MODULATION DIODE

[Opcom]

besides high voltage schottkys, we sell what we call "tandem" rectifiers. These are two bipolar rectifiers in series on one die, and the purpose is more efficient switching due to decreased total capacitance. If you don't want any recovery time, SiC is best, but the common voltages are not as high as we would prefer, maybe only 600-1000V per device. And they are costly.

[WD5JKO]

The problem is where in the heck do you find a 1000V pulse generator? I know somebody will probably come back and say they have something that will work

   Brian,

   To evaluate these diodes you need to use them in a transmitter. For example, take the 3 diode circuit that you like so much, and in your transmitter, run the audio up to 120% positive with a sine wave where the negative cycle will be hard clipped. Then look at the RF spectrum with a spectrum analyzer. A SDR-IQ SDR receiver would be useful here. Look at the sideband distribution of audio harmonics. Then make changes from normal Trr speed diodes to high speed diodes, and compare under identical conditions. You just might say Yikes!, and install a pi low pass filter after that 3 diode circuit.

Jim
WD5JKO

[WD5JKO]

Brian, you are correct in that human speech is not like a sine wave. I will also say that we don't talk in square waves either, except those that remove one sideband and the carrier, i.e. 'square jaw'.  

I just mentioned a test condition using an audio generator, a transmitter, and a spectrum analyzer such that real repeatable data can be obtained, and then comparisons can be made easily when make circuit changes. The waveform used can be anything you want as long as it can be repeated exactly over and over.

Here is something good to look at written by fellow member, John Lyles:
http://lists.contesting.com/pipermail/amps/1999-July/011928.html

I was wondering if you have the book, "Motorola Silicon Rectifier Manual", published 1980? This book has 14 chapters of theory using diodes in all sorts of applications. They go into both the forward recovery and reverse recovery characteristics and show test circuits, scope plots, etc. They also talk about using standard diodes at high frequency. Do you have this book? If not, would you be interested in putting this on your wonderful amforever.com web site? I could lend it to you.

update: one on Ebay, $18.99 buy it now: 350273765073

Jim
WD5JKO

[WD5JKO]

By the way I’m an engineer at Linear Technology here in SV. I worked for Analog Devices before that so I’ve spent the past several years working with this stuff and running tests.

   Brian,

    Why not take LT Spice, and take a stab at modeling the 3 diode circuit? For the RF PA tube, try replacing it with a diode in series with a resistor. Since the tube will likely be a tetrode where both screen & plate get modulated. As we know doubling the plate voltage doubles the plate current at 100% + modulation..So the RF PA should simplify to a diode in series with a resistor. Just wondering out loud...

   Heck the modern engineer doesn't prototype anything! They just Cad the heck out of it, over and over, and over....

  Oh, still have the cover on pool here. The trees are still spewing buckets of reproductive mung over my pool cover and grass. I need to wait at least two more weeks..

http://www.troublefreepool.com/pool-school/
here is where science clashes with pool store marketing..BBB all the way here!!

Jim
WD5JKO

[Opcom]

The problem is where in the heck do you find a 1000V pulse generator? I know somebody will probably come back and say they have something that will work

8-X  

[Opcom]

What’s the turn on time of an 866 or an 872 for example? Do they conduct faster than say like 5nS or so?

From RCA TT-3 (not found in TT-4):

866A:
10000 PIV @cond Hg temp 25-60 deg C for the 866 is given at supply frequency=<150Hz
5000 PIV @cond Hg temp 25-70 deg C for the 866 is given at supply frequency=<1000Hz

872:
10000 PIV @cond Hg temp 25-60 deg C for the 866 is given at supply frequency=<150Hz
5000 PIV @cond Hg temp 25-70 deg C for the 866 is given at supply frequency=<150Hz

Since mercury temperature is a factor, the info (and by extension turn-on/off time) is not complete, but the Hg temp seems more a factor affecting PIV than the supply freq. is. So why have they qualified the 866 data with frequency notes, and why does the 872A have no such high frequency note?

The point is that although the MV tubes seem rather slow "by the book" for audio and the manuals don't push them as audio frequency rectifiers, others have, including the maker of this:  http://amfone.net/Amforum/index.php?topic=20179.0 and the ARRL in the handbook. so, from studying that unit's performance the question could be approximately answered.

866 Switching time has to be short at an audio frequency of 3KHz (167us per half wave) to cover the bandwidth without adding distortion, so how short? And at what real peak inverse voltage?

