we have connected the output I/Q of the AD9854 to ADL5375-05 ,with a 500mv dc ,
the Rset is 2.2Kohm to have a current of app 9mA,
with two 51ohm resistor and a 100 ohm swing limiter resistor, but we dont get 0dBm at the output of mixer,
what can be wrong in our design?
It looks :
baseband input frequency : around 7 or 10MHz
baseband input amplitude : 677mVp_p differential
Full scale current with 2.2Kohm Rset on AD9854 is 18mA and it will swing ideally around 918mVp_p differential with 51ohm termination resistors along with 100 ohm shunt resistor. If its ac swing level is 677mVp_p differential biased at 0.5V on baseband inputs, some loss comes from roll-off in DAC, low pass filter at that 7 or 10MHz, tolerance of resistor values, and not sure if you do some digital back off there.
Output power of ADL5375 would vary over LO frequency. Please refer to Figure 3 on ADL5375 datasheet. No information here about LO frequency. So its output power is around -3.4dB lower than Figure 3 plot with 677mVp_p differential. Figure 3 is typical plot with 1Vp_p differential.
This is a question posted by my colleague,
I would like to supply some extra information.
The output of the DDS is directly connected to the 50 ohm DB biassing resistors and the input of a single chip lpf filter (http://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0112S0119NFL21SP.pdf) with a cutoff frequency of 70MHz.
Followed by a 100 ohm swing limmiting resistor.
Below are some screenshots:
Fiirst is the differential signal, we get 677mv PP
second shows the detail of the AC signalthird shows with DC ofset.