Do you use a waveform monitor ?

[WA3VJB]

I am curious as to whether people routinely monitor their modulation pattern with test gear beyond observing the meter readings of the transmitter.

My original AM rig, the Collins 32V2, has two choices to measure modulation, one is a kickup, the other is 807 modulator current. Neither seemed to provide a reliable way of knowing if I was hitting baseline.

So I began using a cheap oscilloscope, a 10Mhz job that came from the school lab. Been hooked ever since on watching that pattern, and can hardly bring myself to transmit unless I've got a way to monitor the waveform.

If I'm on 75, I use the 40m dipole as a pickup for the scope input, and cut the sensitivity way back. This minmimizes the chance the pattern could be affected by stray fields from other RF sources. Other folks make a loop of wire and hang inside the RF tank of the transmitter somewhere.

There is also a piece of test equipment known as a Modulation Monitor used by AM broadcast stations to provide an indication and a flashing warning at adjustable levels of positive and negative swings. About 25 years ago the FCC changed the Type Acceptance standard for such equipment to allow increased positive swings found in assymetrical modulation.

The new and current standards call for measuring compliance up to 125% positive, with overshoot measurements up to 133% positive.

This meant the older units measuring a maximum of 110 percent positive were no longer a legal standard of determing whether a station was in compliance. Many of these units still sit in racks to provide a clean, low distortion audio monitor to distribute around the station, but others can be had cheaply for AM service on the ham bands.

Anyway, thoughts appreciated here as to use and application of methods to monitor modulation parameters.

 

[N3WWL]

I ALWAYS use an oscilloscope when I'm on the air.  I have a pair of Hitachi model V-212's for each studio.  I home brewed an RF pick-up with combined thoughts from K1JJ and W3BYM.  It samples RF directly in-line with my main antenna switch.  It incorporates a 1N34 diode circuit and potentiometer to also monitor my transmitted audio through quality headphones at any level I please.  It also is used as a pick-up for the Leader digital frequency counter. I couldn't operate without it.  Audio reports can be so varied....this way, I can SEE and HEAR what is transmitted. You can see it in the picture just in front of the mic.

[K1JJ]

Yes, as Jay says, a scope with a good 50 ohm sampling probe is the only real way to know what's going on..

Here's the scope probe pickup description:
http://amfone.net/ECSound/K1JJ8.htm

With the new digital scopes out there, the 100+mhz analog scopes are real cheap now.

After long practice using the scope to monitor your own AM waveform, you can actually adjust your EQ based upon familiarity of your own voice. MUCH information is contained in visual information to a trained eye. Parasitics, hum, frequency response, over and under modulation... it goes on and on.

Remember to use a scope that has a bandwidth at least 5-10 times your fundmental freq. [or better] So, a 100 mhz scope is barely enuff for 10M use.

Two scope channels let you look at the received signal too. Once you have experience watching your own signal, you can let others know about theirs. But, be careful of the effects of receiver AGC and the limiting effects of many I.F. strips that can color a signal.  Many tmes you need to turn off the AGC and just ride the RF gain to get an accurate sample.

73,
T

[WA3VJB]

Tom, explain to me the bandwidth factor in calculating a scope (100Mhz = 10 meters).
Why such a vast multiple from F_o  ?

[W9GT]

I always use a scope to monitor the modulation when I'm on the air.   Now.....if you ask me if I always see what I want to see....or if I totally understand all that I am seeing....well, maybe not always.  It is certainly an effective way to see what is happening with the rig and hopefully reveal problems before they cause trouble with other stations.  I am the first to admit, however, that it doesn't always give me the total picture.  It is a combination of watching the meters and viewing the 'scope, and being familiar with where the rig normally operates properly.  I also find it useful to listen to myself on the receiver.

73,  Jack, W9GT

[K6JEK]

I use a modulation monitor 100% of the time, a scope when I change things.
I find the blinking lights on the modulation monitor much more noticeable than the scope.   Besides who can resist giant backlit meters swinging around?

