3 Super-het (back) to the Rescue

Bringing back the super-heterodyne architecture resolves many of these problems. This is not your grandfather’s super-het architecture! Using the AFE7950-SP RF sampling transceiver as the first stage is a game-changer, specifically because the device supports direct RF sampling into X-band (up to 12 GHz) and is rated for space applications. With a K-band application operating at 28 GHz, a high IF frequency around 8 GHz is ideal. There are two advantages with a high IF frequency. The main one is the image separation. The image and LO bleed-through are very far away. It is very easy to filter these signals off. A similar approach with an IF below 6 GHz would yield a much tighter grouping of the LO and image and make filtering more difficult or impossible.

The second benefit is the reduction of the LO frequency. Finding good high frequency synthesizers is a challenge. While a direct conversion approach needs the LO at the center of the desired band, the super-het approach drops the frequency requirement by the IF frequency amount. Using an 8 GHz IF, the LO needed to hit 28 GHz Ka-band is 20 GHz. That is still fairly high, but is within the capabilities of the LMX2820 frequency synthesizer solution and only uses a two-times multiplier. For space qualified operation, the LMX2615-SP supports up to 15 GHz to cover the Ku-band. Figure 3-1 shows the spectrum of the super-het architecture with an IF of 8 GHz.

Using the RF sampling architecture of the AFE7950 in the first stage offers some additional benefits. The AFE7950 supports very wide bandwidth signals, up to 2.4 GHz bandwidth on the transmitter and up to 1.2 GHz on the receiver. The device integrates four transmitters and six receivers so it is ideal to support multi-channel applications and large antenna arrays. Integrated numerically controlled oscillators (NCOs) facilitate easy IF adjustments for frequency hopping applications. Further, the AFE7950 integrates a high frequency PLL/VCO for generating its high frequency sample clock; no need to route this signal through the board.