If you stick a 200V probe on 400 volts it will breakdown and short 400V into the analyzer. Many scope probes are not rated for high voltage and it is dangerous to use that tiny trimmer cap as a safety attenuator for the analyzer front end. The issue of concern here is old tube radios. People who do this are not looking to measure absolute levels and it has been done for years on SS circuits with their small DC voltages pretty safely. If the probe trimmer is set higher you get less attenuation. When you take a probe set for a 20F input and stick it on the front of an analyzer with a few pf of input C you get around 50db of attenuation just like Mark said. Leigh you should look at a probe schematic before you start calculating signal attenuation.
If the scope was 20 pf the trimmer would be 2 pf when correctly set. the comp signal on the scope front is usually a 1Khz square wave and you adjust the trimmer in the probe until the 10:1 attenuation for the HF matches the resistive for the low. a probe for that is a capacitive divider for all frequencies above a few Khz. "Hell, there are no rules here-we're trying to accomplish something!" It is intended to work with standard scope divider probes on the input, which is a good thing because you'll probably need one to get the IF voltage levels down into range. Note that the input of this buffer amp is 1-megohm, 0.8 volts p-p. That op amp ain't cheap-about $12 bucks-but putting something like it together yourself would be a lot more expensive unless your time is only worth a nickel an hour. Here's a link to a relatively easy to make buffer amp that should get you out to at least 200-MHz, more than enough to deal with anything in an SX-28. You could wait for one of the aforementioned FET probes to come along, but it is also possible to make a buffer amp yourself that will get the job done. So what you need, as others have already pointed out, is a buffer amplifier to convert the high impedances and high voltages of the tube circuitry to a 50-ohm impedance, DC-isolated output for the spectrum analyzer. Spectrum analyzers are wonderful tools, but they are by no means universally applicable in every situation.
In old tube receiver IF amplifier strips, voltage gain was the goal, so they didn't standardize on power and the impedances are uually a lot higher-the point being that the 50-ohm input impedance of the analyzer will almost certainly swamp and distort the response anywhere you connect it in a tube circuit that was not specifically designed to drive a 50-ohm load. Most modern ones are capable of also displaying voltage in some format on the Y-axis, but you're still stuck with that 50-ohm input. Spectrum analyzers are normally meant to give a display of signal power on the Y-axis versus frequency on the X-axis. IF you choose to use the analyzer in ways it was not designed for you need to proceed very carefully and think hard about what you are doing. Spectrum analyzers are wonder tools but they were not made to align the IF of tube amplifiers. I know there are protection diodes but they are not made to take much charge either. for a short time the cap is shorted and will dump dc into the analyzer until the cap charges. The other thing to remember is what happens when you connect the probe to HV. When they fail you will short the input of the probe to the input of the analyzer and it will fail. Many probes are not rated for tube voltages and I have even seen onesįail below the nice rating on the label. Sure the 400V dc will get dropped to safe levels on paper but those probes are really capacitive dividers with tiny caps. I would never use a 10 megohm scope probe as a poor mans divider.
Rigol dsa815 tg spectrum analyzer manual#
you would need the service manual for pinout and voltage. The tek power supply for the probes is getting scarce but you can find the lemo connectors and use a good external power supply.
I use a small coupling cap rated for 1000v DC on the probe tip and I still use a 50ohm dc block connector on the analyzer. They are DC -1ghz so if you connect to 400V DC you will blow the probe amplifier and if you kill the amp you dump DC into the analyzer and kill it. That said you still need to be very careful. They show up on ebay for less than $100 but are not really repairable. They came with diviIder hats that allowed 1x through 100X attenuation and pretty good voltage ratings.
Rigol dsa815 tg spectrum analyzer series#
The best series of probes was the older 6201 series. As Leigh stated Tek has made some very good FET probes over the years but the prices are pretty high. However I have used spectrum analyzers for years in high impedance circuits. You do not want to blow the input mixer as they are not user serviceable. As stated previously the input to spectrum analyzers will not tolerate large signals or any significant DC.