[K1JJ]

Tom, explain to me the bandwidth factor in calculating a scope (100Mhz = 10 meters).
Why such a vast multiple from F_o  ?

Paul,

Well, it's a matter of opinion and depends a lot upon what you want to do with the scope for analysis.

But, lets take for example that you want to blow up the waveform to see the actual sine wave. I've found that difficult to do unless the scope has a much higher bandwidth than the frequency looked at. Also, if you want to be aware of parasitics that can be many times the fundamental, even in the VHF range, a scope with 100mhz+ is required.

Also, to take a look at various harmonics of the fundemental requires this same headroom.

For viewing just a simple carrier and audio AM/SSB modulation, sure, you can get away with a lower bandwidth, but it greatly reduces your ability to pronouce a clean bill of health for additonal culprits... 

Some of the more lab oriented guys like JN and HUZ may have other reasons too, but since getting a cheap eBay 100 mhz scope here, I often wish I paid a little more and got a 200 mhz version... you get the idea.

T

[Glenn NY4NC]

I always use an O-scope to monitor modulation level, pos/neg peaks but that only tells part of the story. Ultimately, I would like to have a spectrum analyzer setup right next to my mod scope to view bandwidth, check for unwanted products, etc...to see the things you can't really see with a scope.
The spectrum scope in my 756 pro II works surprisingly well for checking transmitted bandwidth but a real HP analyzer is in the plans for the future shack.

Yes, I know they can be expensive, but the prices have been coming down and the older models are gettting quite reasonable if you shop around. (e-bay)

[Glenn NY4NC]

I guess that's what you would call "specsmenship" A scope that's spec'd for a 100mhz bandwidth will respond/detect a 100mhz signal, and you'll be able to view changes in amplitude, but you won't be able to view the waveform details (expand the waveform horizontally) like you could with a 400mhz scope.

Tom, explain to me the bandwidth factor in calculating a scope (100Mhz = 10 meters).
Why such a vast multiple from F_o  ?

[K1JJ]

The spectrum scope in my 756 pro II works surprisingly well for checking transmitted bandwidth but a real HP analyzer is in the plans for the future shack.

Yep, no substitute for a good spec analyzer. The scope can see only so much in comparison.

Yes, the ICOM Pro II/III has an unbelievable spec analyzer in it. The Pro III was demonstrated to me by Bob/K1KBW and Ray/N1DVL who both own them. On 6M ssb, they could tell the difference when I was 3.5 or 2.5 kc wide!  It appears to have similar resolution to the previous generation HP spec analyzers, and certainly better than an HP 141T. [that I owned once]

If I ever get rid of my older FT-1000D, I will be looking at a used Pro III years from now. [When they are going for MUCH under $3000... ]

T

[WA3VJB]

Tnx for the explainers on scope specs.

As for the spectrum analyzer, yes, they're great.
I used to have access to a service monitor (Cushman) used in bench work for 2-way FM rigs.
Multi function (did AM) and down to HF, and that was a crude version of a real spectum analyzer.

HEY maybe Pete can sell me his ICOM 756 Pro II and I can use it as a spectrum analyzer.
Get a rig for free.

[W2VW]

HEY maybe Pete can sell me his ICOM 756 Pro II and I can use it as a spectrum analyzer.
Get a rig for free.

You could always put duct tape over the LSB and USB buttons.

[K1JJ]

Oh, one more point about scopes.

Build a class E rig and you will find out quickly how important it is to expand the horizontal scale to study the actual waveform shape. Most conventional transmitters have a nice sine wave and you can forget about it. But, the pre-tank final waveform of class E rigs need to be blown up and the actual waveform tailored to the correct pulse shape. Thus, a scope bandwidth that is much greater than the fundamental is required. I think Steve/QIX recommends using a 50mhz or higher scope for the class E rig on 75M...  along with spotting parasitics.

In general, because of scope evolution, we can expect a better looking, sharper and stable trace from a 100 mhz scope compared to even a 20 mhz scope at any freq. They are so cheap to buy now, why not go with the 100-200 mhz scope.

