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Topic: Why the Sparkfun Si4735 radio shield is all wrong (Read 28125 times) previous topic - next topic


Well Alex, the subject of antenna matching is complex. Since your timetable is so short, perhaps its best for you to put just the required blocking capacitor and mandatory protection clipping diodes at the chip to a pin right there with a valid RF ground directly as a pair of 0.1 header pins near the IC for various antenna matching modules. Also, in addition I would recommend you provide a few pins to this proposed RF input sub board for later versions to include power and power ground along with a pair of pins that are SPI or I2C lines desired to to talk to this front end, so perhaps we can build on this and provide switching to multiple antennas or tuning a band pass filter along with the station selected using this piggy back antenna interface module. Skip holes for .3 pins before continuing along on the header for these utility pins, to give the needed space for the isolated two RF input pins providing the ant in and RF ground in addition to power ground on utility pins mentioned above.
The chip's line output is not a good headphone driver by any means. This subject could take many pages alone, but AC coupling a 32 ohm headphone with only 2.2 uF will roll off the low frequencies drastically. I think you mean 2.2 microfards, which is written as 2.2 µF, or sometimes 2.2 uF, because a 2.2 mF (millifard) capacitor might well be the size of a beer keg.
In order to have good bass response, you need a minimum of 470 uF which gives a -3 dB frequency of 10 Hz into 32 ohms; one octave below 20 Hz. Be aware most electrolydic capacitors do not sound good. Avoid them unless they are bipolar and made specifically for audio and use at least 1 uF of good film cap across them to help with high frequency distortion inherent in electolydics.
It is *much* preferred to use a bipolar power supply and DC couple headphones using a chip designed to drive this low Z- I'll post the part number for you- pinout is standard 8 pin opamp- use standard dual opamp dip 8 part in Eagle to layout. Please do not drive headphones with the Si chip directly. Also this allows you to put in a high impedance volume control in front of the headphone driver chip, either a regular log pot or digital volume control IC as the pot.
Also there are some details of headphone interface that will take too much space right here- no the ferite is unneeded, but an output isolation choke swamped with a resistor is nice for driving headphones as it isolates the FB loop from the low Z load at ultrasonic frequencies, but that's a whole subject alone. I use Eagle and will help you detail this. But my time is limited this next few weeks.You can never have enough time or RF ground vias...


Thank you for your response BillSacks.

I want to keep this board simple and cost efficient so I can sell it cheaper, but at a same time and don't want people to have any problems with it. I do have a question about ac coupling capacitors on analog audio output, I do know if I use low value it will filter low frequency's. Also I read all documentation on Si4737 and to get as familiar with this chip as possible. Question is why they recommend to install 0.39 uf capacitors on analog audio output instead of higher values? It's here on page 15:

I used 2.2 uf ceramic capacitors to be as small as possible and to filter out less of low frequency's, also because most of headphone amps (for example Texas Instruments) recommend to use 2.2 uf as ac coupling on their products.

Also why silicon labs recommends to put 0 ohm resistor (link) before FM antenna?

I do already have completed schematic, but still improving things. Maybe I will post it here so you can maybe suggest improvements or find mistakes if any. I do take my time to carefully check my schematics for mistakes.


Hi Alex; The Si app note is showing it as meant to be connected to a higher impedance than your headphones (which are defined as 32 ohms standard now). In order to get the same 10 Hz -3 dB point, the impedance of the (inferred) input it drives must be over 40 k ohms; you need to drive 32 ohms, and therefor, the capacitor required is 470 uF. There must be a volume control, or users can be harmed by it being too loud. The output of that chip is not likely designed to drive 32 ohm loads, and that should be evident by the 0,39 uF output capacitors which indicate a load of > 40 k being driven.
I'll help you get a headphone amp on this board if you wish, or show you how to produce a line level output for driving an outboard powered speaker or headphone amp. Include just the schematic of a proper headphone amp if the board size will not allow it.
Never ever use a ceramic capacitor to couple audio by the way, they introduce distortion and just plain sound really bad. Use films for up to 1 uF or so and bipolar elecrolydics specifically designed for audio coupling for higher values. Use Nichicon ES Audio rated bipolars and you cant go wrong- http://www.mouser.com/ds/2/293/e-es-3258.pdf
I strongly recommend a volume control into a proper headphone amplifier such as an SSM 2135 (best choice for you).
The zero ohm is just a jumper to allow flexibility. If you just put the pins as I suggested, no jumper is needed. No app note with a short antenna will apply unless you can *tune* the values with RF gear. If the user just puts a one meter wire to the input at the pins, the FM performance will be better than any small ant on the board- they are useless for this frequency; that's why there are not FM radios inside cell phones. FM needs a longer antenna than can be put on a little PCB
AM is another story, with an air core tuned loop being the best option, so leave that totally open with just a connection and allow people to make proper antenna or 75 ohm cable input interfaces to it.


