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 Post subject: Using a spectrum analyzer to align IF's
PostPosted: Mar Wed 20, 2019 11:06 pm 
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I am looking for some info on how to use my new Rigol 815TG spec analyzer to align IF`s in receivers like the R-390A, SP600 etc. instead of using my HP 8644B and an oscilloscope. I have seen it done and it provides a very useful visual image in the time domain. I think this would be more useful to me and easier than using my sweeping RF signal generator and scope.
I am just not sure of the hookup procedure.

Craig W9CLA


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Thu 21, 2019 5:12 am 
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Time domain on a spectrum analyzer?

The typical way to align IF's with a spectrum analyzer is with a tracking generator. It graphs the response on the Spectrum Analyzer in the frequency domain. This looks exactly like what you get when you use a sweep generator and an oscilloscope synced together. You also connect the same way you would with a sweep generator and oscilloscope, IE couple the tracking generator to the mixer grid, and the spectrum analyzer at the end of the IF chain (without loading it too much). Does your Rigol have a tracking generator built in? If not, a noise source is sometimes used.

Perhaps others can comment, but I would not expect this to work very well at the 455khz IF frequency used in the sets you mentioned. It doesn't work well with a conventional sweep generator with a marker and an oscilloscope because the marker is as wide as the bandpass. What works well is a sweeper with either a digital marker, or such precision in the start and stop points that frequency can be read right off the graticule of the oscilloscope. An HP 3336b or similar plus an oscilloscope does the job nicely.

Sorry I couldn't really answer the question.


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Thu 21, 2019 11:10 am 
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Quote:
...An HP 3336b or similar plus an oscilloscope does the job nicely...
That, along with a AD8307 log detector, is what I use also.

Image

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Thu 21, 2019 1:10 pm 
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R390AGUY wrote:
I am looking for some info on how to use my new Rigol 815TG spec analyzer to align IF`s in receivers like the R-390A, SP600 etc. instead of using my HP 8644B and an oscilloscope. I have seen it done and it provides a very useful visual image in the time domain. I think this would be more useful to me and easier than using my sweeping RF signal generator and scope.
I am just not sure of the hookup procedure.

Craig W9CLA

I am not too sure about the Rigol, but it hits a nice price point and is very compact.
I have used my HP 3585A to run IF's and what I have done is use a pair of DC blocks on both the tracking generator and the input. I made the DC blocks and I use the 1 meg input on the HP spectrum analyzer. I assume you only have a 50 ohm input, so you will need an active probe or something of the like so you don't load down the tail end of the IF. I just clip the lead from the tracking generator OVER the insulation of the input lead to the mixer so as to very lightly couple the signal into the circuit and then clip the input to the SA at the output of the IF strip.

Here is a Truetone (Detrola) D723 picture that I have handy. Nothing exciting from a basic radio, and totally unnecessary for a basic radio.
Image


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Thu 21, 2019 4:43 pm 
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As others noted, with a spectrum analyzer you need to use a tracking generator which basically provides the spectral response as "edited" by the IF system.

It really isn't needed for the great majority of vintage receivers unless you are trying to stagger tune or otherwise create a non-standard (for that receiver) bandpass characteristic. Where specific IF bandpass characteristics are needed then it can be used although to me it still seems more straightforward to use a sweep gen and scope as shown by Mike. That is what I use when I need to provide an IF passband display.

I have a wide range Tektronix spectrum analyzer with matching tracking generator but I have only very rarely used the tracking generator with it and my primary use of the spectrum analyzer is for alignment of phasing type SSB transmitters and for checking the spectral output of transmitters and amplifiers. On a couple of occasions I have looked at two tone IMD products for "linear" amps but mostly I use it to check for harmonic and spurious output which is its forte. It made fast identification of unusual behavior from a Johnson Viking Valiant when I was using it for a local 160 meter AM net. I used a solid state no tune transceiver to set up my antenna tuner on 160 and found when using the Valiant on the same frequency the SWR was not high but no longer 1:1. Even odder was the SWR was higher at lower power. There weren't any odd harmonic issues but instead I found when set up for 160 meter operation the Valiant was also producing about 3 watts at 120 khz. regardless of drive level and overall power output. A damaged choke in the control grid bias feed was the cause and it apparently was causing the final to act as some sort of TPTG oscillator only on 160 meters. I made note after that to carefully check spurs from "DC to daylight" when odd symptoms arise.

