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Author Topic: Positive Peak Modulation (again)  (Read 7425 times)
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VE3ELQ
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« on: August 26, 2016, 11:33:45 AM »

Been reading the thread in QSO and other stuff on positive peak over modulation with great interest but some confusion. I'm try to determine just what it is.  Is it just the higher level peaks that get boosted leaving the low levels unchanged?  Or is it like a compressor which pretty much does the opposite? Or something else?  My questions come as I have just finished design and build of a PW Mod generator board based on the LTC6992-1 PWMod chip and a TL072 op amp. It works superbly and is dead linear. I use an external EQ for audio tailoring and band pass limiting but no negative peak limiter so it provides only clean 100% modulation.  I have some ideas on how to re-design it to provide emphasis to the positive peaks if that is what is needed but not sure. If I knew what is required to do I can probably figure out how to do it. Comments appreciated.

73s  Nigel
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M0VRF
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« Reply #1 on: August 27, 2016, 06:51:46 AM »

ALL this talk only applies to 'Mod Tranny' modulated carriers.

With PWM you can modulate to ANY level as the carrier does not have to rest at 1/2 Vcc.

In fact the mod % can be infinite!

I (normally) stick to 150% (40:60 at PWM carrier)

I doubt anyone is using a 'diode detector' these days with the plethora of online SDR's I'm surprised anyone is using an RX!

Oh, nice one for using the LTC chip, you beat me to it! Audio in PWM out, kinda makes it easy.

I use the NE5532 as a audio bpf with UCC35702 PWM, drives a FOD3180 opto, drives a pair of FETs. The opto and FETs have a floating supply.

Whole circuit fits on a PCB an inch square and will mod 50V@10A.



Still have a few PCB's if you ever wanted to try one.

You could drive off board Cree FETs with the same circuit and mod a couple of KW I guess.

 Wink


* IMG_20160827_120353386.jpg (109.66 KB, 1222x643 - viewed 394 times.)
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flintstone mop
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« Reply #2 on: August 27, 2016, 12:12:17 PM »

There is no such thing as Positive modulation being overmodulated. It's a limitation of the receiver. Usually 130% positive is about the max most receivers will handle before distorting. Older receivers will distort sooner.

The NEGATIVE modulation is a serious technical limitation of the transmitter ONLY ! 100% or 95% is the most you can modulate an AM transmitter. Splatter and interference will be caused to nearby Ham ops near your frequency. Or even out of band. And it will shorten the life of the modulation transformer, over time. That is for a plate modulated transmitter.

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Fred KC4MOP
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« Reply #3 on: August 27, 2016, 12:41:26 PM »

Oh!

My understanding was that 'Synchronous Detectors' didnt care?

 Huh

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VE3ELQ
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« Reply #4 on: August 28, 2016, 11:28:59 AM »

Thanks to the responses here and a couple Emails I think I now understand it all.  Clearly its the negative peaks that are the biggest problem.  I currently run 40/60 pulse ratio to provide some positive peak headroom and its the negative peaks that are clipping first.  Got some ideas though for a different ramp generator to decrease the negatives but that will need some experimenting.

73s  Nigel
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W3GMS
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« Reply #5 on: August 28, 2016, 12:14:21 PM »

I doubt anyone is using a 'diode detector' these days with the plethora of online SDR's I'm surprised anyone is using an RX!
 Wink

That is far from true!  

No argument from me on the merits of using SDR receivers since their advantages have been discussed and realized for quite some time now.  

Many folks in the AM and even SSB community like using the older gear.  What we use on the air does not have to be better, just enjoyable to the person using it!

73,
Joe-W3GMS
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N1BCG
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« Reply #6 on: September 14, 2016, 06:39:38 PM »

Also, depending on which F.C.C. rules you acknowledge, the 1500W PEP limit will require carrier reduction if you modulate beyond +100% @ 375 Watts:

There's a formula for this:  Max Carrier W = ((3873 / (MOD% + 100))^2

Some examples of what creates 1500W PEP:

%Mod    Watts

100         375
125         296
150         240
200         167
300         94  (Yep. Full legal limit with just 94W @ +300%)

I can get full legal limit from my 200W xmtr at +174% mod. It will require voluminous amounts of negative clipping to do this with most voices and sound like hell as a result, even on SDRs. That's something that needs a lot more recognition in the hamateur community.

