The AM Forum

Visiting another ham shack is one of the most fun part of the hobby.
Last Friday I visited the Tom Vu antenna jungle and hung out.
Last night Steve Qix stopped by with his new mod monitor and we hooked it to my 160 meter rig. Wow we both sound much better on his audio system.
He made me rethink audio processing when he pointed out the crunchies when the audio goes above 100 %.
Then I showed Steve my RX disease in motion.
A clean shack is the sign of ......... but you do need room to fit visitors

Speaking of junk yard, check this out.

http://coolcatcorp.com/millerauction/MillerAuction.html

Yieks.......nice stuff!

Wow! Hopefully there are still some kin folk or other benefactor alive to receive something from that estate! I'd really hate to see the state or some unrelated 3rd party get every penny.  Especially for what some of that "Junk" is worth! Such a shame to see a car that cost about $500 back in the day go for $12,000 and having it all go into a bureaucrats pocket!

Hi Frank,

Are you talking above 100% in the + or - direction? Of course, in the negative direction it cuts off the carrier and the detected audio sounds like crap, especially on diode envelope detectors. Synch detectors make it much more tolerable.

Above 100% + shouldn't sound any different than symetrical modulation, except of course, louder.  ;D

Cheers

Paul
VE7KHz

Paul,
I went back and rechecked everything and I find the am demodulator in the RA6830 a lot cleaner than the diode detector in Steve's mod monitor. I heard distortion just above 100% on the diodes but no problem on the Racal until I clipped or hit the base line. My point was a little audio processing would have sounded better. I can hit 150% positive on the 160 meter rig.
Steve and I both sound better on his rig with audio processing. I never noticed it until I heard him on my rig.
This makes me think I would sound better with a little audio work.

True, up to a point. After which, the detector and/or the AGC begin to induce distortion.

Hi Frank,

The modulation monitor detector is not going to introduce any distortion to the signal 8)  It's way too clean for that.  I'm wondering if either something changed, or more likely, the receiver IF is filtering stuff out that we were able to hear in the monitor.

Remember we thought you might have some RF getting in somewhere?  Did something change?  The problem may have lessoned due to rain/antenna changes/whatever....    We both heard the transmitted distortion that night I was there (and, we could even see it on the 'scope).  We did think it might be RF, because of the slope change - almost like feedback, but not quite.  It was fairly substantial.  From time to time, I've heard that distortion on your signal, on the air, although it was dramatic listening to it using the modulation monitor !

The mod monitor detector is completely linear up to over 200% positive modulation (I never checked it any higher - I don't have any transmitters that will go much over 200% positive) and there is absolutely nothing in the system or detector that would limit, in any way, the proper demodulation of a heavily modulated signal. 

You will most certainly be able to hear any distortion that you might be transmitting while listening to the modulation monitor output - distortion that you might never hear on the air or even through a very good receiver, due primarily to the extremely wide bandwidth of the monitor, and the flat response to above 20kHz.  A receiver would need to have a 40kHz bandwidth to reproduce that !!! ;D

Check this out:

Here is a modulated triangle wave and the output from the modulation monitor, demodulating that signal.  Note that the output from the monitor is an exact demodulation of the transmitted signal. The output from the monitor is the top trace.  The modulation monitor display is also shown:

[img ]http://www.radioassociates.com/mod_monitor_output.jpg[/img]
[img ]http://www.radioassociates.com/mod_monitor_100_percent[/img]

Below, what things look like at 150% positive - output is an exact demodulation of the transmitted signal:

[img ]http://www.radioassociates.com/mod_monitor_150_percent_output[/img]
[img ]http://www.radioassociates.com/mod_monitor_150_percent[/img]

The detector in the modulation monitor is extremely linear!  The headphone output is flat from a few Hz to well over 20kHz, and the headphone amplifier in the modulation monitor is absolutely superb (and has lots of output, and a phase switch!).  For better or worse, if it's on the signal, you're going to hear it :o

On listening in a local receiver:

I've got what I consider to be a very good hi-fi receiver. It will go to 18kHz bandwidth, with very shallow slopes on either side, so it's REAL wide.  The detector is nice.  Full wave, precision rectifiers.  But it just does not compare to the modulation monitor when I monitor my transmitted signal and switch between the monitor and the receiver.  The receiver is good, but the monitor is much better.

If there's any garbage, distortion, frequency response problems, etc. I hear all of if with the monitor.

I guess the main point of the discussion is, the receiver, by definition, has to color and filter the signal somewhat due to the bandwidth limitations, even if it's really wide  ;)

Talk later and Regards,

Steve

Steve I was listening a wide bandwidth with a long AGC. Yes I could hear distortion when I was cutting off the carrier or clipping. It did not sound as bad as it did on your detector above 100%. That triangle at 150% what does it sound like in the head phones. Would you hear distortion at cutoff?
Hands down we both sound better on your audio system. I wonder if it is due to
processing or something else going on. I sent a 1 KHz triangle through the modulator a while ago that looked pretty good.
OH well we had a good time.
I do need to build an off air monitor though. The Racal may be filtering out some of the crud. 

