The AD9106 is a quad-pack digital-to-analog converter (DAC) that utilizes an on-chip direct digital synthesizer (DDS) for complex waveform generation. The on-chip DDS has a 12-bit resolution and supports sample rates up to 180 MSPS. The low power dissipation of the AD9106 makes this an ideal chip for the Marlin platform. The Marlin platform utilizes two of the four differential output pairs because both the in-phase and quadrature signals are generated. The frequency of the output signals from the AD9106 are between 50-75 MHz. The two unused DACs can be disabled using the power status register.
The in-phase and quadrature signals generated by the AD9106 are differential signals, and have to be converted to single-ended signals prior to sending into the mixer stage. Typically, this would be achieved using a balun. However, off-the-shelf (OTS) balun solutions do not cover many of the lower-end frequencies, especially towards DC. Also, the return loss incurred with OTS balun solutions would necessitate an amplifier stage which would increase power consumption of the chain while also increasing necessary board footprint. To counteract this issue, a custom circuit is utilized that maintains unity gain while also covering more frequencies, especially on the low end towards DC. The circuit and simulated performance are shown below.
Figure 2: Balun Alternative Circuit
Figure 3: Frequency Response of Balun Alternative Circuit
The same low pass filter in the Rx chain is used here in the Tx chain to filter out all noise or excess signal above our max desired IF frequency, which is 105MHz. This filter has a 1dB insertion loss from DC-105 MHz and a 25-55 dB insertion loss between 1-5 GHz. For information about why a bandpass filter was not selected, refer to the part description in Receiver Front End Overview & Theory Of Operation.
The HMC1056 is an image reject mixer/single sideband upconverter that utilizes two double balanced mixer cells and an integrated 90 degree hybrid that converts the I/Q signal into a single-ended RF output signal. An external local oscillator (LO) signal ranging between 9.1-10.1 GHz is mixed with the IF inputs ranging between 75-105MHz to produce an RF output between 9-10 GHz. The integrated 90 degree hybrid allows for power and cost savings along with less necessary board footprint.
Due to the conversion losses of 8-11 dB incurred in the mixer stage, an amplifier is needed to increase the gain of the signal prior to sending it off to the antenna. The HMC564LC4 is ideal for this platform due to its 17 dB gain while still boasting a 1.8 dB noise figure and 25 dBm output IP3 across the 7-14 GHz operating frequency range. Additionally, this part has a low power consumption, low board footprint, and is input/output impedance matched at 50 Ohms.