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50 Ohm Impedance Output — Parallax Forums

50 Ohm Impedance Output

SRLMSRLM Posts: 5,045
edited 2013-02-22 03:00 in Propeller 1
I'm trying to calibrate my oscilloscope, and in the manual it says that for the high frequency compensation I should connect the probes to a 1 MHz square wave with an amplitude of 3V p-p, and an output termination of 50 ohms.

The manual says to use a frequency generator, but I don't have one. So, I want to use a Propeller instead.

I've got a PASM program running on the Quickstart that outputs a 1MHz square wave, but what about the other two parameters? Using just the Propeller and basic components, can I make a circuit that closely approximates the requirements?

Comments

  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2013-02-20 13:18
    Hi SRLM;

    50-Ohm-Proip-Pin.jpg


    The Prop has a finite output resistance, as I recall it's about 120Ω or so.
    This resistance is essentially in series with the 50Ω load resistor.
    This forms a voltage divider so the 3.3V square wave will show up as 3.3V*50Ω/(120Ω+50Ω) = 0.97V.
    What ever the pin resistance is, the combination of the 50Ω coax and the 50Ω load resistor will results in the fastest rise time possible.
    Theoretically the coax can be any length.
    Practically, coax has some resistance so long coaxes will result in a bit lower voltage. However, generally, the rise time doesn't suffer except for very fast risetimes which the Prop is not capable of.

    Note, most scopes have high impedance inputs so you will need to add the load resistance.
    If your scope has 50Ω input impedance the resistor is not needed, (its already in the scope).

    BTW, you could do a similar thing with a 75Ω or higher impedance coax, with a matching load resistor, and get a higher output voltage.

    Duane J
    50-Ohm-Proip-Pin.jpg
    1024 x 264 - 22K
  • Mark_TMark_T Posts: 1,981
    edited 2013-02-20 15:23
    SRLM wrote: »
    I'm trying to calibrate my oscilloscope, and in the manual it says that for the high frequency compensation I should connect the probes to a 1 MHz square wave with an amplitude of 3V p-p, and an output termination of 50 ohms.

    Is this to calibrate the 10x probe itself?
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2013-02-20 15:42
    Hi SRLM;

    I should add:
    To calibrate the voltage across the Load resistor simply have the prop output a steady DC voltage and read it with a voltmeter.

    Duane J
  • RaymanRayman Posts: 14,658
    edited 2013-02-20 16:44
    I think the Prop pin resistance is close to 30 Ohms. If you really want to do the best job, you'll probably need a 50-Ohm line driver chip... But you may get by just driving a 50 Ohm resistor with a Prop pin....
  • jmgjmg Posts: 15,173
    edited 2013-02-20 17:57
    SRLM wrote: »
    I'm trying to calibrate my oscilloscope,...

    What part exactly are you trying to calibrate ?
    1MHz is above probe compensation R * C, so you are really checking the capacitive divider, and usually that is done at a lower frequency, as you mostly want LF == HF gains.
    Once you have probe compensation correct, then a good DC voltmeter can verify your signal swing, which you can apply to the Cal steps.
  • RaymanRayman Posts: 14,658
    edited 2013-02-20 18:13
    I think the test signal for calibrating scopes is usually a 1 kHz square wave. The Tek scopes I've used have this provided on the front panel. But what you're really looking at is the sharp rise of the square wave and you adjust your probe so that it comes up fast and doesn't ring too much. You are really looking at frequencies much higher that 1 kHz when you do this...
  • kwinnkwinn Posts: 8,697
    edited 2013-02-20 18:33
    Rayman wrote: »
    I think the test signal for calibrating scopes is usually a 1 kHz square wave. The Tek scopes I've used have this provided on the front panel. But what you're really looking at is the sharp rise of the square wave and you adjust your probe so that it comes up fast and doesn't ring too much. You are really looking at frequencies much higher that 1 kHz when you do this...

    Rayman is correct. The typical signal generator is designed for and expects a 50 ohm load impedance. With the signal generator outputting a 1 KHz signal, connected to a 50 ohm load, and the scope probe leads connected to the load resistor you should adjust the probe to obtain a perfect square wave, or as close as possible.
  • SRLMSRLM Posts: 5,045
    edited 2013-02-20 19:23
    I'm trying to calibrate an Agilent DSO3202A, and the calibration instructions are on pages 31 and 32.

    http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=662668&lc=eng&cc=US&nfr=-33863.536905611.00

    For the low frequency adjustment (using the front panel square wave, and not part of the original question) I'm adjusting the pot on the probe tip. For the high frequency adjustment, I'm adjusting the two pots in the plug of the probes.

    Thanks everyone for the feedback.
  • RaymanRayman Posts: 14,658
    edited 2013-02-21 03:57
    Ok, I think you'd need that 50-Ohm line driver chip then. Even then, I'm not exactly sure how sharp the rise on a Propeller output pin is...
    Might be good enough though.

    I'd probably run the output of a prop pin into a 1k resistor at the input to a line driver with 50 Ohm termination.
  • Mark_TMark_T Posts: 1,981
    edited 2013-02-21 16:14
    I think the low-frequency adjustment won't be a problem with any realistic output impedance, but the high-frequency
    adjustments will want a matched 50ohm signal I think (well its a BNC connection...)

    You could put a prop pin through a 180 + 68 ohm resistor divider to get 50 ohm square wave at 0.9V p2p (unloaded) 0.45V (50 ohm load).
    Prop outputs should have a risetime of a few ns.
  • LawsonLawson Posts: 870
    edited 2013-02-21 20:42
    The output impedance of prop pins is about 30 ohms. A single prop pin will drive about 2.7 volts down a 50 ohm transmission line that is parallel terminated to ground. (and survive for years at low duty cycles) Add a 20 ohm resistor in series to series terminate for a 50 ohm transmission line. Rise times on the prop are about 1.2nS as measured on the 1GHz scope at work.

    50ohm line drivers are a pain to find. The best "line driver" I've found is 3-6 74AC or 74LVC gates in parallel. (CMOS chip families work best as they current share when paralleled) Rise time is about 2nS and the parallel gates will drive a 50 ohm transmission line with two or three terminated branches if needed. If you can tolerate a 12-15nS propagation delay, surface mount packages, and a lower maximum current then 1ohm SPDT video switch chips also work well.

    Lawson
  • RaymanRayman Posts: 14,658
    edited 2013-02-22 03:00
    The line driver/buffer I've used before is the BUF634, which can drive 3 V into 25 Ohms (what you want). But, it's not the easiest to work with because it needs + and - supplies...

    Maybe you can find a single-supply buffer that works better... A few logic gates in parallel might work too, as Lawson suggested, but you want to be able to drive two 50-ohm resistors in parallel, ideally......

    That's because you get the best performance with 50 ohm resistors to ground on both ends of your 50 ohm cable...
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