yesterday I made some tests regarding that issue with "toggling" bandwidth (
https://www.eevblog.com/forum/testgear/hantek-tekway-dso-hack-get-200mhz-bw-for-free/msg965398/#msg965398):
- removed varactor: not better
- updated to latest firmware (got this directly from Hantek): not better
- tried /tst file with "[filter] 35": not better
- tried /tst file with "[filter] 1000": not better (is that appropriate anyway?)
This must be happening in digital domain, stuck here... As if the scope cannot decide if it wants to be a 60MHz or a 200MHz device. I will try and address this to Hantek, but there is chance that they say that I have bought the 60MHz variant, so where's the problem...
I made some progress to my input stage work (flat pulse response, more bandwidth):
- 50 ohm termination (connected to left side of C01_5 according to "das_oszi_schematic.pdf"): works like a charm and completely eliminates all overshoot at JFET input, but I realized that I lose 1:10 attenuation and self calibration of course, so this is not a practical and permanent solution.
- I found that the LMH6552 wildly oscillates when I short RA01_1/2. A look in the datasheet tells me that it is not capable of driving the high capacitive load of our transmission line. On page 20, the datasheet suggests 21 ohms external series resistance at its output for CL=39pF. Hantek has put RA01_1/2=4x22 ohms at the output in my case. Coincidence? So I put that resistor(s) back in.
- the LM6552 datasheet also suggests to choose not too low resistor values for the feedback network (page 10, fig. 9) when I want gain flatness. So I will probably keep that 440ohms that Hantek put in, maybe lower to 390.
- the LMH6552 still seems to have a hard time driving that transmission line, so I put 1.5pF on top of RX1_3 and RX1_4 (that resistors have 440 ohms in my case). This reduces amplifier bandwidth to ~ 250MHz, and completely eliminates overshoot. Reducing this to 1.0pF raises BW to 360MHz but adds a little overshoot, could be a good compromise.
- after that 1.5pF fix, the reflection due to {missing termination at receiver / unmatched termination at transmitter} is still there, and it shows up as a dip of ~2ns, just after the signal slope has ended. So I added three termination resistors: one across R03_29/R03_32 (not on top of them but such that it terminates the end of that t-line), and one each across the inputs of RL04_1 and RL04_2 (to the bottom of the PCB, soldered directly to the corresponding vias... quite nasty). The traces going from the relays to the ADCs cannot be terminated because of that 500MSPS/1GSPS switching/paralleling. I am not right there yet, but a resistor value of 70 ohms each already shows good improvement. I will need to find the optimum by experimenting more. Later, I will need to raise LMH6552 gain a bit such that it compensates the attenuation that I have introduced here (resistive divider RA01_1/2 <--> termination).
I am still waiting for parts for my 500ps pulse generator, and for my 750MHz active Tek probe, so this task will sleep until then. Keep you updated