EEVblog #529 - HP 35660A DSA Upgrade Investigation - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 29, 2013, 09:41:23 pm
content: A teardown of the front end PCB in the HP 35660A Dynamic Signal Analyser.Dave does some preliminary investigation to see what opamps and FET's are unsed in the analog front end to see if they can be upgraded for better performance.This is more for Dave's record than interesting info.Datasheets:http://www.farnell.com/datasheets/1465563.pdfhttp://www.cotorelay.com/2900.phphttp://www.ti.com/lit/ds/symlink/ne5534.pdf

author: mimmus78
author_rank: Unknown
timestamp: September 29, 2013, 10:12:04 pm
content: Well, I have a similar project for my 2000 counts meter. Still not decided what opamp to use to change it to a 8.5 digits meter.Sorry but i can't stop myself to post it :-)Joking aside, it's a very good idea to improve specs of old equipment this way! Hope to see another follow up from this video.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 29, 2013, 10:30:10 pm
content: Hi Dave,Check the function of J10 and J20. It looks like they can be used to ground the input to the next stage. It would be interesting to measure the spectral noise density with the jumpers in the grounded position. It will give you an idea about the noise contribution of the front end.Jay_Diddy_B

author: EEVblog
author_rank: Unknown
timestamp: September 29, 2013, 10:38:18 pm
content: Quote from: Jay_Diddy_B on September 29, 2013, 10:30:10 pmAh, didn't see those, interesting, thanks.J200 even bypasses the anti-alias filters and other stuff, right at the track/hold input. Very nice. Someone made this thing really easy to measure stage by stage.

author: c4757p
author_rank: Super Contributor
timestamp: September 29, 2013, 10:41:48 pm
content: Completely OT... but I have to say, I noticed in the schematic there that they've bolded the main signal path. I've not seen (or noticed, more likely) that before and I really like it

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 30, 2013, 12:22:50 am
content: Hi,I just did a quick check of my HP 35665A DSA, this is the slightly newer version of the 35660A.With the input terminated in 50 Ohms, I measured a PSD of -153 dBVrms/rtHzwhich corresponds to 22 nVrms/rtHz.Conspiracy? two similar instruments half a world apart, one answerSince full scale is -51dBVThis means that -153dBVrms is -102 dB relative to fullscale.Theoretical SNR = 6.02N + 1.76 dB where N=number of bits.So 102dB SNR corresponds to 16.6 BitsThis with an averaging of 100 samples.So I don't know how much improvement you are going to get from the front end. I am interested to watchJay_Diddy_B

author: EEVblog
author_rank: Unknown
timestamp: September 30, 2013, 08:26:23 am
content: I tried the two jumpers which bypasses the front end. No difference at all. same 22nVrms/RtHz result.I also discovered that my analog board is very significantly different to the schematic and overlay in the service manual. The left input side of the board is the same, but the right and side (by the analog ground plane spit) is vastly different. Doesn't appear to have the same transitor based sample and hold circuit, and no J200 jumper to bypass all the anti-alias filter etc.

author: varno
author_rank: Newbie
timestamp: September 30, 2013, 10:16:29 am
content: The best Low-Noise op-amp that I know of on the market is the LME-49990 at 0.88nv/sqrt(Hz), that amp has noise that is lower than most discreet JFETs, though if you want a low noise FET, one of the best that you can get is the 2SK170, they are hard to get, but if you want a few and are willing to find a good matched pair I'll send you a bunch for you to have at it.

author: sync
author_rank: Frequent Contributor
timestamp: September 30, 2013, 10:57:31 am
content: Replacing a monolithic dual JFET with two discrete ones sounds like a bad idea.

author: varno
author_rank: Newbie
timestamp: September 30, 2013, 11:10:33 am
content: IF you match them for Ids, and then glue them together with thermal epoxy, it works well. so long as the transistors are from the same batch.

author: JackOfVA
author_rank: Unknown
timestamp: September 30, 2013, 11:30:32 am
content: My HP 3562A DSA, one generation earlier than Dave's machine, shows 34.6 nV/sqrt(Hz) at 160 Hz, with 50 ohm termination and 128 sweep average. (Averages must be in powers of 2 with the 3562A).Specs are:20 Hz - 1 KHz <-134 dBV/sqrt(Hz) corresponds to 200nV/sqrt(Hz)> 1 KHz < 144 dBV/sqrt(Hz) corresponds to 63 nV/sqrt(Hz)

author: sync
author_rank: Frequent Contributor
timestamp: September 30, 2013, 11:32:39 am
content: Quote from: varno on September 30, 2013, 11:10:33 amI doubt that. This is not audio stuff. The Analyser goes down to 244uHz.

