AM Modulation Monitor Designed for US!

[John K5PRO]

I use a Harris AM90, which is fairly similar to yours, except it uses the funky neon plasma display for the bargraph. It has cheap switches and thumbwheel switches for the peak flashers.
http://www.broadcast.harris.com/product_portfolio/prod_media/am90.pdf

 I like the features that you had put in yours, like the front panel headphone jack. Also, the multiple RF input selector. What is the RF level it will be optimized for? Odd thing, the old mod monitors (including the AM90 which is a current piece) have higher input impedances and then spec an RMS carrier voltage. I assume that the pickup loop in BC transmitters can drive that fine. I like to see 50 ohm inputs with RF level in dBm for RF test equipment.

What sort of THD/IMD performance can be expected from using your monitor, Steve? This, of course, depends on your demod, and your op amps and their operating point.

[steve_qix]

I use a Harris AM90, which is fairly similar to yours, except it uses the funky neon plasma display for the bargraph. It has cheap switches and thumbwheel switches for the peak flashers.
http://www.broadcast.harris.com/product_portfolio/prod_media/am90.pdf

 I like the features that you had put in yours, like the front panel headphone jack. Also, the multiple RF input selector. What is the RF level it will be optimized for? Odd thing, the old mod monitors (including the AM90 which is a current piece) have higher input impedances and then spec an RMS carrier voltage. I assume that the pickup loop in BC transmitters can drive that fine. I like to see 50 ohm inputs with RF level in dBm for RF test equipment.

What sort of THD/IMD performance can be expected from using your monitor, Steve? This, of course, depends on your demod, and your op amps and their operating point.

Hi John,

The AM90 is a good unit although some features I would like to see are missing.  Last I checked, they were selling for about $2500.00!!  Don't know  if that's changed or not. 

For the REA Modulation Monitor, the lowest RF level required is about 5 RMS volts of carrier or thereabouts.  The RF pickups are high impedance.  The pickups have carbon resistor attenuators for various power ranges, if a direct tap across coax is desired.  For instance, in my own station, I use a pickup that covers the range of 50 watts carrier to about 500 watts carrier.  This is connected directly to the coax cable going to the antenna system.  The carrier level control on the monitor is used to set the reference level at the monitor itself.

The frequency response is 10-20000cps.  Exact distortion measurements will be forthcoming.

Talk later and Regards,

Steve

[w1guh]

In reference to the scopes...don't forget the trapezoid display.  I've never taken the time to do that with the AF on the horizontal input...never got the phase quite right, but I HAD to do an RF trapezoid when I did AM linear.  The envelope display's pretty, but, boring as it is, the trapezoid display is a very good linearity indicator (either of a modulator or an amp), and does an excellent job of showing 100% negative modulation. 

Anyone do this with the AF/RF hookup?

BTW...re: hitting "back" instead of "Post".  I've got this Logitech Marble Mouse (Mouse Ball?) that's got a back button built into it that's really easy to hit by mistake.  It's made me re-type a lot of things (til I figured out to just go "forward").

[K1JJ]

In reference to the scopes...don't forget the trapezoid display. 

Paul,

I've never used a trapezoid pattern for critical measurements.  Could you give me an idea about what definiton level you can determine?

ie, If you were to have a full trap scope pattern to look at and see a very slight bend on the trap's side, what kind of IMD or distortion are you able to see?

When using a standard RF envelope on a scope, I can see when peaks are rounded or flat, and other things. In fact I was told with a well trained eye,  we can see subtle differences in IMD up to -30db 3rd or so. But maybe the trap shows some linearity clues that a tone going thru the rig with an envelope display cannot?

Then again, I know a guy who uses a trapezoid pattern to monitor his AM waveform, but has splatter 30kc up the band at times until someone lets him know... so I don't know... 

Any ideas?

Tnx,
Tom

[John Holotko]

All true to an extent. I can see EVERY peak on the scope. No need for peak hold. I can measure percentage rather closely, not that it's a big deal to me. I just want to be as close to 100% without going over on the negative peaks - all easily seen on the scope. I will not only have a quantitative measure, but a qualitative one too - I can see any hum, some distortions, oscillations, or other non-voice modulation of the carrier. In fact, over time, I've learned what my voice modulation waveform should look like when everything is working FB. If something changes, it becomes evident almost immediately.

All that said, some people do not prefer to watch a scope when they are on the air. I can understand that, although I feel like I'm flying blind without one. For those people, the mod monitor is a good tool. The audio monitor is a nice feature too.

I wonder if it would be safe to say that the modulation monitor is a useful and handy device to complement a working scope and the AM station.  But I don't see it as a replacement or a substitute for a real working scope. Given a choice of having a decent scope or the monitor I'd have to go with the scope. 

[w1guh]

Tom,

Yea...

Understand that RF Trapeziod is a hulava lot easier than an AF trapezoid...expecially with a Heath Monitor Scope and my experience
is only with the RF one, but this is what I saw....

Mostly qualitative.....

If your under-modulated, you'll only ever see a trapezoid.

If your modulated 100% , you'll see a triangle.

If you're overmodulated positive, you'll see...well, a triangle with a rectangloe appended to it.

If you're overmodulated negative, you'll see a triangle with a line extending out from the negative side.

I found that the flat-topping is much, much more obvious with a trapezoidal than with an envelope.  Sometimes it's hard to see
quick flat-topping.