If the 866's failed to turn off fast enough the result would be a short across the mod xfmr secondary. Harmles in some cases, but with the big stuff could be problematic. The application also uses low voltages (~1KV) relative to 866 ratings. The 816 is also proffered in the article for low power applications.

[WD5JKO]

Some LT-Spice circuits, classic 3 diode and 3 diode with soft clip + LPF. See attachments, first the 3 diode mod plot, FFT, and schematic.

Here we have a B+ of 750v @ 200ma, 150w DC input. RL = 3750 ohms
Audio pushed to 140% positive, (1.4 * 750) + 750 = 1800V peak
Keep Alive supply set to 200V

The 3 diode soft clip + LPF will be in the next post..

Jim
WD5JKO

[WD5JKO]

Here I discovered my 4 diode circuit behaved more like a two diode circuit, so I removed two of them. Then added another to keep mod tranny loaded for tube modulator. The LPF remains.

Here we have a B+ of 750v @ 200ma, 150w DC input. RL = 3750 ohms
Audio pushed to 140% positive, (1.4 * 750) + 750 = 1800V peak
Keep Alive supply set to 200V

see attachments

Jim
WD5JKO

[K1JJ]

Here I discovered my 4 diode circuit behaved more like a two diode circuit, so I removed two of them. Then added another to keep mod tranny loaded for tube modulator. The LPF remains.
see attachments

Jim
WD5JKO

Very good work, Jim!

It's impressive how you obtained the soft limiting vs: the hard sqaurewave of before.  

Could you explain the significance of the frequency vs: db curve? I see the effects of the L.P filter on the high freq wispy stuff, but what causes the average roll-off slope in both circuits?  Do you feel that high-freq noise w/o the LP filter is a cause of additional splatter?

I assume that is 0.15 uh for the coil in the LP filter.

I want to try that circuit and replace my simple 3-diode circuit that I'm using now. I will order some high-speed diodes at the same time.  Can anyone recommend a fast diode for say, 1KV and 3A sevice?  I could settle with 1A too.  I assume the in-line diode needs to be full plate current rated, but the three shunt legs can be less current.  Maybe 3A rating for the inline and 1A for the shunt diodes will be OK.

Jim, I wonder how you calculated the three resistor values and wattage ratings?  If it's not too much trouble, could you give me the values for a 4K plate impedance of my 4X1 plate modulated rig?

Thanks, OM.

T

[WD5JKO]

Tom,

   Pass along the 4-1000 RF PA B+ to go along with the 4K RL.... Also what is the design maximum + mod percentage? 120%? 140%

Also the value of R1 is usually too low due to the effect of the keep alive supply. See attachment. The LPF should drop down at 3 X 6 db/Octave (As I recall). Some of the charts look close to 18-20 db/Octave.

The cap values are in Farads, and coil value is in Henries.. That 0.12 is 120mh.

Getting ready for a business trip, time to start packing...

Jim
WD5JKO

[K1JJ]

Tom,

   Pass along the 4-1000 RF PA B+ to go along with the 4K RL.... Also what is the design maximum + mod percentage? 120%? 140%

Also the value of R1 is usually too low due to the effect of the keep alive supply. See attachment. The LPF should drop down at 3 X 6 db/Octave (As I recall). Some of the charts look close to 18-20 db/Octave.

The cap values are in Farads, and coil value is in Henries.. That 0.12 is 120mh.

Getting ready for a business trip, time to start packing...

Jim
WD5JKO

OK, Jim, good on 120mh. That's like a big plate choke, no problem.  I think you meant uf for the capacitance = .002uf.


Figure about 2KV on the final. That's the low tap and I run it there most of the time.  130% positive would be fine for my voice peaks.

Thanks.

T

[Opcom]

zero source impedance or 1-Ohm modulators are not very practical. How can one calculate the source impedance of a real tube modulator?

for example a modulator with two 6L6's running 450V @ 210mA, making 55 watts into 5500 Ohms CT, and is matched via a transformer to a 5500 Ohm load (to make it simpler). Straight from the RCA RC-25.

At some point, an abrupt change in current at the point where the degative load diode conducts is going to reflect in the modulator's voltage waveform. 

The reason for the question is that the spice simulation will need to show distortions in the modulator's voltage waveform to accompany those shown in the modulator's current waveform.

The reason for that is to play around with making a better load for the negative cycle, one that results in the least amount of distortion, which the modulator may have to try and correct for (using neg feedback etc).

[Opcom]

same here. I think I like a rounded negative cycle better. Maybe less harsh? Looks better anyway.
B+ 1000
modulation 2000 peak
V(n012) is the modulated B+ to the RF stage

This does consume more power but the modulator current waveform is not too badly distorted.


