modified heising modulation

[WU2D]

Heising is also helpful in solid state circuits and you can still use the negative peak clipping ideas. The modulation for my mobile is solid state Class AB and it benefits from Heising.

Mike Wu2D

[modulation_apprentice]

Heising is also helpful in solid state circuits and you can still use the negative peak clipping ideas. The modulation for my mobile is solid state Class AB and it benefits from Heising.

Mike Wu2D

any schematic's on using modified heising or any variant on solid state circuits?

[AB2EZ]

The reference I was referring to in an earlier post (and which I highly recommend) is:

Electronic Transformers and Circuits, 2nd edition by Reuben Lee (1955)

The discussion of modified Heising modulation starts on page 192 of this book (section 80 of the book).

It does an excellent job of explaining how to select the coupling capacitor... and also, how to select the Heising choke value so that you don't pick a value that is needlessly large (i.e., a Heising choke that is needlessly large would be one whose inductance is much larger than the magnetizing inductance of your modulation transformer).

The book is available for free download from:

http://www.pmillett.com/technical_books_online.htm

Best regards
Stu

[WU2D]

I built this solid state modulator for my mobile ARC-5 station in 1992. I found a henscratch schematic and have attempted to redraw it with everything that I had in by the time I was done. No values unless I rip it apart! I can not remember if I used the three diode circuit, but at least I had a single diode and a resistive load for some basic negative cycle loading.

The driver transformer was from an old CB. It was something like 32 Ohms to 250 Ohms CT. The Modulation transformer was a backward 6.3 VAC 5 Amp fillament transformer. The Choke was a winding from a Johnson mobile modulation transformer!

Mike WU2D

[k4kyv]

Mine is probably more like the "brute force" method described in the Reubin Lee publication.

I use an RCA broadcast transmitter modulation transformer from a 250 watt transmitter.  The modulation reactor is a UTC LS-103, 50 Hy @ 500 ma.  The coupling cap is 4 mfd, 3000v.  The capacitor goes between the bottom end of the mod xfmr secondary and ground, so it charges up to the 2000 volt plate voltage.  I have run that transmitter at 1 kw DC input up to 150% positive modulation peaks, and had nothing to blow up.

I prefer to ground the mod xfmr secondary directly through the capacitor.  That way, the audio is returned  directly to ground, so that the audio circuit is totally independent of the output capacitor in the HV power supply that feeds the rf final.  Also, any hum from the DC supply must pass through the modulation reactor before it can modulate the final, so the mod reactor serves as additional section of power supply filter for the final stage.  If the modulator output is returned to the HV supply, any hum present at the output capacitor will appear as modulation on the carrier.

If a common power supply is used for the class-B modulator and final, a little known source of audio distortion may be present.  Since the modulator tubes take turns drawing plate current over the course of an audio cycle, two pulses of current are drawn from the power supply by the modulator stage, per audio cycle.  Unless an adequate size filter capacitor is used at the output of the HV supply, these pulses of current  drain will induce an audio modulation of the DC supply, predominantly at the 2nd harmonic of the modulating frequency, due to poor dynamic regulation of the supply.  This modulation will show up at the class-C stage as harmonic distortion if the bottom end of the modulation transformer secondary is returned to the less-than-perfectly regulated HV power supply, whether the modified heising  circuit is used or the modulation transformer secondary is allowed to carry the dc to the final.

If the audio from the mod transformer secondary is returned directly to ground through the blocking capacitor, any 2nd harmonic audio voltage induced at the output of the power supply will be prevented from modulating the final, since it will be isolated by the modulation reactor.

[W9GT]

Thanks guys!  Lots of good info here.  Also good parallel discussion in another thread on Modulation Transformer info.  I checked the schematic for the Bauer 707 BC xmtr.....looks like they used a 5 KV cap for audio coupling with a nominal 3 KV plate voltage on the RF final.


Thanks and 73,  Jack, W9GT

[modulation_apprentice]

tons of good info here. i hope that they keep a good backup of this forum. there is alot of information in here that i would hate to see gone.

[Opcom]

I built this solid state modulator for my mobile ARC-5 station in 1992. I found a henscratch schematic and have attempted to redraw it with everything that I had in by the time I was done. No values unless I rip it apart! I can not remember if I used the three diode circuit, but at least I had a single diode and a resistive load for some basic negative cycle loading.

The driver transformer was from an old CB. It was something like 32 Ohms to 250 Ohms CT. The Modulation transformer was a backward 6.3 VAC 5 Amp fillament transformer. The Choke was a winding from a Johnson mobile modulation transformer!

Mike WU2D

I love that modulator. I see you have it set up there with an op-amp. Do you know how to do a balanced transformerless setup with the MOSFETs that could drive a transformer or speaker to get more power on the same 12 volts? I've dinked around and the "balance" part seems to be the issue. Some annoying DC ends up in the load.

PJ

[WU2D]

PJ,

That design was inspired by some of the old handbook solid state modulators of the late 50's and early 60's but with a straightforward update to more modern parts. It is KISS and it worked at the low power that I run (40 W input power) so I never went any further.

You are right. A better approach would dispense with the interstage transformer and double up on a complementary symmetry output stage as a "pair of pairs" in antiphase to drive the transformer at 2X the voltage swing which means 4X the power! 

And you wonder why Class E is so popular - this stuff gets complex fast.

Mike WU2D

[modulation_apprentice]

Hi !

The 3 diode circuit shown on the am window is not a great circuit.  There have been numerous technical discussions about this circuit.

Here is a better circuit that I used at a many broadcast stations, including a 5kW station.  It will not put undue stress on the modulator or related components    The advantage is that the modulation is controlled exactly, due to the stiff power supply, and the resistor can be very small in wattage.  This circuit also protects the modulation transformer during the entire audio cycle, and keeps a constant load on the modulator.

i am putting my circuit together finaly to do some testing this weekend but i have a question. where it shows to keep alive supply on that schematic does it have to be a rectified dc source of voltage or can it be ac? the way i have it right now is rectified 50vdc going to the connection point. i also have my transformer voltage variable.

[modulation_apprentice]

well here is a picture but now i am second guessing myself on if i should have rectified the ac output of the keep alive transformer. if you look you can see the small block rectifier next to the transformer.

[modulation_apprentice]

well i know i am babbaling but anyways i went ahead and kept the little full wave rectifier on the secondary of the keep alive power supply, what was messing me up is i did not know if it was needed with the 3 other diodes in the negative peak limiter but i am just going to test it with the rectified dc from the keep alive feeding the negative peak limiter. but anyways i was able to put a 56k rheostat on the primary of the keep alive supply and now have variable voltage from 0-50vdc. i am holding it at 10vdc right now to see if any heat is produced from the rheostat. should i put some resistance inline to stop it from being able to pull the voltage all the way down for safety? just a thought i had. let me know if anyone see's anything wrong with what i am doing. i should be able to hook it all up tonight and put up some videos of the scope with it on and off.

have a good weekend everyone.

[modulation_apprentice]

well i got everything hooked up. i have my ummodulated b+ coming into one side of my heising reactor, then on other side of the reactor i have the connection point of d1 and d2 going there. i disconnected my rf final from the modulated side of my reactor and connected it directly to my new negative peak limiter at connection point d2/d3. it is working as far as negative peaks but adjusting the voltage of the keep alive supply i can not see it affecting anything on the scope. i turned power off to the keep alive supply to see what affect that would have and resistor 1 opened up so i replaced it.