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Author Topic: Itsu's workbench / placeholder.  (Read 4853 times)

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Lowering the input signal to 1Mhz shows very nice signals, so this 10Mhz might be to high for this 74AC14 .
Connecting 2 or 4 of these inverters in series will make them respond faster.

Three things to consider:

1) Watch for probing artifacts - see the 1st attached file.
2) Inverters with Schmitt inputs (such as the 74AC14 and 74VHC14) distort the input waveform a little because of their O-->I feedback due to the built-in hysteresis.  The plain input inverters (such as the 74AC04 and 74VHC04) do not affect their inputs so much.
3) Supplying the inverter with 3V might be better than with 5V when the input waveform has a small amplitude. For example: the 74AC14 needs a 3.3VP-P input waveform to switch reliably, when it is supplied with 5.5V ...yet the same chip needs only a 2VP-P input waveform to switch reliably, when it is supplied with 3V *.


* The plain input inverter (such as the 74AC04) needs only 2.2VP-P input waveform to switch reliably when it is supplied with 5.5V ...and 1.2VP-P - when it is supplied with 3V.
   In all these examples it is assumed that the input waveform is centered at ½VCC
« Last Edit: 2019-03-16, 07:31:59 by verpies »
   

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In series and faster he, that seems to contradict in my mind, so lets try that.

It dawned to me to use the RF probing technic right after i wrote about the nice signals at lower frequency.
Will try that too.

I know that last pdf, and used it before, only problem was/is that i cannot find the impedance of the 74AC74
ports, some say its 20 Ohm,  but in my mind it must be higher (KOhms).


Thanks Itsu
   

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In series and faster he, that seems to contradict in my mind, so lets try that.
That's because the word "faster" is ambiguous.
Inverters connected in series have a longer collective propagation time, but in the end the risetime of their output is faster (because their gain multiplies and their risetime is limited only by their slew-rate).
In this application, we do not care about the propagation delay (and the phase shift it causes), since it can be calibrated out in the instrument's settings.
Also, we do not need a low output impedance of the sine2square converter because the FG's ExtRef input has a high impedance, so there would be nothing to be gained by paralleling two or more inverters.

I know that last pdf, and used it before, only problem was/is that i cannot find the impedance of the 74AC14 ports, some say its 20 Ohm,  but in my mind it must be higher (kOhms).
The input impedance is determined by the input capacitance of the inverter so it is variable as it depends on frequency just like the impedance of a capacitor, which for 74AC14 is 25pF per input so its impedance is approximately 600Ω @ 10MHz (the 74VHC14 has 10pF input capacitance so its impedance is 1600Ω @ 10MHz).
The 74AC14's output impedance is approximately 73Ω and the 74VHC14's output impedance is approximately 220Ω.
   

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I put a female bnc connector on the 74AC714 input and changed the input cable from the FG (10Mhz @ 3vpp, sine)
to be a bnc to bnc jumper cable, so without the clipleads etc.

74AC14 supply voltage lowered to 3V.

Using the RF probe tips on the probes i measured the input signal (yellow) and output (across a 1K resistor) signal (blue), see screenshots.

Looking much better compared to yesterday.

Itsu
   

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I put a female bnc connector on the 74AC714 input and changed the input cable from the FG (10Mhz @ 3vpp, sine) to be a bnc to bnc jumper cable, so without the clipleads etc.
Do you have the calculation of FG's output amplitude set for a 50Ω load ? [ Utility->Ch1Set->Imped->Load (50Ω) ]

74AC14 supply voltage lowered to 3V.
So, now the 74AC14 needs 2VP-P input waveform centered at ½VCC to switch reliably. But you have only 1.6VP-P at the input pin of the 74AC14 chip !
So the chip is switch with lower input amplitude than specified.  This could lead to instability later - especially if temperature changes...

Using the RF probe tips on the probes i measured the input signal (yellow) and output (across a 1K resistor) signal (blue), see screenshots.
Looking much better compared to yesterday.
Yes, but you are on the edge.
On the attached scopeshot, I have extended the rising edges of the blue squarewave with pink lines and I have drawn the lower red horizontal line through the points where these pink lines intersect the yellow sinewave. I have done a similar thing with the falling edges of the blue squarewave and drawn the upper red line.

