The AM Forum

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

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

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
N4LTA

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

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

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:

(http://www.classeradio.com/modmon_screenshot.jpg)

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.

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

umm.. a CRT or scope? Although I like the REA products!

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.
 
(http://amfone.net/AMPX/075_1.jpg)

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.

(http://amfone.net/AMPX/075_2.jpg)

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.

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.

Hi Steve,

Will the new monitor have provisions to listen to your transmit audio in real time?  73 Jeff

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

I'm with Patrick, just pick up a cheap O'scope at a hamfest.........

Steve,

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

Thanks

Pat
N4LTA

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.

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?

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

Carl

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

uh!!
OK Steve, I have your REA website book marked and will watch for the availability of your latest.

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.

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

Pat
N4LTA

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

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

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.  :P   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.

Precision active rectifier eliminates worries about junction voltage. Pretty easy to get over 60 dB of dynamic range. The best circuit I have seen is Rob K2CU's circuit.
1N34 is a leaky hos very unstable over tempreature.
Stewve I wonder if you looked into LCD panel displays to make it stand alone.
Now that would be cool and run at a lower power than having a computer running just to write a display. Might take another pic but would br cooler.
Another option would be to drive a flat screen monitor.

The 50MHz Tek 453A and several nice Leader models are under $50. For $200-250 and a little looking you can find a working HP 141T spectrum analyzer. I use the 453A to look at patterns and the 141T to see what is really happening beyond the sidebands as well as their actual transmitted width.

Having a PC display would be nice but if its pricey I can live without it.

Carl

Anything that requires running a computer in the ham shack isn't gg to get off the launch pad here.  I guess I'm comfortable using a scope to monitor my negative peaks--I let the positives land where they may, but I run compression and peak limiting.

Re: Questions from: ssbothwell KJ6RSG on Yesterday at 14:47:16"

1. A scope or CRT circuit is used always while transmitting. I like it because it is always easy to see if 100% negative modulation has occurred. Used CRT-type scopes are pretty cheap for now.

2. I think you still need a dummy load there. Usually a monitor and a load are separate items when any power is involved.

3. It is crude and may leak a small amount of RF but I use a SO-239 "TEE" where the PL259 port is screwed into the amp output and the coax screws into one SO239, and the other SO239's 'socket' has one pin of a dual banana pin connector plugged into it. The small capacitance between the two banana pins couples enough signal to the scope to display well. A X10 probe is hooked to the other banana pin and the probe GND goes to the amp GND. It does not seem to disturb the amplifier output and I don't have to worry about cooking the scope.

http://www.cleanrf.com/ makes a ready to use sampler or demod., could be used with anything.

There are enough choices for doing this monitoring function to pick what you like!

Well I did build a variation of the analog monitor that Steve published.  But I did not use meters...I drove some LEDs....I think 20 per side.  I don't recall the exact drivers but it is all downstairs somewhere.  I built it for my bench as I was tired of unhooking the REA unit and bringing it over.  I did not use the carrier osc circuit and used a voltage divider pick-up to pick up the RF.  Oh and it was a PITA to get it to work correctly.  Having said that, it is nothing like the REA as far as accuracy, but does the job for me when checking out a TX.   For a station monitor....I highly recommend Steve's monitors and suggest either find one of the REA units or wait for his newest. 

The fellow who posted about not wanting to use an REA (if he could find one) for his vintage radios....I completely disagree.  The REA is dead on accurate and will help you to look at your modulated signal.  A scope is always useful, but the REA is the best modulation monitor I have yet to see whether looking at a modern rig or something 60 years old. 

Peter

A mod monitor is fine, but don't even consider getting on the air without a scope!    And a HO-10 or SP-610 is fit only for swinging it around by the line cord and smashing it against the wall  ;D

A HP8901A mod monitor is the bees' knees if you can't afford or can't get a REA.  Built in freq counter/receiver too.  I've gotten several for about a hundred bucks.  It certainly doesn't fit with a vintage station though.

A simple mod monitor is a fun and interesting HB project if you don't want something more complex.  Score a scope first though - not only is a decent scope necessary if you're running AM but it's extraordinarily useful for troubleshooting.   Next to a DVOM or VTVM, a good scope is the most useful accessory you can own.

