Low cost spectrum analyzer <100MHz?

Joerg sez:

I know very little about such test equipment, but I was at one time subscribed to a Google group focusing on such. They were constantly finding used Tektronix scope plug-ins on e-Bay for cheap. There is a constant -- though thin -- supply of several different models (both portable and non-) of spectrum analyzers needing either little or no repair. The group collectively is quite a good resource of parts and repair knowledge when it comes to this.

snipped-for-privacy@yahoogroups.com

or go to Yahoo groups and join the regular way...

Good luck,

Some time ago (say, 10 years or so) there was a series written on a "homebrew" spectrum analyzer that used a TV tuner as an LO. I know the series of articles was googleable, at least a couple of years ago. You could have pretty much any bandwidth you wanted as a function of what ceramic or crystal filter you chose.

Jim

It's this one, isn't it?

Here's the original page:

Agree. TV tuners are among the worst performers in terms of dynamic range. Lately it has gotten worse. Sometimes I have the feeling the young lads at those design houses don't really know what a dynamic range is. After re-roofing our house I had to move the antenna onto a side mast. I figured, well, I might give it the perfect spit shine, a nice amp, distribution and so on. The old set we used to have in the garage really liked that. Tons of new stations. Not so the newer set in the living room. It completely fell off the rocker even after putting a notch on the two strongest channels. I had to attenuate almost 10dB to make that set happy and lost one station in the wake. Pathetic.

Anyhow, back to the analyzer issue: It's for a client and they don't really want to make a science project out of it. What's a nice low cost "staple" among low end analyzers? I only know the top notch equipment but, for example, we do not need any precise frequency indication. It could be one of the old non-PLL analyzers. No tracking gen, just a good dynamic range. I am just not familiar which ones represent a good deal. Any hints?

Having built this circuit many years ago, I can tell you to avoid it completely. The dynamic range is very low (maybe 40db) and the results are unpredictable with frequency. You are much better off getting a used hp on ebay.

Depends. A lazy one, with a fake noise level measuremet (only for some frequency ranges and modulations) is here:

Roughly like this:

Breaking the 1Ghz range in multiple 100MHz ranges, with an overlap between adjacent ranges (see how works Tektronix analyzers)

Designing a narrower filter is difficult, and could be useless.

USB does not mean it will be for sure in "real time"... think at ISM band (2.5Ghz) and how fast are WIFI signals

This will require a stable reference, preferable digitally synthesable.

The maximum available dynamic range is finally a matter of bits numbers of the A2D converter: SNR = 6.02 x n bit + 1.76dB

It means you need a fast ADC with more than 12 bits.

Of course the analyzer arhitecture it may be zero IF or with one (or more) IF stage...

Without a complete calibration (amplitude and frequency) a spectrum analyser is useless. If the frequency calibration could be easy done based on the odd/even harmonics of a stable 10Mhz crystal (you'll see up to 100MHz all harmonics), the amplitude calibration is not easy. An 1Ghz Tektronix scope, DPO4000, has about 2% amplitude nonlinearity, and it's factory calibrated.

I wouldn't shy away from whipping up a filter myself. Done it many times, including really steep crystal filters. One issue with the simpler analyzer models is that they don't have a nice low noise PLL but a YIG oscillator. I found those to be a bit noisy. But OTOH it'll be enough for us.

If it needs a few seconds to transmit it's data that'll be fine. I grew up with Polaroid cameras hooked to side hinges on the screen. Worked. Except when you got the goo from the back foil onto your clothing.

I've done it with really old ones. The trick is to turn it on, make coffee, do some other design work for an hour or so and then it's warmed up. You just have to make sure that nobody touches the thermostat on the wall or opens a window. Would be no problem here. Believe it or not but this office is usually heated by a wood stove. You can keep the room temp at 69.0F and it stays there. To the point where I sometimes thought the digital thermometer had frozen up on me.

No ADCs in the old ones :-)

The impedance analyzer here in the lab uses a slope converter that (in theory) has almost infinite resolution. I get north of 90dB out of that.

True, but my experience with I/Q demod scheme is, well, mixed.

This is how I've done it: Take (or borrow) a generator with known output amplitude over the whole range. Or make one from a few PECL gates. Then plot the amplitudes on the analyzer. Now turn on your 10MHz harmonics thingie and compare. From then on you know the amplitude of the harmonics. In the 80's I did that stuff on a notepad sheet because all they had was a daisy-wheel printer and the accounting folks had that occupied all the time.