T

[W1RKW]

Interesting thread. Out of curiosity, what monitoring methods do people typically use ie. trapezoidal pattern, straight modulation envelope using just the vertical inputs, etc.?

[k4kyv]

To me, running the rig without a monitor scope is like driving a car at night without headlights.

In the scope, I never use the vertical amplifier.  I drive the deflection plates of the scope tube directly with rf.  This assures no nonlinearity due to amplifier distortion.

My homebrew HF-300 rig is set up for either trapezoid or envelope pattern.  The other rigs are set up for envelope pattern only.  I like being able to measure modulation linearity, but the problem with the trapezoid pattern is that it eventually burns a dark vertical line in the middle of the face of the scope tube where the unmodulated carrier trace appears, so I use it only occasionally, and most of the time rely on the envelope pattern. 

To avoid burning a horizontal line into the scope tube, I use a relay connected to the t/r control line to switch in enough DC voltage to deflect the image completely off the screen during standby periods.  It takes only a few days to burn the tube when you keep the bright base line all the time except for when you transmit.  I never attempt to monitor received signals on the scope.

At present my monitor scope is a modified Heapshit HO-10.  The rf feeds the deflection plates directly.  I disconnected the vertical amplifier and pulled out the unused vertical amplifier tube which also doubles as two-tone test oscillator for slopbucket.  The only electronics left in operation is the sweep circuit.  This puts less load on the power transformer, which is prone to crap out on these scopes.  I also modified the circuit to get rid of most of the hum inherent to the crappy design.  I replaced the cheezy rolled up piece of tin can that Heath used for a magnetic shield, with a real magnetic CRT shield taken from a military surplus RTTY monitor.  I managed to collect several carcasses of HO-10's in various conditions of repair, to maintain enough spare parts to keep one going for years to come.

The Heath manual says that the hum  is inherent to the design, but "should cause no problem when using the scope for its intended purpose - monitoring modulation."  The hum comes in two varieties; the vertical trace is 60~ hum modulated, and the Z-axis, that is, the brightness of the trace, is also 60~ modulated.  Part of this is due to hum modulation of the DC power supply; part of it attribuable to the filament line (requires some re-routing), and the rest is due to hum pickup directly from the power transformer to the scope tube hence the need for a real magnetic scope tube shield.  I also modified some of the DC circuitry to the scope tube and was able to sharpen up the image, which is fuzzy in the stock design.

If it weren't for the Heath nostalgia craze that has driven up the collectors' value of those things, they should be plentiful at hamfests for less than $5 apiece, because they are such pieces of crap in their stock condition.  Sometimes you can find one at an attractive price (< $15). But with some work, I have converted mine into a half-decent monitor scope, and have a couple of spare power transformers on hand, just in case.  So far I have gone through only one transformer.

[ve6pg]

...ALWAYS USE MY SCOPES..BUILT A SIMILAR 'SCOPE PICK-UP,AND MONITOR FROM NU9N"s WEBSITE. 'CAN SEE AND HEAR AT THE SAME TIME..I USE TRAPAZOID PATTERN...TIM..SK..

[W2JBL]

 i don't get on the air without a scope on AM or sideband. if i keep a scope on the receiver IF too. this seems lost on the SSB crowd , and if you tune the bands with a scope on your receiver you can see the results of sidebanders not using them- it's pretty ugly on the bands these days.

[Steve - WB3HUZ]

Geez Don! Seems like a lot of work just to use a crappy little HO-10. Maybe it was the challenge.

I'm sure you are aware, but others may not be: Lab grade single (and sometimes dual) channel scopes with 10-20 MHz can be had at most fests these days for less than $50, usually way less [In the last year, John-JN snagged a real clean Tektronix 100 MHz scope for $80].

I've been using one for which I paid $10 for close to 10 years. Never had to do a thing to it. I leave it on all the time. These scopes usually have good vertical amps, and trigger controls. No need to perform major surgery to make them work as a monitor.

Other than being small (which just makes it that much harder to analyze the waveform), the HO-10's don't have much going for them.

