By Per Magnusson, SA5BYZ

This is the second part in the article series about the FoxScope 2m ARDF receiver.

The RF board contains the RF-attenuator/amplifier, local oscillator, mixer, IF-filter and the IF amplifier. It has an SMA connector for connecting to the antenna and interfaces with the processor board via a 32-pin, 0.5 mm pitch connector on the back side of the board. Power and control signals are supplied to the RF-board via this connector while the analog IF signals go in the other direction.

The following files are available for download in a zip archive:

• Schematics PDF
• Assembly PDF
• Layout PDF
• Layout as gerbers
• Pick-and-place file
• Normal BOM (Bill of materials)
• BOM adapted for JLCPCB
• 3D-model (3mf file) for the shield former

Link to a zip-file with the files:

Low-noise Amplifier

The schematics of the LNA is shown below (click to enlarge, or view it in the PDF).

D1 and D2 provide ESD protection, which is probably quite necessary as the input is connected to an antenna that can be touched or otherwise get in touch with charged objects. The parasitic capacitance of the protection is reduced by having two diodes in series in each direction.

L8 and C19-C22 form a pre-selecting low-pass filter. The antenna will be quite effective at picking up signals at some higher frequencies than the desired ones and unless they are suppressed, they may get mixed into the IF band by harmonics of the LO.

D3 is a PIN diode that will attenuate the input signal significantly (by about 16-20 dB) if a current is passed through it. This is very useful as it prevents the input from overloading when one is close to the transmitter and the signal therefore is really strong. The software takes care of deciding when to activate this attenuation step. The actual amount of attenuation can be entered into the software as it will be a little different from unit to unit and the attenuation must be taken into account when displaying the received signal strength. The bias current through the PIN diode is provided from Q1 via R7, which has a relatively large value to not load the signal significantly when the attenuation is not activated.

Q5, a BFU550W, forms a low-noise common-emitter amplifier with two gain settings. R5-C14 provides negative feedback, as does the emitter degeneration resistor R15 (and R14 if the PIN diode is activated). The ferrite bead L7 at the base and de-Q resistor R3 across the collector load L3 help ensure stability. The voltage divider R6-R13 at the base together with the emitter resistor R15 sets the bias.

The gain can be increased by activating the PIN diode D4 by turning on Q4. This boosts the gain by effectively reducing the emitter degeneration. The difference in gain is about 16 dB.

As of this writing (January 2026), NXP has changed the status of BFU550W to End-of-Life. It can still be bought from e.g., Digikey, but a suitable replacement may have to be found at some point.

The output of the common-emitter amplifier has a high impedance and is therefore buffered by two stages of emitter followers, Q2 and Q3 to provide a suitable impedance for driving the mixers. The dynamic range of the amplifier is primarily limited by the bias current of the final emitter follower Q3, so reducing R9 allows the amplifier to handle stronger signals with less distortion. Conversely, increasing R9 saves power while increasing distortion for strong signals.

The total gain from antenna port to the signal labeled “EMIT” is (according to simulations) between -19.5 dB (max attenuation, minimum gain) +17.3 dB (minimum attenuation and maximum gain).

To provide clean power to the LNA, several levels of filtering is used. +3V8 comes from the LiPo battery and is shared with many other circuits, so should be considered “dirty”. C1/L1/C2 provides some rudimentary rejection of high-frequency noise and creates +3V8_RF. The small low-drop linear regulator (LDO) U1 creates the +3V1_LNA rail while eliminating particularly low-frequency fluctuations. A further stage of low-pass filtering is provided by L2/C7. R1/C9 and R2/C10 provide decoupling so that the RF currents through the emitter followers do not pollute +3V8_RF.

Low-Pass Filter and Splitter

The output of the LNA is routed to a combined low-pass filter and splitter, L4-L6 and C15-C16, see below.

The signal needs to be split into two paths as there are two mixers. As this is obviously a narrow-band application, the splitting can be done by reactive components to not waste signal power, as a resistive splitter would have done. The reactive splitter can be viewed as L-networks that match 50 ohms at the mixer end to 100 ohms at the LNA end. Two such networks in parallel give 50 ohms. 56 nH together with 11 pF forms one such impedance transforming link. The remaining 56 nH (L5) and 22 pF (C15) form a simple low-pass filter. The output impedance of the emitter follower is much lower than 50 ohms and later simulations indicate that one would probably gain 2 dB or so by reducing L5 to 39 nH.

Mixers

Since this is a low-IF architecture, we need two mixers driven by LO-signals that are 90 degrees out of phase so that the SDR can later reject the unwanted side-band. The mixers used here are diode ring mixers of type ADE-1+ from Minicircuits. They require a 7 dBm LO-signal and has a conversion loss of about 5 dB.