Hi Frank,

Well, If I were to listen to the clipped, 150% modulated triangle in the 'phones, it would sound - clipped, because it is   ;)  It's an accurate reproduction of what I'm transmitting, which was also clipped.  The output I show on the 'scope pictures above *is* the headphone output from the modulation monitor.   Pretty damn clean  8)

But, I have found pretty much 100% that I can hear a much more accurately in the modulation monitor than any receiver - if for no other reason, no receiver I've ever seen has a 40kHz flat bandwidth.

I think you're right about the receiver filtering out some of the crud.  Same thing happens here, and my receiver is really, really wide.

You know what's cool - I sometimes hook the modulation monitor to the IF output of my receiver and listen around down on the broadcast band through the monitor, and look the the asymmetry (or lack thereof !!) of various stations.  Some of them are BAD - some, out of phase,  some overmodulate constantly.  It's getting to the point where "sounding like a broadcast station" might not be a compliment anymore :-)  :D ;D 8)

It was a great visit with you and the family, Frank.  One casualty - I seem to have lost my blue LL Bean winter jacket.  Did I leave it at your place?

Talk later and Regards,

Steve

Steve,
I'll look around the house and see if you left it at our place. Marie didn't mention anything.
It was raining so I'm sure she didn't put it in the closet.
I think I need to build an off air monitor or take audio off my directional coupler.
I think an audio generator is a better way to check things out since I don't have a compresser.

Steve, Marie said you were not wearing a jacket and she found nothing. fc

Ok, thanks !!!  I must have left it somewhere else !!! I've done this sort of thing before..  Thanks for checking !!

BTW the K2CU AM detector is the cleanest I have ever seen.

I was at that auction, Christies ran it IIRC. Loaded with interesting stuff. Miller was big into anything Stutz or Auburn, or anything mechnical for that matter. He sold the gyro to a museum (maybe NASM? Can't recall). They moved it out under the cover of darkness.

Some of the cars he had were the only ones known to exist, others were one of two, and so on. He even had a Stutz fire engine. One of the Messier brothers (house moving company here) used to go visit him and sit by the woodstove with his foot on an old nail keg full of nails, except the nails ended up being gold or silver ingots, can't recall which. He ended up buying one of the Franklins at the auction.

The wedpage makes light of his rusty VW beetles, but several of them were quite old and apparently valuable, as one rusty greenish blue model brought several thousand dollars in its rusted out state. If you look at some of those pictures, you can see some parts of the cars are restored. He'd take a piece off, restore it to new condition, and replace it. He did excellent work using old techniques and materials. You couldn't tell it from new. The Boattail had some really cool headlights!

The interesting part was that Miller had no SS number for the gov't to track him by, and no bank accounts. He'd flown for the RCAF in WWII, apparently using his flights to and from Europe to bring back more gold, silver, whatever he got his hands on. He'd find out about a car somewhere and be gone to get it, driving it back to arrive home at night.

Alex fell off a ladder in the early 90s fixing his roof and crapped out shortly thereafter. Imogene died a few years later, kicking off the feeding frenzy. The property was well-searched, many a hole was dug. My ex worked at the bank in Barre where they kept his loot. The last time I got a look at it the pile was around 1.3 mil. He had coins, ingots, you name it. This was well before the auction. The gov't ended up with most of his money more because they had no relatives around to claim it. With no means of tracking where he'd worked or how much he made, the IRS just firgured out what he must owe from the proceeds.The Millers did leave a lot of it the SA or some group, but things weren't laid out in a clear enough way to deal with today's lawyers who were circling nearby.

I still have my auction catalog, stashed somewhere safe (translation: AWOL). Only made one full day of the auction and had a great time in the tent watching and listening as phone bidders bought most all of the good stuff. Went back for the 'spare parts and paper' day but it rained cats and dogs so I left. A lot like Hosstraders when it rains, many deals to be had.

The most amusing point for me was kicking off the bidding at $55K on one of his old Rolls Royce projects hidden in a barn. Amusing because my ex damned near passed out (we didn't have remotely close to that kinda money) and the look on her face was priceless. Talk about the color draining from someone's face!  ;D  To her it was a dirty old car that wasn't worth anything, but even my basic knowledge of cars told me $100K or more, so I wasn't worried about winning. It went for $120K, still a deal for a car that old, rare, and well-preserved.

If I remember (or someone reminds me), I'll bring the catalog to Hopkinton in May. Have a distinct feeling I'll still be here.