YIG tuned filter suggestions

[prutser]

For a long time, I am planning to build a Spectrum Analyzer. Target is to go just above 10GHz. It seems the only feasible way is to use an harmonic mixer with a YIG tuned filter. Cost should be reasonable for the YTF (< € 400,-)

Is this feasible (ebay?) or better look for another approach, since we now can get synthesizers over 10GHz?

Any suggestions?

[jjoonathan]

> It seems the only feasible way is to use an harmonic mixer with a YIG tuned filter.

No. I hope my YIG post didn't have any part in giving you this impression, because it is not correct. The most common spectrum analyzer architecture is high-IF, which does not use a YIG. So much so that if someone refers to "the" spectrum analyzer architecture, they are almost certainly referring to high-IF, rather than YIG -> harmonic mixer. Even YIG based spectrum analyzers only start using the YIG filter above several GHz, so they will use the high-IF architecture for 0GHz to 2-4GHz, since the YIG has a minimum frequency. If you have a YIG you'll want to use it, but if not, high IF is the ticket.

The limiting factor on high-IF typically pops out of LO subharmonics and the inconvenience of dealing with a high frequency IF. Your instincts are correct: these problems have been greatly alleviated with modern components, especially that TI 15GHz synthesizer. I recently did a high-IF design at 14.5GHz and I suspect if you survey components you might land in a similar region. I won't tell you exactly what I bought because it was opportunistic. The listings were one-off. If you cultivate an understanding of the constraints, though, you will be able to spot similar opportunities, even though the specifics will be different.

A tutorial on high-IF is out of scope, but if you search around for "the" spectrum analyzer architecture, you will find it. The telltale signature is that the first conversion goes up in frequency. Which is strange, because the eventual point is to go down in frequency! You do it to avoid spurs/images. Once you understand this motivation, you will be able to put together a bandplan that uses available components, and modern components make higher high-IF designs much easier to accomplish.

Good luck 

[Paul B]

hi my advise is not to build a spectrum analyser but to bye a second hand one
a HP going to 22 GHz with yig front end from 2 to 22 Ghz start at about £700

problem with building one when finished it wont be worth the cost of the components to build it
but the item you spend £700 will still be worth £700 next year

[prutser]

Thanks both for the replies.

@jjoonathan The high IF was my intention (initially planning for 1st IF above 8GHz, this might change since it's just a hobby project)
But to go higher  is getting more difficult, so I was considering a 2 band approach with using a YTF for 8-18GHz, which down converts. (due to the YTF no problems with image frequencies)
For this first LO I was considering an YIG (could get a relatively cheap one which does 8-18 GHZ) it might be a challenge to get it PLL locked. Alternative would be a synthesizer e.g. adf5355 with an external doubler, or an adf4371, but it's quis expensive)
From cost perspective the YIG might be an interesting option. Received it today and it works, but currently no idea about the phase noise ( only was able to measure output frequency up to 13 GHz)

Today also received my YIG tuned filter, unfortunately it turns out the heating element is broken. I have no experience with YIG, but the initial idea s was to put a temperature sensor on the housing and use the measured temperature with a calibration table.
Actually no idea whether that is feasible.

 

@Paul B
Here it's hard to find the HP models below EU 1500,- As it looks now, my DIY will probably also cost at least EU 700 but by using a full digital IF with at least 30MHz bandwidth, I will also be able to use at as an SDR frontend.  It's also just the fun :-)
 

[ssturges]

One big problem with high IF designs if your are looking for instrument grade performance is the phase noise scaling on the upconversion and downconversion.  A mixer is an add/subtract function.  It does not scale phase noise with 20log(N) like a frequency multiplier or divider.  All the high IF frequency phase noise will be converted down without 20log(N) scaling on the downconversion if the LOs are not noise correlated. Working round this this requires a deep bag of tricks for upconversion and downconversion to be phase noise correlated and have a level of cancelation.   If you look at typical spectrum analyzers(esp older ones) you will note that they often do not have such good phase noise performance say compared to a phase noise analyzer.

Those TI synth parts sure make design a lot easier!  Not instrument grade but they get the job done. 

Used equipment is getting so expensive, most things have doubled in price in the last year!  Even broken stuff is going for high dollar. 

[Berni]

Yep i also recommend getting that 22GHz HP boatanchor if you want to go fast on the cheap.

Working past 4GHz gets much more challenging. Even just connectors and cables get tricky, as at 20GHz a half meter of regular coax can become a 15dB attenuator. Getting parts that work this high can be hard and expensive as they are very specialized. So the easiest way for getting a YIG harmonic mixer is likely buying the one as a spare part out of an old spectrum analyzer.(Keep in mind these easily get blown up from overload so lots of people might be looking for one to repair a spectrum analyzer)

Then once you do have it there is a lot of work to actually build a spectrum analyzer around it. In this topology multiple parts of the spectrum analyzer have to track together in sync. The amplitude response of the whole thing is also all over the place, so it needs to be calibrated and corrected for. This means that even if you do build one you still need to get signal generator to calibrate it.

Would still make for a great learning experience, but expect to spend quite a bit of money on parts while putting on a lot of work before you get to something that is usable as a spectrum analyzer.