Hi!
Greetings!
It's been few days I have been struggling with these EVM modules. Please have a glimpse of my query and share your suggestions for 0-IF operation of AD8348.
I have doubt in Direct conversion/ Zero-IF operation using ADF4360-9 as synthesizer and AD8348 as IQ demodulator. I have acquired EVM kits for both the chips. I have given 360MHz to LO and 180MHz to MIX inputs for 0-IF operation.
My setup:
>> For ADF4360-9 I have used EVM kit's default connections with very few changes; RFout frequency: 360MHz, PFD frequency: 1600KHz, REFIN freq=19.2MHz, DIV OUT counter =Divede by /A counter, DIVOUT freq=180MHZ, RF output power current: 11mA (0dBm)
>> RFOUT=360MHz from ADF4360-9 is fed to single ended LO input of AD8348 using coaxial cable of length 22cm.
>> The other complement RFOUT' is terminated in 50ohm amphenol standard.
>> AD8348 switch SW11 is in enabled position, SW12 is in MIXIN position (Vgin, input VGA: not used)
>> As my baseband operation is DC, two capacitors COI, COQ connected at IOFS, QOFS are removed.
>> Vref is connected to IOFS and QOFS using jumpers LK5I and LK5Q as suggested in datasheet pages: 18-19 under the section: BASEBAND OFFSET CANCELLATION.
>> No filters are used before baseband application: IMXO is directly connected to IAIN using jumpers LK1I and LK3I; similarly QMXO is directly connected to QAIN using jumpers LK1Q and LK3Q.
>> Vref is directly connected to VCMO using jumper LK11
>> Low pass filters are connected in the differential I/Q outputs. All of them have cut off frequency=159Hz.
>> 180MHz DIVOUT frequency from AD8348 is given to single ended MXIP input of ADF4360-9 using say 'x180' length of coaxial cable
Observation:
ADF 4360-9 output frequency generation is confirmed at 180MHz and 360MHz using spectrum analyzer.
For no 180MHz to MIX input, differential DC outputs of AD8348 are Vi=-409mV and Vq=-364mV
Case-1: AD4360-9 RF output current=3.5mA (output power=-9dBm)
For cable length 'x180'=29cm=> differential DC outputs Vi=1840mV and Vq=-1828mV
For cable length 'x180'=24cm=> differential DC outputs Vi=1846mV and Vq=-1831mV
Case-2: AD4360-9 RF output current=11mA (output power=0dBm)
For cable length 'x180'=29cm=> differential DC outputs Vi=-1780mV and Vq=1604mV
For cable length 'x180'=24cm=> differential DC outputs Vi=-1772mV and Vq=1575mV
Question:
As in the 50ohm coax cables the signal will not attenuate and only phase will change due to length difference, we can calculate the phase difference as follows:
Vi=mag*cos (phi), Vq=mag*sin (phi)
case-1:
phi_29cm=arctan(-1828/1840)= -44.81 deg (in fourth quadrant due to sine component is negative)
phi_24cm=arctan(-1831/1846)= -44.76 deg (in fourth quadrant due to sine component is negative)
hence delta_phi ~0, as I cannot see any voltage change in the response!!!
similarly, for case-2 it turns out to be: delta_phi ~0!
Calculation:
effective length difference= 29cm-24cm=5cm
calculated phase change/cm in coaxial Teflon media of the 180MHz signal=3.13degrees/cm
Expected phase change delta_phi= 3.13deg/cm * 5cm = 15.65 degrees
WHY THERE WASN'T ANY VOLTAGE CHANGE OBSERVED IN THE I/Q OUTPUTS DUE TO PHASE DIFFERENCE?
DO I HAVE IMPLEMENTED THE SYSTEM IN CORRECT WAY? PLEASE LET ME KNOW IF I DO NOT MAKE SENSE OR ELSE IF YOU NEED MORE INFORMATION.
Regards,
Sanket
