View 2.4 Ghz signals on a 1.3 Ghz spectrum analyzer - Page 1
Forum section: electronics / rf-microwave

author: Etesla
author_rank: Regular Contributor
timestamp: August 12, 2020, 01:29:13 am
content: Hi all,I'm a beginner in the world of RF stuff, so bear with me. I recently purchased an HP 8554B spectrum analyzer, which is good up to around 1.3 Ghz. The big problem is that I would like view signals beyond 1.3 Ghz, specifically in the 2.4 Ghz range, without worrying too much about accurately measuring the bandwidth or powers of those signal. Any ideas? I had a couple of my own thoughts on how to do this, but I don't know if they are realistic or not.The first thought was to multiply (mix???) an incoming signal from an antenna with a sin wave at around maybe 3 Ghz, which should put an image of the 2.4 Ghz spectrum around the 600 khz mark to view with the SA. The problem is that I have no idea how to do that in real life. Are there IC's that make 3-5 Ghz sin waves? What kind of mixers can I use for that frequency? Can I realistically implement this with hobby equipment and received signal powers in the microwatts?My other though was to somehow divide the frequency down by two. I think I have a better idea of how to actually implement this, but it still seems difficult. I would basically go from a receiving antenna to a pretty sharp BPF around 2.4 ghz, then I would amplify that signal (which I don't think is trivial), then use some sort of super fast digital logic like a D flop or something to divide that signals frequency by two and feed that divided signal into the SA. I've done this for 100's of Mhz, but I'm not sure how something like this would scale to the 2.4 ghz range...Like I said, I really have no idea what I'm doing, but I would love to hear yall's ideas on how to do this. Just keep in mind that I don't talk RF jargon yetThanks in advance!

author: 0culus
author_rank: Super Contributor
timestamp: August 12, 2020, 02:10:59 am
content: A mixer is exactly the right idea. You'll also want a reasonably good signal source to provide the LO for the mixer. You can go cheap with Analog ADF-based solutions (these can be found all over ebay, if they quote 4.4 GHz you know exactly what chip it uses) but in my experience these are not particularly clean sources and have bad phase noise. I have an HP 8672A that I use for such purposes.  I should add, I assume you are wanting to look at the ISM band. Do keep in mind that the signals here are generally not sinusoidal in nature. I can't comment on how well mixing will work with WiFi beacons and bluetooth spread spectrum type signals as I have an 8566B which easily covers this band with no mixer. I set up a small experiment using a mixer I had handy (not a very good one) and was able to mix the beacons from my wifi router down to around 600 MHz. Quite doable, though there are a ton of losses. You might want to also use a low noise amplifier after the antenna.

author: Etesla
author_rank: Regular Contributor
timestamp: August 12, 2020, 04:49:38 am
content: Thank you so much for the time you put into answering this question! I'm going to look further down the mixer path then. I have a 8640B with the 1 gig option that I can probably triple for use of the 3x harmonic at around 3 gigs to get a pretty clean signal. I'll have to read up on mixers and how to use them correctly. Should be fun. Thanks for the input!

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 12, 2020, 08:16:36 am
content: Erasynth micro provide clean and low phase noise 'synthesized' signal throughout the spec bandwidth min to 6GHz... i also bought china cheap adf chip based earlier 4GHz synthesizer but the gui is tough and signal is bad at low freq. Erasynth Micro worth its $250 price imho, i havent play with GHz china mixer ADL based module though (purchased earlier) for signal downconversion study, no time yet. ymmv.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2020, 08:36:44 am
content: The mixer will introduce artefacts which may or may not be of interest to you.You may need to filter out-of-band signals before they reach the mixer.Some ICs do have low phase noise, e.g. LMX2594, but they are complex to program and you may find it beneficial to buy them ready mounted on a board.If you have time and patience, you can find spectrum analysers quite cheaply. I have two (Tek 492, hp8562b) which go to 20GHz and only cost me £350 each. That's less than a new LMX2594EVM evaluation board

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 12, 2020, 10:19:10 am
content: Something like this would also work:https://www.crowdsupply.com/outernet/morfeus

author: Grandchuck
author_rank: Super Contributor
timestamp: August 12, 2020, 11:51:58 am
content: When I got a DSA815 I wanted to extend it to 3 GHz and came up with this using eBay parts. The oscillator is important. The one I used is a cavity type from an HP unit.

author: Etesla
author_rank: Regular Contributor
timestamp: August 12, 2020, 05:04:16 pm
content: I'm fresh out of funding and space for new equipment, so getting a better SA or a new sig gen probably isn't in the cards. I'm doing this more to see if I can and to learn some cool stuff. Looking at the math, I think that I actually only need a LO that goes up to 1.1 Ghz (2.4 - 1.3) at the lowest. Using a 1.3 Ghz LO would be pretty comfortable and keep any of the LO signal that leaked into the IF output at an arms length from the 2.4 Ghz image at 1.1 Ghz. Is this correct, or is there some real world restriction that keeps me from using a 1.3 Ghz LO?Also, what sort of mixer specifications would I be looking for with this setup?Specifically, If I choose a mixer with a listed maximum frequency of say 2.5 Ghz, would it still work in this use case where I expect there to be images at 3.7 Ghz, even though I don't care about the 3.7 Ghz image?Also, say I get a mixer that needs 7 dbm for the LO, and I've read that it's good to have the RF signal strength be about 10 db lower than that, at around -3 dbm. If the RF signal coming from my receiving antenna is much lower than -3 dbm, say -40 dbm, will I just not see anything? Is a LNA basically required after the antenna, or would a good antenna just work? The noise floor on my SA seems to reside around -90 dbm or lower if that matters. Thanks again for all the good advice!

author: 9aplus
author_rank: Regular Contributor
timestamp: August 14, 2020, 05:48:26 pm
content: Like others already suggestedI am using DBM mixers in combination with EraSynth up to 15 GHz.Have one WJ mixer going up to 15 GHz and one Anaren going up to 4 GHzMorpheus is also useful tool but results are better on lower part of spectrum even low to 10 MHz