Hello Engineer Zone,
We're using an HTG card implementing a DAC conversion fonction based on an AD9172 followed by TCM1-83X+ and SSMC 50-ohms RF output port, therefore very similar to the AD9172-FMC-EBZ reférence design.
We have read carefully the following posts:
- post1 : https://ez.analog.com/data_converters/high-speed_dacs/f/q-a/569040/full-scale-for-ad9176-and-ad9081/491339
- post 2 : https://ez.analog.com/data_converters/high-speed_dacs/f/q-a/570657/no-output-for-ad9175-nco-only-mode/498810
We have configured the device to activate the calibration NCO (instead of the Main NCO) and use it in DC test-mode NCO-only (post 2 helped us a lot because the datasheet was not clear about it), therefore producing a digital sinewave (center frequency fc=1.8 GHz, fDAC=5GHz, 1250 Msps) directly at the input of the D/A converter, and mesuring the analog signal at the RF output (spectrum, fc+/-500MHz):
- for a DC level = -6 dBFS = 0x3FFF (register 148/149), we mesure the test tone at -10 dBm and SFDR in the 1 GHz band at 60 dB
- for a DC level = 0 dBFS = 0x7FFF (register 148/149), we mesure the test tone at -6 dBm and we have clearly saturation (test tone is 2 dB compressed and high-power tones appear in the spectrum)
We do not understand why we are so low in output power, reasoning below with ajustments vs. the posts mentioned previosuly.
Ideally the AD9271 should output 7 dBm at full-scale
At 1.8 GHz the internal impedance is not 100 ohms différentiel but 50 ohms (post 1), that reduces the output power to 3.5 dBm
The TCM is not a 2:1 (100 diff to 50 ohms s/e) but a 1:1 (50 diff to 50 ohms s/e) impedance ratio transformer with an insertion loss of 1.2 dB :
https://www.minicircuits.com/WebStore/dashboard.html?model=TCM1-83X%2B, we should get a bit less power 4.0-1.2 = 2.8 dBm at its output
The connection between the SSMC port and the spectrum analyser account for less than 1.5 dB loss, we thus expected to measure 1.3 dBm
We are short of -7/-9 dB, where do we go wrong?
Thank you for your support,
Eric
