Quote:
Originally Posted by bob91343
Don't expect much gain from a mixer. It seems that everything is working as intended, although you may have to dig to fix the distortion.
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It was almost shocking to me, to hook up the dusty and cold 415, and have the IF response curve pop up on the screen of my 1962 vintage Stark Instruments oscilloscope looking like it jumped off the page of a textbook - same with the S curve. Talk about an easy to use setup! It gives me some confidence about using it to check the colour circuits of my Zenith TV (the reason I got it in the first place), but that`s a totally different topic.
I will have to figure out what you meant in a previous message by using an external signal to frequency modulate my RF generator, I have an ex-military AF sine wave generator which is built like a tank, like a TV-7 tube tester, it doesn`t contain a single electrolytic capacitor and is fully regulated, and employs RCA red series tubes inside. It`s very low distortion - the RF generator is a Stark, but is some cheap Japanese thing from the 60s inside, nothing like the quality of my Stark oscilloscope.
When I loose couple the Stark`s RF output to the antenna of the radio in question, a lot of signal breaks though and I can hear it in the speaker of the radio quite strongly, so the AM rejection must be less than wonderful - again as an FM alignment newbie I am not sure if that is normal or not. The Stark`s internal modulator is way off being a sine wave, not even close.
My interpretation of the S curve is that it is useful for showing only harmonic distortion - i.e. it is the transfer curve of the ratio detector. If it was curved rather than straight, that would represent harmonic distortion. The distortion I am hearing sounds more like IM distortion, which wouldn`t necessarily show up on the S curve since the test signal is probably a single sinusoidal waveform. I really should substitute another speaker, I have wasted time previously troubleshooting strange forms of distortion in radios and TVs and had it end up to be nothing more than a bad speaker. The fact that it sounds okay on AM may be throwing me off, since most of the program content on AM is talk radio which really wouldn`t be affected by IM distortion as much as the music stations on FM.