Hp Spectrum Analyser Centre Frequency error  - Page 1
Forum section: electronics / testgear

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 29, 2018, 07:14:37 pm
content: I have an HP 8568a SA in my workshop that I have been using to calibrate AnHP8662a Signal Generator. If I feed a known frequency into the unit , it and set the centre frequency to that signal it shows it off very slightly.The 8662a requires some frequencies to be set that my counter is incapable of resolving so I was going to use the SA instead.My question is is there a simple cal routine I can try to correct this offset?It seems far worse at low frequencies and I have tried all the cal routines.Anyone any ideas?

author: tkamiya
author_rank: Super Contributor
timestamp: July 29, 2018, 11:19:05 pm
content: Unless there is a way to know which one is accurate, I wouldn't touch either one. As it stands, it could be that SA is off, SG is off, or they are both off. Commonly available GPSDO fed into SG will establish Signal generator is right. Honestly though, I wouldn't use Spectrum Analyzer as basis of calibrating something else.....

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 30, 2018, 07:59:37 am
content: The SG is spot on. It is fed from a rubidium standard, it's defiantly the SA that's wrong.

author: bson
author_rank: Unknown
timestamp: July 30, 2018, 08:16:41 am
content: Looking at the datasheet for the HP 8568A it has a 20MHz calibration output. What if you feed this into the counter, presumably using the same freq ref as the SG? Surely it has a calibration routine for the timebase as it specifies drift/day.

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 30, 2018, 09:18:39 am
content: The 20Mhz is exactly right. The Reference OCXO was checked and is too within 0.1 Hz. The problem is worse at low frequencies. Feeding a 100Khz signal and punching is 100Khz centre frequency and a span of 200Khz shows the peak slightly to the right of the centre graticule and peak search marker shows an herror of around 2kHz

author: bson
author_rank: Unknown
timestamp: July 30, 2018, 10:49:07 am
content: Ouch. Sounds like a/the receiver LO...

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 30, 2018, 04:40:40 pm
content: Is that resulting in an incorrect IF frequency? Sounds like fun!

author: TinkerGDS
author_rank: Contributor
timestamp: July 30, 2018, 07:54:08 pm
content: Simon,I don't know if you got your problem resolved, but I have the same spectrum analyzer and it was just calibrated before I purchased it. Can I test something for you to give you a benchmark?Gerhard

author: KE5FX
author_rank: Super Contributor
timestamp: July 30, 2018, 10:11:03 pm
content: You probably want to use the 8568A's counter marker for this, not the center frequency. The CF and normal marker calibration is derived from a 'lock-and-roll' ramp that is not guaranteed to be perfectly linear.If you are trying to adjust the 10 MHz OCXO in the 8662A, you need to do that with an oscilloscope instead.

author: EE-digger
author_rank: Frequent Contributor
timestamp: July 31, 2018, 12:40:20 am
content: Your center frequency is something like +/- 4kHz on a 200kHz span, plus some other lesser errors. When resolving two points in that span, error is even greater, like +/- 5% of the point separation.This is from the data sheet and not sure if it helps.Re-reading your posts a few more times, I think what you're seeing is within spec.I am wide open to being corrected on this

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 31, 2018, 07:20:54 am
content: Hi Gerhard, could you try A 100 kHz signal and and see what you get with a peak search marker please?I am am also suspicious of the units calibration.Has anyone ever done a calibration on one of these?Loos a bit daunting!

author: TinkerGDS
author_rank: Contributor
timestamp: July 31, 2018, 02:09:22 pm
content: I applied a 100k -10dBm signal from my 8662A. On the SA I started with a Preset, then CF at 100k and span of 200k. I pressed Freq Count then Peak Search. This is what I see. I confirmed the same results from my 3325B.

author: Tony_G
author_rank: Super Contributor
timestamp: July 31, 2018, 03:21:38 pm
content: Quote from: Simon Spiers on July 31, 2018, 07:20:54 amI sold one two years or so ago and FedEx dropped it on delivery so they sent it back to me (3 cheers for insurance) - It's been sitting in my cupboard since then as the entire 10MHz reference section broke off. At some point I'll get around to fixing it and then will need to do the calibration - I bought the full manual set from Artek and it looks like I've collected all the gear needed to do it (assuming that the extenders I have work for it).If you do end up doing the calibration I'd love to see a video or two on the fiddly bits.TonyG

author: Simon Spiers
author_rank: Regular Contributor
timestamp: July 31, 2018, 06:23:15 pm
content: Well that's interesting. Looks like when I press FREQ COUNT I get the correct reading! However the peak is slightly off to one side which bothers me. Is this the LO?

author: TinkerGDS
author_rank: Contributor
timestamp: July 31, 2018, 07:12:43 pm
content: Have you confirmed the misalignment at different frequencies? Now that you're using a different procedure on the SA I wonder if the alignment of the grid is just off if it happens at every frequency?

author: xmo
author_rank: Regular Contributor
timestamp: July 31, 2018, 07:19:51 pm
content: Try changing to a slower sweep time and see if the signal peak shifts closer to the center.

author: Tony_G
author_rank: Super Contributor
timestamp: August 01, 2018, 12:53:52 am
content: Quote from: Simon Spiers on July 31, 2018, 06:23:15 pmTo me it looks like it is triggering early - I'd take a look at Adjustment 25 Digital Storage Display as it mentions the A3A1 Trigger, X & Y gain and tweaking a SWP OFFSET adjustmentTable 3-2 looks interesting as well with a bunch of adjustments for the A3 boards listed as well.TonyG

author: Tony_G
author_rank: Super Contributor
timestamp: August 01, 2018, 01:12:48 am
content: This PDF also has a bunch of information on the IF-Display section troubleshooting.https://literature.cdn.keysight.com/litweb/pdf/08566-90042.pdf?id=821665It clarifies the role of SWP OFFSET to be for sweeps of less than 10ms - HSWP might be what you're after.TonyG

author: Simon Spiers
author_rank: Regular Contributor
timestamp: August 01, 2018, 04:55:07 pm
content: When I change the span it does shift the peak slightly from left to right. Luckily I have the full manual set so all I have to do is work out the problem. I will check the trigger problem but I did replace a board in the display unit last year, I wonder if that's done it? I am also getting intermittent YTO unlock messages at certain spans, but that's a pot that's dirty on the sweep board.TonyG thank you I will have a look at that this evening.