Conversion of 500MHz TDS744A to 1GHz TDS784A

[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 

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.



Regards

[nctnico]

Great hack! I feel so stupid selling my TDS744A earlier this year   

[free_electron]

 

next modification : a self destruct button. counts down from 10 to 0 then sets the tekronix logo on fire.

[Jay_Diddy_B]

next modification : a self destruct button. counts down from 10 to 0 then sets the tekronix logo on fire.

There is no need for a self-destruct button, Danaher Corporation is managing the destruction of Tektronix 

(There is already a emoticon for this !!!)

Regards,

Jay_Diddy_B

[Carrington]

I did something similar with my MSO6034A, i.e I remove a capacitor located at the differential output, and the overshoot decreased slightly.
But no idea about the frequency response, probably not the most adequate.

To achieve a BW of 500 MHz also is necessary to modify the RC network at the input. And to achieve 1GHz the hybrid and the RC network must be replaced.

[Ivan7enych]

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).

Thank you for this investigation!

I have no old GPIB board now. What do you think, if I simply remove 4 capacitors without running Signal Path Compensation, will it make some problems?

[dxl]

It probably won't make 'problems', but the signal levels shown >500MHz are most likely wrong.

[nctnico]

The sensitivity for low signals isn't great. If you look at the specs of the TDS784A you'll see it struggles to get to 1GHz.

[Jay_Diddy_B]

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).

Thank you for this investigation!

I have no old GPIB board now. What do you think, if I simply remove 4 capacitors without running Signal Path Compensation, will it make some problems?

You don't need any external equipment to run the SPC. It is built into the scope. Go to the Utility menu, select CAL, run the SPC. The SPC takes about 5 minutes.

I initially calibrated channels 1 & 2 using the field adjustment software, which requires the GPIB Card and compared them to channels 3 & 4 which I did not calibrate. There was very little difference with my scope.

The calibration is only as good as the signal generator used for the calibration. The Agilent E4425B that I used is very good.

It takes about 40 minutes per channel to perform the HF CAL.

Regards,

Jay_Diddy_B

[Ivan7enych]

For now I'm quite happy with the modded tds754->784, it allows me to see 1.2GHz output from antenna and to use it as a simple spectrometer, see ouput frequency and power. RF output from selfmade cloverleaf antenna is put directly to 50ohm terminated oscilloscope input.

Model type resistor has been removed, capacitors on every channel removed, field calibration hasn't been done yet.

[Jay_Diddy_B]

Hi Ivan7enych and the group,

The modification seems to work well !!

I am waiting on the delivery of a Fluke 6061A RF generator that I ordered from eBay so that I can automate the adjustment process.

Regards,

Jay_Diddy_B

[szhebo]

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.



Regards

Hi dxl?


I also have a TDS754D, and want to upgrade it to TDS784D, but the schematic is different from the TDS744A, I tried to find TDS784D acquisition board photos on the web, but failed.


Could you please tell me how to change the ID resistors of TDS754D, from R52 to R57, which on should be removed and which one should be added/Keeped?


And for the signal path I found two capacitors across each signal leads, one is C781/C782/C783/C784, another is C740/C741/C742/C743, and the second one with two 3.3K resistors in series across the signal leads. My question is if two capacitors(C781/C782/C783/C784+C740/C741/C742/C743) of each path need to be removed or only the previous one(C781/C782/C783/C784) need to be removed?


Looking forward to your reply!


Many thanks!


Regards,


szebo

[Jay_Diddy_B]

Hi,

Somebody asked for the TDS adjustment software.

It is attached to this message.

Regards,

Jay_Diddy_B

[EB5AGV]

Hi all,

Thanks a lot for this thread!

Last year I bought a TDS 744A as part of a test-equipment lot buying. It came with a couple P6139A probes, the pouch and, free of charge, lots of dirt 

I had not done more than powering it up and some basic checks. It worked, but needed cleaning. My usual digital oscilloscope is a DPO 3032, which is a 300MHz, 2.5Gs/s unit. So, except for the 500MHz bandwidth, the TDS 744A was a lesser unit and I didn't find any use for it at the moment.

