Author: Ryan

Termination resistors?

Since the signals going in and out of the PDM microphones are digital, termination  resistors may be needed to reduce ringing and overshoot when the signal is being transmitted over a long distance. TDK’s recommendation is as follows:

When long wires are used to connect the codec to the ICS‐41350, a source termination resistor can be used on the clock output of the
codec instead of a buffer to minimize signal overshoot or ringing. Match the value of this resistor to the characteristic impedance of
the CLK trace on the PCB. Depending on the drive capability of the codec clock output, a buffer may still be needed, as shown in Figure
13.

From: https://43zrtwysvxb2gf29r5o0athu-wpengine.netdna-ssl.com/wp-content/uploads/2016/02/DS-000047-ICS-41350-v1.1.pdf

 

Let’s do some quick calculations if termination resistors are required at these signal speeds. Here’s a paper that discusses when it’s necessary to put termination resistors: https://www.analog.com/media/en/training-seminars/tutorials/MT-097.pdf

The maximum PCB track length is then calculated by multiplying tr by 2 inch/nanosecond. For example, a maximum frequency of 100 MHz corresponds to a risetime of 3.5 ns, so a 7-inch or more track carrying this signal should be treated as a transmission line.

We’re driving the input clock to the PDM microphones at 3.0MHz. At 3.0MHz, the rise time is about 115ns. The dimensions of the microphone array are 1x1ft max, so the maximum FFC cable length is 1ft. 115ns * 2 in/ns is 230in, so we should be at least an order of magnitude safe without termination resistors.

• We haven’t heard back from TDK or Knowles regarding microphone samples, so I’ll follow up next week. Even if we don’t get the microphones for free, we still do have enough budget for them. ($150 for 100 mics)
• We will have our design review on Monday, Feb 24, so we’ll be preparing for the presentation.
• We are on schedule so far.
• To do: Solder wires to the microphones, power it with 3.3V and feed it a 3MHz clock. Capture the output waveform from the mic and develop a script to analyze the waveform. Low pass filtering it should yield the analog waveform.

This week, we’ve evaluated candidates for PDM microphones, which take in a 3MHz clock and output a pulse density modulated digital signal, as shown below: 

Source: https://en.wikipedia.org/wiki/Pulse-density_modulation

The reason why we’re leaning towards PDM micrphones as opposed to analog microphones is because since PDM microphones output a digital signal, we don’t need preamplifiers and ADCs for the analog microphones, and they can be directly connected to an FPGA. The price difference between analog and digital microphones are negligible.  When we’re looking at about 100 microphones, reducing the number of parts greatly reduces cost and complexity.

The two main manufacturers of PDM microphones are TDK and Knowles. I’ve reached out two both companies to see if they’re willing to help us get hold of their PDM microphones. So far, their responses have been positive and willing to work with us.

The main spec that we’re looking in the PDM mics is frequency response. Because we’re interested in 2kHz and beyond, having a relatively flat frequency response is critical to our application. The frequency response information can be found in their respective datasheets. Here’s an example from TDK’s ICS-52000:

Source: http://43zrtwysvxb2gf29r5o0athu-wpengine.netdna-ssl.com/wp-content/uploads/2016/05/DS-000121-ICS-52000-v1.3.pdf

Notice how after 5kHz, the amplitude starts rising and peaks at 12kHz at 18dB. It drops precipitously after that. This is an example with a poor frequency response. This may be good enough for cellphones where frequencies above 8kHz or so are not transmitted, but isn’t ideal for our application.

Here’s the SPH0641LU4H-1 from Knowles. It has a slightly more well behaved frequency response at higher frequencies:

We’ve ordered a sample of of microphones to further investigate their characteristics and identify areas of focus regarding these microphones.

We are on schedule.

To do next week:

Continue communication with Patrick from TDK regarding PDM mics from them as well as logistics. (QTY and shipping to us or the PCB fab)

Receive PDM microphone samples and solder and interface them with MCUs or FPGAs for testing.

Start designing the PCB for microphones. Draft a list of auxiliary components for the mics. (Termination resistors, decoupling caps, LDOs)