Local Oscillator

The local oscillator is an Si5351, a circuit very popular in many radio amateur projects in recent years.

The documentation of some aspects of the Si5351 is unfortunately not ideal. I had hoped to be able to create two LO signals, 90 degrees out of phase, at ~144 MHz, but that turned out not to be possible (it works at some lower frequencies though). After trying many things, it turns out that the best one can do at these frequencies is to set the “multisynth dividers” to 6, which allows for a phase offset of 60 (or 120) degrees. This is obviously not nearly good enough to get any useful sideband suppression, so the extra 30 degrees of phase shift have to be accomplished by an LC-network, L11/C31.

The reference frequency to the Si5351 comes from an inexpensive, low-power, temperature compensated crystal oscillator (TCXO) U4. It provides a “clipped sinewave” output, which mostly means that the output has a high impedance and cannot be loaded much. This is perfectly OK in this application as the input of the Si5351 (which normally is connected to a crystal) is just a few mm away and has a high input impedance. The accuracy of the TCXO is pretty good and should be within about ±4 ppm even after soldering, aging and over temperature. This is much better than normal crystals and probably better than most fox transmitters.

The LO is powered via a separate LDO, U2, to reject low-frequency fluctuations. The supply is further decoupled via the LC-nets L9/C28 and L10/C29. The decoupling both helps creating clean supplies to the LO and prevents the LO from radiating.

Avoiding LO leakage is rather important, particularly in a direct conversion/low-IF receiver like this, as the LO is (almost) at the same frequency as others want to listen to. One of the very few international rules for ARDF receivers is that it must not interfere with other receivers that are at least 10 m away. There are many possible ways for the LO to leak and it is probably a good idea to pay attention to all of them. Another leakage possibility is back through the mixer and LNA to the antenna, but the mixer claims more than 50 dB of LO to RF isolation and the reverse path of the LNA (S₁₂) has about 70 dB of additional attenuation according to simulations, so this path should be pretty well blocked.

The LO leakage path I think is the most problematic is caused by the mixers acting as antennas! They stick up a few mm from the PCB and since the LO signals are thus routed several mm away from the ground plane of the PCB, they have an opportunity to radiate. Maybe the transformers inside the mixers also have a significant field leakage. This radiation can easily be picked up by a simple E-field probe connected to a spectrum analyzer when one lets the probe sniff over the mixers. A sensitive receiver with a good antenna might pick it up from several meters away, so a good shield is necessary over the RF-board. How to construct this shield is described later in this article.

IF Amplifiers

Two identical IF amplifiers (one for I and one for Q) are used.

When receiving weak signals, the LNA has amplified the antenna signal significantly, but it has then been split to two mixers (3 dB loss) and then mixed (5 dB additional loss), so we cannot stop paying attention to noise figure of the amplifiers just yet, but have to use an IF/audio amplifier with very little input referred noise. There are some options, but here I decided to base the amplifier around a low-noise JFET, 2SK3557, Q7, which provides an input-referred noise of about 2 nV/√Hz and a voltage gain of about 3.7. R22 provides proper termination of the mixer output. The output of the JFET amplifier is buffered by an emitter follower to provide a low impedance driving the IF filter.

One objective of the IF section is to reject signals at undesired frequencies, i.e., frequencies higher than the 20 kHz or so the audio codec can digitize. The most important signals to reject are those that are at least 200 kHz away as that, according to the international rules, is the closest frequency spacing between simultaneously operating transmitters during a 2-m fox hunt.

Most of the low-pass filtering is taken care of by L13-L14/C40-C42. This is essentially a Butterworth filter with 200 ohms input impedance, infinite output impedance and a 3-dB cutoff frequency of 25 kHz. It provides more than 80 dB of attenuation at 200 kHz. Some filtering is also provided by L12 together with the input impedance of the JFET amplifier and C37 which bypasses part of the drain resistance of the JFET at higher frequencies.

The output of the filter goes to the opamp U5, which is configured as a non-inverting amplifier with three useful gain setting. If neither of Q12 and Q13 are turned on, the gain is 1 (0 dB). If Q12 is enabled, the gain is 11 (20.8 dB) and if Q13 is turned on, the gain is 101 (40 dB).

The input of the audio codec on the processor board is used in a “pseudo-differential mode” where a single-ended signal as well as an AC-coupled ground reference are needed, so to provide the best SNR and not introduce issues with ground shifts between the boards, both the output of the opamp and an AC-coupled version of the ground potential at the opamp are routed to the codec.