Note, that the lower red line is skirting the bottoms of the yellow sinewave*.  That's how close you are to the switching threshold!
So, consider lowering the supply voltage even further (you can go as low as 2V) or consider using the more sensitive chip, such as the 74AC04 or 74VHC04, which require only 1.2VP-P input, to switch reliably at 3V supply.


* Note that the upper red line is NOT skirting the tops of the yellow sinewave, thus you do not have a problem there.  This is because in the 74AC14 chip, the upper switching threshold is a little closer to ½VCC than the lower switching threshold.

   

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Switched the FG to 50 Ohm (was at highZ).

Lowered the 74AC14 to 2V supply voltage.

FG signal still 10Mhz @ 3Vpp sine wave.


Was thinking on using the (unused here) rear USB plug for its 5v to use as supply voltage for the 74AC14 (pulling 10mA now)
   

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Lowered the 74AC14 to 2V supply voltage.
FG signal still 10Mhz @ 3Vpp sine wave.
Yes, the edges of the blue squarewave intersect the yellow sinewave pretty far away from the sinewave's tops - so the input threshold switching is stable now.

We still do not know, whether the impedance of the ExtRef input on the back of your FG, is really 1KΩ.  Some adjustment might be needed to find an optimal resistance at the output of the converter (R3), that will yield the least sq.wave distortion at the FG's input.

Was thinking on using the (unused here) rear USB plug for its 5v to use as supply voltage for the 74AC14 (pulling 10mA now)
Good idea.  Add an LM317 adjustable regulator (or similar) with a pot for adjusting between +5V an +2V, put it in a shielded BNC box and be done with it.
I suggest an IC socket for the 74AC14 chip since it has the same pinout as the 74AC04 or 74VHC04 ...in case you want to upgrade later.

P.S.
Below is my sin2sq converter based on 74VHC04 and LDK320A in a small SOT23 package. I kept it in the dead-bug stage because air is a better dielectric than PCB...and I was lazy  ;)
I had to put chokes on my supply line (inside the box) because it was picking up EMI and radiating out the square MHz harmonics, at the same time.
« Last Edit: 2019-03-17, 10:15:39 by verpies »
   

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Great, 

i have a dead bug setup now so i will build it up on a proto pcb using a socket
and a LM317 set to 2V from the rear USB plug.

The 1K impedance will probably be very close as thats their specs, so i don't expect much
problems there (now using a 1K load reistor on the output) but will scope the signal when
its ready to be connected.

Sot23 he,  well your eyes are better then your camera then  ;D

Itsu
« Last Edit: 2019-03-18, 09:12:22 by Itsu »
   

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I don't have a camera - I have a plastic thing on my cell phone that pretends to be one.
How is your FG synchronizing with the SA now?
   

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Well, i have not completed my sin2sq converter yet, but presently the FG is running ok with 50 Ohm coax and with the SA sine wave as ext ref input.

I did found out that the FG rear USB B plug has no 5V available, not in PC nor Printer mode.
Same with other (printer) USB B plugs, so i have to find another Vcc source.

There is 3.3V on the green (data) wire, but i guess i cannot use it for pulling 25mA or so.


I have an old Nokia cell phone charger which put out 3.7V dc which i will use probably.

Also building my nano pulser power supply (18V, 12V, 5V, 3V and 200V).

Itsu
« Last Edit: 2019-03-18, 20:55:21 by Itsu »
   

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Finished my sine2sq converter, running on 2V (via LM317) from a 5.6V wall transformer, see picture.

Measuring the output directly from the out bnc shows a nice square wave, but when attaching a piece of coax
or attaching it to the FG in/out port shows some distortion.


I suspect the used inbetween coax cables are not up to the 10Mhz task or were damaged in earlier nano pulse
(1.8kV) experiments, so have ordered some good UHF 50 Ohm coax.

I have the 74ac14 output pin (pin2) directly attached to the out bnc plug, so without any resistance (50 Ohm/1K).