But testerday, while looking for other info on a different project (a Tektronix CSA 8000 mainframe I am trying to get back to work), I stumbled upon this thread. Wow!!!. I was amazed to find I had a hidden treasure  , as my 744A came without 1M option... which seemed to be also a configuration hack (and, yes, I have managed to activate it using just Keysight IO libraries and GPIB  )

So I took it out its casing and carefully cleaned it, including the keypads, as some keys were not working fine. The HORIZONTAL POSITION control also failed, so I took out all the front panel, dismantled and cleaned it. Now the oscilloscope looks like a close to new unit! (I will put some pictures later)

Well, once I performed the resistor mod and the capacitor removal, yes, the unit is shown as a 784A and now it reaches 4 Gs/s. I have run the SPC after a warm up period, and it has also worked fine. But there is a problem when the unit works at 4 Gs/s, which I think it is a matter of calibration. The test is simple: I put a 250MHz signal and activate only Channel 1. What the unit shows is a distorted signal. Then, if I activate Channel 2, so the sampling rate lowers to 2 Gs/s, the signal is an almost perfect sinusoid.

I need to setup a PC with a NI GPIB card to run the calibration software which I hope will solve this problem.

Regards,

JOSE

[Jay_Diddy_B]

Hi Jose and the group,

I think that distortion will go away when you perform the calibration. At 4 GS/s all four ADCs are being used and the interleave timing has to be correct to get an undistorted sine wave. At 2GS/s the scope was calibrated when it was TDS 744A.

Regards,

Jay_Diddy_B

[cncjerry]

I did the same upgrades to my 744, regularly run high frequencies into it and not only sine waves.  I haven't done the calibration and don't see any distortion like the new poster.

[EB5AGV]

I did the same upgrades to my 744, regularly run high frequencies into it and not only sine waves.  I haven't done the calibration and don't see any distortion like the new poster.

Yes, this is not unusual. Note that the compensation among samplers could be better in your unit without correction. Anyway, I doubt it is perfect, but probably lots less noticeable than on my unit.

All in all, I am happy with the experience. I hope I can fix it by calibration. If not, well, I can always revert the mod. It has been so far an interesting learning experience and a way to get back to life an old and tired unit 

BTW, just adding Option 1M is already worth the effort 

Regards,

JOSE

[tford]

After reading the TDS744A to TDS784A conversion thread, I purchased a used TDS7444 in good condition. It appears to fully operational. Gave it a cleaning and removed the 4 capacitors (C1266, 67, 68, 69) and jumper R1064. The results were the addition of option 1G, (1GSPS Ch1, 1 GSPS CH2). The bandwidth did increase to 1 GHZ for all 4 channels, but no TDS784A 4 GSPS conversion. Any ideas of what I can do to complete the conversion? Also, I have FV:v1.1e instead of v1.0.1e. 
Thanks

[nctnico]

Perhaps the jumper is wrong one. Option 1G limits the samplerate to 1Gs/s instead of 2Gs/s (for export purposes) so this is BAD!

[tford]

I checked my R1064 removal, all was good. I then removed all resistor jumpers, attached small wires to the R1061 through R1064 pcb pads and ran the wires out the board to a dip switch. With R1064 shorted and R1061,62,63 open, my power on self test screen displayed a TDS784 and the sample rate jumped to 4GSPS. Yeah! With all jumpers open, no TDS scope model was displayed on power up. Possibly the firmware version is set up to read the jumpers differently.

[nctnico]

So the setting is the other way around: R1064 must stay!

[tford]

I would like to capture my TDS744A scope screen display image and store it on my computer for reference. What is the TDS7xx hardware/software setup you all are using to post these perfect scope screen images?
Thanks, Tom