GenRad RCL-Digibridge 1687B active probe replication

[madao]

Hello people.


 I have buying two  GenRad 1687 (B) RCL Digibridge, but without active probe.

Schematic tell me, it contains a transformer.
It must transfer 1MHz signal without phase shifting.  On my works is  TC4-1T RF minatur transformer using for TTL-ECL Clock shifting 
I have tried with him (in backwards) and 12 Ohm at secondary winding: a bit about 1V RMS, it is very good.  A big luck, that it fit with 12 resistor very well (a bit much, 70mVrms to much).  Obiviously , zero phase-shifting condition with TC4-1T  is satisfied.
https://www.minicircuits.com/pdfs/TC4-1T+.pdf


active probe with manhattan style. After few trial & error with capacitor at input for  impedance amplifier runs it  with pausible value on display.    Impedance amplifier is a  MAX4477 , of course is it too slow (to much phase shifting)
I'll trying  with  video-buffer.    Sziklai-pair  is just a ordinary emitter-follower, typical early  ciruit for using in fast signal acquisition.
But, my result with MAX4477 tells me: This is why is using  transistor with fT of  gigacycles.

Cable is  high quality VGA cable, it was accessory of BARCO monitor.

[madao]

Next trying with   BUF634 as buffer is a success. This amplifier is fast enough for  zero phase shifting.

I am needed  very good capacitor as  GenRad 1404 for verifity of result.
Luck, i have a GenRad 1404-A, it is out of service  (Reason: whisker)   I must searching him in storage.

Pic shows C measurement  in series and parallel mode.
Capacitor is a  styroflex-typ, 301pf 1%.

series

parallel


Measurement with  33µH choke tells me same: pausible result 

It looks very good with  TC4-1t transformer and BUF634 as buffer.


regards
matt

[madao]

PCB from china is arrivied.


But, i must search correct value of input  capacitor-Divider. (It doesn't run with orginal value, too high amplitude)

now is value  10pf /100pf.

Regards
matt

BTW:  a little teardown while  washing of case

[madao]

good news:

I am finished with probe-replica.
  I have needed much time, because GenRad 1687B hasn't high priority and i haven't workplace for mechanic stuff.

Friends from me have turning a pieces brass for me.

Turned brass


It fits well.


Finished :-) (check with 39pf NPO ceramic capacitor )


Check with capacitor with extreme low loss  D = 0.0003


early  concept and schematic ( only  input capative divider is other, acutally 10/100pf)
Transistor-type is  BC817-25
Buffer/amplifier, of course: BUF634 (or high speed op-amp )




If you need PCB and  TC4-1T, pleas wrote me a PN. You got it for donation and shipping cost (from germany to other country (EU 8€, outside EU  16€  )

[madao]

GenRad 1404A Capcitor  is found by digging in pile :-) (I have only part of GR 1404A, it is written of, Whisker)

After  whisker removing with russian style ( 30V 4A PSU) is  GR 1404A useable.