Suitability of Propscope for RF work in the 3 to 10 MHz Range.

I've been doing some Googling and getting conflicting answers. I also asked this question in an unrelated thread and figured it would be more likely to be answered if it were a separate thread.

What is the highest frequency the Propscope can realistically work with? If it samples at 25 Mhz, then I assume it must be used for frequencies well below that. For example, the Nyquist limit would be less than 12 MHz, but that would barely resolve the wave form. I would think more samples are needed with some interpolation. So suppose ten samples are required peak to peak. That would mean 2.5 Mhz would be the ceiling.

So I think this means it would work fine for audio waves, AM broadcast band waves, an 1 MHz digital, but not work for 10 MHz frequencies. Is this correct?

Comments

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  • I think it should work just fine in the 3 to 10 MHz range. I purchased it just for that type of frequency response.

    I had a DSO138 scope that didn't cut it when trying to trace the signal going to a WS2812B LED chip.

    The Propscope worked just fine.

    Mike

  • well - it got sold as a 20 Mhz scope, why doubting?

    1 cog, using the conduit, is transferring hub to PC at fastest serial speed, the other ones are sampling into the hub, so you will not get a continuous sample rate at 20 Mhz, just for about 17 seconds.

    Mike
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  • Martin_H wrote: »
    I've been doing some Googling and getting conflicting answers. I also asked this question in an unrelated thread and figured it would be more likely to be answered if it were a separate thread.

    What is the highest frequency the Propscope can realistically work with? If it samples at 25 Mhz, then I assume it must be used for frequencies well below that. For example, the Nyquist limit would be less than 12 MHz, but that would barely resolve the wave form. I would think more samples are needed with some interpolation. So suppose ten samples are required peak to peak. That would mean 2.5 Mhz would be the ceiling.

    So I think this means it would work fine for audio waves, AM broadcast band waves, an 1 MHz digital, but not work for 10 MHz frequencies. Is this correct?

    Depends on what you are trying to do, exactly ?

    The Prop and PropBASIC can make a 0.5Hz~ 40MHz Reciprocal Frequency Counter, so you can measure 10MHz frequencies with good precision.
    http://forums.parallax.com/discussion/123170/propbasic-reciprocal-frequency-counter-0-5hz-to-40mhz-40mhz-now

    You are right that the samples per sine cycle are low at 10MHz, but are you worried about the points on every single sine wave ?
    Provided the sample rate and sampled sine are not frequency locked, you will sample differing points over many cycles.


  • @All, thanks for the replies
    jmg wrote: »
    Depends on what you are trying to do, exactly ?
    ...
    Provided the sample rate and sampled sine are not frequency locked, you will sample differing points over many cycles.

    I hadn't thought about the differing sample points over several cycles. That should fill in most wave forms, even if the sampling rate is a bit low for a single cycle.

    I mostly want to use it to learn more about building VFO's for radio receivers, as well as what sorts of frequencies pass though specific front end band pass filter. I've built a few kit radios, and radios following cookbook instructions. But I don't have a good feeling why certain circuit elements are what they are. So noodling around and seeing what happens should be instructive.

    I may also want to try my hand at generating video signals. So that requires about 6 MHz of bandwidth as well.

    For digital work I plan to monitor things like I2C bus traffic, or home brew eight bit machines, but it should be fast enough for that.
  • jmgjmg Posts: 10,760
    edited November 2017 Vote Up0Vote Down
    Martin_H wrote: »
    I mostly want to use it to learn more about building VFO's for radio receivers...

    For that, you just need a RF probe (to confirm it is oscillating) and a means to measure frequency, see the link above.
    A HC4060 (or similar) gives a means to amplify & also divide smaller signals, so the Prop can count them
    Martin_H wrote: »
    ....as well as what sorts of frequencies pass though specific front end band pass filter.

    for that, seems you just need a means to automatically sweep the VFO you made above, and measure a RF probe out.
    The Frequency counter can measure & the prop can vary the VFO to sweep, and also capture the RF probe output.


