LTC5599: why are these baseband inputs passed through 49.9 ohms and the capacitor + SPI

Question

I'm designing a radar transceiver board using the LTC5599. In the datasheet for the part, there is a schematic that shows the circuit configuration for the evaluation board. Here is an image of the layout given.

Here is a link to the datasheet:

https://www.analog.com/media/en/technical-documentation/data-sheets/5599f.pdf

Two questions:

1. For the SPI lines, why are there 1k resistors and 2.2pF capacitors? My best guess is that it is to reduce emissions, since the SPI bus can operate at a maximum 20 MHz and is likely to leak interfering signals. Is this correct? If so, should all SPI interfaces have the resistors and capacitors attached to them in a multidrop configuration?

2. Why are there 49.9 ohm resistors and a 100 nF capacitor to ground on the BBPI/BBPQ differential pair? For context, this pair of wires is the differential baseband input to the chip. The quoted baseband bandwidth for the chip is around 28 MHz. Are these resistors there for impedance matching purposes? And if I only plan on using baseband signals that are a few kilohertz in bandwidth, can I potentially ignore this part of the circuit?

Any help would be much appreciated.

Answer 1

For the SPI lines, why are there 1k resistors and 2.2pF capacitors? My best guess is that it is to reduce emissions

Your RF output frequency is going to be around 1 GHz and that can easily couple onto lines used to control the chip (such as SPI) and so it's quite likely that the presence of the 1 kΩ resistors and 2.2 pF capacitors is to prevent this happening to a large extent.

Why are there 49.9 ohm resistors and a 100 nF capacitor to ground on the BBPI/BBPQ differential pair?

I expect that these values can be increased but remember one important statement in the data sheet that relates to all the specifications that it contains: -

Test circuit is shown in Figure 13.

So, if you drift away from the recommended test circuit you jeopardize the specified performance. The price to pay for the inclusion of these components with the specified value is uncertainty. I would need to have a really good reason not to use the values specified and then I'd want to check with the supplier (AD/LT) the validity of any changes.