Author Topic: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value  (Read 15815 times)

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Offline Adal

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Hi everybody.
Recently I acquired an used INSTEK GSP-827 spectrum analyzer. During the initial test, I discover that the displayed level are -6 dBm lower.
For example the calibration level is:
-30 dBm @ 100 MHz   

It´s appear
-36 dBm @ 100 Mhz.     ???
The same level appears  if I put external RF Level.   

Fortunately, this machine include a input offset in order to compensate the "user" gain/loss.  So I use this machine as is.
I hear your recommendations or if someone have the service manual for this thing, I can attempt a correction. 
Best regards
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #1 on: March 06, 2015, 11:02:01 am »
Hi,
I seem to have the same problem with my unit too!
Let me know if you find a solution or a source of service manual and I will do the same.
Best regards
Andrew
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #2 on: March 06, 2015, 11:46:44 pm »
Hi Andrew.

I think is a software issue, not hardware.  Because if I lower or increase the input RF level for example in 10 dBm, the spectrum follows the variation.
I write to Instek for a solution. Maybe you can write them too in order to "pressure" for a solution.

Best regards. 
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #3 on: March 08, 2015, 03:22:13 pm »
Hi, I've written to Instek too so hopefully at least one of us will hear back!
Will let you know if I do,
Best regards
Andrew
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #4 on: March 10, 2015, 10:29:12 am »
I heard back from the manufacturer today - trying to point me to an authorised service centre and not responding to the question of whether the -6dB is a known issue:

Thank you for using GSP-827.
 
Could you please let us know the serial number of GSP-827?
We could trace and find the proper distributor to assist you soon.
 
As the GSP-827 need to be proceeded the complete calibration by ATE after repairing (include the components swapping),therefore, from this case,  we would have the authorized distributor to assist you solve the problem


Regards

Andrew
 

Offline ruffy91

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #5 on: March 10, 2015, 11:52:03 am »
-6dBm seems like a impedance mismatch. (-6dBm means exactly half the signal amplitude)
 

Offline KJDS

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #6 on: March 10, 2015, 12:10:08 pm »
Is the 6dB flat across the band?

Is it possible to turn the calibration off?

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #7 on: March 10, 2015, 02:20:01 pm »
The most damaged components in a spectrum analyser are the first mixer, pre-amplifier and, in extreme cases, the input attenuator. The input socket centre contact is also a potential victim of abuse.

Mixer death is usually accompanied by a larger signal loss than you are seeing. A MMIC gain stage at the input of the analyser would be a possibility as if can continue to operate but with no amplification. You then effectively lose its stage gain but all other functions such as the attenuator and mixer work normally.

One of the greatest errors that some spectrum analysers make is to connect the RF input to an open circuit type antenna. ESD and static present themselves to the sensitive front end electronics and cause al manner of damage. It is a good idea to always have a 10dB attached to the input of an analyser and only removing it when you are absolutely sure it is safe to do so. Many Spectrum Analysers need a button sequence to select 0dB input attenuation for just this reason. The manufacturer likes to keep a pad in circuit to protect the sensitive RF stages that follow.

As to a software fault, sorry, I don't buy that idea. It would initiate a recall from Instek and much talk in the industry.

My advice would be to carefully investigate the RF front end circuit.... they are usually pretty easy to understand and are often laid out in a linear fashion with interstage screening, stripline interconnects and 'weird' pcb filters  ;)  If you get lost, just post a picture and I may be able to assist in identification of possible culprits.

Aurora

Additional comment. The unit has front end protection..... normally a couple of fast diodes and a pad. Worth checking that this has not been destroyed.

On many spectrum analysers you can see the 'Zero' marker at 0MHz (its actually the the first L.O).
Is that at the correct amplitude ? Issues here can mean mixer damage.

Also, check your noise floor. Is it still within spec ?
« Last Edit: March 10, 2015, 02:41:43 pm by Aurora »
 

Online AndyC_772

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #8 on: March 10, 2015, 08:01:53 pm »
Are you absolutely sure that the cal output isn't specified as an open circuit voltage, rather than the voltage into a terminated load?

Exactly 6dB across the range sounds too convenient to be a hardware fault.

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #9 on: March 11, 2015, 07:24:23 pm »
I heard back from the manufacturer today - trying to point me to an authorised service centre and not responding to the question of whether the -6dB is a known issue:

Thank you for using GSP-827.
 
