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 Post subject: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Wed 06, 2017 6:37 am 
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I usually do FM alignments using a distortion meter, but I'm doing one using my SA+TG on this Scott LT-110 tuner...I just wanted opinions on whether the pass-band I've come up with looks reasonable.

The hookup is the TG output of my HP 3585A SA through some attenuators and a DC block producing a -35 dBm sweep into the primary of the first IF transformer. SA input through a scope probe on the + diode after the ratio detector.

I'm going to go through this again when I get a couple of other attenuators in, I think I'm still overloading it a little bit, so this is just an early check-in.

IF Before:
Image

IF After:
Image

Audio quality does seem a bit improved...I think it's a bit too much of a peak, though?


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Wed 06, 2017 8:54 am 
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I can't comment on the passband of the IF alone. Since the thing that really matters is the combination of the passband and the behavior of the FM detector, what you really want to see is the straightness of the center section of the S curve, and to see this at a variety of signal levels. If a limiter is present, strong signals tend to get a longer flat section (since it effectively decapitates the IF passband and makes it into a flat plateau), so that also has very interesting effects as a function of signal strength.

Are you able to show both the IF passband that you're measuring by this method, and the S curve as measured by a sweep generator (ideally at several input levels)? And throwing in the results from a distortion analyzer would also be very instructive.

We can learn a lot from looking at how the outputs of these various analytical techniques gives different perspectives on the performance of the FM IF and detector.

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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Thu 07, 2017 12:35 am 
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I'm not sure about an S-curve, my RF/sweep generator is one of the more modern 0-10V X-Ramp types (HP 8643A) rather than having the horizontal driven by a 60 Hz since wave to make the S-curve figures. I've never tried to do the S-curve method to be honest, although if you think that sweep generator can do it, I'm willing to give it a shot with some pointers. I'll add some distortion measurements tomorrow, though.

I'm referencing this book while performing the alignment for the Scott: http://akdatabase.org/AKview/albums/use ... gnment.pdf

A second read-through confirms my first thought, actually, it's a bit too steep of a peak. The 10.6-10.8 points should be "within 90% of the center marker amplitude", which would be less than 1 div down instead of close to 3 divs at this scale.

More tomorrow!


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Thu 07, 2017 12:40 am 
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The more modern sweep generator should actually do a better job than the older ones with sinusoidal sweep. Main thing is to figure out how to couple it to a dual-channel scope with X-Y mode. Once you've got that figured out, you're in business. I think you will find generating swept transfer functions (S curve) for the FM demodulator is very helpful. The straightness of the center portion of the curve should correspond very well to your distortion measurements, and will probably tell you a lot more than you can discern from distortion measurements if something is going wrong. The S curve combines many things at once in a visual display that really tells you a lot about both the IF pass band, the symmetry of the transfer function, and the tendency to distort.

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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Thu 07, 2017 11:10 pm 
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What spec/an are you using?

Just curious and maybe my understanding is incorrect, but if you convert the dbm values to voltage levels, wouldn't the 10.6 and 10.8Mhz points be within the 90% of 10.7Mhz?


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 6:00 am 
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Well, it looks like I'm not going to be able to generate an S-curve after all. My sweep generator's minimum sweep duration is 500 ms, which is certainly not workable with a fast phosphor scope like my Tek 2246, and my digital scope doesn't work well in XY mode. I'm not actually sure why it even has one, to be honest. It's fine for Y-T work at least....

I did re-do the alignment with the SA+TG, my HP 3585A. It was overloading; I added another 30 dBm of attenuation on the output to where the limiter stopped kicking in. I did finally get it a bit flatter at the top, and this helped to improve stereo reception. After doing a full MPX alignment, it now sounds really good both mono, and stereo separation is about 25 dB.

At 1 kHz, THD in Mono comes out to about 0.8%; it's about 1.5% in Stereo. (The tuner itself is rated to <1% in Mono and <2% in Stereo per the manufacturer). Tuning is symmetrical on either side of the station.

Going to call that a complete success. (Also: with the SA displaying reference in Volts, apparently the GPIB plotter software has a bug where it places the cursor icon at the wrong point, although the numerical report is accurate. Go figure.)

Image

Instruments used were HP 3585A SA+TG, HP 5534B counter for adjusting the 38 kHz oscillator, HP 3324A for arbitrary 19/38/67 kHz signals, Tek 2246 scope, Sencore SG80 stereo signal generator for tweaking separation, an Audio Precision System One for distortion measurements, and a Sencore PA81 for level meters.

Image

Image

Image


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 7:33 am 
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That's too bad that your digital scope won't do the job. Slow sweeps are what most digital scopes excel at. Anyway, it would have been instructive to see how the different techniques compare. I don't have a distortion analyzer, so I've never had a chance to try that. I do have a spectrum analyzer (which I have lent out to my work place, but could bring back) so I may try that one of these days, since it has a mode specifically designed for taking nice transfer functions in frequency space.

Nice to see that your tuner is performing well. Those Scott tuners are really excellent.

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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 7:57 am 
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My digital scope is a cheap one, unfortunately. The nice horizontal line on the Tek with X hooked up only is 8 blotchy dots about 1 div and change apart on the Rigol. I bet a nice new Tek or Keysight digital scope would do much better. My older HP digital scope doesn't even do XY at all sadly.

What SA is it you have?


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 9:04 am 
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A Marconi. I don't recall the model number. It's a cast off from my old work place. They kept the nice HP ones, naturally. My Marconi is temporarily at my current work place.

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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 3:37 pm 
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jkoebel wrote:

A second read-through confirms my first thought, actually, it's a bit too steep of a peak. The 10.6-10.8 points should be "within 90% of the center marker amplitude", which would be less than 1 div down instead of close to 3 divs at this scale.

More tomorrow!


Would you mind explaining the math there? I am fairly new to the frequency domain world and am still trying to wrap my head around it.


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 Post subject: Re: Scott LT-110 Tuner - FM IF Alignment Question
PostPosted: Dec Fri 08, 2017 8:33 pm 
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Location: Seattle 98125
It's mostly that the pass band needs to be wide enough and flat enough so that the higher frequencies, containing the stereo information, make it through the chain with everything at about the same level. (Making the peak too narrow, as I did originally, meant that the higher frequencies weren't getting through and so the MPX decoder didn't have the stereo information to work with.)

For an actual explanation of the math underlying frequency modulation, I'm afraid I don't understand that well enough to teach it to someone else.


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