AM modulation depth

A modulation scope. Plenty of circuits were printed in the ARRL 'Radio Amateur Handook' over the decades.

Air band? (Navigation 108 - 118 MHz, Communication 118 - 138 MHz).

The ICAO rules a calibrated instrument, but a good oscilloscope will do. Please do not exceed 90 % in the negative direction.

I've done this up to 30 MHz. But would it also work on VHF with not too fancy CRTs ?

This is not a classroom homework helper group.

The RF is applied directly to the plates in a modulation monitor.

What makes you suspect this has to do with homework??

I agree. So, there's not to worry about amplifier bandwidth. I simply wondered if wire and electrode length, parasitic capacitance aso wouldn't play havoc above 100 MHz. This method must have a practical frequency limit with simple CRTs often used on HF bands?

See chapter 2.

The simplest method is to fabricate a diode detector (I prefer germanium, but schottky will work). The DC level will give you the reference and the audio above and below the carrier level will give you depth of modulation.

I"m presuming you are wanting to measure the DM in the aircraft band as it is the last bastion of VHF AM modulation. AM double sideband full carrier. Any other AM modulation detector will be different than this trivial method.

THanks,

Jim

And if you don't have a high frequency scope, then use a diode detector/envelope detector and look at it with a slower scope.

Look at the I.F. output of an aircraft receiver with an oscilloscope.

Since you are a ham radio operator I assume you'd like to do that with equipment you either have or can borrow from someone in town. Three methods come to mind:

a. If there are at least occasional tone modulation phases you could switch to your CW filter or at least a nice SSB crystal filter, move to one of the sidebands and read the S-meter. Then move to the carrier and read that. If you haven't calbrated your S-meter yet this would be a good time to do that :-)

b. Hang a scope to the IF of the receiver. Whether that's 10.7MHz or maybe a 2nd IF of 455kHz doesn't really matter, both should be well within the realms of a low cost oscilloscope. >100MHz may not be unless you have a fairly new scope.

c. Mix it down to 10kHz or so, feed it into the soundcard of a PC and then it into software like this:

If you use the last method make sure your PC or laptop audio settings are such that any sort of noise canceling is turned off. Otherwise you might have an unwanted lowpass in there. A quick test: When you touch the input line with your finger everything up to 20-25kHz should move up and down. If there were an earlier roll-off you'd see that.

Now if someone had a nice spectrum analyzer and you could offer a cold big bottle of Trappiste, that would also solve the problem :-)

Indeed Jim. I'd like to check the MD of the Dittel VHF COM XCVR I use in my ASH25E Schleicher sailplane. TKS for all the suggestions.

Willy

The question sounds like it comes straight out of the test or the workbook.

What makes you suspect that it doesn't?

Thanks, Rich

Now, after seeing your call letters in your nym, I do not.

Then use power pads at the output of the Dittel so that the input to the diode detector will be 10 milliwatts or so. Keep in the lower power ranges of the diode so that the curve is at least approximately linear.

Or you could use the old light bulb trick. Load the transmitter with an incandescent light bulb approximating the output power of the transmitter and key the transmitter. Then speak normal tone into the microphone. If the light bulb shows no flicker then you are 70% modulation or less. Medium flicker between 70 and 85% and severe flicker over 85%.

Ain't a hell of a lot of us in here that specialize in that particular segment of the spectrum, but I've spent nearly my whole professional career there. Not a lot of chips for VHF AM work, so we bend the FM chips to our purposes.

Thanks,

Jim

I guess lots of different people haunt this newsgroup. And not everyone is an electronics professional, nor an english native speaker... English is only our third language in Belgium.

I happen to be an MD with some eclectic other interests. Been ARRL Life Member for some 40 years and flying for 20 or so. COM XCVR problems are recurrent at our airfield, and I'm assembling some sort of a 'portable lab' to diagnose and fix those problems 'in the field'.

Willy

You could simply use an oscilloscope.