author: varno
author_rank: Newbie
timestamp: September 30, 2013, 12:18:18 pm
content: You would be surprised as to how badly monolithic dual-jfets can be matched.If he wants to go and get a single matched part the 2SK389, is there as an option, or the obvious course of action would be to use the op-amp on it's own. it is actually very good,

author: Salas
author_rank: Frequent Contributor
timestamp: September 30, 2013, 06:53:30 pm
content: Quote from: EEVblog on September 30, 2013, 08:26:23 amFrom the schematic's areas you passed over in the video it seems there is no gain where you focus, it looks like just a combo buffer. The gain stage is somewhere next.

author: Salas
author_rank: Frequent Contributor
timestamp: September 30, 2013, 09:10:41 pm
content: Could be the 2N5546 JFETs.

author: EE59
author_rank: Newbie
timestamp: October 01, 2013, 12:43:21 am
content: Quote from: EEVblog on September 29, 2013, 10:38:18 pmI don't think J10 and J20 ground the input to the next stage. They enable the bootstrap circuit to dynamically follow the output and adjust power to the FETs, allowing them to handle higher voltage swings. With them grounded you could overdrive the FETs. They shouldn't change the spectral noise density.Most of the noise is from the series input resistors R9, R10, and R20.That bootstrap circuit would make a good fundamentals Friday video.

author: casinada
author_rank: Frequent Contributor
timestamp: October 01, 2013, 08:41:09 pm
content: Now Dave is Famousor other people are running out of ideas.http://www.wirelessdesignmag.com/videos/2013/10/hp-35660a-dsa-upgrade-investigation?et_cid=3512503&et_rid=210453744&location=top

author: tecman
author_rank: Frequent Contributor
timestamp: October 02, 2013, 02:52:55 pm
content: I would try AD797 op amps. Internally compensated, more than enough bandwidth and 0.9 nV/rt-hz. Should drop in as a replacement, just remove the existing comp cap.paul

author: hobbs
author_rank: Contributor
timestamp: October 04, 2013, 04:03:45 am
content: I have the 35665A. There's an outfit (glkinst.com) that has reverse-engineered the options system in that one, and offers an All Options EEPROM to turn them all on. Works great, and it was pretty cheap at $65 including a new CMOS backup battery.CheersPhil Hobbs

author: vk2hmc
author_rank: Contributor
timestamp: October 08, 2013, 05:36:31 am
content: Dave,here is a hint!If the JEDEC part number is not listed in the products part list description, use a HP Xref to convert HP part numbers to JEDEC (www.jedec.org) part numbers.For your Part, 26-0715 (18 is dropped).Therefore, the HP part number for the Op-Amp is 1826-0715.Using one of the HP Xref's, this equates to a NE5534!The Original HP cross reference was posted in HP bench briefs regularly so there are quite a few different scanned copies around.I also triple check part numbers before ordering/replacement, just to be sure!This is my main Xref -http://www.vk2hmc.net/blog/?wpfb_dl=7(!warning 21Mb)Be aware here are a number of different cross reference lists present in the file.If the part is not in the 1st list, be persistent and keep searching.Also, I find that this searchable excel spread sheet is pretty quick, accurate and fairly complete -http://www.vk2hmc.net/blog/?wpfb_dl=6Actually, once you get used to the HP part system, it is quite easy to both remember and find the JEDEC part number if you have to.--marki

author: JoeO
author_rank: Frequent Contributor
timestamp: October 08, 2013, 01:29:14 pm
content: Marki:Thanks for the lists. They are great.You forgot to mention that the first list IS searchable in a PDF viewer.I did look for the 1826-0715 in the PDF file. It is listed in there 6 times as a NE5534AN.Joe

author: vk2hmc
author_rank: Contributor
timestamp: October 08, 2013, 02:01:21 pm
content: triple!I got it into my head the part is a 1826-0711 for some reason even though I _wrote_ 1826-0715 in the post - go figure...

author: innkeeper
author_rank: Unknown
timestamp: August 17, 2017, 01:05:28 am
content: I found a cross reference for the 55-0460 / 1855-0460 it showed a 2n5545anyone know of a less noisy replacement for it?this fet's noise is typically rated at: 20 nv/(sqrt)hz2n5545 data sheethttp://www.vishay.com/docs/70253/70253.pdfI'll post the link to the cross reference when I find it again, but it confirms someone suspicion from earlierThis topic caught my eye as I just scored an HP 3562Ashould be arriving shortly.the service manual does have a bom and part numbers, and seems to share a very similar front end to the HP 35660A and uses the same part numbers.I'm more then willing to give upgrading the opamps and fets it a try once I do the initial checkout of it.Anyone think there would be any reason not to socket the opamps with machined sockets like mill-max?

