When a single tone is fed into an AM rig, the pattern on a monitor scope resembles what you would see when a 2 tone is fed into a ssb rig. What does the monitor scope look like when a 2 tone is fed into an AM rig ? My 2 tone gen is offline right now, and so is the scope.
Tnx... Jim VE7RF
Hi Jim,
When transmitting an AM carrier, one tone will show THD, total harmonic distortion. Two tones will show IMD, inter-modulation distortion.
The object is to get the THD peaks down below -25 dB and the 3rd order IMD peaks down at -30dB or better.
Anything poorer than the above numbers will generally start to cause QRM splatter to adjacent stations when running legal limit.
It's easy to give out THD or IMD reports to another station when he injects a tone(s) and you watch the results on an SDR spectrum receiver. I've done it several times by request in real time on the band.
Jim/JKO: Yes, IMD measurements don't take prisoners. It takes a lot of work to get most rigs to show decent numbers at full modulation. I usually have to work hard to get decent numbers on my homebrew rigs, but it's worth it in the end. The overall modulation system is the usual troublemaker in addition to simple improper final loading (especially with a linear) or flat-topping of various degrees. The exception are the pre-distortion RF feedback loop SDR rigs.
I think the cleanest signal I've ever seen was when I tapped the 100 mW pre-driver on my FT-1000D and fed it into a class A one watt lab amp. It showed -70 dB 3rd order. I could hear absolutely no splatter in the RX up the band. That then drove a super linear class A 4CX-350J at 40 watts. What a driver for the 8877 class AB1 linear!
T