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| Bode plot up to a few 100 MHz |
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| rf-loop:
--- Quote from: 2N3055 on October 04, 2023, 09:49:02 pm --- --- Quote from: tszaboo on October 04, 2023, 09:17:57 pm ---So instead of a vector network analyzer, you need a scalar network analyzer. There are some old HP units on ebay that would do this, but only if you can get a deal. Depending on your amplifiers, a VNA could be a bad choice. They expect 50 Ohm input and output, plus you can blow them up with just a bit of power. Considering you already have a nice 300 MHz scope, I would get a used signal generator, and program the thing up to make sweeps. Probably a Z0 probe would help on the scope side. Or you get the bonus round: Get a RF white noise generator to 1GHz, and record the bode plot with the FFT of the scope. --- End quote --- Allow me a bit of pedantry here: If it is only magnitude plot (no phase info) than it should not be called Bode plot but simply frequency response plot... Just saying-- --- End quote --- Just saying... Whole Bode plot: - - Bode magnitude plot - - Bode phase plot We get (least with Siglent Bode Plot (hopping swept frequency response analyzer)) both but in some cases Bode magnitude plot is enough. Scalar analyze (Bode magnitude plot) is still good for many things. But example with Siglent oscilloscope Bode Plot we can not do just scalar plot only. (what can also be bit faster) This is one thing what is in wish list... so that user have also some faster options depending his variable needs. |
| tszaboo:
--- Quote from: rf-loop on October 06, 2023, 07:12:10 am --- --- Quote from: 2N3055 on October 04, 2023, 09:49:02 pm --- --- Quote from: tszaboo on October 04, 2023, 09:17:57 pm ---So instead of a vector network analyzer, you need a scalar network analyzer. There are some old HP units on ebay that would do this, but only if you can get a deal. Depending on your amplifiers, a VNA could be a bad choice. They expect 50 Ohm input and output, plus you can blow them up with just a bit of power. Considering you already have a nice 300 MHz scope, I would get a used signal generator, and program the thing up to make sweeps. Probably a Z0 probe would help on the scope side. Or you get the bonus round: Get a RF white noise generator to 1GHz, and record the bode plot with the FFT of the scope. --- End quote --- Allow me a bit of pedantry here: If it is only magnitude plot (no phase info) than it should not be called Bode plot but simply frequency response plot... Just saying-- --- End quote --- Just saying... Whole Bode plot: - - Bode magnitude plot - - Bode phase plot We get (least with Siglent Bode Plot (hopping swept frequency response analyzer)) both but in some cases Bode magnitude plot is enough. Scalar analyze (Bode magnitude plot) is still good for many things. But example with Siglent oscilloscope Bode Plot we can not do just scalar plot only. (what can also be bit faster) This is one thing what is in wish list... so that user have also some faster options depending his variable needs. --- End quote --- IMHO he is right. But I found phase plot to be less usefult, unless I have a sine and cosine wave I/Q data for example. I see an 1 GHz white noise source for about 30 USD (don't know how good quality is) so OP really needs to consider whether paying a lot more for phase information is worth it. Even if the white noise source is not flat to 1 GHz it's possible to measure it and compensate the results with the source magnitude difference. |
| joeqsmith:
Without all four S parameters, how do you measure the amplifiers stability? It's another shortcoming of the low cost VNAs out of China, 2-port 1-path. My work around was to add a transfer relay. Best thing would be to buy better equipment. You could have a look at Picotech's low cost 4-receiver VNA. https://www.eevblog.com/forum/testgear/vna-suggestions-for-10k-usd/msg5072017/#msg5072017 |
| alm:
The VNWA 3SE (latest version) also incorporates a transfer switch so it can measure all four S parameters without changing connections. I believe there's support for using an external bridge to deal with power amplifiers. See this recent topic for some pointers. I'm skeptical about the ability to get useful measurements at hundreds of MHz using a high impedance environment, since there is no such thing as high impedance at higher frequencies. Similar to using 10x scope probes for hundreds of MHz. They work, assuming you are have a very well behaved low impedance source. The impedance fluctuations over the bandwidth would be huge, likely affecting the measurement. |
| joeqsmith:
Does their software also calculate stability? Hard to say about working with high impedance's and RF without knowing more of what the goals are. Waiting for lasmux to release their active probe. https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4973839/#msg4973839 |
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