Could you please let us know the serial number of GSP-827?
We could trace and find the proper distributor to assist you soon.
 
As the GSP-827 need to be proceeded the complete calibration by ATE after repairing (include the components swapping),therefore, from this case,  we would have the authorized distributor to assist you solve the problem


Regards

Andrew

Andrew.
I don´t receive yet any response from Instek. So I will attempt the repair. Remember, I acquire the GSP-827 used, so maybe the thing are damage.
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #10 on: March 11, 2015, 07:28:26 pm »
Hi Aurora.

Ok, let me take out the covers and check for damage.   :-BROKE

Somebody has a service manual?
Best regards.
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #11 on: March 11, 2015, 07:54:38 pm »
Sadly GW INSTEK service manuals are as rare as hens teeth. I have been in touch with the support team previously and they will not release service information to the public.

Aurora
« Last Edit: March 11, 2015, 09:12:18 pm by Aurora »
 

Offline KJDS

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #12 on: March 11, 2015, 08:51:01 pm »
You should be able to get an idea of where the issue is from some testing.

If the -6dB is flat across the frequency range then the damage is after the first mixer.

If there is an auxillary IF output you can also check the level there.

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #13 on: March 11, 2015, 10:54:55 pm »
Unfortunately the GSP-827  don't have a IF out. So I need teardown the thing and looking for damages in the input path.  I take some pictures of my equipment and put here.
Best regards
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #14 on: March 11, 2015, 10:56:18 pm »
Yes. The -6dBm error are flat across the frequency range
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline G0HZU

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #15 on: March 11, 2015, 11:22:33 pm »
I had a quick skim of the features/modes in an online doc and this analyser can operate as 75R and also 50R.

For example, I think you are meant to calibrate this analyser in 75R mode with an external 75 to 50R attenuator inline? The manual calls it the 'ZZ adaptor'. This zz adaptor will have a loss of just under 6dB. If you calibrate in 75R mode and forget to fit the (ZZ adaptor) 5.9dB matching attenuator during the calibration then you could end up with an analyser that appears to be deaf by about 6dB after calibration.


« Last Edit: March 11, 2015, 11:35:38 pm by G0HZU »
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #16 on: March 12, 2015, 08:27:50 pm »
Thanks for all the comments. It is some kind of hardware fault, not a 75 /50 mismatch or anything like that. It's behaving in a similar way with external input. I've also found out that the previous owner of my unit had to return it for repair on several occasions for this very fault. Hopefully when its skin is removed, it should be straightforward to find the problem !
Regards
 

Offline KJDS

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #17 on: March 12, 2015, 10:30:26 pm »
Yes. The -6dBm error are flat across the frequency range

Then it is very unlikely that the problem is before the first mixer.

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #18 on: March 13, 2015, 05:46:55 pm »
Thanks to all for your comments.

Andrew, I agree with your diagnostic. In fact yesterday when I turned my GSP-827, the internal 100 MHz -30 dBm calibration signal appears -34 dBm, only -4 dBm  lower  :-//     
And another fault, when I put an 100 MHz AM -30 dBm signal with 100 % modulation, I can´t resolve clearly the two side band, only the carrier.  :-- 
I make a video in order to show us the test

My strategic for fix it will be
Check the receive path.
Check the IF train and the digitizer.  I hear you suggestion.
 

 
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Instructions: The main board is probably defective. Replace it.
 

Offline videobruce

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #19 on: March 14, 2015, 04:39:02 am »
Sadly GW INSTEK service manuals are as rare as hens teeth. I have been in touch with the support team previously and they will not release service information to the public
Aurora
The Chinese are tight lipped. I guess they are paranoid about the competition coping their design. Rigol is the same way. I tried more than once to get a real SM, but there isn't any (so I'm told). Both have something they call a "Service Manual", but all it is is a "verification' manual. The 827 is a older SA, there maybe a possibility doing a search to stumble across something outside of Instek.  ;)

Funny, with the real US companies, most had/have SM's available. Actually with both importers, you have to the equipment serviced in China.
 