Rodger WQ9E


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Fri 22, 2019 1:45 am 
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Greetings to the Forum:

Note that Craig mentioned he had a Rigol DS815TG. The "TG" stands for tracking generator.... and in this case is a built-in option. I have one of these instruments and it works very well. It should work quite well for the IF alignment application. Loosely coupling the spectrum analyzer input to the IF output should work nicely; the Rigol is quite sensitive and also has an input amplifier that can be switched on for low level signals. Thus, it is not necessary to worry about loading the radio IF stage; simply bringing a coupling loop near the detector should suffice. The Rigol, being a digital instrument, has no need for markers; you can put a cursor on the display at any point and it will tell you the frequency. Also, the graticule is quite accurate. The resolution BW can be set as low as 300 Hz, so there is plenty of resolution to see what is happening even in a 455 KHz IF. The analyzer only goes down to 100 KHz or so, so if you are trying to align a 60 KHz IF in a Hammarlund or Hallicrafters, you are out of luck.

Have fun,

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Fri 22, 2019 12:51 pm 
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Hi All,

I have, and use, the Rigol 815TG for a lot of my boat anchor restoration work. Something to keep in mind is the limit on the input signal and DC levels. I sure don't want to have to send it back for repair because I have burned out the input or output.

For most alignment work, I simply use the tracking generator as a signal generator and peak the tuned circuits for maximum output. I go from the TG output through a 100db step attenuator and then to a shielded probe with a blocking cap to prevent any DC from passing. With the adjustable output of the TG and the attenuator, I can get a signal level down to -102dbm, enough for Signal to Noise measurements.

I do use the spectrum analyzer along with the tracking generator when I want to look at a bandpass circuit, such as the response through an IF filter, or a bandpass circuit in a mixer chain. I generally use a 10x scope probe for the input to the spectrum analyzer, but this requires care in where to probe to avoid too high of a DC voltage level. The scope probe will detune a tuned circuit so keep this in mind. When this is an issue, I couple through a 1pf capacitor right at the probe end of the scope probe to minimize the capacitive loading.

One interesting thing that I have run into with the Rigol is that when you set the Resolution Bandwidth to the narrow ranges (like 300 cycles), you need to change the sweep rate to something like 1 second in order to get a useful display.

I would suggest looking for some tutorial videos on YouTube to help give you an deeper understanding of how to make the most of your analyzer.

Bob, AB1MN


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Fri 22, 2019 1:32 pm 
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ab1mn wrote:
One interesting thing that I have run into with the Rigol is that when you set the Resolution Bandwidth
to the narrow ranges (like 300 cycles), you need to change the sweep rate to something like 1 second
in order to get a useful display.
This is true for all spectrum analyzers regardless of type.

It's a function of the response of the filters and detectors in the SA.
If the sweep is too fast you get an asymmetric passband display when it should be symmetric.

- Leigh

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Fri 22, 2019 1:52 pm 
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Bob makes an excellent point about protecting the "front end" of a spectrum analyzer because it is easy to damage and expensive to repair/replace. I use a Tektronix 7L13 spectrum analyzer and the manual starkly warns that +13 dbm (just under 20 milliwatts) with the input attenuator set to minimum will destroy the first mixer which is why I don't use my analyzer when another instrument will perform the same task. When you need a spectrum analyzer it is worth its weight in gold but mistakes with it can be extremely expensive.