And for linearity...well, a straight line is obvious opposed to sometihg other than a straight line. But if there's a slight curve I don't have the experience to translate that to per cent distortion.  I'm sure it can be done, though.

When I ran AM linear, I could see the pinch-off distortion on the negative side.  It's not clear to me if that pinch-off was in the linear or the modulater (can anyone clue me in?).

Hope that helps.  All qualitative, but as boring as it is, I found the trapezoid to be a better alternative.

The dowside is that unless you get the AF phased correctly the display's a mess.  I know there's good scope photos of this in old handbooks, and also in the Monitor Scope manual.

Paul

[K1JJ]

Interesting, Paul.

On your question about seeing pinch off...   I wud say it's the modulator, since a normally functioning  linear's output is simply a "perfect" reflection of what it gets at its input. Other than flat topping from saturation, or severe crossover distortion caused by running below cutoff, linears are pretty transparent to whatever goes in.

One day I will try a trap pattern when I start testing the new projects coming up. Usually I do either a single tone envelope on the scope for AM and/or listen carefully on the RX for the bandwidth IMD of a linear.  The trap may be another technique to add to the arsenal.

Anyone else have observations about the finer points of using a trap pattern to see distortion?

T

[w1guh]

Yea, in thinking about it I'd guessed it's probably the modulator. 

One of the points I didn't say directly is that it's much easier to tell a straight line from a non-straight line or judging if a sin wave is good.

Oh, what the heck...this reminds me of getting that 4x1 on the air.......

I first tried to tune it up with an envelope....that was impossible...just COULDN'T get a pattern that looked right.

So...it was time to make the jumpers to try the trap...didn't know what else to do.

So...didi that and tried her out again....

EUREKA!  The problem was perfectly obvious immediately....I was flat-topping (dummy load) because AM Linear requires HEAVY loading on the final...e.g., the load cap is at max loading.  Viola!  Had a good traingle.

Reminded me of screen modulation...which also requires heavy loading.

And some other notes about that workhorse.

So I got everything right so the Coaxial Dynamics meter said I had 500 watt of carrier.  Cool!

Then I did the math with the 4x1...EGAD!!!!!  2100 Watts!!!!!!  Oh F-------!

Hey, nothing like dissapating 1600 watts on a plate rated for a KW, right?  The tube soldiered on wonderfully.  (for how long, I don't know).

Um...technical question.  Is that effiency par for the course?  It surprised me.

Another thing the amp did was that after a while, the loading cap shorted.  It's a multi-section broadcast type.

I fixed that and went back on that air.

But, everytime I fixed it, it didn't take much AM operation for the problem to happen again.

I'm guessing is that what was happening was the loading cap, while perfect for SSB/CW service just didn't have the current capability that I was subjecting it to...the plates were heating up and warping.

Hey, experts, is that a good diagnosis?

Paul

[w3jn]

A good way to see distortion is to set your oscilloscope (NOT a Heathkit monitor scope, for god's sake) to X/Y and look at the gozinta and the gozouta of the amp, mudulator, or whatever.  A perfect linear relationship will be a STRAIGHT diagonal line.  Distortion will show up as bends in the line or little curly cues at the ends.

[VE7 Kilohertz]

Hi Steve,

Nice job!! Reminds me of the Harris AM-90. Will it work similarly to a modern commercial monitor? i.e.  10 Vrms in broadband, "no tune" etc?

Is this your company? Cool. I tried making a go of a business from one of my hobbies. It was alot of fun and a good learning experience.

Good luck!!

Cheers

Paul

A Modulation Monitor designed for the AM Community!

This is a peak reading instrument, showing your true modulation percentage
at all times.

The monitor features separate high resolution (100 LEDs total)
positive and negative peak meters, and both meters include a peak-hold function.

An adjustable negative peak flasher is also included.

Unlike most commercial modulation monitors, the AMM-HF1 will accurately display modulation
levels up to 150% positive, a modulation level achieved by many good-quality amateur transmitters.



Click on picture or here to view larger image

For more information, including a complete list of features, check the Radio Engineering Associates' Web Site

Regards,

Steve

[w1guh]

" (NOT a Heathkit monitor scope, for god's sake)"

Why?

Are there problems with the amps that introduce non-linearities?

That's about the only reason I can think of...

But wait,

The Y input has no amps that I can recall...

And you just can't beat the convenience of the switching...you leave it in the line all the time and have a full time monitor.  There's
no, "Well, the 'scopes being used for something else" excuse.  You don't need any calbration at all for waveform interpretation.

[w3jn]

 A Heathkit monitor scope generally won't have the necessary sensitivity to look at the input vs output of a relatively low level stage.  That, and they're 10X what a decent scope will cost, have no amplifiers, can't be used to troubleshoot anything, are prone to HV crapout, have very limited sweep range, have no triggered sweep, have terrible sweep linearity, and are generally hammy hambone junk. 

Uhh.... did I miss anything...?

Seriously, a decent scope will do everything a Heathkit monitor scope will do, do it much better, and can be used for a myriad of other uses around the shack.  A Heath monitor scope is light years ahead of nothing.... but $5 spent wisely for a used bench scope at a hamfest will reap many benefits.

73 John

[WA1GFZ]

Think you covered it John. My 7904 cost me $75 and $100 for plug- ins I can easily
see a 1 GHz signal. I have the heath spectrum display which is another toy HOJ.