OH2NLT AD9952 400MHz DDS test board for JUMA-TRX2
04.09.2008
Background
JUMA-TRX2 LO is generated with AD9851 DDS chip. The chip is clocked from 30MHz oscillator. Internal multiplier (x6) is used to rise the DDS clock frequency up to 180MHz. JUMA-TRX2 QSD/QSE mixers requires clocking at 4xRF (listening frequency). The clock is divided to four phases for mixer switches drive. AD9851 comparator true and inverted output is used to drive IC7A and IC7B flip-flops which are used to form I-LO and Q-LO signals. I-LO and Q-LO signals are are overlapping so that four phases are formed. Because both DDS output comparator edges are used, DDS output frequency needs to be 2xRF. General rule of thumb is that DDS can generate usable output signal up to about 40% of its clock rate. 180MHz clock gives us 72MHz and when we divide this with two we have about 36MHz max RF. JUMA-TRX2 RX is specified up to 30MHz. AD9851 and the mixer clocking logic is working at its limits when listening to 30MHz. Of course LO signal quality gradually lowers before DDS limit is reached. Imperfections starts to show up when listening frequencies above 14MHz band.
Tests with high performance AD9952 DDS
To be able to investigate problems and compare different performance DDS circuits I made a test board with Analog devices AD9952 DDS chip. AD9952 can be clocked with 400MHz clock rate. I-LO and Q-LO quadrature signals are generated from 4xRF DDS output with IC6 and IC7 (see test board schematic). This approach gives us better quadrature timing. We have also more headroom in upper frequency limit. 400MHz x 0.4 = 160MHz / 4 = 40MHz.
I designed the test board so that it is connected behind the JUMA-TRX2 DDS board place which is originally reserved for voice memory board. Small modification in the JUMA-TRX2 main board allows both (original and test board) DDS chips to be used to feed mixers. I made special version of TRX2 firmware which controls AD9952 test board and allows user to switch between original AD9851 and test board AD9952 DDS on the fly. This makes testing and comparing things easy. In the test software there is also frequency calibration for AD9952 test board oscillator.
Test Board
JUMA-TRX2 main board modifications
Test software for JUMA-TRX2/AD9952 test board version 20.10.2008
Test board pictures Top | Bottom
Note the plated hole under AD9952 for AD9952 bottom pad soldering.
Board installed
Top | Back
First tests with the board. 100MHz signal without filtering looks promising.
Things need to be done
Test board oscillator is ordinary 20MHz crystal. 20MHz XTAL frequency is multiplied by 20 inside AD9952 chip. Test board needs higher frequency and higher quality oscillator before real measurements are done.
