From a H/W point of view all you need to do to get started is to connect your RFzero, via the USB port, to a USB port on your computer. Of course a GPS antenna will also be a good idea, but a basic start doesn’t require it.

Later you can populate your RFzero board both when it comes to the way the LEDs are mounted, which headers are mounted, GPS back-up, RF shielding, two-tone or I/Q-output and adding an output filter etc.


If you later on decide to use a shield it is a good idea to mount the headers carefully and standing straight up. Otherwise you may find it difficult to mount a shield later on.


You can choose to mount the 3 mm LEDs directly on the RFzero board straight up or bent so they fit through holes in a front plate. You may even mount a 2 pin header per LED, and run wires to the front plate further away.

All the LEDs have to be connected/mounted correctly. On the RFzero the positive side of the LEDs (the anode is the long terminal) have to be mounted to the left on the board when the USB connector faces you. There is also a small plus sign “+” printed on the board. If you mount the LEDs the wrong way they will not light up. So please remember this phrase: Long Leg Left.

You may verify if your intended way to solder the LED in place is correct by putting the LED terminals through the holes, but without soldering them, and connect power to the RFzero. Then gently, using a finger, apply force to the LED terminals to make sure that there are physical contacts between the terminals and the holes in the PCB. If the LED is lit then you have connected the LED in the right way.

Examples of mounting the LEDs where the PPS LED is connected through a pair of wires to a two pin header instead of directly on the board.

If you are in doubt, after soldering the LEDs to the board, you can easily verify if you did it in the right way. If you look closely at the LEDs you will be able to see if the big part of the internal of the LED, the cathode, is to the right. You can see this clearly in the green LED in the picture above.

LCD connections

The RFzero can use both 3,3 V and 5 V liquid crystal displays (LCD), but, if using a 5 V LCD the logic communication has to work on 3,3 V level. Most modern 5 V LCDs are able to do this but if you have an old 5 V LCD it may not work. If you are to buy a new LCD please ask the seller specifically for a 3,3 V LCD.

RFzero, and Arduino in general, supports LCDs that are compatible with the Hitachi HD44780 specification. You can use other standards but in this case you may have to find a third party display library or write it yourself.

The RFzero supports other displays than the HD44780 in parallel mode. Please see the displays page for more information.

LCD voltage jumper

Before you connect any LCD to the LCD header please ensure you have set the correct drive voltage for the LCD on the LCD voltage select header – JP13. Alternatively, if you know that you will always be using one of the voltages you could short the relevant jumper position with a small piece of wire that is soldered in place.

The LCD voltage header JP13.

The LCD voltage set to use VI

Please note that if you run the LCD on the VI level the contrast and backlight will change if the VI is changed.

The LCD voltage set to use 3,3 V (3V3).

LCD header and connections

The RFzero LCD header JP12 is prepared for LCDs that comply with the Hitachi HD44780 specification. Using two cables with six wires in each you can easily connect the LCD to the RFzero board. One cable should go to the left side of JP12 (GND, V LCD, CON, RS, R/W and ENA) and the other cable to the right side of JP12 (DB4, DB5, DB6, DB7, Anode and Cathode).

The LCD header JP12.

On the backside of the LCD connect the two cables to “each end” of the 16 pads/pin header leaving the four in the middle unused.

Two six wire cables connected to a LCD pins 1-6 and 11-16.

The RFzero LCD header is designed to run the LCD in four bits mode, i.e. LCD data on DB7 to DB4. If you need to run your LCD in eight bits mode you will have to take the remainder four bits from some of the other pins available on the RFzero and connect them to the DB3 to DB0 pins on the LCD.

LCD contrast and backlight

The contrast of the LCD can be controlled on the R15 trimmer. Please note that if you run the LCD on the VI level the contrast will change if VI is changed.

If you think that the backlight is not strong enough for the place where you will use your RFzero you can add a blob of solder to the SJ1 solder jumper. Doing so shorts R16 so the backlight resistor goes from 20 Ω to 10 Ω allowing more current to the LCD.

The R15 LCD contrast trimmer and SJ1 solder jumper.

GPS back-up

You have the possibility to connect a back-up battery or supercapacitor to the RFzero (does not apply to PCB v1.0). If you don’t want to back-up the GPS almanac it is a good idea to short JP7-1 (+/VB+) and JP7-2 (VB) using the supplied header and jumper.

