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MODULATION DIODE




 
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WA1GFZ
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« Reply #50 on: April 15, 2010, 10:13:58 PM »

why not just put balance resistors and caps across each diode. What is the purpose of the simulation?
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WA1GFZ
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« Reply #51 on: April 16, 2010, 05:47:16 AM »

Adjust the simulation to show it because I don't see what you mean
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WA1GFZ
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« Reply #52 on: April 16, 2010, 11:11:53 AM »

it does not help I see no value in a diode in series with the B+
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WA1GFZ
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« Reply #53 on: April 16, 2010, 03:09:33 PM »

I tried a bunch of 3 diode simulation and bottom line any time you conduct / clamp you generate harmonics. The higher harmonics can be pulled down slightly by putting a resistor in series with the bias supply to modify the slope of the waveform going through the switch point but the change wasn't that great. The simulation second harmonic was down 50 dB when not clipping and only down 20 dB when clipping in my best configuration. So the only thing you could do is add a brick wall filter out at 5 KHz or so to limit wideband crud and live with the harmonic energy in band. So best to avoid hitting the limiter with low level processing. Adding a bunch of extra parts didn't change the performance.
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K1JJ
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« Reply #54 on: April 16, 2010, 03:22:33 PM »

Frank,

Let's say we use the circuit strictly as a limiter to prevent hitting -100% negative and as a safety net for mod transformer damage.

I wonder how the side splatter crud compares when using this limiter vs: simply hitting -100% negative without it?  

I ran some tests yesterday into a dummy load while listening 7kc off freq to my voice audio. As expected, when I hit the limiter hard, I cud hear some slight crud generated. Though I didn't try it with the limiter completely out of the circuit yet.  It does seem transparent when not leaned upon.

Based on this test alone, I plan to run my audio with no leaning on the limiter at all - just used as a safety net, especially for damaging negative 100% abuse to the mod xfmr.

BTW, I also have a low level limiter inline that effectively keeps the audio below -100% neg.  (Frank, I just set it up since your visit the other day)

T
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WA1GFZ
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« Reply #55 on: April 16, 2010, 03:43:25 PM »

Low Z bias source 1KHz 65 ref, -20dB second harmonic, -40 dB at 12 KHz
High Z bias 1 Kohm 1KHz 65ref, -23 dB second Harmonic,-42 dB at 12 KHz
zero bias V           1 KHz 65 ref, -25 dB second harmonic, -44dB at 12 KHz

so you do get a slight advantage with a high Z bias source. I used 1 K in series with the bias and ran the voltage up a bit. Still there is less crud as the bias voltage is reduced.
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K1JJ
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« Reply #56 on: April 17, 2010, 02:27:43 PM »

I have some test results with Fabio: (single 4-1000A plated modulated by 4-1000A's)

As Frank suggested, I added a 1200 ohm resistor in series with the positive "keep alive" bias supply. With a HV probe on the limiter output I could see the effects of the high level modulator audio waveform having smoother negative peaks instead of clipping hard.

In addition, I added the 40K resistor across the 4th diode that Brian proposed.  So the circuit now looks exactly like Brian's last circuit post using 4K and 40K resistors, but has the 1200 ohm resistor included as described above. I tried different resistor values, but 1200 ohms worked out best for me. Too high and there was too little limiting - too low and the limiting was hard.

With Fabio running into a dummy load and using normal voice modulation, I could see the negative peak limiting on the RF waveform to be at about 97%. It was about 94% without the 1200 ohm resistor.

The good news is the side crud products. I first tried the rig with the limiter turned OFF. I  modulated at positive 140% and hit -100% negative.  My receiver showed S9+40 carrier on frequency. Up 8kc I could see S9 crud on the S-meter as I hit -100% negative. Below -100% modulation level there was nothing.

I then engaged the limiter and did the same test. There was NO crud at all when I hit the limiter hard (-97% negative) and the positive peaks were at 140%.  

So, I have proven to myself that the limiter circuit, as described, is definately better than having none at all when hitting the baseline at -100% negative. In fact, when leaning on it lightly for higher positive peaks, I could hear almost no change in side crud.   The only limitation was when I ran out of audio headroom and the positive peaks started to flatten - this caused just as much crud as hitting -100% negative, as expected.

I would say the limiter is a worthwhile addition to a high level plate modulated rig, in addition to some low level limiting.

