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Author Topic: Modulation Monitor?  (Read 6897 times)
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KZ5A
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« on: March 27, 2012, 09:48:51 PM »

I am interested in building a homebrew AM modulation monitor but don't feel qualified to design one from scratch.

Digging around on the internet hasn't turned up anything useful.  Can anyone point me to a schematic or plans for one?

73 Jack KZ5A
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K6IC
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« Reply #1 on: March 27, 2012, 10:01:22 PM »

Hi Jack,

Steve,  WA1QIX (a member of this Forum)  has a write up and schematic here:

http://amfone.net/21stAM/mod_monitor.htm

For a time,  he made these and sold them from his company,  REA.

Vic
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N4LTA
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« Reply #2 on: March 27, 2012, 10:10:45 PM »

Interestingly - I am assembling a prototype PC board for Steves circuit tonite. I got all the IC sockets and bypass caps on the board and the power supply connections tested as well as tested the calibration oscillator.

It's an analog experimenter's dream - lots of trim pots and adjustments. I will use two 2 1/2" Simpson VU meters.

Pat
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« Reply #3 on: March 27, 2012, 10:15:35 PM »

Hey Pat,

VU meters might be close enough for ham stuff, but real mod monitors of the past had extremely specialized ballistics to track properly.  

Too bad no more are being made today.

73DG
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N4LTA
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« Reply #4 on: March 27, 2012, 10:18:42 PM »

Yes - Steve also used VU meters and has an analog ballistic adjustment built into his circuit to make the conventional VU meter somewhat adjustable.


Pat
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« Reply #5 on: March 27, 2012, 10:25:51 PM »

Hi Jack,

Steve,  WA1QIX (a member of this Forum)  has a write up and schematic here:

http://amfone.net/21stAM/mod_monitor.htm

For a time,  he made these and sold them from his company,  REA.

Vic

That circuit is old and has problems.  The monitor that R.E.A. sold was considerably different.  There is another monitor coming quite soon from R.E.A. that uses a software based display (there is a hardware component, but it is fairly small).  This one is very accurate!  The pricing is going to be very attractive.

Here is a screen shot showing all of the displays available:





* mm_screenshot.jpg (190.7 KB, 887x747 - viewed 209 times.)
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« Reply #6 on: March 27, 2012, 10:31:50 PM »

I should mention, the screenshot above is of the monitor playing back a "transcript" of a session recorded off a receiver I.F.  In addition to being connected to a transmitter, the Monitor can be connected to any receiver (with the addition of a simple interface), and you can watch the modulation of other stations, AM broadcast stations, etc.  The monitor allows you to record everything that is coming in from the monitor's hardware interface.  These transcripts can then be "played back" through the monitor as if it were live.

The station being shown at the moment is Frank WA3JBT, who has considerable asymmetry in his modulation.
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K6IC
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« Reply #7 on: March 27, 2012, 10:34:32 PM »

WOW!!

Sorry Steve,  did not realize that the linked circuit was  revised.

That new monitor looks great.  Some day,  will just have to get a computer at vintage operating positions.

73  Thanks,  Vic
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« Reply #8 on: March 28, 2012, 01:39:06 AM »

umm.. a CRT or scope? Although I like the REA products!
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WA3VJB
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« Reply #9 on: March 28, 2012, 05:08:06 AM »

I am interested in building a homebrew AM modulation monitor but don't feel qualified to design one from scratch.

Digging around on the internet hasn't turned up anything useful.  Can anyone point me to a schematic or plans for one?

73 Jack KZ5A

Jack, this schematic is from a little Sylvania device, an example of which I have.  The audio quality is very good from the diode detector involved, and the meter is adequate to observe negative peaks and avoid overmodulation.  That helps the adjustment of any processing you might want to run to increase your modulation index through peak limiting and compression. I recommend doing so.

Not sure from your post whether you're strictly interested in aural monitoring of your air signal (such as for making recordings or to drive your headphone amp), or whether you may want that plus a measuring device as well.
 


There's another version here.  This addresses the somewhat high coupling needed to provide adequate drive to what otherwise is a passive circuit (unpowered, no amplification stages).  No experience with this one.



