EEVblog #391 - Rigol DSA815 Spectrum Anayser Teardown  - Page 1
Forum section: electronics / general

author: Winston
author_rank: Regular Contributor
timestamp: November 27, 2012, 04:36:19 pm
content: I wonder if the presence of etched LC filters on the PCB is due to a possible intention to use the same PCB at higher frequencies in some future model via component changes. If I'd spent the incredible amount of effort to design a beautiful PCB like that, I'd try to enable sort of option that if technically feasible.

author: LaurenceW
author_rank: Frequent Contributor
timestamp: November 27, 2012, 08:51:36 pm
content: What, you mean make one product but then release different bandwidths with just a software key? Rigol wouldn't dothat, surely?(slightly miffed owner of a genuine DS1102E 100MHz Scope...)BUT, if I was Agilent, Tek or any of the other first tier names, I'd be getting worried for my company's long term viability about now...

author: tinhead
author_rank: Super Contributor
timestamp: November 27, 2012, 09:22:36 pm
content: Quote from: Winston on November 27, 2012, 04:36:19 pmthis is IF filter, it does not have anything to do with frequency range of a SA.

author: EEVblog
author_rank: Unknown
timestamp: November 27, 2012, 09:24:16 pm
content: Dave.

author: almoehi
author_rank: Contributor
timestamp: November 28, 2012, 11:07:41 am
content: Hi Dave,did I spot some JBC tips in this video?

author: EEVblog
author_rank: Unknown
timestamp: November 28, 2012, 11:32:45 am
content: Quote from: almoehi on November 28, 2012, 11:07:41 amYou did.I have a JBC CD series iron now.Dave.

author: JackOfVA
author_rank: Unknown
timestamp: November 28, 2012, 12:48:22 pm
content: Dave:The classic analog spectrum analyzer design is multiple conversion - with an up-converting input stage.The input runs through a low pass filter, attenuator, etc. and then is up-converted to a frequency above the highest input frequency. For a 1.5 GHz high limit, the 1st IF might be in the 1.8 to 2 GHz range. A rather wide fixed bandwidth bandpass filter is used after the 1st mixer. Up-conversion is used for several reasons, one of which being that if the 1st IF frequency is someplace inside the operating frequency range you have a major signal leakage problem. Another one being that it's much easier to build a LO that spans 1 octave, say 2-3.5 GHz where the 1st IF is 2 GHz, than to build a LO that covers 9 KHz to 1.5 GHz. And, of course, image rejection becomes difficult with a lower frequency 1st IF. (Some of these problems can be solved with digital processing and I-Q samples, but there are still excellent reasons to go with an up-converting input design.)In your tear down, the 1st IF bandpass filter is likely the one formed by the |unun| shaped traces. Probably 200 MHz or so bandwidth, centered at the 1st IF.In a classic analog spectrum analyzer, this high IF is then amplified and run to a second mixer where it is down converted - perhaps to an IF around 300 MHz where some very wide resolution bandwidth filters can be used, and then a third conversion to a much lower frequency such as 21.4 MHz where crystal filters with resolution in the tens of Hz or less are feasible.Looking at your video, the design seems to follow the classic analog approach with up-conversion to the 2 GHz range or so, followed by a second mixer / LO to a lower frequency. Then into the ADC for the rest of the signal processing.Jack

author: free_electron
author_rank: Super Contributor
timestamp: November 28, 2012, 07:14:53 pm
content: the little transformer around the adc is a balun , or more precisely an unbal. they go singl eended to differential. that adc most likely has differential inputs.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 28, 2012, 08:33:13 pm
content: It's a bit hard to see, but it looks like a bad soldering job...?

author: gregariz
author_rank: Frequent Contributor
timestamp: November 28, 2012, 09:08:45 pm
content: Quote from: PA0PBZ on November 28, 2012, 08:33:13 pmit just looks nasty

author: tom66
author_rank: Super Contributor
timestamp: November 28, 2012, 09:47:38 pm
content: It looks like not enough heat has been applied to those parts -- probably because they're metal and bonded straight to the ground plane and applying too much heat would lead to damage to the pads. However, they could have helped the situation by using thermal relief on the through holes -- it doesn't look like they have.

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 28, 2012, 11:18:21 pm
content: A bit of preheating could make a big difference.

author: robrenz
author_rank: Super Contributor
timestamp: November 29, 2012, 01:39:52 am
content: A JBC would help

author: almoehi
author_rank: Contributor
timestamp: November 29, 2012, 06:47:48 am
content: Quote from: EEVblog on November 28, 2012, 11:32:45 amCool! After some research I decided to buy a CD-B because of their tip technology and the easy way to change them. I found it somewhat difficult to get my hand on one as most dealers only sell to businesses but one Austrian ( not Australian) dealer also sells via eBay. It would be very interesting to hear about your impressions in a video blog.Cheers,Armin

author: crispy_diode
author_rank: Newbie
timestamp: December 01, 2012, 06:31:32 pm
content: This is indeed a classic RF spectrum analyzer design. I've traced out some of top-level blocks in the RF deck. The engineering of this unit is incredible for the price. My hats down to the engineers who designed this. Very nice product.I wonder what are the actual LO frequencies used in this project and cutoff frequencies of the IF filters. If I remember correctly, HP in their older SA models used 321 MHz and 21.7 MHz for the 2nd and 3rd IF filters respectively.

author: EEVblog
author_rank: Unknown
timestamp: December 02, 2012, 02:18:45 am
content: Quote from: crispy_diode on December 01, 2012, 06:31:32 pmAwesome work, thanks. Looks good to me.Dave.

author: G0HZU
author_rank: Super Contributor
timestamp: December 16, 2012, 11:19:54 pm
content: I had a quick look at the front end section as this is quite a critical area for performance and I'm not sure you have it labelled correctly.Item 4 looks to be a basic diode based wideband RF detector and I suspect one of its roles is to rapidly sense input overload at high RF power levels that could cause damage to the front end attenuator or mixer.This will presumably rapidly trigger the opamps (comparator?) that sit nearby and this will toggle the input RF switch (item 1) to dump the RF power into those 4 x200R SMD resistors arranged in parallel to give a 50R dummy load rated at about 1W or so (item 2)So I think item 1 is an RF switch (for damage protection) and not an LNA as labelled in the earlier image.Item 3 looks to be another RF switch and I suspect that this switches in a test signal? Does the analyser have a front end test signal selection in the menu? eg something like 100MHz and -10dBm would be typical for an analyser.Item 5 is a 5 bit digital step attenuator and at a guess this is a Peregrine part (looking at package and pin order etc)If I had to guess further I'd say it was a PE4306 (0-31dB in 1 dB steps) but it looks like the label has been sanded away by Rigol