Offline videobruce

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #20 on: March 14, 2015, 04:41:56 am »
Quote
We could trace and find the proper distributor to assist you soon.
Since when distributors repair equipment??  :--
 

Offline videobruce

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #21 on: March 14, 2015, 04:51:50 am »
Quote
-6dBm means exactly half the signal amplitude
Isn't -3db half the level??
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #22 on: March 15, 2015, 03:33:00 am »
Here's  the RF PCA
I've not had time to do a thorough circuit analysis yet but here's a brief description (Refer to GSP-827 Front end photo...
RF input through capacitor into resistor pad R91,92, R93 (75R, 120R, 93R respectively). Then there's a cap to ground and what appears to be 2 x SOT23 back to back diodes marked 'EOF'...probably limiters.
The signal then feeds via C103,101 into U22 marked S169. This is a Skyworks Phemt GaAs SPDT DC-2.5GHz switch. Signal continues into a second S169 (U23), a third S169 (U24) and then into U21 marked S131 418. I think this is another Skywork's switch. Then onto another S169  (U25) into U26, another S131 418. The signal then gets +16dB gain from U18, an NBB400 MMIC. Looking at this components solder seems to suggest it has been the subject of some attention before!. I plan to isolate it, power it and feed a signal into it and check how much gain it is giving.
I'd be surprised if it was this component, being so far down the chain. I guess the first things to check are the protection diodes.
Regards
Andrew
« Last Edit: March 15, 2015, 03:44:45 am by G4JMO »
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #23 on: March 15, 2015, 12:29:03 pm »
Do not be fooled by the position of the MMIC. I have seen many such amplifiers destroyed by excessive signal level. I have also seen partial failure as some are multi stage inside the package and suffer failure of one stage. The fact that this has received attention in the past increases my suspicions surrounding this component. It is worth checking how much current it is drawing and comparing the treading with the manufacturers recommendation. This can help identify failure and (when repaired) check that the MMIC is correctly biased in the design.

I managed to cook an MMIC in a test piece by passing broad band noise through it. It is not just the peak RF signal level that can be harmful, it is also the combined energy of a multitude of signals in a broadband signal source that causes high dissipation in the amplifier.

If this does turn out to be the failure point, I recommend some further testing after repair to see whether some modifications are in order to protect the MMIC.

Some interesting reading from NASA:

https://electrondevices.grc.nasa.gov/files/T-06-Dec-1996-JPL-96-25.pdf

NBB400 datasheet is here:

http://www.rfmd.com/store/downloads/dl/file/id/29224/nbb_400_data_sheet.pdf

http://www.farnell.com/datasheets/1499972.pdf

Aurora
« Last Edit: March 15, 2015, 12:52:49 pm by Aurora »
 

Offline grumpydoc

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #24 on: March 15, 2015, 12:48:56 pm »
Quote
Isn't -3db half the level

-3dB is half the power, but if you reduce voltage by a factor of two then power is quartered which is a 6dB drop
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #25 on: March 15, 2015, 01:22:35 pm »
Looking at the pictures of the MMIC gain block stage, there is evidence of rework in the MMIC supply rail and associated bias components (R67,R69 & R71) The MMIC may not be running in optimum conditions and suffering as a result.

As a side note.... I do not know whether the rework was carried out by a service centre, but if so, I would send the tech for retraining ! A pretty poor soldering job on the reworked components. The MMIC installation is very untidy and still has flux residue present. Not good tradecraft at microwave frequencies. Worth removing that flux with IPA even if the stage is working. I would also tidy up the soldering but that may not really be necessary if the stage is operating correctly.

Additional comment: I raise the possibility that R67, 69 & 71 are selected on test in order to 'calibrate' the gain stage and this may be why the bias resistors have been reworked.

Useful app note here:

http://www.avagotech.com/docs/5967-5924E

http://www.hittite.com/content/documents/mmic_amplifier_biasing_procedure.pdf

http://www.ece.ucsb.edu/~long/ece145a/Lab2_145A/BiasingMMICAmps.pdf

Aurora
« Last Edit: March 15, 2015, 01:43:55 pm by Aurora »
 

Offline videobruce

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #26 on: March 15, 2015, 02:34:13 pm »
Quote
Isn't -3db half the level
-3dB is half the power, but if you reduce voltage by a factor of two then power is quartered which is a 6dB drop
Not to get too far OT or hijack the thread, but since you brought it up;
Most SA's that I have seen seem to have a default max 50vdc input level, But the RF levels are different between models with the same max 50vdc. Why?
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #27 on: March 15, 2015, 02:45:29 pm »
An RF spectrum analyser is an ac device and uses capacitor coupling at the input.

