Electronics > Beginners

Volts to dBm - sanity check?

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elroy:

--- Quote from: radiolistener on April 03, 2019, 06:35:18 pm ---Please note that Chinese DDS such as FY6600 shows invalid voltage.

--- End quote ---

Incidentally, my friend echoed the observation that the Chinese DDS has severe shortcomings. I knew from the outset that it wasn't a real quality instrument, but I wanted to see if I could get some use out of it and hoped it wasn't a complete loss.

If I want to replace it with a better signal generator, are there any makes/models you would recommend? Knowing that I'd need to spend more, but cannot afford, say, thousands of dollars.

radiolistener:
As I know, regarding to amplitude, FY6600 works pretty good. But you're need to take into account that it shows Hi-Z voltage for open output (with no load) and when you connect 50 Ohm load it will be divided by two. But this is true if your gen has 50 Ohm output. Just check it and if it is 50 Ohm, then just setup twice higher voltage than you needed and it will works.

There is some noise on FY6600 output from switching power supply, but it can be fixed with replacement switching power supply with linear regulated one.

The main problem with FY6600 is a jitter due to FPGA code implementation. It leads to parasitic spurious components in the signal.


--- Quote from: elroy on April 03, 2019, 08:12:12 pm ---If I want to replace it with a better signal generator, are there any makes/models you would recommend?

--- End quote ---

Siglent SDG2042 nice gen and it shows voltage as expected, you can select Hi-Z or 50 Ohm indication. But it is more expensive.

If you wan't something more cheap, I think it's better to buy some used but brand name equipment

radiolistener:
Here is input impedance for genuine RTLSDR Blog v3 (bought in official store). PCB version 2018  9  13.
https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

The dongle is powered and switched into direct sampling mode. LO tuned at 5 MHz.

As you can see it is not 50 Ohm and significantly varying depends on the frequency.

Impedance for 1 - 81 MHz:


Impedance for 0.5 - 17 MHz:


VSWR for 1 - 81 MHz:


VSWR for 0.5 - 17 MHz:

elroy:
Looking at the plots reminds me of the old saying: There are two times a day when a broken clock has the correct time ...

radiolistener:

--- Quote from: elroy on April 04, 2019, 12:31:15 am ---Looking at the plots reminds me of the old saying: There are two times a day when a broken clock has the correct time ...

--- End quote ---

something like that ;D 
But if you look at SWR plot, it is not so bad actually :)
And it works with pretty good sensitivity for it's price.

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