These are part of an article in a 1989 edition of the AM Press/Exchange this website kindly has archived. Scroll down near the bottom.

http://amfone.net/AMPX/075.htm

You might want to poke around the internet a little too. A number of articles popped up when I was looking for the Sylvania documentation.

http://www.radiosparks.com/schematics.asp?UID=AM+Modulation+Monitor

For my time, and for not much money, I would search for a commercial AM broadcast modulation monitor. They're usually a rack mount chassis, with tubes !!! yay, and without modification can typically reach through 40 meters (General Radio 1931-A).

These come on the market fairly often, long since retired from AM radio stations. The FCC in the 1980s changed the type acceptance standard for required measurement of modulation levels, such that units not reading positive peaks to 125% (with overshoot to 133%) no longer were acceptable.

As AM stations bought new, compliant modulation monitors, the old units which measured only to a registered 110% were either discarded, or kept in the technical room of the station to provide a feed to the house audio system of the station's air signal. In the years since then I will guess most of them have been taken off the inventory lists.

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« Reply #10 on: March 28, 2012, 06:44:44 AM »

Steve
As usual, I'm drooling here with your latest and I always enjoyed the pos and neg readouts always available on your monitors.
Sorry to slightly hi-jack the thread, but what latency is there in the display to what is output from the computer and input from the analog source? Moving meters and wave form scope movement of incoming audio would be a challenge. How would you let us know of availability?
Sorry Jack, KZ5A.......I'm a lazy sort and building a monitor is out of my league. I have had wet dreams about Steve's AM monitors and lost out from the last production models.........They are rarely for sale on eBay.
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Jeff W9GY
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« Reply #11 on: March 28, 2012, 06:45:32 AM »

Hi Steve,

Will the new monitor have provisions to listen to your transmit audio in real time?  73 Jeff
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« Reply #12 on: March 28, 2012, 07:10:28 AM »

Hi Steve,
I'm looking forward to the release of the new modulation monitor. Is this new system going to have transmission line samplers based on transmitter power output like the last monitor? I'm also wondering if the samplers will have an AGC? Will I need a separate sampler to use with a receiver?

Since I am a Smug old codger with Flex radio and a computer in the shack the new setup will fit right in. Thanks for the effort. I like the OB meters. What about latency?

Mike
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The Slab Bacon
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« Reply #13 on: March 28, 2012, 08:40:06 AM »

I'm with Patrick, just pick up a cheap O'scope at a hamfest.........
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« Reply #14 on: March 28, 2012, 08:41:04 AM »

Steve,

Can you elaborate on the problems that the published circuit has? I have spent quite a bit of time building it.

Thanks

Pat
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« Reply #15 on: March 28, 2012, 10:03:36 AM »

Hi Steve,
I'm looking forward to the release of the new modulation monitor. Is this new system going to have transmission line samplers based on transmitter power output like the last monitor? I'm also wondering if the samplers will have an AGC? Will I need a separate sampler to use with a receiver?

Since I am a Smug old codger with Flex radio and a computer in the shack the new setup will fit right in. Thanks for the effort. I like the OB meters. What about latency?

Mike

Hi Mike,

The new monitor has AGC built into the hardware interface (and the software, too).  So, no "carrier level set" is necessary.

There are different RF pickup units for different power level ranges like 3 to 30 watts, 30 to 300 watts, 50 to 500 watts, 200 to 1000 watts, etc.  The numbers are for carrier output and up to 200% modulation.

There is a real-time audio output that comes directly from the hardware interface.

The display latency is very low (essentially real time).  The hardware interface uses USB which operates at 1000 packets per second.  The hardware interface sends 96 bytes per packet and this results in a sample rate of 48000 samples per second.  The monitor displays the result of each packet in real time, as it is received.  USB may buffer a very small number of packets, but the monitor takes packets as soon as they are available and operates on them and displays them as they come in.

Generally, for real-time modulation monitoring, the monitor will be used as a display, and the audio will come directly from the hardware interface.  This is how virtually all modulation monitors are used and how I use it myself most of the time.