To answer your question .... DC blocking capacitor voltage rating versus the sustained ac signal overload capability of the input protection.

Aurora
« Last Edit: March 15, 2015, 02:47:26 pm by Aurora »
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #28 on: March 15, 2015, 04:04:41 pm »
Thanks Aurora for your input. Yes, the MMIC re-work does look untidy work for an official re-work doesn't it!..and the biasing does look like select on test from the resistor chain appearance.
So, I spent the morning checking out the gain from the input through to the output of the MMIC.
Accounting for external cable losses etc, the test setup had 0dBm at 2GHz injected into the input N connector. I then removed the interconnecting coupling caps to measure the signal down the chain.
With C101, 103 removed and coax tapped onto the output of the Pi attenuator, I measured -19dBm. Looking at the Pi components, this seems high..and coincidentally 6dB more than I calculate it should be. But I'll be upsetting the match somewhat with my soldered coax tap and the removal of the coupling caps. I tried to keep the coax tap as short and tidy as possible.
I put the caps back in and then took out C102,104 and tapped onto the output of the switch, U26. I measured -22dBm. This suggests a loss of about 3dB through the six switches. At 0.5dBm per switch, this seems about what I'd expect.
I put the caps back in and then removed the two caps from the output of the MMIC and tapped onto this output. I measured -10dBm, suggesting the MMIC is providing 12dB gain. This is about 3dB lower than the data sheet suggests, but again, I'll be affecting the matching with my coax tap and removal of the coupling caps.

So here's the big surprise. I refitted all the components, and decided to re-check the error on both the 2GHz signal and the 100MHz internal cal signal....
Weirdly, the 2GHz signal was now showing only about 0.9dBm error (reading about -1.9dBm) taking into account the external cable loss of about 1dB.
I switched on the internal 100MHz calibration signal and measured 1dB error.
With all the screening covers screwed back on, the unit is still showing only about 0.9dB error, which is within the tolerances of my test kit and the unit.
Very odd... but I seem to have a correctly working unit after all of that! At least I learnt something about my analyzer :)  ...but not why it had a 6dB error before stripping down!   :-//

« Last Edit: March 15, 2015, 04:27:50 pm by G4JMO »
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #29 on: March 15, 2015, 04:27:18 pm »
Interesting news.

A possible interstage coupling capacitor dry joint ?  The 6dB error in the pad is interesting. In such a situation I would have lowered the test frequency to one less affected by cable mismatch etc. 2GHz is one heck of a tricky frequency to carry out a path loss test. I would have plumped for 100MHz or 50 MHz myself as you are looking for an error that is evident at the calibration output running at 100MHz  ;) Far less demanding of your coaxial cables and test equipment  ;D

I hope she keeps working Ok for you

Best Wishes

Fraser
« Last Edit: March 15, 2015, 04:29:19 pm by Aurora »
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #30 on: March 15, 2015, 04:36:01 pm »
Thanks Fraser. Yes, you're absolutely right, 2GHz is making things hard for myself. The reason for the choice of frequency is that I'm without a sig gen that goes below this at the moment. My Giga-tronics 6100 only goes down to 2GHz. Anyway, all seems good. I guess you're right, it could have been a poor interstage  capacitor joint...who knows!

Regards
Andrew
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #31 on: March 15, 2015, 04:47:08 pm »
...having  warmed up for an hour, the calibration signal is now showing 0.6dB error  :)
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #32 on: March 15, 2015, 06:40:53 pm »
A further development!

I left the unit running for several hours. On checking the -30dBm 100MHz internal calibration level, it now showed -36dBm !!   :palm:
I tuned the unit off and immediately back on, switched the cal signal on and.......-30.6dBm!
I should have checked the external 2GHz signal too but I switched it off too quickly!
I'll leave it running again and see if it changes and if it does, check internal and external sources.
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #33 on: March 15, 2015, 06:54:58 pm »
Please advise what equipment you are using to RF measure levels.

Grateful if you will confirm that you have tested the level of the calibration output port and ensured that it is stable in amplitude.

Maybe a quick restart from scratch.

Choose a confirmed accurate output signal source. Test the spectrum analyser by viewing the RF source and noting the measured level on different attenuator and IF filter settings. Leave the unit to warm up over several hours and repeat tests exactly as before (including the level of RF source). Compare notes. If an offset is present, switch off SA, restart it and carry out tests again. Note readings Now switch off SA and leave it to cool down for several hours. Switch on and repeat tests.

