#8: Two-tone test of SSB transmitter output 

Nice idea!

Yes, it is simply a resistive splitter/combiner. Not the best way to do it, but it works. Here's more detail than you'll ever need: www.microwaves101.com/encyclopedias/resistive-power-splitters

@@w2aew Hey Alan, thanks for the reference. On that page, I'm assuming this is the Wye version? Then: R1=Z0/3. So if my function generator is 50 ohms output, I assume my R value is about 17 ohms? The speaker would be only 4 or 8 ohms though. Am I in the right neighborhood, what did you use for the R values?

@@patrickbouldinkm5l143 In this case, it was a 16.7ohm Y-combiner. Since we're talking about audio and impedance mismatches don't matter, I used it to drive the speaker anyway.

@@w2aew thanks Alan, I reproduced this with success, thanks for the fun learning!

If the function generator has a mod system then u can produce a carrier signal at what ever frequency u wish ..then u can modulate it all on same channle ..or u can use both channle and achieve this as well ..but most new function generators allow u to produce a carrier plus a audio signal together ..some don't ..then u would have to have my ore than on channle available to produce more than one signal .hope this helps

I generator..depends on its function if it's just one channle used or not ..

You really can’t tell linearity accurately with a two tone waveform.

​@@w2aewthat's just the opposite of what you said in the first 10 seconds of this video

You can get a decent idea of non linearity, but not a precise measurement. A good discussion can be found here: preciserf.com/wp-content/uploads/2012/03/Appnote-2-Two-Tone-Tests2.pdf

@@w2aew thanks , good info and scope examples in that discussion

What are you using to listen to the SSB transmission?

@@w2aew Ahh!... Hadn't thought of that. Does SSB require a particular type of receiver?

@@andrewferg8737 Yes it does! It just sounds like a garbled mess on an AM receiver. You need to use something like a product detector, since the carrier needs to be re-inserted for demodulation.

@@w2aew I guess my next project will be an SSB receiver! Product detector... a bit like an FM receiver? Would I need a mixer to re-insert the carrier or could I insert it directly into the receiver's antenna amplifier with a simple resistor?

@@andrewferg8737 Not exactly like FM. It's typically done in the final IF stage. Mix the SSB signal with the "missing" carrier frequency - in this case, the IF frequency. This is much the same as you'd do in a CW receiver where you heterodyne the CW signal with another tone to create a "beat" note that you can hear. This mixing with an LO at the IF frequency for detection of CW and SSB signals often requires that the LO be tunable so that you can adjust for the best audio quality. The adjustable LO for the product detection mixer is often called a BFO (Beat Frequency Oscillator). You can sometimes "couple" the IF-freq oscillator into the IF stage to get it to sort of work. In the days before SSB capable receivers were widespread, people used to place another receiver in close proximity to their main receiver so that some of LO that is radiating from the 2nd receiver would couple into the main receiver...

Yes, of course two harmonically related tones will create the two fundamental signals as well. The main issue with keeping the two tones non-harmonically related is to ensure that the higher order intermodulation products don't overlap and give false results via constructive/destructive interference.

@@w2aew Consider if the second tone is three times the frequency of the first, where they just combine to give a (no-problem) first "tone" having a third harmonic, much as might have a person's voice. Constructive/destructive interference refers to identical signals adding or subtracting from each other. Note that this is an informative experiment to try. Of course non-harmonically tones work perfectly, while harmonically-related ones are difficult to generate. I don't want to beat this minor point to death because yours is a very nice video. We can just agree to disagree. K6KV

@@garygordon2530 Not just the fundamentals, but the higher-order intermod products can also interact in/out of phase - and since you're typically doing this to look at linearity - IMD generation is one of the things you want to look at without any interaction. The degree of interaction will depend on phase, which can vary, thus give you inconsistent results.

Hi Alan, I didn't particularly want to drag this out, except for feeling that your post begs a response. I urge anyone interested in this subject to explore the books by Orr and Terman,which are much more authoritative (and insightful) than those by ARRL. Of course these authors never mention the so-called "harmonic-related restriction". They only say the obvious, that what is transmitted is identical to transmitting two closely-separated carriers separated by the difference in the frequencies of the modulating tones. Anyone can view this on a spectrum analyzer: one will see two carriers and nothing else, no imagined intermod frequencies whatsoever. For example, consider transmitting USB on a 7200 KHz SSB carrier, with modulation tones of 1000 Hz and 2000 Hz. Looking at the output of the transmitter on a spectrum analyzer, one will see two carriers, one at 7201 KHz, the other at 7.202 KHz. For someone listening on a receiver they won't know and cannot determine the carrier frequency. All they can say is that the difference between the two modulation tones 1000 Hz, and they won't hear any "intermod" products of course. If the transmission instead was on a carrier of on 7000.5 KHz with modulation at 500 Hz and 1500 Hz a receiver would present them identical as for the previous example; again they would not be able to ascertain the carrier frequency. There's no reason to test with harmonically related modulation tones, and no reason not to, other than their not being convenient to generate with separate audio oscillators. Of course you're welcome to have the last word, however I assure you that I know of what which I speak. Best regards, Gary

@@garygordon2530 Hi Gary, Everything you state is abolutely true - with the assumption that the modulator, the gain stages, the PA, etc. are all distortion-free. Assuming they're all linear, no harmonics will be generated and no IMD products generated. You'll only see the two tones, whatever they are. And yes, these two tones will of course appear as two individual unmodulated carriers in a receiver or on a spectrum analyzer. No argument from me - all 100% correct, I have no doubt that you know what you're talking about. My only point is that if the transmitting path has some distortion or non-linearity in it, then there will be potentially be harmonics of the two tones, and thus potential higher-order IMD products - where the 3rd order IMD products are usually the most troublesome as they will appear on either side of the main tones, at an offset equal to the original tone spacing. If harmonically related tones are used, then the IMD products could overlap making it difficult to determine actual IMD levels.

You can put a resistor in series with each output (like a few kOhms), and that will isolate them enough from each other.

thank you for great videos

You want them both to be the same amplitude at the RF output, so you may need to tweak one or the other depending on the frequency response of the tx path of the radio.

I found the explanation in a comment below.

Just that you eliminate the frequency response of the microphone (from affecting the amplitude of the two tones).

+wb8vqv Each is a 16.8 ohm resistor - so that when each port is loaded with 50 ohms, all ports have 50 ohm input impedance.

Mainly because 2-tone is most useful for measuring and calculating peak envelope power when you only have an average-reading power meter. For AM, you can easily measure carrier power and then estimate PEP from that.

@@w2aew Thanks.