Downconversion for spectrum analyzers - Page 1
Forum section: electronics / beginners

author: spasmodic1
author_rank: Newbie
timestamp: December 21, 2013, 11:10:04 pm
content: Hi,i have a rigol DSA815, im no million air and dont want or really need to spend that money on a scope to cover up to the 5.8ghz region.What would be the best way to frequency divide a signal while maintaining a somewhat close to original amplitude so one could display the signal on a lower bandwidth SA?

author: DrLuke
author_rank: Contributor
timestamp: December 22, 2013, 02:30:41 am
content: Well, one way is to mix it with a similar frequency, resulting in one very high frequency signal, and one low frequency signal. That's how software defined radios work. For more, see this:http://en.wikipedia.org/wiki/Heterodyning

author: Unknown
author_rank: Unknown
timestamp: December 22, 2013, 03:46:09 am
content: (Disclaimer: by no means a microwave expert, though I've dabbled...)Yeah, a classic IF mixer/downconverter would be a typical choice (e.g. the venerable MC13154, although it's only good to 1.8GHz). At those sorts of frequencies, something like a RFFC5072 (just found from a quick search) would get you in the ballpark of your DSA815's 1.5GHz, or you could do a double conversion.Would it be good enough for dicking around and looking at stuff? Sure.Would it be good enough for side-by-side relative comparisons/measurements? Probably.Would it be good enough for absolute or long-term measurements? Hmmm, not without some serious thinking applied to the design.

author: spasmodic1
author_rank: Newbie
timestamp: December 22, 2013, 04:48:18 am
content: Thanks for the replys,i have a double balanced mixer, but it just a matter of having a vco or tunable oscillator for the LO that would cover a decent range of frequencies.Although that rffc5072 did look very useful hard to obtain in small quantities though.But i thought there might be something out there that could give you a clean signal without all the harmonics of a mixer. But i will give the RFFC5072 a go if i can get one.cheers.

author: mhzghz
author_rank: Contributor
timestamp: December 22, 2013, 08:34:40 am
content: Quote from: spasmodic1 on December 21, 2013, 11:10:04 pmMake a downconverter with a quality Watkins-Johnson or M/A-Com M1H double-balanced mixer and a 5 GHz local oscillator signal at +7 dBm.You'll need to attenuate your 5.8 GHz signal down to -10 dBm max. Apply it to the mixer's RF input.Apply a +7 dBm 5 GHz signal to the LO port.The DC-800 MHz IF output will correspond to 5.0 - 5.8 GHz. You should add a 3 dB attenuator on the IF output.You could probably use a 2.5 GHz LO and the second harmonic in a pinch.Losses will be the mixer loss + IF attenuator loss, which will be around 10 dB total.

author: spasmodic1
author_rank: Newbie
timestamp: December 23, 2013, 03:04:01 am
content: Thanks quys,I will have to work on an oscillator suitable for those frequencies.

author: notsob
author_rank: Frequent Contributor
timestamp: December 23, 2013, 06:16:08 am
content: minicircuits may be worth a visit to see what they have available

author: KJDS
author_rank: Super Contributor
timestamp: December 23, 2013, 08:50:01 am
content: If you're going to want a useful measurement out of the spec ana, then the oscillator should be phases locked back to the spec ana reference. I'd also want a level 13 mixer as a minimum, do some research of mixer third order products and work out what dynamic range you'd be happy with. To get the best dynamic range from the mixer, drive it harder than nominal, so if it is spec'd for 13dBm +/-2dB, aim for 14 to 15 dBm.The output of the mixer needs to be terminated in 50 ohms at both IF frequencies. If you have a 5GHz LO and a 5.8GHz RF signal then the IF needs to see a good 50 ohm at the wanted 0.8GHz IF and also at the 10.8GHz IF that you don't need.

author: spasmodic1
author_rank: Newbie
timestamp: December 29, 2013, 01:06:50 pm
content: Quote from: KJDS on December 23, 2013, 08:50:01 amthanks KJDSThe IF is going to the SA input which has a 50 ohm input so termination isn't needed right?. Or are you saying at .8 ghz the IF will need to be impedance matched to 50 ohms?

author: KJDS
author_rank: Super Contributor
timestamp: December 29, 2013, 01:36:48 pm
content: Quote from: spasmodic1 on December 29, 2013, 01:06:50 pmThe spectrum analyzer will provide a good match at 0.8GHz. It is very likely that it will provide a bad match at 10.8GHz which is where the other mixing product will be made. It's a non-trivial area of design. If you're not interested in accuracy and just want an idea that the modulation is ok then you can ignore it, but it will mess up conversion loss accuracy and do funny things with the higher order mixing products which may end up appearing when they otherwise wouldn't. The easiest way round it would be to put a 3dB SMA attenuator on the mixer output. That will provide enough loss at 10.8GHz to prevent any nasties occurring.

author: Mr Simpleton
author_rank: Unknown
timestamp: January 02, 2014, 12:10:33 am
content: You will have to add filters! If you do look at the mixing products and what frequencies your oscillator will be at, you may find a suitable bandpass filter to help.Old analyzers had a nifty feature that you could see how the spurs moved, and thus decide that is was a spurioius not something to be bothered about... still it is a painSo get a good bandpass filter at the input of the mixer, and keep your 815 at highest possible frequency to move the mirror signals away. And do not forget to add some gain to recover from mixer and filter losses... Looks quite like a major project