Question of the Month Answers, June 2006

Charles Werner from Gamma Remote Sensing AG has submitted this month's question:

1. How good are direct digital synthesizers at generating FMCW signals? I am looking at a system that uses a frequency translation loop Analog Devices AD9858 to get output in the range of 2.1 to 2.3 GHz for an FMCW radar.

2. What is the best way to get this signal heterodyned to the range of 4.1 to 4.3 GHz and what configuration of mixer/LO is recommended/ required?

From: Earl McCune, Panasonic

Dear Charles,
Direct digital synthesizers (DDS) are excellent at generating extremely linear frequency ramps. The largest error source in the output frequency ramp linearity comes from any variation in group delay through the DAC output low pass filter (LPF). My recommendation is to keep the maximum DDS output frequency below 15 percent of its clock frequency (fc), and then to use a wider LPF (around 25 to 30 percent of fc) to keep the group delay fairly flat across the signal bandwidth. Depending on the FM sweep rate you are using, you may find the design problem much easier to multiply from the DDS output to your final output frequencies using a PLL. A PLL will be needed anyway to synthesize the additional upconversion LO. If you choose to heterodyne the output, you must keep in mind that the sense of the sweep will invert if you use a high side LO (6.4 GHz). In general, by using high side injection it will be much easier to filter out the heterodyne spurious products.

From: Kaixue Ma, MEDs Technologies Pte. Ltd.

Dear Charles,
It can only be used up to 2.15 GHz. If you need 2.1 to 2.3 GHz, you need to add an external mixer, which can modulate the 2±0.15 GHz signal up to your required 2.1 to 2.3 GHz. This 150 MHz signal can come from the AD9858.

There are two ways to get this frequency: one is to use the frequency doubler in the output of the VCO. In my opinion, the best way is to use the sub-harmonic mixer connected to the VCO output to up-convert to 4.1 to 4.3 GHz.

From: Mujfarmiya Shaikh, Cranes Software International Ltd.

Dear Charles,
1. DDS are preferred for generating FMCW signal by radar designers due to its precision frequency tuning, phase offset control and linear "chirp" capability. AD9858 can be used with an external VCO and loop filter to generate a 2.1 to 2.3 GHz signal. 2. The best way to heterodyne the signal in the range 4.1 to 4.3 GHz is to use LO of 2 GHz, which beat with 2.1 to 2.3 GHz and produce 4.1 to 4.3 GHz without spectrum inversion. LO of 6.4 GHz also can be used, but it will give spectral inversion. Choose a mixer whose 1 dB compression point is 5 to 10 dB lower then the LO output (input to mixer at LO port). Mixers are available from Hittite, HP avantek, Maxim, Magnum microwave and many more manufacturers. ZComm VCOs offer good phase noise, which can be used in this design.