Modifications

You can modify your own RFzero board both when it comes to the way the LEDs are mounted, which headers are mounted, the RF shielding, I/Q-output connectors and adding an output filter.

LEDs

You can choose to mount the 3 mm LEDs directly on the RFzero board straight up or bent so they fit through a front plate. You may even mount a 2 pin header per LED and run wires to a 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 them the wrong way the LEDs will not light up. So please remember this phrase: Long Leg Left.

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

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.

Jumpers/headers

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.

Air and temperature shielding

The short term stability of the Si5351A 27 MHz crystal oscillator is good enough for most applications. However, if your RFzero is subject to rapid temperature changes it may be beneficial to shield the crystal oscillator e.g. using a piece of NON-CONDUCTING foam.

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.

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.

GPS USB

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.

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 equivalent.

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 that 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 a ADP-2-1W combiner.

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 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 a 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.

fminfmaxZ1Z2Z3Z4Z5Z6Z7Z8Z9Z10
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.

fminZ1Z2Z3Z4Z5Z6Z7Z8Z9Z10
400 MHz6,8 nH130 nH6,8 nH27 nH6,8 nH39 nH8,2 nH10 pF12 pF12 pF

I/Q-output

If you want to use your RFzero as a VFO with I/Q-outputs you will have to remove T1 and 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.

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 but also increase the second harmonic.

Location of C40, T1 and Z1 on the PCB.