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Author Topic: Itsu's workbench / placeholder.  (Read 3492 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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