I would like to clear up something about % modulation to avoid confusion on my part.
Is this the way it is being discussed?
100% positive mod means the peak envelope voltage seen on the scope goes to 2x the carrier level. (4x power)
200% positive mod means the peak envelope voltage seen on the scope goes to 3x carrier voltage (9x power)

[K1JJ]

Brian,

As per your schematic, I added a 1K 200w resistor across the main diode on Fabio. Hopefully this will clean up any crud generated by the slower power supply diodes I use.  It still works FB, though I have not run any tests to see a difference.  At least I won't be tempted to tear the exisiting limiter down and buy high speed diodes now...

T

[WD5JKO]

Hi All,

  Just waking up here now in Thousand Oaks, Ca. I hate traveling the time zones...

Anyway, it seems to me that adding R2 at a low resistance will have the unintended consequence of boosting the keep alive current demand, and also will cause the modulator current (V1) to by very high when V1 voltage drops below the keep alive supply voltage. This means the Modulator RL will drop, and drop by a bunch.

On the soft limiter I presented I had to increase R1 almost 3X because it was influenced by R2, R3. I had to play with the ratio of R2/R3 for the desired soft limit without clip, and then play with R1 until the V1 current was symmetrical, and virtually constant over the 360 degree modulation cycle.   

Maybe tonight I can focus on this again with Fabio specific thoughts.

Jim
WD5JKO

[K1JJ]

OK on all, guys -

So far the circuit in Fabio is working FB and hanging in there.

I liked the circuit posted that showed a soft, rounded limiting to the negative peaks. It required a .15H inductor and .002 caps in a LP filter.  I wonder if this soft limiting can be done without this LP filter?  I think that the hi freq attenuation will occur from the plate bypass cap and plate choke anyway, so an additional LP filter may not be needed. It will only make the hi freq curve more complex.

I'll stick with the 1K across the main diodes for now. The mod load is about 4K which wud mean 400 ohms, but I'll stay at 1K until the modeling shows otherwise.

Jim, with your new circuit would your negative lead of the mod transformer/diode connection be equivalent to the heising choke negative lead on my rig?


BTW, I only have about 55 volts as the keep-alive voltage. It produces a very thin line at about 98% negative. I know it's working cuz the line gets thinnner when out of circuit. I don't see any reason to limit at 90% or even 95% prematurely, do you?  It's just limiting the depth of modulation for no reason. Taken to an extreme, what if we set the limiting at 50% negative?   No reason I can see not to get as close to 99% as possible, unless I'm missing something here..  There does not seem to be any momentum to spill over past 99% with this circuit, no matter how hard it is hit.

T

[WA1GFZ]

please include spice directive for diodes in the simulation
I want to see what you are using for Trr
My guess the problem is unmatched reverse recovery times from diode to diode.

[W2PFY]

Yo Tom, when you get it all figured out and satisfied with the results, let us know your findings. I'm going to hopefully have the Maytag, belching out fire before winter sets in again.

[WA1GFZ]

Tom,
Remember the forward voltage drop of the diodes reduces the keep alive voltage.

[K1JJ]

OK, Brian -

Yes, I could make up that circuit. I already have an extra string of diodes mounted on the Plexiglas and have an extra 400K 200w resistor available.

First I will wait to see what Jim comes up with too, before proceeding with the changes.

Thanks for the effort.

T

[WA1GFZ]

Thousand Oaks are there any left?

[K1JJ]

Brian,

I built up your latest 4-diode circuit with the 40K resistor. It appears to work fine with no problems.  I see the same negative peak limiting as with the original 3-diode circuit, but have not run any detailed tests to know if there are any improvements.

W9AD did do a spectrum look at my signal tonight and felt Fabio looked clean. I ran my audio from 1.5 kc to 7kc wide and he said the bandwidth tracked well with no splatter. I leaned into the limiter at times, so it appears to do the job.  Again, I had no side splatter crud before with the 3-diode circuit, but the new circuit is probably a better bet for the slower power supply diodes I use here.

Tnx for your efforts.

Terry: Well, there ya go, OM.  Unless the circuit gets changed, this is the final one for me. Back to the class E project...


T

[Opcom]

Is Brian's 4-diode simulation there for a 4-1000 PA running 2KV at carrier? Just trying to figure out what I am studying.

[WA1GFZ]

extra parts don't appear to do squat

[WA1GFZ]

The waveform slope hitting the clamp voltage is not modified so the level of crud generated will be the same.