The monitor also can function as a network server, and allow remote computers running the R.E.A. modulation monitor software to connect to it.  This is very useful in a broadcast station situation where the station engineer or others can monitor the modulation from any location (assuming sufficient network bandwidth) and also listen to the audio.  This type of use results in latency *at the remotely connected monitors only* because you are streaming data over the network, and a certain number of packets must be buffered to avoid drop-outs.

Connection to a receiver involves amplifying and demodulating the IF output.  I use a simple amplifier that brings the IF level up to about 3 to 4V p-p of carrier, which then drives a detector that is the same as the detectors in RF pickup units.  I may create a PC board and kit to facilitate connection to a receiver if there is sufficient demand.  In any case, the schematic will definitely be published.

A stand alone hardware display similar to the existing R.E.A. modulation monitor will most likely be produced as an add-on.
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ssbothwell KJ6RSG
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« Reply #16 on: March 28, 2012, 03:47:16 PM »

i have a few basic question about how all modulation monitoring is actually done in the shack.

1. do you guys constantly monitor (either with a dedicated mod monitor or an o'scope) during all transmission or do you just check it when calibrating your rigs?

2. is the modulation monitor, such as steve's design, 50 ohm terminated so that it can be used like a dummy load in place of your antenna when calibrating your rig?

3. if you want to monitor during regular operation, how do you get low power rf out to the modulation monitor/o'scope from your final amplifier that is separate from the main rf out? do you use a power splitter/combiner?
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« Reply #17 on: March 28, 2012, 04:10:12 PM »

I would think that a SB-610 or HO-10 would be a natural for this?

Carl
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KZ5A
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« Reply #18 on: March 28, 2012, 04:39:12 PM »

The "Circuit 1" from the ampx article looks pretty close to what I was looking for, no stinking IC's or transistors, and gives selectable monitoring of pos or neg peak levels.   Looks like it could be easily modified for two meters.   Might grit my teeth and include an LED for peak monitoring.

The REA stuff is nice looking, too expensive, and not appropriate for a vintage station IMHO.   If I ran AM with my K3 (which I don't) the REA box would fit in nicely.

I have a SB-610 and a B&K shop scope,  the SB-610 has focus issues still remaining after replacing the HV rectifier and filter caps to get the trace back so it's only marginally useful, don't really want to drag the "real" scope to the operating position.   Even if it was working correctly, I would prefer meters.

Thanks to everyone for the input.

73 Jack KZ5A
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flintstone mop
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« Reply #19 on: March 28, 2012, 05:23:54 PM »

uh!!
OK Steve, I have your REA website book marked and will watch for the availability of your latest.
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« Reply #20 on: March 28, 2012, 08:33:20 PM »

i have a few basic question about how all modulation monitoring is actually done in the shack.

1. do you guys constantly monitor (either with a dedicated mod monitor or an o'scope) during all transmission or do you just check it when calibrating your rigs?

2. is the modulation monitor, such as steve's design, 50 ohm terminated so that it can be used like a dummy load in place of your antenna when calibrating your rig?

3. if you want to monitor during regular operation, how do you get low power rf out to the modulation monitor/o'scope from your final amplifier that is separate from the main rf out? do you use a power splitter/combiner?

Good questions, all.

I constantly monitor my air signal with a bench quality oscilloscope, not a hammy hambone type.  Analog 30MHz scopes (to get you through 10 meters) are a few hundred bucks these days, and well worth it for air signal and bench work.  I connect whatever antenna I'm not using to the BNC of the scope, and turn the input sensitivity to the minimum needed for a triggerable good pattern. This avoids stray RF pickup that a broadband input otherwise might detect on a scope. Adjust sweep speed for a few iterations of the waveform, and you've got a picture.

The RF pickup for the mod monitor is a simple "gimmick" coupling coil near the RF tank. Not enough to affect tuning of the tank, but enough to provide drive to the modulation monitor, which is also in use whenever I'm on the air. It's not critical, since the units have input level adjustments. I have a 1956 RCA BW-66 that is tuned and calibrated to one band, and a 1962 General Radio 1931-A set for another band.  Saves having to re-adjust everything for band changes. Same pickup coil. I move the coax. Never drive the transmitter directly into the unit.

