Author Topic: HP 5350B - Noise on input  (Read 274 times)

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Offline W4PJBTopic starter

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HP 5350B - Noise on input
« on: October 22, 2024, 07:48:22 pm »
I have an 5350B with a LOT of electrical noise coming from the input of my 5350B counter. As in, sweeping signals approaching 0dBm! Something tells me this is not normal for these counters. I'll include a pic below. I noticed this when I was having issues with accurate readings on some frequencies, particularly with one SG. I have to really crank up the amplitude to get over the noise or it reads hundreds of MHz low at 4GHz. It's fine around 2GHz, but as you can see from the photo, the noise is much lower in that range also.

I'd be interested to hear others experience on this. Please note, I had the 8566B SA set at -10dBm, and a 10dBm pad in line. My pad was only rated for 4GHz, so it's likely the noise is happening at much higher frequencies, but it's not making it through the pad. So if anyone tries this I'd put some attenuation in line to be safe as the signals are strong.



 

Offline Orange

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Re: HP 5350B - Noise on input
« Reply #1 on: October 22, 2024, 11:56:59 pm »
The signals that you see on the SA are coming from the synthesizer comb generator in the 5350.
This is not normal for a good 5350B. I'm afraid the sampler bridge (U1) in the 5350B has at least one damaged diode (probably short) which shorts the LO of the comb generator to the input to the counter.
Counter sensitivity is compromised as well if this is the case

This difficult to repair, best is to look for a replacement microwave module.
 

Offline W4PJBTopic starter

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Re: HP 5350B - Noise on input
« Reply #2 on: October 23, 2024, 12:38:10 am »
Hi Orange,


Thank you for your reply. I had suspected this was from a "leak" in the sample bridge and replacing the A12 module would be the fix. I was just hoping it wouldn't be, as they are neither cheap nor easy to find! And yes, it is absolutely killing the sensitively!
 


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