Regards Itsu   
   

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Finished my sine2sq converter, running on 2V (via LM317) from a 5.6V wall transformer, see picture.
Double female BNC sockets ?

I cannot see chokes on the supply line. Am I wrong?


I suspect the used in between coax cables are not up to the 10Mhz task or were damaged in earlier nano pulse (1.8kV) experiments...
You can determine the quality of the coax by making a S11 (return loss) or Z<-S11 measurement with a VNA (or with a SA + VSWR bridge) while the end of the coax is terminated with a 50Ω resistor.

Of course, calibrate the instrument first by attaching the 50Ω termination resistor directly to its port1 - you should get a flat trace after the calibration.
Next, insert the coax to be tested between the instrument's port1 and the 50Ω terminating resistor - for an ideal coax the trace should stay flat as all the energy transmitted by the instrument get perfectly realyed to the resistor and absorbed in it (IOW: no energy should get reflected and come back to the instrument).

...any losses in the coax will make the S11 or Z<-S11 trace squiggly. The higher the amplitude of these squiggles, the worse the coax is.

so have ordered some good UHF 50 Ohm coax.
LMR-240-UF ?
   

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Double female BNC sockets ?

Yes.

Quote
I cannot see chokes on the supply line. Am I wrong?

There is a 150uH choke on the + lead, nothing on the - lead.


Quote
LMR-240-UF ?

No, RG174,  but your LMR-240-UF seems way better.

Itsu
   

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This is the 10Mhz square wave signal at the input of the FG (via a BNC tee) using an RF probe tip.
The FG accepts it, but its rather different compared to the unloaded output signal from the sine2sq converter,
see post #205 screenshot (blue)
I have a 6dB 50 ohm attenuator at the sine2sq output.
 
Itsu
   

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Well, that is not surprising because:
- the sin2sq converter has ~73Ω  output impedance
- the coax has 50Ω impedance (...or 75Ω )
- the FG has 1kΩ input impedance.
- your scope probe has ???Ω  impedance (could be too low).

So you are bound to have some kind of impedance mismatch or the low impedance of the probe is perturbing the signal too much.
How does the sq.wave distortion at the FG's input change, when you put a series resistor at the sin2sq output and vary it?  ...without the 6dB attenuator.

P.S.
If varying the series resistor does not help, then maybe this arrangement will work:
SA's output -> 50Ω coax -> 50Ω terminator -> sin2sq -> 75Ω coax -> ~75Ω terminator -> FG's input
The 50Ω terminator can be in the box with the sin2sq converter. The ~75Ω terminator should be adjustable.  Take measurements with a Tee at FG's input using a 10MΩ probe with RF probing arrangement (no long ground leads!).
   

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The probe has a 10MOhm impedance.

The used 75 Ohm coax jumper cable shows the below response when using the SA + VSWR bridge.
The used 50 ohm terminator was first normalized and shows a flat response across the frequency range.

I will change the output bnc female socket for a male one so a can loose the output cable (box direct attached to the FG)

Then play around with a 1K series pot at this output to see how it changes the FG input signal.

Thanks   Itsu     
   

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The used 75 Ohm coax jumper cable shows the below response when using the SA + VSWR bridge.
Oh yes! Such mess is to be expected when using a 50Ω instrument to measure a 75Ω coax with a and 50Ω termination resistor at the end of it.

However, the sin2sq converter's output impedance is closer to 75Ω, so a 75Ω coax should be a better match for it at the transmitting end.

The used 50 ohm terminator was first normalized and shows a flat response across the frequency range. 
And it should stay that way even if this terminator is measured THROUGH a coax ...if the coax is close to ideal.
You should not see high amplitude squiggles with a good 50Ω coax terminated with a 50Ω resistor.

I will change the output BNC female socket for a male one so I can lose the output cable (box direct attached to the FG)
Excellent choice ;)

Then play around with a 1K series pot at this output to see how it changes the FG input signal.
Looking forward to seeing how the signal changes with the variations of this resistance !


