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Author Topic: IF Alignment and sweep generators  (Read 6464 times)
N8CMQ
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Posts: 353




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« on: January 28, 2011, 10:53:13 PM »

 For years I have used a standard generator to align a radio. Never had any problems doing it that way, but I felt like I was missing something.
The books have always shown curves that the IF response should look like, and I could graph them out with careful measuring and drawing it out, but that was tedious...
 I have seen sweep generators, but have never had one, so I bought one and tried it out. But it was a "universal" style and did not have a rate control, and swept at 60 Hz, too fast for a 500 Hz filter. Useless.
 I recently purchased a Wavetek 180 sweep generator to see if having more control over the settings would help, and, it did!
By setting the width to narrow, and the rate to slow, I was able to watch my scope display the curves for the 500 Hz selectivity setting!
Then, I did the second IF stage, and I was able to see the upper and lower side band response!
 It was a pleasure to see directly what I had to plot before!
I still had to plot the frequency and width, as there are no marker generators, but that is second nature for me to do anyway.
What was also very cool to me, was the ability to check the 5KHz setting, and turn on and vary the notch filter settings and see what it did!
On my SX-101, the IF is tuned for 50.5 KHz. That is the start frequency, then as the selectivity is set wider, it adds to that frequency. That is why the notch filter adjust knob starts at 50 and goes to 54. And you can see the notch go through the response curve just like the book shows.
 While I am not saying you have to have a sweep generator, I am saying that the price of some of the better units are coming down, and make it affordable for the budget minded of us!
And what got me to buy the Wavetek 180 is the fact that it has other uses as well! It has the sine, square and sawtooth waveforms, DC offset, Hi and LO output, a TTL output, and a bunch of other features!
 While the test equipment is not state of the art any more, it does the job, and it is affordable. Keep your eyes open and you may find a jewel for your test bench!
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G3RZP
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« Reply #1 on: January 29, 2011, 01:49:39 AM »

I find the tracking generator that goes with my HP141 spectrum analyser useful for that job. It has reasonably good phase noise too, which many sweepers don't have.
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AA4HA
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« Reply #2 on: January 29, 2011, 05:35:11 PM »

There are some receivers (R-390A) that uses offset IF frequencies in the three stages of the IF deck to give a somewhat flat bandwidth across the spectrum that the widest filter runs at (16 KHz). A sweep generator is useful for aligning these types of receivers. You may see one stage set to 453 KHz, the next to 457 KHz and the third to 455 KHz.

For many receivers you can just peak them all at 455 KHz (or 10.7 MHz, whatever your IF setup is).

Setting them all the same will give you a sharper performance curve on the IF stages but it may actually be down by several dB if you were to flip over to SSB or some other receiving condition where the signal is offset from the carrier.

It really depends on how sharp the individual IF stages are to begin with.

A sweep generator is pretty neat to use if you are looking at filter performance. For most hams it is not a "must have". You can accomplish the same thing with careful measurements at different offset frequencies and a spreadsheet application.

Ms. Tisha Hayes
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AC5UP
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« Reply #3 on: January 29, 2011, 07:45:45 PM »

If you look at magazine adverts for consumer radios of the 1930's you'll find an occasional reference to 'sideband clipping' as a drawback of the new superhet designs. They reduce your listening pleasure, cheat you from hearing all of the music, and is something you won't need to worry about with a tried & true Neutrodyne, Synchrodyne, TRF (or whatever the advert was selling) radio. No kidding... Excessive selectivity as a negative sales point.

Back when I started rebuilding and restoring junque receivers I did my best to peak an IF for maximum gain so I could dig the weak ones out of the soup. Took me a long, long time before I realized that on AM the music really is in the sidebands and tweaking for a flatter, fatter IF passband was the way to go even if it did cost a fraction of a dB in gain. The noise floor invariably limits AM BCB reception, and the radio has more than enough AGC to compensate, so it's more a case of gaining fidelity than losing signal.

If your listening is mostly SSB or SWL stations spaced 5 kc's apart, tweak 'em right down the center for a tight IF. If it's a five tube AM superhet you're working on, consider adjusting the top slugs to 453 kc's and the bottom slugs to 457 kc's so the IF has a little spread. 

If you don't like the sound you can always re-tweak it dead-on @ 455 and call it good.  Cheesy

BTW: Tisha is correct... Better than 30 years ago I scored my first R-390, wired the audio to a small amplifier with a semi-decent speaker, and was completely blown away by how good the radio could sound at the wider IF settings. Who knew that sibilance and AM weren't mutually exclusive terms ?
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AC5UP
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« Reply #4 on: February 17, 2011, 02:05:26 PM »

This is an FYI to the sleep and sweep challenged among us:

If you should happen to have a signal generator similar to an HP 8640B and a function generator of modest capabilities you have the potential for a swept IF signal source. The HP 8640B is capable of external FM modulation at a variety of preset deviation settings. The BNC modulation input is DC coupled which means that if a person were to set the HP for FM on 455 kc's and modulate it with a sawtooth or triangle wave from the function generator it should sweep +/- the deviation setting. Choose a low frequency on the function generator. 500 to 1,000 cycles should be plenty. Any signal generator capable of FM with an external modulation signal should be able to do the same. A sawtooth or triangle wave is preferred for linearity but a sine wave will give you sweep.

'Scope sets up as X-Y with one side to the output of the IF strip and the other to the external modulation feeding the HP.

BTW: On Tektronix scopes like a 2213 it's not exactly obvious how to set an X-Y display. Rotate the horizontal sweep rate to "zero", which is labeled as X-Y in a place you won't see when your hand is on the knob, then note how Channel 1 vertical becomes X and Channel 2 vertical becomes Y. Adjust gain & position on both channels as needed.
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W5JO
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« Reply #5 on: February 17, 2011, 07:55:00 PM »

Another option if you want a sweep generator is to watch for high quality function generators.  Some of them have a sweep function included and many tune from almost zero up to 10 MHz.
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AC5UP
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« Reply #6 on: February 18, 2011, 09:24:41 AM »

About six years ago I was given a modest stash of test gear that derived from an SK. A no-name sweep / function generator of early 80's vintage was included and since that time I have yet to use it for a sweep IF alignment. Should be easy enough to figure out but so far I've had nothing but cinderblock moments with it. Can't locate any documentation, not sure if it works properly (even though it does have signal coming out of the right places) and my freq counter tells me the dial is 'ballpark' accurate at best. It's also way too drifty...

Sooner or later I'll probably cut my losses and pick up something with a better pedigree, but for now I use it as a source for signal tracing and not much else.
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W5JO
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« Reply #7 on: February 18, 2011, 12:20:42 PM »

I have one, the brand I forget, made in Israel, that is solid state with a digital readout.  I didn't receive a manual with it either but managed to work my way through it and understand its set up.  I wonder if the one you have doesn't have some real problems if it is drifting?  Generators like that are not supposed to drift at all.   On mine the frequnecy can be swept at a variable rate and it is good from 10 cycles up to 10 Mhz.  I highly recommend this route rather than looking for something that is just a sweep generator unless you need markers with it.
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