[SOLVED] ADL5380 (Demodulator) Conversion Loss

[@YY]

Hi,

From the Circuit note,
https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0374.pdf

It's stated that the conversion loss would be around -4.64 dBm. But no matter how i try it still gave me a -8 dBm loss. Does anyone has any ideas? where should i troubleshoot? I really ran out of ideas.

Besides, I have tried both the 1:1 and 1:4 balun but both gave the same results. I would aspect a lower power loss for 1:4 balun but i failed. Is there anything I should concern?

 

[mtwieg]

Hi Vfone,
Thanks for the advise.

Hi mtweig,
Thanks for the information.

For a 1:1 balun, I would see a 50ohms input and 50ohms output. For a 1:4 balun, I would aspect a 50ohms input and 200ohms output?

Depends which way the balun is oriented. The adt4-6t is 1:4 with respect to primary:secondary. So you want the secondary to be connected to the IF port of the ADL5480, and the primary will be the output. Having it reversed would certainly explain the issue...

have you tried just measuring the voltage at the Qhi/Qlo/Ihi/Ilo pins directly with an oscilloscope, without any other load on those pins?

 

[@YY]

Hi mwtieg,

I have something to ask about balun, from the datasheet,
https://www.analog.com/media/en/technical-documentation/data-sheets/ADL5380.pdf
at pg 25 regarding the baseband output, it's stated that
"Each baseband pair has a 50 Ω differential output impedance. The outputs can be presented with differential loads as low as 200 Ω (with some degradation in gain) or high impedance differential loads (500 Ω or greater impedance yields
the same excellent linearity) that is typical of an ADC."

i am using a 1(primary):4(secondary) balun, that's mean the balanced signal is 4 and unbalanced signal is 1, i am wondering is the output impedance 200ohms?

I have also measured the voltage at the baseband, Ihi = ~450mVpp, Ilo= ~450mVpp. This is the best i can get.

 

[mtwieg]

Hi mwtieg,

I have something to ask about balun, from the datasheet,
https://www.analog.com/media/en/technical-documentation/data-sheets/ADL5380.pdf
at pg 25 regarding the baseband output, it's stated that
"Each baseband pair has a 50 Ω differential output impedance. The outputs can be presented with differential loads as low as 200 Ω (with some degradation in gain) or high impedance differential loads (500 Ω or greater impedance yields
the same excellent linearity) that is typical of an ADC."

i am using a 1(primary):4(secondary) balun, that's mean the balanced signal is 4 and unbalanced signal is 1, i am wondering is the output impedance 200ohms?

If connect the balun the way you show in your schematic (secondary connected to the downconverter, primary to the outputs), then connect those outputs to 50ohm loads, then that should present 200ohms to the downconverter ports, which is fine. Keep in mind, the output port won't be impedance matched, but that's okay.

I have also measured the voltage at the baseband, Ihi = ~450mVpp, Ilo= ~450mVpp. This is the best i can get.

Those numbers don't mean much without also knowing the RF input power or voltage...

 

[@YY]

Those numbers don't mean much without also knowing the RF input power or voltage...

Hi mtwieg,

The output Vpp is referring to an RFin with a power level of 0dBm. In the circuit notes, it is expecting a Vpp at IF output with ~1.1Vpp. However, I the best i could get is only ~850mVpp.

- - - Updated - - -

Besides, I found that the 1:1 balun has the better performs compare to 1:4 balun. 1:1 balun has an ouput of IF with ~850mVpp but the 1:4 balun has only ~450mVpp.

 

[mtwieg]

Hi mtwieg,

The output Vpp is referring to an RFin with a power level of 0dBm. In the circuit notes, it is expecting a Vpp at IF output with ~1.1Vpp. However, I the best i could get is only ~850mVpp.

Okay, so if you get 450mVpp on each IF pin, then your differential output voltage is 0.9Vpp, input is 0.63Vpp at 0dBm, so your voltage gain is ~3.1dB, not far off.

Besides, I found that the 1:1 balun has the better performs compare to 1:4 balun. 1:1 balun has an ouput of IF with ~850mVpp but the 1:4 balun has only ~450mVpp.

Then you probably have something mixed up, there's no way the 1:1 balun should give higher output voltage at the ADL5380 pins, unless you're putting in the 1:4 backwards. 850mVpp or 1.1Vpp (at each pin) implies conversion gain even higher than stated in the datasheet.

 

[@YY]

Hi mtwieg,

thanks for the explanation!

I am using mini circuit T4-6T 1:4 balun for the IF.
https://www.minicircuits.com/pdfs/ADT4-6T.pdf
I have done a simple checking where I only input a 400mVpp (50ohms impedance) to both the secondary port. However, I only archive a ~800mVpp at the output port (1Mohms impedance). I think would it be the balun problem?

Please advice.

 

[mtwieg]

Hi mtwieg,

thanks for the explanation!

I am using mini circuit T4-6T 1:4 balun for the IF.
https://www.minicircuits.com/pdfs/ADT4-6T.pdf
I have done a simple checking where I only input a 400mVpp (50ohms impedance) to both the secondary port. However, I only archive a ~800mVpp at the output port (1Mohms impedance). I think would it be the balun problem?

Okay so now you say you get a voltage gain of 6dB. So I guess you solved the problem?

Please advice.

It's really hard to give advice when your description of the "problem" changes every post.

 

[@YY]

Hi mtwieg,
Thanks a lot for the helps. I have understand. I have a gain around 2.49 dB.

Thanks everyone for the help..! I have learn a lot, gain a lot of knowledge now..! I will work hard on it.