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Title: 2 tone into an AM TX ? Post by: VE7RF on October 25, 2020, 11:49:28 AM 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 Title: Re: 2 tone into an AM TX ? Post by: W1TAG on October 25, 2020, 08:19:17 PM Jim,
Take a look at: http://www.w1tag.com/2_Tone.htm Should answer your question. Good was to check positive peaks. John, W1TAG Title: Re: 2 tone into an AM TX ? Post by: WD5JKO on October 25, 2020, 10:30:46 PM I attach IMD plots of two of my rigs. Getting low IMD at 100% modulation is not easy.
The first image shows the math relation of each blip on the pan adapter. You might recall a QST article on the K7DYY Super Senior. They specified the IMD but they only used one tone! Jim Wd5JKO Title: Re: 2 tone into an AM TX ? Post by: K1JJ on October 26, 2020, 01:20:31 AM 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 Title: Re: 2 tone into an AM TX ? Post by: VE7RF on October 26, 2020, 12:18:41 PM 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 Tnx. What I want to know is.... what does the pattern look like on a scope (not a sa) .....when a 2 tone is fed into the mic jack. Jim VE7RF Title: Re: 2 tone into an AM TX ? Post by: WD5JKO on October 26, 2020, 01:35:40 PM Jim, The scope waveform is complex, and hard to trigger the scope without jitter. I attach one image that I had of a 2 tone test signal. Each tone has to be cranked down in amplitude with respect to the value needed to 100% modulate 1 tone. Oh, My G76 was decent at 50% modulation, and much less so at 100% modulation. This is due in part to a Negative Peak Limiter used at a high level on the modulated B+. Fortunately I do not speak with sine waves! Jim Wd5jko |