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 Post subject: using a sweep generator with digitial o'scope
PostPosted: Mar Fri 02, 2018 8:27 pm 
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Joined: Mar Fri 02, 2018 7:58 pm
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I am sure this topic is rather simplistic but.... I have a WaveTek 1001A sweep/marker generator and an Tec 210 digital o'scope. How do I connect these devices to align the RF front end of a FM stereo receiver?

As a starting point I have connected the outputs on the back of the WaveTek to the X and Y inputs of the o'scope and the front output of the WaveTec to a RF detector, which is then connected to the front input of the WaveTek. I set the WavTek to 120MHz and minimal sweep. I set the o'scope to X-Y and I expect to see just a DOT on the o'scope. I cant seem to find a signal on the o'scope.....

I must be doing something wrong. Could use some expert advice on connecting and configuring these two pieces of test equipment so I can align the RF section of a AM/FM receiver????

Thanks
Wayne


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Fri 02, 2018 10:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Tucson AZ
Looking at pictures of the 1001a, there appears that there is an output on the rear for the vertical output. This needs to be connected to the scope that runs the "Y" channel.

Why are you using 120Mhz? FM is from 88 to 108mhz.


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Fri 02, 2018 10:52 pm 
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Joined: Mar Fri 02, 2018 7:58 pm
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Hi

I am trying to follow the setup instructions in the WaveTek manual. I have connected the WaveTec to the o'scope as described in the manual. I have connected the RF out to a detector and the detector to the WaveTec and adjusted the WaveTek as instructed in the manual. [According to the manual I have the RF is adjusted to around 100-150MHz - dont have the unit in front of me].

With all that done..... I cant seem to find a signal on the o'scope. The o'scope is a 2-ch Digital 60MHz Tektronics. I am new to the Digital scope, I suspect I am not configuring the scope properly......

Hopefully one of you can steer me in the right direction ;-)
Wayne


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sat 03, 2018 2:23 am 
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Joined: Mar Thu 29, 2012 8:51 pm
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Location: upper midwest
I also have a Tektronix TDS210...

If you push the DISPLAY button on the front panel, there will be a option for FORMAT. Press the button next to that to change from "YT" to "XY". If you already did this and still no dot on the screen, the other possibility is your input positions are set to move the dot off screen. I would start with nothing connected to the front jacks and then check the settings of the position knobs. You should at least get a spot you can center.


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sat 03, 2018 2:48 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7889
Location: Long Island
This is probably not the steer you were looking for, but you might consider putting the sweep generator on the shelf and reading the alignment instructions for the receiver instead. Almost no known FM receiver requires a sweep generator to align the FM front end. The oscillator and RF tracking are simply adjusted against fixed frequencies which may be over the air stations, same as AM radios.

Where sweep has any use at all is in IF alignments, but even there it depends on the vintage and design of the receiver. From the mid-1970s on, manufacturers tended to use non-adjustable filters rather than IF transformers. They may also have recommended a specific alignment procedure, specific sweep curves to achieve, and/or equipment since textbook perfect sweep curves are seldom correct for minimum distortion or maximum stereo separation.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sat 03, 2018 5:03 pm 
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Joined: Mar Fri 02, 2018 7:58 pm
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Hi Chris

I am using the WaveTek and o'scope to align the radio as these are the instruments I have. I am working on a 70's vintage Sansui whose front end is very tunable. While cleaning eons of grime from the receiver board I have inadvertently de-tuned things. I also had to disassemble/clean/reassemble the large multi-gang tuning capacitor, which itself has adjustable mica-based capacitors. I want to adjust things to get the best filter passband performance I can. The FM receiver has three filter stages: initial RF and two IF stages. The AM is a little simpler with only two filter stages.

Clearly, I can adjust things by ear..... but sadly my ears are old and are well out of tune ;-)

And beside, using the sweep/marker gen and o'scope to tune the front end is an interesting challenge.

I am not all that familiar with the DIGITAL o'scope. So, as noted previously, I suspect that I am not configuring the o'scope properly to work with the WaveTek sweep/marker generator.....

I want to also adjust the FM Stereo portion but need to find something I can use to generate good quality FM Stereo source signal to use to align things.... But. One step at a time. Receiver filters first.
Cheers
Wayne


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sat 03, 2018 5:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1168
Location: Tucson AZ
Your initial post, you stated that you set the Wavetek to 120mhz, which is outside the FM band, and, beyond the measurement capability of your scope. You need to set it to what the RF frequency is called out for in the manual for the receiver.

For the RF, you won't be using sweep. For the IF, you will be at 10.7mhz and that will be a sweep that is modulated to whatever the manual calls for.


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sat 03, 2018 5:42 pm 
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Joined: Mar Fri 02, 2018 7:58 pm
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Hi

I am not trying to measure the RF signal output by the Sweep generator with the o'scope. Rather I am measuring the Sweep generator's sweep signal vs demodulated signal (measuring x-y). The rate of change of these signals is quite slow and well within the 60MHz BW of my o'scope.

