Enjoy wideband audio on your analog FM repeater with this simple-to-build op-amp filter system! The 4KFM Audio Filter
BY DWAYNE KINCAID,* WD8OYG
ew things are more irritating to me The bandwidth standards of today’s Unfortunately, there is no modern than listening to an analog re- repeaters are excellent. Nearly all of specification to define what “Hi-Fi” VHF Fpeater with low deviation and low them have good frequency and devia- is or could be. The old ham radio spec- audio bandwidth; it’s just grating on the tion response up to about 5,000 Hz, the ification for the audio bandwidth was ears, all midrange and no fidelity. Low FCC limit. Often, problems are caused 300 to 3,000 Hz, left over from the land- deviation also makes you constantly by the users. Today’s inexpensive VHF line telephone system days. Over the adjust your radio’s volume for different handhelds have inconsistent deviation; years, radio equipment has continu- users. This article describes a repeater they’re all over the place. Many clubs ously improved, and most of today’s audio accessory that allows you to have a “deviation adjustment night” repeater systems can easily transmit 60 increase the bandwidth of an analog FM every year or so to get users’ radios set Hz to 5,000 Hz with 5-kHz deviation. repeater to 4,000 Hz, while adding an correctly. The extra bandwidth is essential to extra layer of protection against over- deviation. It also eliminates any resid- ual CTCSS (PL¨) tones. The dual filter features a 5-pole low- pass filter set to 4,300 Hz, strongly attenuating audio above 4,300 Hz. The slope of the filter is such that it provides an additional 5 dB of rejection at 5,000 Hz, the FCC allowable limit for narrow- band FM deviation. The 4,300-Hz fre- quency will give repeater operators room to adjust the deviation somewhat higher, as well as increase the audio quality of the repeater. The second filter is a 7-pole high-pass filter set to 300 Hz. This filter removes any residual CTCSS tones that may be present in the signal. The 7-pole filter will reduce any tones in the standard 60- to 250-Hz sub-tone band. The end result is a high-performance Photo A. The circuit board for the filter is tiny — just 1 by 2 inches. audio filtering system that provides a clean signal from 300 to 4,300 Hz in a small 1-inch by 2-inch PC board, and Receiver Discriminator gives 4,000 Hz of available bandwidth 50 to 5000 Hz, 1V p-p Out 4300 for the “4KFM standard.” Why Do We Need It? Today’s repeater systems are mar- 4 3 vels of amateur radio engineering. Who would have thought, just a few short years ago, that there ever would be a time when a club could Receiver Discriminator operate a first-class repeater system 50 to 5000 Hz, 1V p-p Out 300 without a resident repeater guru? Everything has become plug-and- play, like the BridgeCom series, and Yaesu’s Fusion repeaters. 6 5
* 1445 Parran Rd. St. Leonard, MD 20685 email:
34 ¥ CQ ¥ August 2019 Visit Our Web Site creating a great-sounding repeater; having 4,000 Hz available (4,300-300) instead of 2,700 (3,000-300) makes a
HPF IN HPF OUT LPF IN LPF OUT +POWER noticeable difference. The more natur- al sound is much more pleasing to the ear over a long conversation or net. Having the filter reduce signals above 4,300 Hz also helps keep the repeater within specifications for channel sepa- ration from your frequency coordina- tors. Most places in the U.S. have 20- kHz channel spacing on 2 meters, so with 5-kHz FM deviation you could the- oretically have 5 kHz of audio signal and stay within the minimum 20-kHz spac- ing for adjacent channel separation. (Some crowded metropolitan areas have 15-kHz separation between re- peater frequencies; in those areas, these bandwidths would be tight but still within limits. Ðed.) How Does it Work? The filter uses operational amplifiers1 (op-amps) that can be used to amplify and/or filter signals. In this case, the Figure 2. Parts layout and wiring connections. amplifying characteristics are config- ured as a circuit with 0-dB gain. That is, the output signal is the same as the 0 input signal except at frequencies atten- uated by the filters. The block diagram in Figure 1 shows -10 the twi filter sections that are cascaded in series to provide high rejection. The db Loss quality of an op-amp filter is measured by its number of “poles” — the more -20 poles, the better the response. Typical filters have 3, 5, or 7 poles, and mainly follow the convenience of the PC board -30 layout. More poles means more op- amps, which means more integrated circuits (ICs) on the board. Our 7-pole -40 filter uses four op-amp sections, while 100 1000 10000 the 5-pole filter uses three. Frequency Photo A and Figure 2 show the filter’s tiny size; you can easily mount it inside a repeater or repeater controller. Dual Figure 3. A graph of the 300-Hz and 4,300-Hz filter output shows the response. 1/4-inch mounting holes allow easy stacking if more than one filter is need- ed. 100-mil headers allow for soldering Audio or using a connector for attachment. Signal Generator The graph in Figure 3 shows the per- formance of the filters, usually called “response curves.” Looking at the tone filter, we see that normal voice signals 4KFM Scope above 300 Hz are passed with virtually Filter no loss. The curve shows that the clos- er the signal gets to the sub-audible tone range, the more the signal is blocked. The 4,300-Hz filter shows the curve attenuating signals above 4,000 Hz, and reducing them even more as the signal approaches 5,000 Hz. To test, we used an audio signal gen- erator to drive the filter, and a Rigol Figure 4. Signal generator, scope, and 12-volt power supply are all that is need- MSO-1104 oscilloscope (Figure 4) to ed for testing. measure the response, plotting the www.cq-amateur-radio.com August 2019 ¥ CQ ¥ 35 Figure 5. The schematic shows both filters and the header for interfacing.