    Addit: If you want to use it on radio receivers, it could be useful to read more than one Frequency.
    For that, I'd suggest grab a few of the new 74AHC1G4210GWH from Mouser (SOT23-5, 125MHz min Fi)
    If you wire X2 & Q to a prop pin each, you can measure X2 for low frequencies, and Q for > 40MHz or lowest level signals.
    Put a couple on a PCB with std leads, (8 COGS to play with, so 2-3-4 channels of Freq Counter are simple )

    & maybe also make a compact RF probe PCB with 1 x 74AHC1G4210GWH, and Low-C Schottky Diodes.
    One diode can connect to X1 for direct RF probe, another can connect to X2 for amplified RF Probe, and X2 can also feed a DC offset to the Prop for ADC Zero.

    A fine 6 wire cable, with signals for
    * Power,GND
    * Q (shielded?)
    * RF Diode_X1 (shielded?)
    * RF Diode_X2 (shielded?)
    * Resistor to X2

    would make a quite nifty Prop-RF probe, able to measure Voltage and Frequency, over good ranges.
    As you know frequency, you could even include a calibration table for the RF probe & amplifier stage.


  • Martin,

    There's really no substitute for a good, stand-alone o'scope. Save your pennies, and spend the money on something with at least a 100 MHz bandwidth, such as these:

    https://www.bkprecision.com/products/oscilloscopes/2190E-100-mhz-1-gsa-s-2-ch-digital-storage-oscilloscope-dso.html
    https://www.alliedelec.com/rigol-technologies-ds1104z-plus/70788928/

    And don't be afraid to buy used. But stick with Agilent (HP) or Tektronix, if you do.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • @jmp, thanks for the ideas about the RF probe.

    @Phil, thanks I'll wait and get a standalone model.
  • PublisonPublison Posts: 9,873
    edited November 2017 Vote Up0Vote Down
    Martin,

    I bought a Rigol DS1054Z 50 MHz 4-Channel scope from Amazon for $399.00. There are free software upgrades on the web to bring it up to 100 Mhz with other freebies. It was a great purchase and I am enjoying having a stand alone O'scope again. Software upgrade went without a hitch.

    https://www.amazon.com/Rigol-DS1054Z-Digital-Oscilloscope-Channels/dp/B012938E76/ref=sr_1_1?ie=UTF8&qid=1510673038&sr=8-1&keywords=rigol+ds1054z


    Infernal Machine
  • For reference, the PropScope box says:

    Capture sine waves up to 5MHz or square waves up to 12.5MHz
    James https://github.com/SaucySoliton/

    Invention is the Science of Laziness
  • I'm sure the Propscope is a great gadget. Especially for classroom use or if you really need something small and portable.

    Otherwise I'm with Phil, save some pennies and get a proper scope. Scopes with amazing capability can be had amazingly cheaply now a days.

    Not sure about the BKPrecison though.

    The go to to budget scope at the moment is the RIGOL DS1054z as suggested by Publison. 4 channels, 1GSa/s, a ton of useful features. And you are not going to blow it's inputs by applying more than 20v. It's good to 300v!

    But, Publison, there are not free upgrades on the net. The upgrades are pay for items. More correctly the unlocking codes have been hacked and one can unlock features for free. Including enabling 100MHz bandwidth when you buy the cheap 50MHz scope. I have done that, works a treat. I think Rigol is happy with this as they have been selling a lot of scopes as a result!

    My rule of thumb is that I want at least a ten times higher sampling rate that the frequency of the signals I want to look at.

    For example, when I connect my hacked Rigol to a 100Mhz clock from a DE0 Nano FPGA board I do at least see it as a sort of sine wave of about the right amplitude. Which is sort of useful, at least tells me I have a signal coming out. And the hack worked!



  • I bought a used 60 MHz analog Tektronix for $70. I've heard if you look around long enough you can get them for $50, or even free, but my time was worth something.

    It seems to work fine, and has let me measure what I need so far. I am still fighting a learning curve with it thought, and have watched several videos about scope usage, and probe calibration. What I have learned so far is the Colpitts oscillator I built has some nasty harmonics in it. It also has stability problems as well.

    So my oscillator is the pits.
  • Martin_H,

    How old is the calibration if ever?

    Also crappy probes will give you crappy results.

    How is your Oscillator powered so you can eliminate power noise as a culprit?
    Is your Oscillator shielded since external noise can also affect it?
  • The oscillator is battery powered by a 9 volt power supply, but not shielded.

    I haven't done anything to calibrate the scope, but a reference square wave looked pretty good, so I don't think it is that far out of whack.
  • I've an old Tek too. 300mhz. It's within a couple percent on everything I could compare to. That has held over the 10 years I've had it.

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