T


This was my test set-up:


* The First TimTron Mobile.jpg (23.38 KB, 300x455 - viewed 682 times.)
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Steve - WB3HUZ
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« Reply #57 on: April 17, 2010, 02:41:49 PM »

Quote
The only sure way to eliminate flat-bottoming (without controlling the carrier level, because controlled carrier stinks) is to build a transmitter using Hoisy's old upside down tube circuit if you want greater than 100% modulation.

That is but one way. There are many others.
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K1JJ
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« Reply #58 on: April 17, 2010, 05:11:35 PM »

When the clipping starts near 95% negative, does the bias supply draw more current as the % of positive modulation increases -  or does current stay constant at this turn-on point no matter how far the audio level goes past the turn-on point?

T
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K1JJ
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« Reply #59 on: April 17, 2010, 06:20:46 PM »

Yes, more or less.  I was thinking that the ideal situation would be if there was some "sag" to the bias supply caused by the series resistor voltage drop as the current increased, then there would be a tendency for a more rounded negative peak.   Just like low level limiting, the softer peak will generate less crud and harmonic frequencies when pressed.

I'll have to get in there with my HV probe again and look at the high level audio waveform coming out of the limiter with and without the resistor. I thought it softened things, but I need to recheck this to be sure.

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« Reply #60 on: April 17, 2010, 09:02:50 PM »


Hi All,

I'm back looking at this thread after traveling to W6 land, and working a 65+ hour week in 6 days. Still here too, but i got Sunday off.

I see great progress was obtained with the modeling, Fabio test results, and some differences in preferred approach. I'm trying hard to see what that diode in series with the B+ does, other than take us back to the beginning of this thread.  Wink

Also, take the spice simulation and add the pi LPF like my earlier post (.002uf/.12H/.002uf) and then run up the frequency to 3 Khz, and then 5 Khz. The edges of the clip point get progressively more rounded which I think is a good thing since the upper audio frequencies in our voice will not create as much crud as the lower frequencies.

You guys are advancing the state of the art with 'Advanced Modulation'.

Jim
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WD5JKO
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« Reply #61 on: April 17, 2010, 10:57:01 PM »


Brian,

The resistors R2, R4 form a voltage divider when diodes D1, D3 conduct and when D2 is reverse biased (when waveform is below the blue line). For R4 at 1.2K to do much, then R2 needs to me much lower than 60K. Also when below the blue line, resistor R1 needs to be higher than RL because R1 is effectively in parallel with (R2 + R4).

If you don't see it this way, look at the AC current from the audio source. It will not be symmetrical until you increase R1 above RL.

Jim
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KA2DZT
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« Reply #62 on: April 18, 2010, 03:40:12 AM »

Tom,

I use a single diode keep alive circuit.  You do not need to use three diodes.  I've mentioned this many times on 75M.  I use a keep alive voltage of about 10% of the plate supply voltage.  The difference is that the keep alive supply has a series of extra RC filters.  3K with about 10ufd to ground followed by another 3K with about 3ufd to ground followed with another 3K with about 0.5ufd to ground.  This high impedence supply allows the voltage to sag on neg peaks, thus rounding the edges of the clipping.   I believe it is the hard clip that causes the crud you talk about.  The output from the supply runs through a string of high speed diodes to increase the PIV.  Each diode has a 1/2meg resistor accross it without any caps.  The cathode end of the string is connected directly to the class C end of the mod xfrm.  There is no need to use the other two diodes.  Any load you think the mod iron needs to see is presented by the power supply. 

I use a 100ma fuse between the supply and the anode end of the diode string, in case the diodes fail.  Also, I have a LED in series, which will flicker when I hit the neg peaks hard.  I leave the keep alive supply on at all times.  On standby (no plate voltage),  the LED will remain on, as the PA is drawing a little current from the supply.

My xtrm, as you know, is only 50W with a plate voltage of about 650V, but I think this circuit will also work with ur big rig.  You may have to play around with the RC values.

You can place a voltmeter at the output of the supply (before the diodes) to measure the keep alive voltage.  On standby the PA will drag the voltage down to some resting point.  This will be the lowest voltage the supply will reach when transmitting.

Fred
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K1JJ
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« Reply #63 on: April 18, 2010, 10:17:55 AM »

Fred said:

"The difference is that the keep alive supply has a series of extra RC filters.  3K with about 10ufd to ground followed by another 3K with about 3ufd to ground followed with another 3K with about 0.5ufd to ground.  This high impedence supply allows the voltage to sag on neg peaks, thus rounding the edges of the clipping. "


Interesting approach, Fred.   Maybe one of the guys here will add that to the model to see. I'd be curious.