36 ¥ CQ ¥ August 2019 Visit Our Web Site Discriminator Repeater this filter has plenty of headroom, which Audio from RX Filter Controller Transmitter helps keep the circuit noise very low. What is Needed to Complete? After completing the filter, test it on your bench with a scope, and then install in your repeater system. The filter goes in line with the discriminator audio from the repeater receiver to the controller or Figure 6. Inserting the filter in the repeater audio chain. repeater transmitter (Figure 6). No adjustments are needed, as 1-Vp-p in produces 1-Vp-p out. Once installed, send a 4,300 Hz signal result on a log chart. You could also use The DC power supply is an interesting into the repeater and measure the devi- an audio spectrum analyzer with a circuit that should interest you if you work ation at 5,000 Hz. I like to keep my devi- tracking generator. with audio circuits. It includes a DS9096 ation at around 4,500 Hz. Keeping the voltage regulator, adding a second max deviation and max modulation at What the Geeks Want to TLE2426 “rail splitter” that divides the about the same level most efficiently pro- Know: Schematics, Theory, output of the regulator by two. Following vides a wideband signal for analog FM. References the schematic (Figure 5), you can see In future articles, we’ll talk about We designed both filters using the that the V/2 voltage is used to bias the repeater linking, voters, and RF power Texas Instruments FilterPro; it’s avail- op-amps so they can “pretend” the sig- levels. able on the web and works very well. nal varies from 0 to 9 volts, with “no sig- Note: You simply pick high-pass or low-pass, nal” being 4.5 volts. With that configura- 1. An op-amp is a two-input amplifier whose out- tion, you have a signal range of ±4.5 volts put is proportional to the difference between its the cutoff frequency, and the number of inputs. For more discussion of op-amps, see poles. Choose “Butterworth” type for the at the input of the filters. Most repeater KL7AJ’s “Analog Adventures” column in this issue best audio response, and choose audio signals are in the 1-Vp-p range, so of CQ, as well as the May and June 2019 issues. “Sallen-Key” design. Use 1% tolerance resistors and 5% How to Do a 15-30 Minute Audio Filter Demo at a Club Meeting capacitors; these are commonly avail- The audio filter makes a great demo at a club meeting. Set up a signal generator and able at Mouser and Digikey. In our final a scope, and vary the output of the signal generator between 50 Hz and 5,000 Hz for design, some required values were not each filter. available, so we used two capacitors in Draw a 5,000-Hz wide signal on the board. Show where a CTCSS tone lies between parallel for better precision. 60 and 250 Hz, and show the tone used on a local repeater. Then show the voice fre- We used the TL-084 IC quad low- quencies between 300 and 4,300 Hz. Note that there are three main voice formants, noise op-amp chip with a bandwidth of frequencies containing more voice energy: 300 Hz; 2,200 Hz; and 3,500 Hz. At the low about 5 MHz, plenty of headroom to end, half of the 300-Hz range is cut off, but there is not much we can do about it. The handle audio-frequency signals without entire 2,200-Hz range is there; in the old 300-3,000-Hz range, the top of the voice range introducing much noise. For the 5-pole was cut off. Increasing the range to 4,300 Hz adds the entire third voice formant, and filter, the input section provides one makes the audio much more natural, understandable, and pleasant. pole and the remaining two sections For a more elaborate demonstration of the importance of bandwidth, place two HTs provide two poles each. The 7-pole fil- on a simplex channel, and run the signal generator into the mic of the first HT. Then ter has one more op-amp section for an place the scope on the speaker port of the second HT. Adjust the signal generator and additional two-pole section. have the audience help plot the graph. You may be surprised at just how bad it is.
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