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| Review: Hantek DDS 3X25. Anyone own one? |
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| saturation:
Typically a Hi-Z switch on a FG, effectively replaces the 50 ohms in series with 1 megaohm resistance. To emulate the same simply put a 1 megaohm resistor in series with the Hantek 3x25 output. The only reason to have this is so the function generator output voltage matches the output scale of the FG, otherwise, it typically 2x larger than the FG scale. It uses 1 megaohm to match what its commonly used for; to see on a scope. In theory, Hi-Z can mean infinite impedance such as in tristate buffer ICs. But if that were so, anything in series with it will see zero output. The 3x25 is best used by someone who enjoys hacking and exploring how to stretch gear without fear. Its not the ideal instrument for teaching, because it does peculiar things that had to be hacked around. |
| alm:
--- Quote from: saturation on January 14, 2012, 01:11:03 pm ---Typically a Hi-Z switch on a FG, effectively replaces the 50 ohms in series with 1 megaohm resistance. To emulate the same simply put a 1 megaohm resistor in series with the Hantek 3x25 output. The only reason to have this is so the function generator output voltage matches the output scale of the FG, otherwise, it typically 2x larger than the FG scale. It uses 1 megaohm to match what its commonly used for; to see on a scope. --- End quote --- Do you have an example of a function generator that actually does this, preferably with schematic? This sounds like a dumb thing to do. Where do I buy the 1 Mohm coax cable that matches the impedance of this supposed 1 Mohm output impedance? Assume I use a 1 meter RG-58C cable to connect the function gen to the scope, and the function generator is set to its max (3.5V) amplitude. This unterminated piece of coax will have a capacitance of about 100 pF, so the impedance at 75 MHz will be about 20 ohms. This will load the function gen and will reduce the amplitude at the output of the function generator from 3.5 Vp-p to about 70 uVp-p. Plenty of function gens have the ability to switch between 50 ohm and 'hi-Z' load, the only thing it usually does is multiply the displayed voltage by two, since it was calibrated for a 50 ohm load. The output impedance is usually a constant 50 ohms. Some may be able to switch to 600 ohms for audio applications, but not 1 Mohm. |
| Mechatrommer:
novel construction. |
| saturation:
Sorry alm and mecha, brain faxt on my part, although it was partly factious in that the Hantek hack was made for people to play with, and get more value from it, not getting something very basic spoon fed to everyone. What I meant effectively is it forms a 1:1 voltage divider with the scopes impedance, fixing the voltage but not its effect as a transmission line. Adding 1 megaohm could work at low frequencies, but it will play havoc as frequencies rise for all the reasons alm already makes clear, the impedance is still mismatched at least for the cable and the connectors. Hi-Z as an abbreviation for "high impedance" is an ambiguous term ; tristate on one hand, non-50 ohm devices on another, anything over 10kohms for microphones; there could be more. I've heared the term 'hi-z' used in conjunction with scope probes meaning specifically 1 megaohm probes. http://www.home.agilent.com/agilent/product.jspx?nid=-34025.0.00&lc=eng&cc=US There was once a signal generator with adjustable output impedance, as in mecha napkin'd schematic: http://www.emchire.co.uk/products/product-code/506 Finally hi-z switch on many function generators just means the electronics adjusts the output so the dial scale is what is actually outputted if the DUT is not 50 ohms, it does not really adjust the output impedance of the function generator to 'high impedance'. It can just blatantly halve it, or better, may have a voltmeter at the output that will sense, feedback and adjust the output regardless of what the DUT impedance is. --- Quote from: alm on January 14, 2012, 01:38:28 pm --- --- Quote from: saturation on January 14, 2012, 01:11:03 pm ---Typically a Hi-Z switch on a FG, effectively replaces the 50 ohms in series with 1 megaohm resistance. To emulate the same simply put a 1 megaohm resistor in series with the Hantek 3x25 output. The only reason to have this is so the function generator output voltage matches the output scale of the FG, otherwise, it typically 2x larger than the FG scale. It uses 1 megaohm to match what its commonly used for; to see on a scope. --- End quote --- Do you have an example of a function generator that actually does this, preferably with schematic? This sounds like a dumb thing to do. Where do I buy the 1 Mohm coax cable that matches the impedance of this supposed 1 Mohm output impedance? Assume I use a 1 meter RG-58C cable to connect the function gen to the scope, and the function generator is set to its max (3.5V) amplitude. This unterminated piece of coax will have a capacitance of about 100 pF, so the impedance at 75 MHz will be about 20 ohms. This will load the function gen and will reduce the amplitude at the output of the function generator from 3.5 Vp-p to about 70 uVp-p. Plenty of function gens have the ability to switch between 50 ohm and 'hi-Z' load, the only thing it usually does is multiply the displayed voltage by two, since it was calibrated for a 50 ohm load. The output impedance is usually a constant 50 ohms. Some may be able to switch to 600 ohms for audio applications, but not 1 Mohm. --- End quote --- |
| alm:
--- Quote from: saturation on January 15, 2012, 02:04:59 pm ---Hi-Z as an abbreviation for "high impedance" is an ambiguous term ; tristate on one hand, non-50 ohm devices on another, anything over 10kohms for microphones; there could be more. --- End quote --- No worse than high frequency or high voltage. High frequency may be anything over 20 kHz to an audio guy, anything >> 60 Hz for a power supply designer, ~30 MHz for a ham or 100 GHz for someone working in the microwave spectrum. High voltage may mean anything > SELV for many low power electronics people, but may be > 1 kV for an electrician or > 35 kV for a lineman. In this context it means >> 50 ohm (assumed to be infinite), similar to the open terminal voltage of a Thevenin circuit. --- Quote from: saturation on January 15, 2012, 02:04:59 pm ---I've heared the term 'hi-z' used in conjunction with scope probes meaning specifically 1 megaohm probes. --- End quote --- With scope probes they basically mean >> 50 ohm, although the essential difference between hi-Z and lo-Z is that lo-Z are resistive transmission lines, while hi-Z are essentially capacitive dividers at higher frequencies. A Hi-Z probe may have an input impedance of 10 Mohm // 10 pF (10x probe), or 20 kohm // < 1 pF (active probe connected to a 50 ohm scope input). A lo-Z probe may be 5 kohm (100x probe). --- Quote from: saturation on January 15, 2012, 02:04:59 pm ---There was once a signal generator with adjustable output impedance, as in mecha napkin'd schematic: http://www.emchire.co.uk/products/product-code/506 --- End quote --- A fair number of them have a switch between 50 and 600 ohm. Some HP generator were available in both 50 ohm and 75 ohm versions, although changing this requires you to replace some components. --- Quote from: saturation on January 15, 2012, 02:04:59 pm ---Finally hi-z switch on many function generators just means the electronics adjusts the output so the dial scale is what is actually outputted if the DUT is not 50 ohms, it does not really adjust the output impedance of the function generator to 'high impedance'. It can just blatantly halve it, or better, may have a voltmeter at the output that will sense, feedback and adjust the output regardless of what the DUT impedance is. --- End quote --- The majority of them will just double the voltage, external leveling is usually reserved for specialized applications that require it, like the Tek SG504. |
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