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| INSTEK spectrum analyzer GSP-827 -6 dBm down than the real value |
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| G4JMO:
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. |
| Adal:
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. |
| Fraser:
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'. |
| Fraser:
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 |
| G4JMO:
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 |
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