Using such a test protocol will give us a firm baseline against which to offer advice.

As a side note, most SA's carry out a power on self test diagnostic that also checks the signal path and levels in the IF chain. Not sure whether you unit does this ?

Aurora
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #34 on: March 15, 2015, 07:08:41 pm »
There is no calibration output port to be able to externally verify the unit's calibration signal level.
External source from my Gigatronics 6100 is being validated on an HP8565A spectrum analyzer.
Since my last post 40 mins ago and restarting the unit, the internal calibration is unchanged at -30.5dBm. The unit has been on continuously for at least 3 hours now, apart from the few seconds to restart it.
 

I've just realised in my previous tests I could have stolen the 100MHz calibration signal from my 8565 instead of using the 2GHz source. Oh well!

If I don't see the reduction again in the next 2 hours, I'll let it cool off for a few hours and then restart from cold and log the amplitude of an external cal signal from my 8565A, which I know is stable, removing one of the variables.
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #35 on: March 16, 2015, 12:26:22 am »
Latest test...after the analyzer had been on for 3 hours but briefly powered down to 'reset' the internal cal reading to -30.5dBm as previously described.
Left untouched with the internal cal level being displayed:
(Analyzer settings 10MHz span, RBW 300KHz, VBW, 300KHz Ref level -20dBm)
+1 hr : -30.3dBm
+2 hr : -30.2dBm
+3hr  : -33.9dBm
+4hr  : -34.3dBm
+5hr  : -35.9dBm
+6hr  : -34.0dBm

So a -6dB drift occurred again. but error seemingly reversing and coming down by 2 after the 6th hour.
Disabled internal cal and connected HP8565A cal output to the analyzer. Ext 100MHz reading -15dBm (should be ~ -10 from the HP) indicated a similar ~5dB error to the internal cal source.

Turned the unit off and straight back on and re measured the internal and external cal sources. Internal now back to -30.5dBm and the external reading -11dBm (had to change the ref level to 0dbm to be able to measure the external signal. Changing the ref level from -20 to 0 seems to contribute perhaps 1.5dB of variation)

So this suggests that the calibration amplitude isn't drifting but the analyzer amplitude measurement accuracy is drifting when left running for 3 or 4 hours. But a power recycle corrects the error immediately. Have to assume that some kind of power-up re calibration sequence corrects some parameter for it to regain its accuracy. Perhaps on power-up, the internal cal level is used to characterise and compensate for the gain of a stage that drifts over time?

It will be interesting to hear from Adal as to how his unit varies over time and whether it is similar to  mine and recovers accuracy if power cycled when warm.



« Last Edit: March 16, 2015, 10:29:40 am by G4JMO »
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #36 on: March 16, 2015, 03:52:51 pm »
Hi Andrew
Thanks for your reply and test. Let me do my own test in order to compare result. I'm out and return the next week to my home lab. :-/O
Best regards.
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Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #37 on: March 16, 2015, 04:49:06 pm »
If this SA was on my bench I would proceed to tap the IF's at various points in the signal path and monitor them as the level error appears. It would then be obvious at which point the error is introduced. I would then investigate the suspect IF stage and look at all signals entering it, including the Local Oscillator level. If all inputs remain constant, yet the stage output drifts, I would start checking individual parts of the stage for the cause.

If the IF stages are fine right up to the final stage I would suspect a Detector/ADC issue, or the processing that follows it.

I treat SA's little different to a Superhet receiver as that is, after all, what they are in terms of RF stages.

I regret that you cannot really diagnose a fault in the IF chain without tapping it at appropriate points. The effect on the IF levels by introducing the coax taps is not an issue as you are interested in Delta IF level and not normal operational levels per-se. Some SA's have IF level monitoring and will throw up an error if the levels are not correct at boot. Really good SA's have diagnostic monitoring capabilities that permit closed case level measurement to spot problems in the RF stages. Sadly I would not expect the GW Instek to have such a capability.

Also, as you suggest, some SA's (like my Advantest 3132) have dynamic calibration that operates at boot, adjusting levels against the accurate calibration signal to provide the correct gain at various points in the IF. A drifting stage will be compensated within a certain range at boot but will continue to drift afterwards as the gain is not retested unless rebooted.