Addendum:  Ooops, just noticed this topic was posted by a VE. Your rules may vary...
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VE3ELQ
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« Reply #7 on: September 15, 2016, 07:13:51 AM »


I can get full legal limit from my 200W xmtr at +174% mod. It will require voluminous amounts of negative clipping to do this with most voices and sound like hell as a result, even on SDRs. That's something that needs a lot more recognition in the hamateur community.

Addendum:  Ooops, just noticed this topic was posted by a VE. Your rules may vary...

Here in VE land the physics are the same but my license allows me 2250 W PEP but few VEs run that much.  My big linear makes 1600W PEP max on 40M SSB.

Thanks for your post here and in the other thread, great reading. I have a nice PW modulator now working with no peak limiter or any other enhancements and it sounds great. Will leave at that.

73s  Nigel
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AB2EZ
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« Reply #8 on: September 15, 2016, 02:09:59 PM »

Nigel
et al.

Attached is a.wav audio file of an AM test waveform that I use.

It consists of a series of low duty cycle composite pulses that repeat every 200 milliseconds

Each 8 millisecond long composite pulse consists of a 2 millisecond positive-going rectangular pulse, having amplitude A; followed by 2 milliseconds off, followed by a 4 millisecond negative-going rectangular pulse having amplitude A/2.

Therefore, each 8 millisecond long composite pulse has zero average value, and the overall waveform has no DC content.

The fact that the positive-going pulse has twice the amplitude of the negative-going pulse is useful for checking the positive peak modulation capabilities of the transmitter.

Stu


* AM Test Waveform 3.WAV (2432.04 KB - downloaded 172 times.)
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
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« Reply #9 on: September 17, 2016, 08:51:18 PM »

It looks, from this excerpt from the Canadian Government website, that Advanced licensees can run 1kW DC input.


Advanced Qualification:

access all amateur bands below 30 MHz
use maximum transmitter power of 1000 watts DC input
build and operate transmitting equipment
establish repeaters and club stations
remotely control fixed stations, including the use of radio links

Class E is a good thing to run there, because power is limited by DC input - so, the higher the efficiency, the better !!!
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VE3ELQ
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« Reply #10 on: September 18, 2016, 08:23:41 AM »

It looks, from this excerpt from the Canadian Government website, that Advanced licensees can run 1kW DC input.


Advanced Qualification:

access all amateur bands below 30 MHz
use maximum transmitter power of 1000 watts DC input
build and operate transmitting equipment
establish repeaters and club stations
remotely control fixed stations, including the use of radio links

Class E is a good thing to run there, because power is limited by DC input - so, the higher the efficiency, the better !!!

Here is a copy of the regs from here:
http://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf01226.html#p10

10.2 Amateur Radio Operator Certificate with Advanced Qualification

The holder of an Amateur Radio Operator Certificate with Advanced Qualification is limited to a maximum transmitting power of:

    (a) where expressed as direct-current input power, 1,000 W to the anode or collector circuit of the transmitter stage that supplies radio frequency energy to the antenna; or
    (b) where expressed as radio frequency output power measured across an impedance-matched load,
        (i) 2,250 W peak envelope power for transmitters that produce any type of single sideband emission, or
        (ii) 750 W carrier power for transmitters that produce any other type of emission.

73s  Nigel
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« Reply #11 on: September 19, 2016, 07:57:02 PM »

Wow, it looks as if you have a choice.  Obviously, the best choice for AM when using class E is 1kW DC input.

Pretty cool.  It certainly gets rid of any ambiguities in measuring power.  Even the 750 watts of carrier output makes sense if you're measuring your power in the output, rather than the input.
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wa1knx
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« Reply #12 on: September 22, 2016, 04:27:42 AM »

    I remember running pwm, you had the two power supply rails, your voice, and a carrier you could
set anywhere in between the rails and fit your voice as you see fit!   I had a HB negative peak gain limiter
I set at 99% and let the plus peaks go.

    I just finished a 811 modulator mod on my dx-100 with 1250-1300v on them.  it will do 150%+ positive. sounds
great, just my natural asymmetry but I have a DOD gated limiter on the audio drive and still adjusting it in.  I love
the multiband limiters to.

73 deano
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« Reply #13 on: September 22, 2016, 02:34:42 PM »

I like DOD stuff as well ....usually good value for the bux
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« Reply #14 on: September 24, 2016, 12:28:27 PM »

Canada is the one country to which I would ever consider moving from the USA.
Not that I ever consider moving from the USA.
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