Accurate display of amplitude and proper spectral display of the signal require using a bandwidth setting, frequency span setting, and sweep speed that are correct. Sort of like the exposure triangle in photography, with a spectrum analyzer changing one of these parameters may require also changing one or both of the other to preserve accurate measurement. I use the 7L13 analyzer in a Tektronix 7633 storage scope mainframe which provides an adjustable variable persistence display mode so that a very low sweep rate can be used when needed and a single trace "snapshot" storage mode is available for capturing an image for photography. Using its best resolution bandwidth of 30 hz requires a very slow sweep speed with the phase lock engaged to keep residual FM to below 10 hz. Most spectrum analyzers will warn you with an "uncalibrated' warning if you choose a sweep speed and/or frequency span that exceeds the instrument's capabilities at the bandwidth you have chosen.

And on edit I see Leigh has already jumped in with a great explanation of RBW.

Rodger WQ9E


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Sat 23, 2019 1:14 pm 
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Jthorusen wrote:
The resolution BW can be set as low as 300 Hz, so there is plenty of resolution to see what is happening even in a 455 KHz IF. The analyzer only goes down to 100 KHz or so, so if you are trying to align a 60 KHz IF in a Hammarlund or Hallicrafters, you are out of luck.

Hi Jim.
I was just surfing E-bay and stumbled across one of these Rigols. The specs say they go down to 9 Khz with a RBW as low as 100 Hz (up to 1Mhz). Maybe they have improved the specifications since you purchased yours? They actually look pretty darn good for the price point.


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Sat 23, 2019 4:23 pm 
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The "scan speed" problem can be eliminated by sending in white noise from
an (if needed, amplified by one transistor) Zener diode noise generator, and
looking at the output on the display of an SDR radio such as my Airspy.
This works very well. You still have the same problem coupling to the
50 ohm input of the Airspy (or upconverter) without risking burning it out.
You just set the display to time average FFTs.

And you can use this method with AGC on. Frequency response can change wildly
with AGC changes. This is frequently recognized in TV alignment instructions
by specifying that you must clamp the AGC bus to some specific voltage.

For IF alignment, you don't even need the noise generator ... if it's a boatanchor,
just find a nice signalless spot on the dial, above the frequency of any
lightning crashes, and use noise picked up by the antenna.


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Mar Sat 23, 2019 11:27 pm 
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Greetings to Scott and the Forum:

The one drawback to the Rigol is that there is no printed manual. I was too lazy to dig out the CD ROM and try to wade through it to find the actual specs, so I went from memory. The instrument is at least as good as I said, and perhaps better?

Regards,

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Apr Fri 12, 2019 6:24 am 
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"Visual Alignment of Radio Receivers R-390/URR and R-390A/URR"
https://www.r-390a.net/ST-32-152.pdf


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Apr Fri 12, 2019 4:21 pm 
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Its interesting to find that even the latest digital SAs aren't all that much faster than an old 141T when looking at narrow bandwidths.

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Apr Fri 12, 2019 5:42 pm 
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A DC blocking adapter is also mandatory for a SA, and also for signal generators. Many are DC coupled right the attenuators and mixers.

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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Apr Sat 13, 2019 12:14 pm 
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Although the document ST-32-152.pdf on the R390A disk was written in 1969, it does not take into account that T501 and T502 in the R-390A IF deck need to be stagger tuned for most decks. I think it would be best to use Chuck Rippel's procedure at the very beginning of the 'IF deck' section of the 'Pearls' on the R390A disk here:

http://www.r-390a.net/Pearls/IF_deck.pdf

But, it does not use a sweep generator.

Regards, Larry


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 Post subject: Re: Using a spectrum analyzer to align IF's
PostPosted: Apr Sat 13, 2019 7:45 pm 
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As info, cheap Analog Device log amplifier detector boards are being sold on eBay for about 14 bucks. Many use the older AD-606, and there are some newer designs with newer 8300 series AD log detector chips. Not a bad deal for a few bucks.

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