Having the GPS backed-up may result in slightly faster satellite acquisition, thus valid GPS data. However, if the GPS almanac is more than two weeks old the back-up has no practical relevance.

The JP7 header and back-up connections.

The RFzero has built-in components for charging an external supercapacitor. According to the datasheet the typically back-up current, I_BCKP, is 15 μA. Thus, a 1 F supercapacitor with an ESR of 100 mΩ should provide about 15 hours of back-up time.

The below table shows how to connect the jumpers vs. back-up type.

Back-up typeJP7-1
No back-upShort with JP7-2Short with JP7-1Not connectedNot connected
Supercap. (3,0 V to 3,3 V)Short with JP7-2Short with JP7-1Positive terminalNegative terminal
Battery (3,0 V to 3,3 V)Not connectedNot connectedPositive terminalNegative terminal

The GPS data out (GDO) and GPS PPS out (GPO) connections on JP7 are not used for GPS back-up purposes.

Example of a battery holder for two AA batteries, Ø14 mm x 50 mm.

Ground loop

The ground loop, JP15, to the right of the RFzero is a good signal ground and makes it very easy to attach an alligator clip to if you want to measure something on the RFzero.

The ground loop wire soldered in place.

To mount the wire please strip the wire supplied and bend it using a pair of pliers. Then cut the two legs to the same length; 12 mm to 15 mm. Since the wire is rather thick and it is to be soldered to the ground a fair amount of heat from the soldering iron is required. The easiest way to solder the wire is to start from the top side. Then you can align it properly before soldering it from the bottom side too.

Air and temperature shielding

The short term stability of the Si5351A 27 MHz clock is good enough for most applications. However, if your RFzero is subject to rapid temperature changes it may be beneficial to shield the clock, e.g. using a piece of NON-CONDUCTING foam. Doing so can improve the stability typically by a factor of eight.

The non-conducting foam can e.g. be glued to the PCB or attached to a couple of header pins soldered to the ground track surrounding the RF section.

Example of headers soldered to the ground track before mounting the foam cover.

Example of the finished work after mounting the foam cover.

If you also put the RFzero in a box you will have an even better frequency stability.

Steve, W4NSF, has made a very professional RF and temperature shielding for his RFzero.

The RF and temperature cover over the RF section.

Inside view of the cover.

RF shielding

If your RFzero is to be used in a harsh RF environment shielding the RF part of the board or perhaps the entire RFzero may be relevant.

The PCB fits into a standard metal sheet box (Weissblechgehäuse) 102 mm x 82 mm x 30 mm or taller. An additional L-shaped sheet of metal can also be fitted inside the metal sheet box to shield the RF section completely from the PSU, GPS and digital circuit.

Metal sheet box (Weissblechgehäuse).


The TP12, TP13 and TP14  test points on the bottom side can be used to access the u-blox NEO-7M directly via an USB port, e.g. for synchronizing a PC to the GPS. By default this optional GPS access is disabled. To enable it simply short the SJ2 solder jumper on the bottom side with a solder blob.

The bottom side of the PCB showing the optional GPS solder points where G = Ground, M = DM, P = DP and the SJ2 solder jumper pads to the right of the P test point.

Output filter

You can design an output filter, e.g. using the free Elsie by Tonne Software Jim, W4ENE, using the Z1 to Z10 pads (SMD 0805) on the PCB. This way you can tailor a filter, high or low pass, to matches your specific requirement.

The Z1 to Z10 pads for a custom on-board filter.

The value and type of each Z# depends on the design criteria, e.g. for a low pass filter Z1, Z3, Z5 and Z7 are capacitors and Z8, Z9 and Z10 are inductors and if Z2, Z4 and Z6 are used they are capacitors. Not all Z# have to be used. If so they can be omitted or shorted whichever applies.

Below are series of possible low pass filter designs where the second harmonic is attenuated at least 10 dB and the third harmonic is attenuated at least 55 dB by the low pass filter itself. When used in combination with an RFzero the harmonics will then be attenuated at least 60 dBc but often more. If you go below fmin in the table below the attenuation values no longer apply.