A little bit of low-pass filtering is also accomplished by C60 and R33/C56 to further reduce unwanted high-frequency components that may still be present in the signal, despite the low-pass filtering earlier in the signal path.

The supply of the JFET amplifier must have very little noise as the signal at the drain of the JFET is referred to the supply. This rail, +3VIF, is created by a capacitance multiplier based on Q14:

The supply to the opamp U5 is less critical, so here the battery voltage +3V8 is used directly.

Connector to Processor Board

The RF board receives power and control signals from the processor board and sends the IF signals to the codec via a small 32-pin board-to-board connector, J2:

The PCB

The RF board PCB has four layers and is 83.5 x 40 mm in size. All components, except for the connector J2, are on the top side. See assembly drawings below.

The top copper layer has most of the routing:

Layer 2 is a solid ground plane:

Layer 3 has some power a little routing while the rest is a ground plane.

Layer 4 is also almost completely ground, except around the connector:

An assembled board looks as follows:

I let JLCPCB assemble the top side and to save some cost, I soldered J2 on the bottom side myself. Trying to do this myself was a bit of a mistake as the 0.5 mm pitch connector turned out to have a bad habit of sucking up solder between its pins, forming bridges under the plastic body that were almost impossible to get rid of. The solution I ended up with was to cut out a piece of and old solder stencil with apertures for another 0.5 mm pitch component, masking off part of the apertures to reduce the amount of solder being deposited, and then using it to print a tiny amount of solder paste on each pad. Here is a photo of the stencil with some Kapton tape masking off parts of the apertures:

After placing the connector on the board, I then used a soldering iron with a very fine tip to solder pins at opposing ends of the connector to fix it in position and then carefully soldering the rest of the pins using only the printed solder paste. All of this was done under a stereo microscope. In hindsight it would have been better to let JLC also populate the bottom of the board. If I had planned for that, I might have put some of the other components on the bottom side as well and saved some space/weight.

In a previous version (from 2023) of the receiver, I used a 0.8 mm pitch connector to connect the two boards and that was enormously easier to solder by hand. Unfortunately, it had become obsolete when I updated the design and I was unable to find any other suitable 0.8 mm pitch connector.

As mentioned previously in the section about the LO, preventing LO leakage is rather important. This is largely accomplished by placing a shield over all components on the top side. Making a shield with tight tolerances out of sheet metal is difficult, so I came up with a better solution. I 3D-printed a shield form in PETG (PLA melts too easily and is therefore not particularly suitable) and covered it with a piece of wide copper tape:

After testing that the board works as intended, the shield can be soldered to the exposed ground plane around the circuitry of the board.

If I were to redo a design like this, I would make sure to not require the indents around the screws in the middle of the long sides of the shield. These add some additional complexity and assembly time.

Issues

I had some issues while building these boards. Some have already been mentioned, but here is a summary.

PIN diode polarity

Both PIN diodes on all of the assembled boards did somehow end up assembled with the wrong polarity. Either I gave the wrong instructions to JLC, or they screwed up. This means that the input attenuation (used when very close to a transmitter) was not effective and that the LNA had very little gain. This is incidentally the setting that is used most of the time during a race, so the receiver works pretty well regardless. I did not discover this problem until after I had built several receivers. A friend of mine even won the 2025 Swedish Championships using a receiver with this defect! The highest possible sensitivity is obviously not of utmost importance in all cases. Anyway, I had to take back all receivers, dismantle them, including desoldering the shields, and then rotating the PIN diodes.

BFU550W Obsolescence

The RF transistor BFU550W used in the LNA has become obsolete. It can still be bought (January 2026) at e.g. Digikey, but a replacement should ideally be found.

Soldering J2

As mentioned above, it was a real pain to solder J2 manually. I normally do not have trouble manually soldering 0.5 mm pitch FPC connectors or TQFP ICs, but this connector has very tall (relatively speaking) pins between which solder apparently likes to get sucked in between. Most of each pin is also hidden by the plastic housing, so it gets very tricky to remove the stubborn solder bridges. It is almost not possible to not get too much solder when applying solder manually, so screen printing minute amounts of solder using a stencil was the only workable (although still difficult) solution I found for hand-assembling these.

Board Manufacturing Details

There are some traces on the board intended to have an impedance of 50 ohms. To make them end up close to this impedance, it is important to use the correct stackup. Also, the board needs to be 0.8 mm thick to fit the mechanics. Both of these requirements are met with the JLCPCB stackup called JLC04081H-7628. To comply with the EU RoHS directive, lead-free HASL should be selected as the surface plating (ENIG would also work, but is much more expensive). Asking that the impedance be checked/controlled by the manufacturer is not necessary and would cost extra.