[W2VW]

Some guys know better than to use a scope:

http://www.eham.net/forums/Amplifiers/3163

[Steve - WB3HUZ]

This was some what useful.

[K1JJ]

This was some what useful.

Hmmm... funny the way the forth example looks like a balanced modulator doing its thang... [notice the tiny wavelets] Normally an over modulated plate modulated AM rig wud show the carrier cut off.  That waveform actually is not overmodulated except for maybe receiver detector distortion.  Guess this is the sign of the times using a ricebox for demonstrating AM.... :-)

Actually, the major peaks on #4 should be higher - when the wavelets appear like that - compared to example #3.  A form of peak limiting is occuring, though no flat topping observed. Must be the ALC coming in... [sigh]   Get out the soldering iron.


T

[WV Hoopie]

After using a scope to monitor the station, I don't think I'd ever transmit without one. Would have to shut down if the scope malfunctioned or broke.

I've been using a cheap fester 5 MHz scope for years and it was the only piece of equipment kept after selling my station back in the 80's. A rf pickup as described by K1JJ & W3BYM is used plus a slight twist. The rf is fed to a project box large enough for two sections; one for waveform viewing and another for a trapezoidal pattern. The rf goes to a band switch which is connected to a 100pf receiving type air varible cap in parallel with toroidal cores selected by the band switch. The cores are wound to be resonant with the 100pf cap mid range of operating bands used. Tuning the cap also varies the waveform height on the scope, output of this circuit goes straight to the vertical deflection plates of the CRT, bypassing the vertical amp. Include an off position for the bandswitch, IF output of my R-390A goes to the regular vertical input of the scope and when receiving, switch to off to watch the other end of the QSO.

Modulated B+ coupled by a 0.01uF 6KV ceramic cap goes to the other side of the project box where it feeds the standard voltage divider circuit describe in 1950-1960's ARRL handbook for watching a trapezoidal pattern. Its output goes to the horizontal input of the scope. I've included a switch to turn it on when desired.

So with this setup; turn the bandswitch to the band desired and adjust the cap for the height on the scope. When receiving either select the off position to monitor the other end of the QSO or turn down the intensity control as not to burn a horizontal line on the CRT. Flip the switch for a trapezoidal pattern and don't leave it here long if not transmitting or a nice little dot will burn in the CRT.

More switches, knobs, dials, and displays to impress visitors.

wd8kdg
Hoopie,

[Bacon, WA3WDR]

Scope!  Once in the morning does it!  And my breath is fresher all day.

(One might have to be a retired Flower Child to remember that commercial.)

[Glenn NY4NC]

Brill cream... A little dab will do ya!

I want one of these on my bench!!!.... (about $2k used)

Scope!  Once in the morning does it!  And my breath is fresher all day.

(One might have to be a retired Flower Child to remember that commercial.)

[Todd, KA1KAQ]

I want one of these on my bench!!!.... (about $2k used)

Hey Glenn, there was a guy selling these at Hosstraders for $79.95, 2 for $100 or Buy 1, Get 2 Free - your choice! I told you to come to the 'fest. but nooooooooooo

Okay, all kidding aside, I must confess that I was one of those fools who never used a 'scope while AMing 15 years back. Fortunately for me I was using a 32V-2 also and with a crappy antenna, wasn't much of a nuisance. But between Don's 'without headlights', suggestions from Paul over the years, and (the clincher) JJ's incredibly simply scope pickup/pick off project, I will be prepared the next time I light off a rig at home. I've picked up a couple of those older, simple scopes about the size of a shoebox and hope to be able to make use of them as a simple monitor. It would be nice though, as pointed out by Glenn, to be able to see what the signal looks like overall and if any nasties exist on it.

One of the 'scopes is an old Navy model, the other is a Philco - along the lines of the old Pocketscope by Waterman. I know these things are not great, but has anyone had luck using them before? No substitute for a 'real' scope (I have a Tek 545 around somewhere, complete with cart ), but for something handy along the lines of the little 3 inchers in the old CE 100/200V they should be okay?

Along with making good sense, it's yet another cool thing emitting light and jumping around to add to the radio experience.