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Doubt in Direct conversion/Zero-IF operation using ADF4360-9 and AD8348

Question asked by ssp on Jun 6, 2016
Latest reply on Jul 5, 2016 by stanis

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

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