Thanks
Wayne


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sun 04, 2018 3:13 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7889
Location: Long Island
If the goal is to fix the radio, your best bet is still to get the manual for it and follow the alignment instructions given. Forget about sweeping the front end or adjusting its passband. Unless you are in a position to analyze circuits and design hybrid networks to match the input and output impedances exactly, there is no way to make such measurements in a meaningful way with a scope and a sweep generator. Passband in tuned circuits depends on loading, and if the loading is thrown off by mismatched impedances at either end, you won't get the right results. Just set the oscillator frequency and antenna tracking per the manual and let the front end go at that.

In the factory the front end was probably aligned using a sweep method but on a custom jig which matched the input and output impedances of the front end circuitry, supplied particular marker frequencies, and had a template showing a custom response curve to adjust for. You don't have the factory jigs or templates, so it is not possible to know or replicate what they did originally. In any event, the passband of a superheterodyne receiver is determined predominantly by the IF stages, and those can be aligned or at least verified for correct operation with a sweep generator and a scope. The receiver service manual will tell you how to do it if it is practical.

If the goal is to get a sweep generator working with your scope, see page 26 of the TDS-210 manual. It instructs you to put the display into X-Y mode. When so selected, the channel 1 input and controls become the X axis and the channel 2 input and controls become the Y axis. The scope channels should be set to DC coupled. Do not expect miracles, as they tell you on the next page, in X-Y mode it operates as a sampling scope and the sample rate is only 1-Ms/sec. A plain old analog CRT scope would work better in this instance.

The horizontal and vertical outputs of the Wavetek would be connecte to the channel 1 and channel 2 inputs of the scope. As long as the generator is sweeping, this should produce a horizontal line on the scope. You may have to fiddle with the controls on both devices to find the right combination of settings.

Next thing you need is a test demodulator. There is a circuit in the Eico 369 sweep generator manual for a simple demodulator you can build, or you can find others online. RF detector probes for signal tracers use the same circuits. They basically consists of a VHF detector diode, a couple of resistors, and maybe a capacitor or two. The test demodulator connects between the RF output of the Wavetek and its demod input. If it is working right, the horizontal line on the scope should deflect upward when the RF output of the generator is increased. Once you get the sweep generator and scope working together with a test demod you are ready to try it on a piece of equipment.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Sun 04, 2018 11:07 am 
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Joined: Jul Mon 08, 2013 2:48 pm
Posts: 253
You should also keep in mind that though the sweep alignment method is simple in principle it's not only science but alo an ART which requires some training and experience to be mastered. Getting some curve on the screen is easy but the correct interpretation of it much less, and you'll very seldom get the nice pictures shown in the service manuals (even tried to adjust for a -4.3dB drop on a little scope screen at a specified frequency ?). You'll soon discover that allmost ANYTHING can dramatically change the curve shape: injection method, signal level, take-off point, demodulator probe, impedance (mis)match, AGC bias, markers injection, even moving the position of the test wires... and (last but lot least) spurious oscillations ! These techniques must be fully understood and mastered before you start or you'll end up with poorer results than when you started (or worse: a totally unaligned receiver). There were some very good (old) books about this topic (sweep alignment) from SAMS and RIDERS which can still be found and are a recommended (essential ?) read, they'll tell you everything you need and won't find in the service manuals. (but of course nothing about using digital scopes !)
Still, the best way to learn is to try and practice. Good luck and have fun.


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 Post subject: Re: using a sweep generator with digitial o'scope
PostPosted: Mar Mon 05, 2018 2:12 am 
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Joined: Mar Fri 02, 2018 7:58 pm
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Guys

Many thanks for the advice. thyratron26 was spot on. I found the "Display" topic on pgs 72-72 of the scope manual - describing the xy vs xt mode. I had assumed that ch1-ch2 was the setting I needed but turns out on the digital scope I needed to set the Display to "xy" mode..... Ah,HA!!

Chris108 pointed out the XY mode as well (his manual must be different to mine as couldn't find any reference xy on the page he cites) and offered a heap of additional thoughts. I've got a RF detector good to ~2500MHz. The xy sampling rate of 1 MSPS is more than adequate for use with the WaveTek - its fastest sweep rate is limited to 10ms. And, fortunately, I have an aligmnment manual for the radio - although it calls for using an unknown version of "Genscope". I don't have a Genscope of any type but I do have the WaveTek and Tek Digital o'scope.

Turbologic has some interesting thoughts. The WaveTek has a 50 ohm output - the radio has a 75 ohm (FM) and 300 ohm input (AM) input. So it is easy to make a resistive matching pad. As to the RF detector; I am referencing the 'test points' identified in the radio tuning manual. I think my RF detector input impedance is quite high (M ohm) and I plan to use a small capacitor to couple it to the test point so I dont create a dc load. I will play with the detector "load' to see if/how it corrupts the measurement.

In general, I don't think 'absolute' filter measurements are required. Basically, as I understand the radio tuning manual, the objective is to optimize the collective passband shape of the front end filters and seek minimum overall passband attenuation. Should be an interesting exercise.

I think I am good to go!

Thanks again
Wayne


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