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Author Topic: Modern AM Modulation ???  (Read 1656 times)
WA2ASB
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Posts: 39




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« on: June 20, 2013, 07:20:14 AM »

OK, I'm an old fossil, but I noticed that the K3 has an AM modulation feature.  Of course you need a different filter than what I have, but it got me to wondering how AM modulation works with solid-state rigs.

When I got my ticket in 1957 plate modulation was the way to go.  There were other forms like screen-grid and even cathode, but plate modulation was the best.  However, it required a big modulation transformer if you were to run any kind of power.

Since I didn't see any big transformers in the K3 when I was assembling it, it got me wondering how they modulate.  Can anyone give me a short, simple explanation?
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KE3WD
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« Reply #1 on: June 20, 2013, 07:39:28 AM »

Bing search, "low level AM modulation" keywords being "low level".  

Basically, the AM happens before the PA, which, of course, must be linear.  

One way for the beginner to possibly better understand and get a handle on it is to examine the schematics of solid state CB transceiver modulation schemes, you will find some with modulation transformers, these emulate the old plate situation but with solid state output device(s), others you will find the Low Level Modulation scheme being used.  Big key there is no modulation transformer to be found.  

Perhaps the biggest mistake modern hams make when attempting to work AM mode with the modern solid state transceiver is to turn the RF output UP instead of down.  Running the RF all the way up or even too high leaves no room for good solid modulation.  For most of the modern rigs, setting the Carrier to be about 1/4th the full output power available is a very good starting point.  Then turn RF power DOWN to get more modulation into the sidebands and a nice, full sounding AM on the air.  Don't fixate on the Carrier power, realize that the greater majority of what's happening with AM is in those modulated sidebands. 



73
« Last Edit: June 20, 2013, 07:42:38 AM by KE3WD » Logged
WX7G
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« Reply #2 on: June 20, 2013, 08:13:59 AM »

With properly adjusted equipment plate modulated AM, collector/drain modulated AM, and low-level AM produce exactly the same RF waveform. One monitoring the signal with a spectrum analyzer or other instrumentation cannot tell you which is being used.

On AM the K3 runs 25 watts carrier hitting up to 100 watts PEP. If one wants to run the legal limit the amp is set up for 375 watts of carrier and no more.
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WA2ASB
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« Reply #3 on: June 20, 2013, 07:25:26 PM »

Thanks guys, I really appreciate the information.  I remember when they took 11 meters away from us and gave it to the CB'ers.  The CB'ers frequently bought bootleg linears for running AM on 11 meters.  (This resulted in my Drake L4B not covering 10 meters.  Another case of don't punish the wrong doer, but everyone else.)

Anyhow, from what I read, running AM into a linear amplifier was not very efficient.

In spite of my difficulties building the K3 because of my big paws, I was so impressed with the KPA500 that I saw at Hamcom in Plano, TX, that I have one on order.  I'm assuming that running this on AM is a feature, but efficiency would be very poor.  I remember my late Elmer's (W2ZOL) BC-610 and what a monster it was, but it was AM.  (It had a sticker on it with the call sign W2BIB, which was his late wife.  If any reader has it, I'd be interested to hear from you).

Having retired from IBM having spent 30 years directly working on hardware and later software, I must admit that I'm a little overwhelmed by today's radios with their computer interfaces and operations.
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WA3SKN
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« Reply #4 on: June 21, 2013, 04:03:48 AM »

"Anyhow, from what I read, running AM into a linear amplifier was not very efficient."

AM is not efficient... period!  Thats why we go to the complexity of SSB.  Costs more, but worth it!
73s.

-Mike.
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WX7G
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« Reply #5 on: June 21, 2013, 04:30:25 AM »

Looking at the KXPA500 manual on page 15 it reads "Power Output 500 watts PEP/SSB/Data" and "Duty Cycle at 500 watts 10 minutes key down / 5 minutes standby."

This indicates it is up to the job of running AM. At full RF output the efficiency is about 50% (more or less depending on the load impedance and the drain voltage). When running 125 watts RF carrier with AM the efficiency will be about half (25%) of full power efficiency. Drain dissipation will be almost as much in AM as in RTTY, around 500 watts and the amp will do fine. The K3 will be set to about 10 watts of AM carrier.
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AA4PB
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« Reply #6 on: June 21, 2013, 05:15:37 AM »

With the K3 all of the modulation takes place in the DSP chip. The resulting low level signal is then amplified by the linear PA. For AM its not as efficient as plate modulating a class C amplifier but given the current technology doing it in the DSP makes it a lot more versatile.
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WA2ASB
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« Reply #7 on: June 21, 2013, 05:40:03 AM »

I agree with Mike, with modern equipment it doesn't make much sense to run AM.  The K3 covers 6 meters as well, and that got me thinking about 6 meters where I used to run a Tecraft xmitter on AM as a teenager.  I don't currently have an antenna for 6 meters and I haven't even been listening on the band, but I'm betting most activity there is probably SSB.

I remember that Dick, my Elmer, had a transmitter, or maybe it was a transceiver, and it was DSB rather SSB.  I don't recall who made it, but it sure was small compared to the BC-610.
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