Gain drift could be due to a failure in a gain control circuit that drives a gain variable amplifier stage or there could be an AGC circuit issue on one of the local oscillators. There remains the unpleasant possibility of a a fault in a local oscillator causing level drift at its output.

Kind Regards

Fraser

P.S.

I suggest you do IF signal level monitoring with the SA centred on a known RF test source and the span set to 'Zero Span'.
« Last Edit: March 16, 2015, 04:52:13 pm by Aurora »
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #38 on: March 16, 2015, 04:55:00 pm »
Just had another random thought..... if any of the local oscillators are drifting in frequency you could be seeing the IF signal drifting into the skirts of the IF filter and so reducing the level passed to the next stage. Just a thought.

Aurora
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #39 on: March 16, 2015, 07:40:37 pm »
Thanks for your thoughts Fraser. All very sensible ideas.

Unlike my old HP8565A analyser which is straightforward to work on (helps having a service manual too!), the Instek is of the 'Crunch n FFT' variety. Given the construction (photo attached) it's not that easy to work on either.

Without a service manual and beyond the accessible front end RF stages, it's a bit of a Black Hole. Being pragmatic,  given the nature of the problem, I can work around it once I've  characterised the  drift, and it proves to be predictable.

As long as it has warmed up, a quick power cycle seems to keep it within a dB for at least 2 or 3 hours. I can put up with that! 

I've just started a new test from cold tonight. It started out at -34.5 dBm and after an hour  it's sitting at -30.9 dBm. We'll see if it drifts up again.

Best regards
Andrew
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #40 on: March 17, 2015, 05:02:51 pm »
Results from the 'start from cold' test...

The same drift profile occurred. After settling at ~ -31dBm the level stayed more or less constant for about 3 hours. Then in the space of about 1 hour, the level drifted to ~-35dBm. Power cycling returned the reading to -31dBm. So as long as I remember to power cycle after its normal stabilisation period, it's manageable.

The question remaining is whether Adal see's the same kind of drift profile and recovery or whether his unit is suffering from a different problem.

 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #41 on: March 20, 2015, 10:54:46 pm »
Hi everybody.
Here are the result of my own test
After 6 hours my GSP-827 Shows -35 dBm in the calibration signal. No changes if I cycle on-off the spectrum analyzer.
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline KJDS

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #42 on: March 20, 2015, 11:10:31 pm »
Hi everybody.
Here are the result of my own test
After 6 hours my GSP-827 Shows -35 dBm in the calibration signal. No changes if I cycle on-off the spectrum analyzer.

That's an odd looking shape. Can you halve the bandwidth but leave the RBW the same, then check the level with each of the RBWs, you'll probably need to reduce the span for the narrow bandwidths.

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #43 on: March 20, 2015, 11:21:20 pm »
KJDS here are the result of one test. No matters what RBW use, the error level remains the same. I need check the input path and check again
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Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #44 on: March 20, 2015, 11:30:06 pm »
Here are the result using the same settings of Andrew spectrum analyzer. ????
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Offline KJDS

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #45 on: March 21, 2015, 12:14:59 am »
When I saw the first plot I thought it possible that one of the secondary LOs had become unlocked, causing the IF to miss the filter, hence the odd shape, but a wider RBW would have shown lower loss and that's not happening.

Unless you can probe down the IF path then you won't work out where the issue is.

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #46 on: March 21, 2015, 02:40:04 am »
Ok KJDS.  Thanks for your comment.
Andrew, could you explain me how take apart the covers? I can't take over the covers????
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Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #47 on: March 21, 2015, 02:44:06 am »
All the screw are out, but the cover does not slip. I can't identify the cause. Maybe Andrew can help me
Instrument behavior: Instrument does not turn on
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Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #48 on: March 22, 2015, 08:39:12 pm »
Hi Adal,
I was initially puzzled by the cover too! ...then I discovered that the carrying handle end caps push off and reveal two more screws. Undo these and then the cover slips off.
Best regards
Andrew
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #49 on: March 23, 2015, 06:10:21 am »
Here are the inner's of my spectrum analyzer. The input path and the components that Andrew check first. I will testing tomorrow and let you know
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Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #50 on: March 24, 2015, 07:32:19 am »
Bad news.
I made the same test than
Andrew and noting appears. Additionally I lost the tracking generator option because I check the backup battery and the machine doesn't remember the installed options  |O
Maybe is time to call Instek México distributors for help
Instrument behavior: Instrument does not turn on
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Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #51 on: March 24, 2015, 06:08:39 pm »
Oh no! Sorry to hear of your problems.That is unfortunate. I was lucky I suppose that I have no options fitted otherwise I would have shared the same fate!
I guess you have no other option but to contact the service centre.   
Best regards
Andrew
 

Offline Fraser

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #52 on: March 24, 2015, 06:42:10 pm »
I truly hate designs that use a volatile memory to hold calibration data or option keys.

