5 Pole CTCSS or Splatter/Ultrasonic Filter Board
For Two-Way Radio Applications
Actual Size FL-10 PC board.
(Click image to enlarge)
(Click image to download a PDF copy)
The FL-10 is a 5 pole active filter using two op-amp sections. The circuit board has been designed to allow the filter to be configured either low-pass (LP) or high-pass (HP). Switching between the two is done by the existence of R3, the position of jumper J2, and by the selection of the components which have a letter designation. "Lettered" components can be resistors or capacitors (or absent) depending on the desired configuration.
Please note you cannot change the configuration of the filter from LP to HP (and vice versa) with a mere change of J2's position. As such, this wire jumper is soldered into the appropriate location when the board is populated, and no header is provided.
Components A - E - F get tied to ground through J2 when configured for LP operation.
Conversely, components A - E - F get tied to 1/2Vcc (bias) through J2 when configured for HP operation.
When used with the RA-35, the FL-10 configuration is LP, with the knee at approximately 15 kHz, or 4 kHz depending on your needs. Good engineering practice tells us it's desirable to LPF the output of the CM119A when driving a naked FM modulator not having any protection from high frequency energy; preventing spurious emissions. 15 kHz was chosen so the RA-35 would work with most digital deployments, but still protect against spurious emissions beyond the filter's cut-off. When the RA-35 is intended for use in analog only systems, the knee of the filter is set to 4000 Hz to insure adjacent channel splatter doesn't occur. This requires other part values as compared to the 15 kHz filter.
The use of this filter is optional in the RA-35. Some people will be feeding a MIC input that is (already) protected from generating spurious emissions by default of the existing limiter/filter.
When used as a CTCSS reject filter, the FL-10 is configured HP, and the knee is set at 350 Hz. The supplied op-amp is a high-performance TL062, and when run from 10V or more has excellent dynamic range and low noise. Of course, the IC is socketed in case the builder chooses to use some other op-amp they consider superior.
These filters were optimized by Robert Meister WA1MIK. Bob reworked the part values for the best performance.
The CTCSS filter was designed to give a good compromise between maximum rejection of CTCSS tones below 186.2 Hz and minimizing
the voice roll-off below 400 Hz.
Many thanks go to Bob for his time and effort to help bring you these quality filters. Be sure to check out his evaluation article.
If you purchased our kit, refer to the parts list and make sure you have all of the components you need to build it.
Click here for Parts List.
Assembly can be done by personal choice, but you may find it easier to install the resistors first, followed by the capacitors, jumper(s), IC socket, and header socket.
The resistor color bands can be difficult to see without good lighting and strong magnification. If in doubt, use an ohmmeter and double check the value before soldering resistor components in place. Please refer to the additional notes about component identification in the parts list.
This device is intended to be installed inside a radio or project box for protection. When used with the RA-35, the mating 4 pin in-line socket allows the FL-10 to simply be plugged onto the radio adapter by first removing the shorting jumper on the audio header. Refer to the notes in the low-pass section of the parts list for additional installation instructions when this filter is used with the RA-35.
Refer to the board silkscreen when this filter is used with equipment other than the RA-35.
GND = ground
O = output
I = input
+V = supply voltage
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Product of Kevin Custer - Masters Communications, all rights reserved.
Performance optimization of these filters by Robert Meister - WA1MIK.
Specifications may change without notice.
Images property of Kevin Custer - W3KKC