Here's another thread discussing the same issue http://amfone.net/Amforum/index.php?topic=15743.0

I built Steve's analog monitor (before the LED one hit the market),  Its in use every day...Bought a bunch of cheap dual op-amps, and tacked it together on a hunk of perf-board. I calibrated the unit somewhat differently than the procedure Steve outlined.  Basically adjusted things using voice waveforms and a scope looking at the modulated output of the transmitter. Also, changed some time constants to use a big buzzardly VU meter.  Audio output on the unit is indeed high fidelity and, with a little more amplification, drives headsets for constantly monitoring your signal quality.

From the information in that thread, I built several variations of the ECS monitor mentioned by Jeff W2NBC. Stu AB2EZ suggested a few changes to allow use with higher power and I ended up with this circuit:

http://www.w1aex.com/756AM/RFsamp.jpg

The audio output will drive a pair of headphones fine, but at the Flex operating bench I run the audio output into one of the 2 inputs of a set of Bose Companion II Series speakers with the other input of the powered speakers connected to the Flex Radio. That gives me an automatic T/R type setup so with headphones plugged into the jack on the Bose speakers the receive audio comes through and then switches over to the transmit audio as the Flex receiver is muted with PTT. The scope pickup works fine running into my $40 Tektronix 485 flea market scope. At the Flex operating position the sampler is inline between the Flex and the amplifier as I didn't think it would handle the +250 watts of carrier and the +1400 watts of positive peaks for very long when running AM. When I want to sample the output amplifer's RF rather than the Flex Radio's output with the scope I simply switch the scope over to a 2 meter rubber ducky and it sniffs plenty of RF from 160 meters to 10 meters to display a clean envelope pattern.

At the Viking III operating bench I use the same circuit with a relay to handle the PTT switching of audio from the receiver during RX and from the RF sampler during TX. The scope output is connected to a Textronix 453 scope. The sampler has no trouble handling the +150 watts of carrier power on AM and whatever positive peaks that the Viking produces.

I ended up using an NTE equivalent of the 1N34 in both RF samplers, but I have to agree with the other comments that there's probably something that would work much better out there. I did stick a 1N914 in there at one point and would have to say that 1N914 switching diodes definitely suck in this application!

Rob W1AEX

Or if you want to homebrew a mod monitor completely analog, that was ready for FCC approval in 1979 and never produced, here is a circuit I designed, being just out of school. It required an RF detector stage in front, all I had was the Dc/audio circuitry to get very accurate metering and peak flasher. I had even prototyped most of it at the time, including the meter peak hold circuit. The op amps chosen are ancient JFET types, no longer desirable. This was the way many commercial mod monitors worked from that vintage, like Belar, TFT, Harris, etc.

Jack,
       The little greifkit scopes are POSs for the money people want for them. But for a dedicated do nothing else but a mod level monitor they aren't all that bad. The biggest problem with them is the prices that people want for them, for what they bring you can easily buy a descent older lab grade scope.

They all seem to have an asthignatism / focus problem. The chinese replacement CRTs, I'm told are even worse. There is a pretty simple mod out there to add an asthignatism adjustment pot to them that really helps out with this problem. I did it to a couple of them years back and it worked pretty well. It's been so long that I cant find the paperwork but it was pretty simple to do.

As far as an RF pickup for a lab grade scope goes, almost anything will work. I usually just use a couple foot piece of wire hanging off of the vertical inpoot jack. Or if it makes you feel better you can just jack in the coax from whatever antenna you have and are not using at that particular time.

just my $.02 worth................

Slab,

Fortunately, I didn't pay too much for my SB-610 because it had "no trace".  I gambled on it just being the HV filter caps/diodes and lucked out.  Replaced those and now it has a strong trace that doesn't focus.   So there's a good chance that a the other HV coupling caps are also bad and one of them is known to cause the "out of focus toward the right" problem.   So I'll replace those before giving up on it.  It's cosmetics are about 9.5 so I'm hoping I can use it.  I'll look around for the astigmatism mod.

There is also a CE MM-2 in the restoration queue but I'm thinking more along the line of using it as a RX IF monitor for the 75A4.

However the whole modulation monitoring issue has taken a turn for me since I just picked up a RCA BW66F.   Needs major TLC, but is complete and looks quite restorable.  Hey and just the thing too compliment the "new" RCA BTA1R1.

73 Jack KZ5A

Jack,
        I'll look around (no promises that I'll find it) I may have the asthignatism mod written down somewhere. IIRC it was rather simple, just a couple of resistors and a pot. I remember that it only took a few minutes to do. If I can find it, I'll post it to this thread.

If you have no focus at all, check ALL of the resistors in the focus circuot and connecting to the CRT, they are notorious for the high Voltage opening them up or causing them to go way up in value. I had to replace a bunch of them in my 610.