400 kHz550 kHz3,3 nF300 pF8,2 nF1,2 nF8,2 nF1 nF2,7 nF18 µH18 µH15 µH
1,6 MHz3 MHz1 nF120 pF1,5 nF560 pF1,5 nF470 pF820 pF3,9 µH2,7 µH2,7 µH
2,9 MHz5,1 MHz680 pF68 pF1 pF360 pF820 pF220 pF470 pF1,8 µH1,5 µH1,5 µH
5 MHz9,3 MHz390 pF56 pF560 pF220 pF470 pF150 pF330 pF1 µH820 nH820 nH
9,1 MHz16,4 MHz220 pF39 pF270 pF120 pF220 pF82 pF120 pF560 nH470 nH470 nH
16 MHz30 MHz100 pF12 pF180 pF56 pF150 pF39 pF56 pF330 nH270 nH270 nH
30 MHz56 MHz47 pF2,2 pF100 pF12 pF100 pF8,2 pF47 pF180 nH180 nH180 nH
54 MHz72 MHz33 pF2,2 pF68 pF15 pF82 pF10 pF47 pF150 nH120 nH120 nH
100 MHz150 MHz15 pF1 pF33 pF3,9 pF33 pF3,3 pF12 pF68 nH68 nH68 nH

Attenuation at Fmax is typically 1 dB. The above values take into account the additional capacitance in the filter PCB.

Filter characteristics of a 52 MHz low pass filter from 1 MHz to 400 MHz. The insertion loss is 0,7 dB at 52 MHz. Red is transfer function (10 dB/div), blue is input return loss (10 dB/div) and green is input SWR (At 1 MHz it is 1:1).

Below is a possible high pass filter design that can be used to extract third and higher harmonic above 400 MHz, e.g. to get a 432 MHz signal.

400 MHz6,8 nH130 nH6,8 nH27 nH6,8 nH39 nH8,2 nH10 pF12 pF12 pF

T1 combiner

The RFzero is factory mounted with a Coilcraft PWB-2-BLB transformer (T1). But if the primary use of your RFzero is for making two-tone signals we recommend replacing the standard T1 transformer with a combiner like the Mini-Circuits ADP-2-1W+ or similar.

Mini-Circuits ADP-2-1W+ combiner.

Example of mounting the ADP-2-1W+ combiner. Please note that the combiner has pin 1 at the top left in the picture (black dot) and thhe two pins in the middle on each side are not soldered to the PCB.

Replacing the default transformer with a combiner greatly improves the two-tone performance of the RFzero with up to 40 dB.

10 MHz two-tone signals 1 kHz apart using an ADP-2-1W combiner.

Dual RF output

If you want to use your RFzero with two outputs, e.g. a  VFO with I/Q-outputs, you will have to remove T1. Please be aware that removing T1 affects spectrum performance most noticeably the second harmonic, but also the spurious level.

You can mount two U.FL PCB connectors on the free pads next to T1. Never mount the U.FL connectors and the T1 transformer at the same time because the two U.FL outputs can see each other RF-wise. You may have the U.FLs mounted if the standard T1 is replaced with a combiner.

Two U.FL connectors mounted on the RFzero board and T1 removed.

U.FL connectors seen from the bottom and top.

Instead of using U.FL connectors an alternative way is to run a short piece of semi-rigid cable from the right side pad of the missing T1 to a connector at the edge of the RFzero PCB. Also short the left side of the T1 pads, i.e. effectively connecting C40 to the left side of Z1.

Alternative dual output using a piece of semi-rigid cable and SMA socket. Please don’t pay attention to the component numbers, since it is of an RFzero prototype PCB with different component numbers.

Very low frequency output

If your primary frequencies of interest are in the low range of the spectrum, say below 100 kHz, you may remove T1 since the default transformer is for high MF to VHF use. When T1 has been removed please short the left side of its pads, i.e. effectively connecting C40 to the left side of Z1. Alternatively you can mount two U.FL sockets on the CON4 and CON5 pads. In either case this will increase the signal level to 3,3 V peak-to-peak, and also increase the second harmonic.

Location of C40, T1 and Z1 on the PCB.

For the lowest range of the Si5351A frequency spectrum, 2 kHz to 50 kHz, you may increase the value of C40 and C41 or even remove them. The nominal value of C40 and C41 is 10 nF which is not a lot of capacitance below 50 kHz. Thus the transfer is limited.