SignalSurge

This is an open source (check licensing here) Bandpass filter for VHF and UHF amateur radio bands, with a selectable LNA and a sequencer. Projected using Eagle CAD 9.

Why?

I live in a metropolitan area and very RF-polluted. One of these days, only FFS I have added a FM bandstop filter and I found that, contrary to my belief, my reception was way better! That means:

1. The front-end radio filtering is less than great (even being an expensive FT-991A)
2. Strong out of amateur band signals (and there's plenty of them) will desensitize your radio
3. By filtering out unwanted signals, you give more dynamic range to your radio, improving your reception.
4. See for yourself here.

That said, the objective of this project is to remove everything outside 2m and 70 cm bands (like: FM broadcasts, air band, commercial VHF & UHF, and everything else between), ensuring that the precious front-end gain on the radio built-in amplifier focus on amateur band signals only.

I could buy a 300 EUR filter, but... man, that's expensive. So why not expend EUR3,000 man hour-worth of my time to build one?

Features

• Two selectable sharp amateur band (2m and 70 cm) bandpass filter weeds off signals, ensuring that the transceiver front-end focus its gain figures on inband signals only
• High performance and selectable low noise 15 dB amplifier after the bandpass filter, adding some oomph to weak signals
• The amplifier is shut down when the radio is transmitting by powering off its 5V rail (by cutting the regulator "Enable" line)
• Specialty RF relay allowing 50W in VHF/UHF frequencies to flow to the antenna with low loss, while keeping unparalleled isolation to the downstream components (north of 60 dB)
• Relay default state (NC) is to bypass radio directly to antenna, allowing the board to be safely powered off
• TX Inhibit functionality to prevent the radio to transmit while the relay isn't positioned
• Static bleeding on antenna side when switched to BPF/AMP
• Small surge protection on antenna and radio sides
• Co-Planar Waveguide (CPW) design on RF lines for impedance control and good RF performance

Library

Eagle custom component library here

Bill of Materials (CSV format, DigiKey format) here. Notice that there's a full RF Inductor and Capacitor kit specified in the BOM - and that is costly. If you have the suitable RF L/C components, fine! Otherwise (like me), buy a proper one.

Schematic:

Preliminary board layout

For full board image, click here

BPF theoretical performance

Here's the calculated theoretical filter performance, for VHF and UHF bands:

We shall see how that work out in the real world.

Amplifier performance

Here's the theoretical BFP460 gain figures as per the application notes:

Maturity level / Status

• Feb 07: Bought the needed parts! Previously:
• Reached the "minimally lovable project" stage.
• I think that with regards to the base components and schematic, this is pretty much it.
• Still untested, unbuilt - and yes, it is still green - but slowly making its way through.

To Do

• Assemble and build the board
• Improve the SO-239 connector
• Assure the filter operation

Relay Logic

BPF and Relay control

Truth table (NOR)

RADIO_TX	FORCE_BYPASS	Signal Level	Behavior
0	0	1	VIA BPF
0	1	0	BYPASS
1	0	0	BYPASS
1	1	0	BYPASS

RX/Via BPF states:

Circuit	Connected to	Power
Relays	NO	+
Isolation	NO	+

TX/Bypass states:

Circuit	Connected to	Power
Relays	NC	-
Isolation	NC	-

LNA

Truth Table (AND)

BPF_ON	LNA_ON	Signal Level	Behavior
0	0	0	OFF
0	1	0	OFF
1	0	0	OFF
1	1	1	ON

LNA On

Circuit	Connected to	Power
V.REG.	EN	+
LNA SW	NO	+

LNA Off

Circuit	Connected to	Power
V.REG.	EN	-
LNA SW	NC	-

CHANGELOG

• Rev. A: Initial release (in the making)

ACKNOWLEDGEMENTS

I'd like to thank the following contributors:

• The fine r/rfelectronics community for helping me square out a RX mute/isolation (tip: it was right under my nose) issue and how to select the large signal switch and a critical review of the bandpass filters
• The r/AskElectronics community for the sanity checking on using a PNP transistor to switch a ground signal.
• The mighty and fine Gordon Hudson (AD5GG) for: guiding me on component layouts, deciding for a proper impedance control, his primer on CPWs (coplanar waveguides) and assorted awesomeness.