P.S.
In US the female socket is the one, that has a hole at the MOST CENTER part of the connector. In EU, I noticed that people refer to the MOST OUTER part of the connector to make that determination.
I can't help to notice, that the US analogy is closer to real life.
   

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Quote
P.S.
In US the female socket is the one, that has a hole at the MOST CENTER part of the connector. In EU, I noticed that people refer to the MOST OUTER part of the connector to make that determination.
I can't help to notice, that the US analogy is closer to real life.

 ;D


Quote
Looking forward to seeing how the signal changes with the variations of this resistance !

Ok, using a BNC male output socket and a BNC knee to directly connect the sin2sq box to the FG.
I have a 1K pot in series with the chip output and the bnc and measure with my RF probe across the bnc connection.

It seems that at 123 Ohm the impedance is best matched, see screenshot.

Going through the 1K range from 0 to 1K and back see the video:
https://www.youtube.com/watch?v=Zbni3VzLdkg

Guess i put a 120 Ohm fixed resistor inbetween and leave it this way.


Itsu
   

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Hmmm,  i received my RG174 50 Ohm coax cable and a 1m piece with bnc connectors shows the following response when using the SA + VSWR bridge with a 50 Ohm bnc terminator  see screenshot 1.

Replacing the 50 Ohm bnc terminator with a known good induction free 50 Ohm resistor does not change much.


Doing a SWR measurement shows screenshot 2


Itsu
   

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It seems that at 123 Ohm the impedance is best matched, see screenshot.

Going through the 1K range from 0 to 1K and back see the video:
https://www.youtube.com/watch?v=Zbni3VzLdkg

Guess i put a 120 Ohm fixed resistor in between and leave it this way.
Yes, it looks like you are done.  Most likely the remaining small imperfections are caused by probing artifacts and the pot's inductance.
Thanks for the video.
« Last Edit: 2019-03-27, 20:01:54 by verpies »
   

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Hmmm,  i received my RG174 50 Ohm coax cable and a 1m piece with bnc connectors shows the following response when using the SA + VSWR bridge with a 50 Ohm bnc terminator  see screenshot 1.
If you got a flat SWR trace with just the 50Ω termination resistor connected to the measurement port of your instrument, then this result must be accurate.

Below are my measurements of various coaxial cables terminated with a 50Ω resistor.  The units on the vertical axis are Ohms of effective impedance.
Effective impedance is related to SWR by Z=SWR*50Ω ...so it is just an SWR scaled by your system impedance Z0.

Note, that the longer the cable is, the more compressed the ripples are horizontally  :o

FYI:
SWR = (1+|S11|)/(1-|S11|), where S11 is also known as the "Reflection Coefficient" or "Gamma" ...and the Return Loss = 20*Log(1/|S11|). More is here and here.
The Reflection Coefficient (S11) is small for good matches. "Return Loss" - just the opposite.

In 50Ω systems, the real Reflection Coefficient is:
−1 for shorts ,
Negative for loads < 50Ω,
0 for perfect matches,
Positive for loads > 50Ω,
+1 for open loads.


Sorry about the colors. I forgot to set a black background.
« Last Edit: 2019-03-27, 19:59:42 by verpies »
   

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Thanks,  very nice.

Yes, the 50 Ohm terminator was flatlined by the normalisation process, then the coax was inserted.

So the RG174 i have behaves as expected (similar as yours).
But the winner is the LMR240 cable.

Nice to see the 75 Ohm of the RG59 standing out this way.

The LMR240 cable was by far the shortest length i think, followed by the RG174, the RG59 and then the RG58
judging by the number of ripples.


Itsu
   

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The LMR240 cable was by far the shortest length i think, followed by the RG174, the RG59 and then the RG58
judging by the number of ripples.
Yes
   

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Re. your sin2sq converter:  I noticed that the duty cycle of the output is not 50%.
This is caused by the 74AC14 chip not having its switching thresholds centered evenly around ½VCC.
This will become fixed when you upgrade to the 74xx04 chip.
   

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ok,  i have ordered some 74VHC04, so that will improve the signal further.

I have it running now with a fixed 120 Ohm series resistor for a few hours, and looks good / stable.




   
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