RF Dongle: Schematic Update

I made some changes to the schematic, and wanted to fill you all in.

Balun

Above is the application circuit for the RF transceiver, NRF24L01P. The chip transmits and receives RF on pins ANT1 and ANT2. This chip has a balanced output, and is designed to use a balanced antenna:

I find it helpful to think of it like a differential signal in electronics. If a signal is single ended, then it is referenced to ground; if it is differential, it has positive and negative halves, which are equal and opposite. Similarly, the NRF24L01P outputs equally and oppositely to the two legs of the dipole antenna. However, I don’t want to use a dipole antenna; I’d rather use the rubber ducky antenna found on walkie-talkies, since they’re more compact and work equally well in all directions. This presents a problem: the NRF24L01P wants a balanced antenna, but the antenna I want to use is unbalanced. I’ll need some sort of interface or translator.

Fortunately for us, the application circuit shown above does exactly that. L1, L2, L3, C5 and C6 form a matching network, turning the balanced ANT1 and ANT2 signals into a ground referenced signal, which is compatible with a rubber ducky antenna. This was the approach I showed in my original schematic, but I have changed to a more compact and higher performance solution.

A balun, a combination of BALunced and UNbalanced, transforms a balanced signal into an unbalanced one. It can also be very, very compact; the one I’m using (2450BM14A0002T) is just 0.8mm x 1.6 mm! With this, the schematic becomes much simpler:

The balun, on top of changing the signal, provides filtering:

The parameter we’re interested in here is IL_dB, the blue trace. IL stands for insertion loss, which indicates how much of the signal is transmitted or reflected. Think of it like shining light onto a piece of glass; some of it goes through, some of it is lost as heat, and some of it is reflected.

In the graph above, we’re interested in the frequencies around 2.5 GHz, which is the frequency that the NRF24L01P operates at. Let’s look at the blue trace:

• 2~3 GHz: insertion loss is almost 0 dB. This means that most of the signal goes straight through the balun with very little attenuation.
• > 3 GHz: insertion loss falls rapidly. This means that most of the signal doesn’t make it through the balun

In other words, the balun acts as a low pass filter, which is very helpful. Now, instead of accidentally transmitting high frequency signals, we’ll only transmit the signals we want. The balun is also bidirectional, which means signals going through the balun to get to the NRF24L01P will also be cleaned up, which will hopefully improve communication.

RF LPF

I made a very similar change on the low pass filter that is attached to the PA/LNA. Previously, I used two pi filters to clean up the RF signals, coming and going to the antenna. Now, I’m using a single component to perform that task. This is not a balun; the signal comes and goes as unbalanced. But, like the balun, it only lets certain frequencies through:

S11 and S22 shows how much reflection occurs, while S21 shows how much of the signal goes through the LPF. S21, the blue trace, is at 0 dB for frequencies below 3 GHz, which means signals go through the LPF mostly unchanged. However, after 3 GHz, S21 rapidly drops off, which means that the signal no longer makes it through the filter. S11 and S22, meanwhile, increase rapidly. This means that any signal coming into the filter will see a brick wall and just bounce back, like shining a light on a mirror. The LPF will, therefore, only allow our RF dongle to receive and transmit signals in the frequencies we’re interested in!

USB Shield

Apparently, how GND and the USB shield are connected is a controversial topic. Some recommend connecting the two directly, some suggest no connection, and some say connect the two through a capacitor or resistor. Yet another school of though says to connect the two through a capacitor and resistor in parallel. I decided to go with the one that makes most sense to me, which is the last one.

• In theory, the shield and GND are already connected together on the host side (if I plug my dongle into a computer or laptop, then the computer or laptop is the host). This isn’t always the case, though, due to non USB standard compliant designs. To make sure that GND and shield don’t develop a DC difference in voltage, R16 was added to connect the two. No significant amount of current can flow through such a large resistor, but the resistor will gradually bring the GND and shield to the same voltage if a difference exists.
• The capacitor is for EMI purposes. Think of a dipole antenna and how it functions. The dipole antenna transmits a signal by creating a time varying voltage across its two legs, which allows it to propagate electromagnetic signals. It’s possible to make a dipole antenna accidentally; all you need is two large pieces of metal or copper. GND tends to be very large, almost always the largest piece of copper on the PCB, and shielding or chassis can be very large as well. If a time varying voltage appears across GND and shield, then the two pieces of metal essentially form a dipole antenna, emitting electromagnetic radiation, which can be disruptive to other electronics. C27 prevents this by having very low impedance at high frequencies; to an RF signal, GND and shield are shorted through C27.

I’m almost done with the layout, so I’ll present that next time.