So far I've changed my circuit three times and I'm happy with present results. It appears to work as I desire at this point, but I'm always open to further changes as new ideas come about.

I'm going to replace the 1.2K and try a 40K resistor for R4 and take some scope readings of the HV waveform. 

Jim, your comments are always appreciated to keep us all thinking and trying different approaches.


T
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« Reply #64 on: April 18, 2010, 03:46:11 PM »

OK, Brian, I will check out the before and after.  Though, I cannot detect any distortion with the rig as-is. 

I raised my keep alive voltage to 138V last week. So hopefully it will be enuff when I go with a 40K resistor, but if not, I can go higher.

Here's a shot of the prototype limiter board I've been working with.  There's been lots of changes on it and it shows... Wink

T


* 4X1 Rig 750.jpg (330.54 KB, 1280x960 - viewed 710 times.)
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« Reply #65 on: April 18, 2010, 04:43:17 PM »

Yo Tom,

I looked at ur limiter circuitry.  Watch out that u don't hurt yourself.  I looked at all those posted curcuits.   You don't need most all of that stuff.  If u look closely at it,  the 6K load is not in any way across the sec of the mod iron,  u have a 60K resistor in series back to the bias supply.  Where is that 6K resistor presenting a load to the mod iron?  Its not. The forth diode in series with the plate supply to the B+ end of the mod iron does what?  Nothing that I can see,  of course we now should bridge it with another resistor,  1.2K or maybe 40K HuhHuhHuh  U also don't need the other two diodes or any of the resistors.  I would but two or three 2K resistors in series with the bias supply.  Add some caps to ground after each added 2K,  connect it to the one diode string that connects to the class C end of the mod iron.  Remove the rest.  The 2K resistors don't have to be high wattage as there is not much power dissipated in them.  There is only a little current drawn on neg peaks.
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« Reply #66 on: April 18, 2010, 06:38:53 PM »

W5hro,

I've looked at all the pics and graphs.  I see the difference with and without R4.  I don't use the 3 diode limiter, as I've mentioned.  I use a single diode setup as I've posted.  It seems to work fine in my xmtr.  IMO there is no need for the two other diodes.  While I see the difference with R4,  IMO u don't need D4 or R4, same for D2 and D3. (hope I didn't mix up D numbers, I'll check)

Fred
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ka3zlr
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« Reply #67 on: April 18, 2010, 06:45:49 PM »

and....if our audio is processed prior to application and presented correctly we wouldn't
need a three diode limiter...sheezzzzz... Undecided

mo junk.


73
Jack.

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ka3zlr
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« Reply #68 on: April 18, 2010, 07:20:43 PM »

Ah,... you said you modulator is, I said when your audio is presented correctly.

Presented correctly...you create your overdrive on purpose to achieve some
Psycho Acoustic effect..or is it "Presence" . oh strapping yea....

To wit your responsible for your emission bad construction practice
to leave your machine dependent on a silicon scheme....go get'em tiger  Smiley

When one has Plenty of Good Clean Signal we don't need over driven anything...

Alas Less Audio problem more signal quality....Listen to Don's advice....

73

Jack.

 

 
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K1JJ
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« Reply #69 on: April 18, 2010, 07:26:10 PM »

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
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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There's nothing like an old dog.
ka3zlr
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« Reply #70 on: April 18, 2010, 07:31:15 PM »

Make more fire in the wire an Less audio Junk....

73

Jack.
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Steve - WB3HUZ
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« Reply #71 on: April 18, 2010, 09:06:36 PM »

2010 Resolution:  Build less - Operate more     Huh
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WA1GFZ
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« Reply #72 on: April 18, 2010, 09:08:54 PM »

DZT I agree.
All this extra bullpucky does nothing. A diode in series with the B+ supply is USELESS. A resistor across a useless component is also useless.
All I see in simulation is stay with the simple 3 diode circuit. Soften the blow with a series resistor on the keep alive supply to round the edges a bit. All the other changes are BS.

Send me an email to work in the morning Tom and I will send you my Spice file so you can see for yourself.
Glad you added a NPL, actually Friday night I was thinking of a way to add a NPL to SS the driver
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WA1GFZ
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« Reply #73 on: April 18, 2010, 09:37:37 PM »

fourth diode doesn't do anything but drop your B+ supply a few volts.
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Steve - WB3HUZ
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« Reply #74 on: April 18, 2010, 09:47:22 PM »

LOL. The simulations don't do anything. They are inert and inanimate. Your interpretation of the data is at issue.
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