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Conversion of 500MHz TDS744A to 1GHz TDS784A

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Jay_Diddy_B:
Hi Group,

In this thread: https://www.eevblog.com/forum/testgear/enabling-option-1m-extended-acquisition-memory-tektronix-tds754a/msg565086/#msg565086

I documented the process for enabling the extended acquisition memory option on TDS7xx scopes. In this thread I am going to show how I successfully converted a 500 MHz TDS744A to the 1GHz TDS784A.

Tools and Equipment needed for this conversion

You need a PC fitted with a GPIB board to run the Tektronix Field Adjustment software. This is described here:

https://www.eevblog.com/forum/testgear/tektronix-tds700a-field-adjust-software-help-needed/msg563939/#msg563939

You need an accurate voltage source. Tektronix use a Data Precision (Analogic) 8200. You need +/- 9.5V, +/-0.95V and +/- 0.095V.
You need a signal generator to 1.005 GHz. Tektronix uses the SG504 with the levelling head. I substituted an Agilent E4425B

Base line measurements

I used my Transmission line avalanche pulse generator to measure the risetime of the unmodified scope:



The pulse generator is documented here:
https://www.eevblog.com/forum/projects/transmission-line-avalanche-pulse-generator/msg185639/#msg185639

The measured risetime was 650ps this is consistent with a 500 MHz scope.

Configuration Resistors

Tinhead documents the configuration resistors on the back of the Acquisition board in this message:

https://www.eevblog.com/forum/testgear/tek-tds-3054b-vs-tds-744a/msg539818/#msg539818

So I removed the Acquisition board. Here is a picture of the board configured for a TDS744A:




And after I removed R1064, the board is configured for a TDS784A:




I put the scope back together and the boot screen changed to this:




Excited by this I measured the risetime again, there was no change:



 Input Hybrids

I started researching eBay to see if the TDS784A used different ceramic hybrids in the input attenuator. I found some listings for the H2462G (pulled from a TDS744A) and H2462J (pulled from TDS784C). I was thinking that may the input hybrids were limiting the frequency response. These are the photographs from eBay:





They looked very similar to me. Somehow it didn't make sense to use the same part number with a revision code for different bandwidths


To be continued ...

Regards,

Jay_Diddy_B

Jay_Diddy_B:
Continuation..

Not really satisfied that the input hybrids were causing the bandwidth limitation I decided to look closer at the Acquisition Board. I found this picture of a TDS784A Acquisition Board on eBay:



If you look really closely you will see that C1266, C1267, C1268 and C1269 are not populated.

I looked at the schematics in the TDS520B Component Level Service manual to see what these capacitors are used for:





In the second schematic these capacitors are connect across the differential signal leads.

I suspected that these capacitor are there to reduce the bandwidth. I had a look at the Acquisition Board in my TDS744A and they were fitted:





So I carefully removed these capacitors and saved them so I could put them back.

After removing the capacitors you have to run the Signal Path Compensation (SPC).

I checked the risetime again and was significantly faster.

To be continued...

Regards,

Jay_Diddy_B





Jay_Diddy_B:
Continuation ...

Having modified the hardware I used the Tektronix Field Adjustment software to calibrate the scope. The software calls for the use of a SG504 levelled signal generator. I don't have one of these. I used an Agilent E4425B RF Generator and very high quality cable. I found I had to increase the generator output by 5% when the software called for frequencies higher than 500 MHz to compensate for losses in the cable.

After a couple of false starts I was able to calibrate the scope.

Risetime after Modification and Calibration

Here is a picture of the risetime:



The risetime is 360ps, this is consistent with a 1 GHz scope.

Sine wave measurements

Here are some sinewave measurements confirming the increased bandwidth:
 
I started with a 6 MHz signal and adjusted the output of the signal generator for a reading of 100mV RMS on the scope.




I then increased the frequency to 100 MHz and the scope read 100mV



At 500 Mhz still reading 100mV



At 760 MHz the reading dropped to 86mV (-1.3 dB)



At 1 GHz the reading dropped further to 76.6 mV (-2.3dB)




It looks like a successful conversion  :D :D :D

Enjoy !!!

Jay_Diddy_B


dxl:
Hi,

i can confirm the modification. I have a TDS754D, which i upgraded to TDS784D by changing the ID resistors. However, the calibration failed, because freqency response was out of spec (1 GHz was low by ~-4 to -5dB). After removing the capacitors, the calibration runs now flawlessly, and the Field Adjustment software
says it passed calibration.

:D

Regards

nctnico:
Great hack! I feel so stupid selling my TDS744A earlier this year  |O  |O

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