author: innkeeper
author_rank: Unknown
timestamp: August 19, 2017, 08:14:26 pm
content: Looking at the schematic, it appears to me the 4 2n5545's on the front end of the high and low buffer side would introduce the most noise earliest in the input at 22nV/rtHz.Someone correct me if i am wrong, but it would seem if one can't make any noise floor improvements by upgrading those, that likely any otter op amp upgrades later in the circuit will likely not net much.i do notice they drive an offset adjustment. using (A33/A35) R212 and 1412 in the HP hp 3562A. I couldn't find the procedure to adjust those, does anyone know how thatch done?Also any commendations on good replacements for the 2n5545?

author: chuckb
author_rank: Frequent Contributor
timestamp: August 21, 2018, 02:18:36 pm
content: Linear Integrated Systems has a very low noise dual JFET the LSK489. It seems to have about 4 times less noise than the 2N5545. The capacitance seems similar also.Then the noise limit would be the NE5534 op amps. An LT1028A would be a good replacement for that. the Compensation may need to change to keep it stable. Or use the LT1128A.That's just in the front end buffer, I don't have a schematic to identify other downstream opamps that may introduce noise.I have an HP35665A that I want to reduce the noise on. Let me know if any of this helps!Mill-Max sockets should be fine at these frequencies.

author: innkeeper
author_rank: Unknown
timestamp: August 24, 2018, 02:46:12 am
content: I would be very curious about what you see.from what I gathered from earlier statements though there might be practical limits on the ADC.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 24, 2018, 03:18:29 am
content: I ordered a spare input pcb for my HP35665A and I will first characterize that and then modify it over the next few weeks. The 35660A schematic shows 7k of resistance (11 nv / rt Hz) for protection before the JFETs. So that will set the white noise lower limit. The 20nV / rt Hz flicker noise corner will be moved from 10 Hz down to 0.1 Hz with the new JFET and op amps. The white noise floor will probably be reduced from 22nV / rt Hz to 11nV.The LT1028 has to much current noise for the given circuit impedance. The LT1007A or another may work better. The first 4 NE5534s will be upgraded because they provide up to 24 dB of gain and establish the noise floor. There are many more NE5534s in the Anti Aliasing filter but they should not be impacting the noise floor because they work with a larger signal.I would think the ADC sets the dynamic range (14 bit) of the system but the first gain stages should set the noise floor. We will see.

author: precaud
author_rank: Frequent Contributor
timestamp: August 24, 2018, 12:52:00 pm
content: Quote from: chuckb on August 24, 2018, 03:18:29 amI thought HP promoted these as 12 bit systems, is it really a 14-bit ADC?

author: chuckb
author_rank: Frequent Contributor
timestamp: August 24, 2018, 01:39:17 pm
content: Looks like my memory was wrong again.The specs for the 35665A have the noise floor at -76 dbfs which would be 12 bits. The resolution is 0.016% which is between 12 and 13 bits.It works great for what I need.

author: precaud
author_rank: Frequent Contributor
timestamp: August 24, 2018, 01:48:29 pm
content: Yeah, 12 bits is fine for a lot of uses, especially with averaging.My recollection is, in terms of resolution, the HP DSA lineup was:35660A/35665A = 12 bit3562A = 14 bit3567A/3565S = 16 bit (I have one)35670A = 16 bitAmazingly, the 35670A is still in production, but is scheduled for discontinuation later this year. It's had a very long run.

author: chuckb
author_rank: Frequent Contributor
timestamp: August 24, 2018, 08:35:02 pm
content: Today I'm using the 35665A to plot the performance of a 4 pole 10Hz low pass filter. The DSA source is monitored on CH 1 and the filter output is monitored on CH 2. This is with 800 lines resolution and 0-400 Hz span. After 1000 averages the CH 2 maximum rejection displayed is -89dB below the 8 Hz reference amplitude.The FREQUENCY RESPONSE display shows -110dB of rejection in the filter at 200 Hz referenced to the amplitude at 8 Hz. After 10 averages I could read -100 dB of rejection. I'm not sure how things are processed for the FREQUENCY RESPONSE function vs just looking at the CH 2 amplitude. That's a pretty good dynamic range for a 12 bit ADC! Maybe dithering helps a lot here.It works great for what I need but, I will try some modifications to reduce the front end noise.