Author Topic: Teardown, Repair & Analysis of a Rohde & Schwarz FSH3 3.0GHz Spectrum Analyzer  (Read 19442 times)

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Offline Adal

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Hugoneus Thanks for your video!
I have a FSH-3 that has the same damage that you show. Unfortunately mine have the PCB carbonized. I attach some pictures of my machine in order to ear your suggestions for fix my spectrum analyzer. For the service info, I use the FLUKE scopemeter 190, they use the same  battery charger power control.
Best regards. 
Instrument behavior: Instrument does not turn on
Instructions: The main board is probably defective. Replace it.
 

Offline tonyM

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Hello everyone,

I have TSMU unit, which has the same RF-block as FSH3.

It is broken, i.e. it can't measure correctly. I will be very gratefull, if someone help me to repair it.

My TSMU-RF-board PCB is older than FSH3, just a little difference at the input (other SPDT RF-switches=AS179-92LF, SMD marking S79) and other/older TCXO. Attenuator IC is also missing.
2xRF-switches + 1xBFP520 are making the circuitry of Pre-Amplifier, placed between missing Attenuator-IC and 3GHz-LPF. Pre-Amplifier can be switched ON or OFF/by-passed by the software.
RF-transistors used are: BFP420 (SMD marking AMs) and BFP520 (SMD marking APs).

Below is the progress until now on my attempt to repair the unit.

Due to the info, found in FSH3/6 service manual and also in this thread, I've made some investigations and measurements. I think there is a problem with PLL-synthesizer: it can't lock correctly.

I made that decision by measuring the signal at the output of IC=HMC434 (divide-by-8 module, placed between 1st-LO and SYNCON-synthesizer, i.e.
between 2xBFP520-transistors and IC=uPB1509GV/divide-by-2 or 4 or 8 module) and the signal at the output of IC=HMC433 (divide-by-4 module, placed between 2nd-LO-3200MHz (1xBFP420-transistor) and 3rd-LO-800MHz (1xBFP520-transistor).

The signal at IC=HMC433 output is OK: exact 800 MHz, with strong level. But the signal at IC=HMC434 output is not OK.
Due to RF block diagram, 1st-LO-frequency is between 4031...7031 MHz, and after HMC434 it must be between 503...879 MHz.
So:
1. if I want to measure signals with frequency=925...960 MHz (=signals from BTS->Mobile at 900MHz-band), 1st-LO-frequency after HMC434 must be between 619 and 624 MHz. In my case this is not true, I don't observe increase in levels of this spectrum piece.
2. if I want to measure signals with frequency=2110...2170 MHz (=signals from BTS->Mobile at 2100MHz-band), 1st-LO-frequency after HMC434 must be between 767 and 776 MHz. In my case this is not true, I don't observe increase in levels of this spectrum piece.

For the measurements, I'm using Tektronix NetTek Spectrum Analyzer YBT250, with frequency range 30...2500 MHz.

I also found, that 1st-LO is made by 3xVCOs: 1xBFP420, 1xBFP520, 1xBFP520, and each one is responsible for a spectrum piece of about 1000 MHZ (VCO-1=4000...5000, VCO-2=5000...6000, VCO-3=6000...7000 MHz). Each VCO is switched-on by applying Vcc to respective BFPx20-transistor, and the exact frequency depends on Vtune, applyed to the varicaps (a pair for each VCO, Vtune rail is common for all VCOs). I observed the above facts by using ROMES software to measure at 3 different frequencies: 500, 1500, 2500 MHz, which in turn pulls into operation the respective VCO. Scanning with ROMES software at full band 15...3000 MHz didn't give any increase of levels for spectrum 503...879 MHz at HMC434-output.
It seems that VCOs are working (Vtune is ramping to about 20V when full band 15...3000 MHz is selected for measuring), but to be 100% sure, I need spectrum analyzer with higher bandwidth- at least up to 7...8 GHz.
So, at this moment, I don't know if the problem is in HMC434.

The only thing, which I can try at the moment, is to swap the places of HMC434 and HMC433. In such way, I can test if HMC434 works properly: at HMC433's place (after 2nd-LO-3200MHz), the output of HMC434 must be 400MHz.

Any suggestions?
« Last Edit: April 09, 2020, 08:56:10 am by tonyM »
 

Offline tonyM

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I just swapped HMC434 and HMC433.
At HMC433's place (after 2nd-LO-3200MHz), there isn't any strong 400 MHz-signal at the output of HMC434.

So, the problem is in HMC434.
Do you agree with me?
 

Offline tonyM

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I just swapped HMC434 and HMC433.
At HMC433's place (after 2nd-LO-3200MHz), there isn't any strong 400 MHz-signal at the output of HMC434.

So, the problem is in HMC434.
Do you agree with me?

Just changed HMC434.
Result: my TSMU is fully operational again :)
 

Offline dianzimi

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I have a fsh3 ,it is not work and missing  some resistence  and  capacity  ,show the picture,can you tell three resistence  and one capacity  value   ???thank you
I can not load the picture ,can you tell the email??my email is dian.zi.mi@163.com
 

Offline hgl

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In 2018, I looked into the R&S TSMU and also saw the excellent FSH3 teardown video.
A German dealer had gotten a whole pallet of them from a bankruptcy and offered them for about 25€ on Ebay.  I then bought 2 and started a thread in German microcontroller forum.

https://www.mikrocontroller.net/topic/454625

The RF boards of TSMU and FSH3 look the same but the assembly is different.
The rest of the construction is completely different.


 

Offline tonyM

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I have a fsh3 ,it is not work and missing  some resistence  and  capacity  ,show the picture,can you tell three resistence  and one capacity  value   ???thank you
I can not load the picture ,can you tell the email??my email is dian.zi.mi@163.com
First, help us identify PCB version of your FSH3.

