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Old 12th Mar 2021, 3:27 pm   #116
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Spectrum Analyser TR4172

Quote:
have mounted the IF1 module back in the analyser but unfortunately I’m still missing 12.5 dB? Jeremy what do measure if you do the following:

- Master reset
- Cal -20 dBm 50 MHz at input 1 (level checked with power meter)
- CENT 50 MHz and SPAN 10 MHz
- ATT 0 dB
- ZERO SPAN (Shift, Arrow Up)

At the CRT I see -32.5 dBm. At the IF output at the back I measure -38.6 dBm at 3.33 MHz, see picture. It is wiggling a bit. The calibration screw at the front is at max.
I cleared some space in the spare room and set up both the TR4172 and my old HP8566B analyser. They have been running about an hour now and both are working fine. The TR4172 still has its previous cal corrections stored and it is running with the cal screw at its usual part setting. From memory this is probably at 3/4 full. It's not a good idea to try and do a system cal if this is turned to an extreme setting. I think it causes problems with the 95dB logamp and this manifests itself as an incorrect log response near the top of the screen after the calibration is complete. However, this is all remembered from a long time ago.

With your settings it shows -20.1dBm at 50MHz on screen with 0dB attenuator setting. If I set the attenuator to the usual/default 10dB I see -20.0dBm on screen.

With 0dB attenuation I see -24.5dBm at the IF out at about 3.34MHz.

If then connect it all up again and look at the noise floor at 50MHz on a 500kHz span with the ref level set at -40dBm I see a -146dBm/Hz noise floor with the 1Hz noise marker enabled and 0dB attenuation. I think it used to be -147dBm/Hz typical at VHF so maybe I need to leave it running a bit longer and do a recal. It hasn't run the automated cal for a couple of years at least.
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Regards, Jeremy G0HZU
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