The AM Forum

I had a Rigol 100 mHz digital scope before the fire that destroyed everything here and had a problem using it to display the modulation of an RF carrier.  I don't recall what the problem was but it seems to have been something to do with digital scopes in general.

I had older scopes and a Heathkit mod monitor and a QIX software type so the fact that the Rigol didn't do that job didn't bother me much.

I now want to purchase a scope but if it won't display AM modulation, it won't do what I want to do right now if I go digital again.

Someone told me that newer scopes have analog inputs so it's not a problem with these.  I suspect he was confused as it seems that analog input is pretty fundamental to a scope.

Can someone clear this up for me?

Thanks,

Jack

Jack,

   I am so sorry to hear about your fire.

As to scopes for AM modulation, I like the fully analog Tektronix 2215, and others in that series. Those often need re-capping of the power supply electrolytics. This is not too hard.

The digital scopes like what you mentioned all have a degree of latency. This depends a lot on the scope settings, but to me, always too long of a delay. I have the scope similar to yours. It is a Hantek DS05102P 100 Mhz. When I tried it as a modulation monitor, I recall at times a delay of 1 second or more. Say. "Hey", and then wait for the scope to display "Hey"!  Still useful, but the delay to me is not something I want to have on a monitor scope for AM modulation.

Hope this helps,
Jim
Wd5JKO

That's a fine instrument the Tektronix. I'd second that suggestion 100% because I use the same thing and get 100% good results all the time with no fiddling.

A 100-150MHz analog scope is perfect. Why 100MHz and not 40MHz? because if you have some almost-suppressed VHF oscillation, you can usually see it clearly on the 100MHz analog scope with the sweep frequency turned up to view the actual RF waveform of your carrier.

I think the issue you might have seen with a digital scope as a mod-monitor is that the sampling rate can be in some way related to, or close to a harmonic or sub-harmonic of the carrier frequency, and this creates false envelopes on the display. It is called aliasing. The big dollar digital scopes today have less issue with this but still require a knowledge of the digital scope operating principle to avoid it, and the settings may have to change as you change bands unless you spent oodles for it. So it is not very convenient.


BTW Also very distressed to hear of your fire. I hope you are insured. I got my whole shack building and contents included in my homeowners through Farmers and it was not hard to do and cost very little extra.

Any scope will be fine to measure AM mod.

I use a SIGLENT with absoltely no problem.

Just a matter of setting up the trigger correctly.

I'd have another go if I were you.

Old 'scopes are nice but not reliable anymore and have given up with old test kit, overpriced anyway unless it's given away!

J.

I still use the TEK 475 , many many years without a problem. When I had my RF lab, TEK came along to offer new models digital scopes, but they did not give the possibilities the 475 offered. In addition, when I switched the 25 kW 27 MHz generator on, the 475 worked fine, both digital scopes with their plastic cases were completely out, one was even dark...
The old unreliable ? 475 offers me more than 30 years reliable service by now  ;D

Tek scopes have become almost "free" the price has come down so much - especially at hamfests.

One problem with digital scopes that I don't like at all is the "jaggies" on the waveform display.

I'm using a 2235 Tek that I got at NEARFEST for $20. It does have an intermittant (mechanical)
in the horizontal width, I'd need to open it up to fix it. Might just be needing some deOxit on some
contacts...

Before that I used a Phillips scope (no fan, no fan noise). Another hamfest almost free...

                 _-_-bear

Just went to the -bay. wow what a ton of bargains on CRT based TEK analog and hybrids.

The trigger has nothing to do with a trapezoid modulation monitor display.   

As for reliability, I have, and have had Tek scopes going  back to the50s and 60s.   All were/are perfectly reliable. 

Good info. I have two older CRT scopes that would work well as monitors. How do you get the RF sample? I'd be using it on a Viking 2.

Thanks, Rich

The classic "trapezoid pattern" is one thing, watching the modulation envelope for peaks and negative "baselining" is a different
thing. The latter requires a scope with a trigger and timebase (I'm sure you know).

                      _-_-

Could be as simple as wrapping some turns of wire around ur coax, or draping a wire near the tank or finals, even outside the
cabinet depending on how much RF is there.

You could put in an explicit "RF Sniffer" box in the feedline too...

I don't mute my receiver and use the IF out into the scope during transmit and receive... YMMV.

                               _-_-

Those phenomenon are readily seen on a trapezoid display also, as illustrated in most any ARRL handbook of the 50s and 60s.   

I realize triggering on the envelope is another monitoring techinque, but it is no better, and in some respects less revealing than the trapezoid display. 

Can't watch the received signal with a trapezoid only scope though...

True, but aren't we talking about monitoring one's own transmit audio here?

Wrapping turns around the coax shield can give false or weird waveforms if anything is not perfect.

this lash up works great for me. placed after the LPF and before the wattmeter, serves as a nice HV cap coupling signal to scope.

Good picture. I was thinking of using a tee but was not sure of the best way to sample the RF.

Rich

Get a t with two so239 and a PL259 for the T part.

Get a good pair of needle nose pliers.

Turn, counterclockwise, the pin in the Pl side.

Remove, grind it down.  Reinsert.  Use a double so239 barrel to connect another piece of coax to the test equipment, or any other mating connector to whatever.

Change the amount of sample voltage by grinding the pin down shorter and shorter.

--Shane
KD6VXI

thanks for passing that along .... just scored a nice Tek 465M that seems like new .... now to find some more probes