line with room to spare.  To do the test I was hitting the audio really heavy, way overmodulating.  Under normal transmitting conditions I wouldn't be hitting it anywhere near that heavy.

Looking at the wave envelope pattern, the negative peaks look real good, they're rounded and they never hit the base line.  This is with hitting the audio fairly heavy.  Positive modulation is hitting 150% and yet the negative peaks are not baselining.  What more could one ask for with AM.

I thing there is no reason why this single diode circuit wouldn't work with higher power xmtrs.

I did look over ur other post on this subject and the waveforms you posted.  I see why you're using the forth diode.  I guess you're using a modeling program,  something that I'm not able to do hear.  I see there are issues with the switching timing between the 3 diode.  You show how the forth diode and resistor someway corrects for this.  Good reason to give my single diode circuit a try in ur next xmtr .

What's the worst that could happen, you blow up ur mod iron,  they're cheap enough to replace

Fred