My TSMU has older PCB, slightly different from FSH3. It has:
- missing Downconversion section for frequencies above 3GHz on the back side of the PCB
- missing Attenuator at the input signal path
- slightly different input signal section
- different model of 10MHz-TCXO
1565113-0

Newer TSMU PCB is the same as FSH3 PCB. It has:
- Downconversion section for frequencies above 3GHz on the back side of the PCB, but without components (populated in FSH6)
- missing Attenuator at the input signal path
1565119-1
1565125-2

FSH3 PCB is the same as newer TSMU PCB. It has:
- Downconversion section for frequencies above 3GHz on the back side of the PCB, but without components (populated in FSH6)
- Attenuator at the input signal path
1565131-3
1565137-4

All above PCB versions have the same Tracking generator part on the front side of the PCB, where the missing parts have to be placed:
1565143-5

Please check and identify your PCB.
And also provide the numbers on the front and on the back side of your PCB, marked by the blue rectangles in the above TSMU-ver.2 pictures.

Where are the missing parts on your PCB? At the input signal section (marked in the above pictures as "Pad")?
« Last Edit: August 15, 2022, 04:28:32 am by tonyM »
 

Online pdenisowski

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The FSH3, which btw was the first generation of handheld SAs from R&S.

The FSH3/6 series was a very popular product: one even made a trip to the International Space Station (although it did have to be slightly modified - see link to article)

https://www.rohde-schwarz.com/file/n184_fsh3.pdf

The FSH Series is still made, however from what I know the hardware for the old FSH3 and the newer FSH4 and above is different

Yes, that's correct.  In addition to the FSH4/8/16/20 we also currently manufacture an even newer (!) portable spectrum analyzer, the FPH.
« Last Edit: August 14, 2022, 02:20:09 pm by pdenisowski »
Test and Measurement Fundamentals video series on the Rohde & Schwarz YouTube channel:  https://www.youtube.com/playlist?list=PLKxVoO5jUTlvsVtDcqrVn0ybqBVlLj2z8

Free online test and measurement fundamentals courses from Rohde & Schwarz:  https://tinyurl.com/mv7a4vb6
 
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Offline dianzimi

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I want  to  know  Attenuator component   value  at the input signal path
 

Offline tonyM

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I want  to  know  Attenuator component   value  at the input signal path
Do you mean the components in the red rectangle bellow?
1566937-0
 

Offline dianzimi

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yes
 

Offline dianzimi

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yes
 

Offline tonyM

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I think these 3 black parts (excluding input capacitor) at FSH3 input signal path are resistors, which are making Pi-pad attenuator.

As you can see, TSMU (ver.1 and ver.2) lacks Pi-Pad attenuator. Only one Zero resistor (see pictures below):
1566943-0
1566949-1

If you put Zero resistor in the right place (as in TSMU-ver.2), I think your FSH3 will work (if next components/ICs are OK), but the measured levels will not be correct.

As I don't have FSH3, I will propose the following experimental approach for calculating the Pi-pad resistors values (and calibrating the instrument):
1. Put zero resistor at the input signal path
2. Plug-in signal generator with known level at RF-input
3. Check the level, measured by FSH3, and calculate the difference in dB
4. As you know the difference (and input impedance=50 ohm), use calculator https://www.pasternack.com/t-calculator-pi-attn.aspx to find the values of the resistors.

« Last Edit: August 17, 2022, 09:39:15 am by tonyM »
 

Offline hgl

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There is an interesting detail, the pin 2 of the synthesizer chip is isolated on the TSMU version 2. (Not properly visible in the photo, you can see only a small piece of brown foil under the pin). This is not the case with version 1 and FSH3.  On the photos you can not see whether the synthesizer chips of FSH3 and TSMU are really the same.
« Last Edit: August 16, 2022, 09:25:04 pm by hgl »
 

Offline dianzimi

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  who  knows   the components  detail  information  in the red rectangle bellow??[img][img]
 

Offline tonyM

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  who  knows   the components  detail  information  in the red rectangle bellow??[img][img]

As others nearby with SMD marking "512", probably it is SPDT switch QPC7512: https://www.rfmw.com/products/detail/qpc7512-qorvo/676832/
 

Offline dianzimi

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Do you know the components in the fsh6 pcb,with SMD marking "534"?
 

Offline tonyM

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Do you know the components in the fsh6 pcb,with SMD marking "534"?
Could you provide Hi-Res pictures of FSH6 PCB?
« Last Edit: November 18, 2022, 06:24:29 pm by tonyM »
 

Offline dianzimi

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ok[/img]
 

Offline dianzimi

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Offline tonyM

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SMD parts with marking "534" are SPDT switches up to 6GHz.
In order to check if next sections are operational, you can bypass the input secton (Attenuator + Amplifier) by removing its first and its last SPDT switches, and soldering thin semi-rigid coaxial cable:
« Last Edit: November 20, 2022, 10:42:42 am by tonyM »
 

Offline tonyM

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Only attenuator by-pass (by the same way it is originally done in TSMU-ver.2):
« Last Edit: November 20, 2022, 11:23:57 am by tonyM »
 

Offline scopeman

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I can also describe the signal path for the 6 GHz option if anyone wants to see it?

I realize that it has been a while but I would love to see the FSH6 even though I think I know how it works. I do have both a FSH3 and a FSH6 both with blown attenuator chips. It's been a while but I think that the one in the FSH3 that I have is a SOIC packaged part but I don't remember the part number. I think it was made by Macom.

After I get my E4407B fully squared away perhaps I'll work on the two FSH's assuming I can find the attenuator chip for the FSH3.

Thanks!

Sam
W3OHM
W3OHM
 


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