I have several pieces of quality equipment that contain lithium memory batteries with a predicted life of 10 Years. the service manuals just specify a p.d check. One manual provides details of placing a temporary supply across the lithium battery contacts while the cell is removed and a new one fitted. Anyone owning older test equipment should really check that such does not have a battery backed memory containing essential data. If so, check the battery every year and fit a new one if the p.d drops significantly from its stated output. I change 3.6V lithium batteries when the reach 3.0V. Some volatile memory suffers corruption issues at voltages less than 2.6V. Remember to apply a 'holding' voltage across the lithium battery terminals though. From memory you need two AA alkaline cells (approx 3.2V p.d) supplying the memory via a 10K resistor.

There is little excuse for volatile memory being used for mission critical data inside modern equipment. Blimey, eeproms have been around for so long now, why would anyone want to have a flippin battery backed memory except maybe for an RTC module.

Rant over, but I really feel for Adal. He is in a situation that now makes OEM support essential and that could be costly and inconvenient.

Aurora
 

Offline G4JMO

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #53 on: March 24, 2015, 09:32:26 pm »
I completely agree. It's a poor way to design. I've just faced a similar issue with my  Gigatronics 6100 sig gen. It must be at least 20 to 30 years old. Whilst fixing it recently, I got hold of the service manual and discovered that a Dallas NV Ram held critical calibration data that if lost would render the machine useless. Gigatronics refused to even discuss the unit because of its age so they wouldn't have been any help if the worst had happened. Fortunately I could still read out the contents using my EPROM programmer so it's all safely backed up now and I've also replaced the Dallas device. In a way, I'm lucky the unit needed fixing, otherwise I'd have never discovered this Achilles Heel until it was too late.
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #54 on: March 25, 2015, 03:44:19 pm »
Dear all, thanks for your help and comments.  Yes, I have bad luck with my GSP-827 due the design of the configuration info. I attach some pictures  of the inners for your info.
Best regards
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Instructions: The main board is probably defective. Replace it.
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #55 on: March 25, 2015, 03:52:01 pm »
Now I have the demodulation option. ??? Lesson earned, don't acquire "high performance" used test equipment without support or technical information. Maybe Agilent/HP
Best regards
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline Adal

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #56 on: May 29, 2015, 06:37:34 pm »
Hi.
Some news.
Recently I found the service manual for my machine:
http://www.melchioni.it/moduli/assistenza/ricercaRiservatoDownload/1/?&page=2
For calibration or activate the options, I need a "service password" only available by Instek.  :--
So I inquiry for that an Instek Mexican distributor, I´m still waiting for a quote.  :-//
Best regards.
 
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline G0HZU

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #57 on: May 30, 2015, 11:12:10 pm »
Quote
The signal then gets +16dB gain from U18, an NBB400 MMIC. Looking at this components solder seems to suggest it has been the subject of some attention before!. I plan to isolate it, power it and feed a signal into it and check how much gain it is giving.

Looking at the various images of the RF front end I'd guess that the first RF amplifier U18 (marked N4) is an old Stanford Microdevices  GaAs HBT part. The logo looks right for the NGA-486 but it's many years since I did any design work with these devices as they went obsolete a long time ago.

RFMD did take over Sirenza a few years ago so the NBB400/NLB400 may be descended from the same technology. The NGA-486 was quite a fragile part and required careful attention to thermal management and also they can be easily damaged from minor overload and transients. I've seen a lot of dead NGA-486 parts in my time :)
« Last Edit: May 30, 2015, 11:55:21 pm by G0HZU »
 

Offline xmicron

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Re: INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value
« Reply #58 on: June 25, 2015, 08:34:34 pm »
Hi Andrew,

Have you managed to solve this issue?
It looks like I am having the same problem!

Regards,
Frank
 


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