On really old analyzers I sometimes let the harmonics generator run into the input line via a 20dB splitter, to see where I am on the frequency scale. I mean, in the olden days they navigated big airliners in a similar fashion by following headings to regular AM band stations. It always got them home.

Anyhow, looks like we might just buy a real oldie like the HP141. I've used that a lot when I was young and it has never failed me. It is just very big and heavy. And no automatic stuff whatsoever. That was why the others back then hated this thing so I could have it as long as I wanted to ;-)

If you were in the UK I'd strongly suggest a Marconi TF2370.

I've only seen one of those for sale. Unfortunately it is in Australia (I'm in the US) and it is in pretty deplorable shape. H-deflection looks dead, digits missing etc. We'll probably go for the HP141T mainframe with the respective modules in there. That'll feel like stepping into my old Citroen again.

While at it I was looking for Tek 7L13 modules for the 7704 mainframe here in the lab. Surprisingly those are still rather expensive, almost $1000. Same for the older 7L12. Something like that would be just fine for the task at hand. But you can get a complete HP141T set-up for less money and it comes with the display mainframe.

With those specs (80dB DR) you won't find nothing under $3000.00 even on ebay That said, this looks nice 50K-3.3GHz for abound 3 grand brand spanking new...

It's been too long ago but I remember guys in the 80's saying that I'd be lucky to squeeze more than 60dB or so out of the old HP141. Then I did. They still didn't want to use it because everything had to be hand-dialed and hand-tuned and it was drifting a bit. AFAIR you just had to be quite careful not to fry its input.

That is indeed nice. Thanks, might be an option for us.

Build your own? This uses a PC for some of the work.

He mentions Wes Hayward who has directions for a project of one without a PC.

Another guy puts out kits based on his designs here:

Looks like the Spectrum Analyzer is about $205

and the Tracking Generator to go with it ~$49

Robert

Joerg, I have bought an old YiG Tektronix T494. Is useless for most of the digital signals requesting to be measured nowadays. Forgot about that and focus on:

This sort of old spectrum analyzer, like T141 is good only to see the TV stations and repetitive signals. You can't analyse anything is digitally transmitted and is switching fast (like WIFI, WIMAX, WCDMA, GSM, etc)

I have no problems with DDS. One of my beginning designs with DDS is here:

And infinite laziness as well. I think you are familiar with all single-dual-multiple slope converters, right ? Then you know how much it takes for one measurement and why.

Ha-Ha ! We've just returned to Agilent a new bought $350.000 network analyser just because it comes from factory with a problem above the 22Ghz range... As long you can't prove the generator is working well, you have no reason to believe is working well... a calibrated certificate means NOTHING.

Or make one from a few PECL gates. Then

Anything of interest here?

martin

We are only after low level noise measurements. It's all going to be plain vanilla AM and phase noise but for the latter we already have external discriminators. Can't tell you details here but they operate in the Tera-Hertz region.

Thing is, my client needs to make measurements for maybe a couple hours per week, then optimize a production process which requires days of re-tooling, measure again, re-tool again and so on. I doesn't make sense to spend $30k for an analyzer or shell out $500/week in rent if the job can be done with a vintage analyzer. In the same way that I wouldn't use a Ferrari to pick up lumber at the hardware store.

I've built a few, too, but not at frequencies that high ;-)

Time is usually not such a big factor. My impedance analyzer can take 15 minutes on some scans. But it's programmable and does it while I can continue on a schematic, or take the dogs for a walk (they love that).

$350k? Wow. Well, this ain't a big or gvt funded company but a start-up. They don't have that kind of budget :-(

And I am absolutely certain we can do it with a 141T.

Maybe but I don't know how well they'd do noise floor measurements. I know the old machines from HP and some from Tek would. The only one from Aaronia that would be able to measure the required range is the big one (1MHz-7GHz). And they still don't seem to have a dealer in the US as far as I could see. On the online form it says overseas order can take 6-8 weeks. Too long, and if it doesn't do noise floors for us the return could be a hassle.

Check

Oliver

An ordinary UHF tv tuner can be converted into a low cost spectrum analyzer.

Construction projects appeared a decade ago, possibly in RE.