Hello, I just want to post an update on the radio breakout board.

Board will consist of two half's, first is a main radio board, and second half is a customizable sub-board.

Main board is set to I2C mode and got a jumper for setting one of two I2C addresses, also it got built in logic level shifter for SDA, SCL, Reset, and Interrupt capable of speeds up to 400kHz selectable by jumper for 5v and 3.3v operation.Combination of 32.768 kHz crystal and inverting logic making a single pin clock generator to provide clock for CLK pin, making clock more stable and digital audio output usable. Voltage regulator to provide clean 3.3v power to the logic as well as sub-board.

Sub-board is like a plug-in shield for main board that is fully customizable for your needs, it got antenna inputs, optional FM trace, optional digital audio output, optional analog outputs, and optional headphone amp.
Headphone amp got Panasonic low ESR film decoupling capacitors on input, preset gain (volume control is inside Si4737 for I2C control), and MAXIM Integrated capacitor-less audio output architecture to keep board small size.

I am delaying for a week (in 3 weeks) release date because I am also designing other small boards for Arduino and want to put them all on the same panel.

Board will be compatible with Si4735 Arduino library with following simple modification to it:
1. Use external clock source
2. Set to I2C mode
3. Interrupt pin is inverted (change interrupt trigger state)

Let me know what you people think about this Si4737 Radio Breakout (more like a shield).


Oct 08, 2013, 05:36 am Last Edit: Oct 08, 2013, 05:56 am by Alex_K Reason: 1
Sorry for not updating status on Si4737 Radio Module, I was really busy with other things. Board was made and works flawless. Didn't had time to write code for it, so I modified some things in sparkfun SI4735 library to make it compatible, it does need some work to make easy use of weather band. Board have all the features described in previous post. Beats hell out of sparkfun version. Here's some pictures of the final product:


MCU Radio Modules available for sale.  if anyone needs it for their projects, this is great working design. Can be used with Sparkfun's library with small modifications or without any library,  I can provide sample sketches and plenty of help. Boards already been bought and tested by other people thru this forum. It's $30 via paypal or $35 thru ebay, standard shipping in USA is $3. 
Send me private message and I will respond. All boards made and shipped from USA Michigan.
Thank you.


I can't find it on Ebay.
It is okay if you post a (commercial) link to your Ebay item, after puttting so many work in it.


Hi, Krodal. Sorry but I run out of them already. I need order some parts to assemble more of them. You are 100% right about the link to the item, and I will post it, as soon as I put some more on ebay.


Jan 23, 2014, 08:22 pm Last Edit: Jan 23, 2014, 08:29 pm by ZBDunham Reason: 1
Hello all!
We've made a bit of progress on our radio project I posted about last year and currently have our hardware and software files for our second revision on github here -> https://github.com/The-Public-Radio, for anyone who's interested. Occasional updates are also being made here -> http://yourpublicradio.tumblr.com/ where you can learn more about what we're building.  We're still trying to improve our FM reception though, and would appreciate any feedback anyone might have with regards to the board layout and antenna design.


Alex are you still making the boards either populated or not?  Would love to get one.


If the supply voltage for the Si4735 is too low, it could be provided by Arduino D8 + diode + diode + a transistor like BC547 with basis to the 2nd diode, collector to +5V and emitter to Si4735 p21 (VDD).
That should provide ~3.2 V with sufficient current.

Alternatively, on the 4050, p14 (6A) could be connected to p3 (1A), and p15 (6Y) could be connected to p2 (1Y). The supply for the Si4735 would then be shared over 2 4050 output ports in parallel, each providing half of the current.

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