They provide both an audio monitor (e.g. you can put your audio on a channel of your recording machine, and the received audio on the other channel, and you also can hear if you are working the mic properly), and for measurement and indication.

The mod monitors came from broadcast service and don't cover above 40m, so I don't use one on 15 and 10. Wish I could find a Gates HF modulation monitor that they sold with their shortwave AM transmitters, but I've never even seen one for sale.

Hope helpful.

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« Reply #21 on: March 28, 2012, 09:35:41 PM »

Has anyone esle tried to build the AM modulation monitor mentions in the second post in this thread? Also is the best diode for detecting AM still the 1N34A or are  the hot carrier diodes, PIN diodes like the 1N5719,  or shotkey diode a better way?

http://amfone.net/21stAM/mod_monitor.htm

Pat
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« Reply #22 on: March 28, 2012, 09:54:59 PM »

i have a few basic question about how all modulation monitoring is actually done in the shack.

1. do you guys constantly monitor (either with a dedicated mod monitor or an o'scope) during all transmission or do you just check it when calibrating your rigs?

2. is the modulation monitor, such as steve's design, 50 ohm terminated so that it can be used like a dummy load in place of your antenna when calibrating your rig?
9
3. if you want to monitor during regular operation, how do you get low power rf out to the modulation monitor/o'scope from your final amplifier that is separate from the main rf out? do you use a power splitter/combiner?

1. Like Paul says, all the time. This can be your first warning if something is going wrong.
2. Steve??
3. Look in forum section Hints and Kinks for a post named Scope Pickup (IIRC) for a very simple pickup that can be very cheaply made. I think the post is by K1JJ.

Bill
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« Reply #23 on: March 29, 2012, 12:29:50 AM »

I use the CleanRF AM 5k sampler and an tek 2245A scope here. I also have a very nice Gates mod monitor with an external second Mod meter thanks to my friend W7TFO.  Here is a video of the gates

http://youtu.be/iI-OoBbgXY4

 Steve, Put me down for a monitor please. I cant wait to run that on the flat panel in the shack and get rid of the Scope.

C
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« Reply #24 on: March 29, 2012, 12:42:35 AM »

Has anyone esle tried to build the AM modulation monitor mentions in the second post in this thread? Also is the best diode for detecting AM still the 1N34A or are  the hot carrier diodes, PIN diodes like the 1N5719,  or shotkey diode a better way?

http://amfone.net/21stAM/mod_monitor.htm

Pat
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As far as I know, I have the only successful implementation of the circuit, and in the implementation I have, there are many deviations from the original.  I finally removed the circuit and article from my web site completely because I was constantly answering questions about adjustment, accuracy, etc.  Tongue   Several years later, when I completed the design and built the final version of the R.E.A. AMM-HF1 modulation monitor, I had redesigned almost every circuit, and added others.  

The upcoming modulation monitor referenced elsewhere in this thread has virtually nothing in common with its predecessors - there are almost no analog circuits at all, using A to D converters, a microprocessor and digital circuits for the implementation.  This results in a more accurate monitor, and there are no calibration pots or other analog adjustments needed.

As far as diodes, a number of diodes can be used.  The key is a very low forward voltage drop, and the ability to operate at high frequencies.  I can't cite specific part numbers right off the top of my head, but I know there are other diodes that will work just fine.

RF pickups - A resistive voltage divider works quite well, as will a current transformer.

Oscilloscopes - Personally, I like a scope because it shows the shape of the modulation envelope (notice the new modulation monitor has a scope display), but have never found them to be overly useful for exact or even close measurements of dynamic waveforms.  This is because the modulation envelope moves so fast, most of the time you miss the event you're looking for (overmodulation).  If the overmodulation is very severe, it will be visible on a scope for sure, but trying to see an every-half-second overmodulation event (or even a more regular, but very short duration event) is certainly difficult.  A storage scope would work well but most people don't have one.   Scopes work extremely well when looking at stable, repetitive waveforms.
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