Audio Filters for Classic Radioteletype Application

Audio Filters for Classic Radioteletype Application

D.R. Sentz

Started July 11, 2015

A long time ago I acquired several of the "phone company" type

toroid transformer/inductors that are commonly referred to as

44/88 millihenry or 22/88 millihenry toroids. I still have four

of them available for use. Radioteletype (RTTY) hobby articles

refer to these toroids as being very useful for the mark and

space tone filters in homebrew "terminal units" (TUs). The

classic ham radio RTTY signaling conventions were;

1. High frequency (HF) RTTY

Modulation Frequency-Shift-Keying (FSK)

Shift 850 Hz

Mark signal nominal RF carrier frequency

Space signal RF carrier 850 Hz

2. Very High frequency (VHF) RTTY

Modulation Audio-Frequency-Shift-Keying (AFSK)

Shift 850 Hz

Mark signal 2125 Hz

Space signal 2975 Hz

A single terminal unit design can employ two filters, one for

each of the VHF AFSK tone frequences, and then employ a

"reversing switch" to choose the HF signaling mode or the VHF

signaling mode. If the inductor is exactly 88 millihenrys, then

the capacitor to go with it, for the 850 Hz shift "standard"

mark and space tone frequencies, should theoretically be as

follows;

Tone Filter Capacitor

2975 Hz 0.0325 uF

2125 Hz 0.0637 uF

I do not have an audio signal generator, but I can still easily

breadboard the audio filters, and use a "select-in-test"

procedure to arrive at the proper capacitor choices. The

procedure requires only the following equipment, that I have;

Short-wave U.S.Army receiver model R-174

Fluke digital multimeter model 79

8 ohm to 500 ohm audio transformer, and 600 ohm headphones

clip leads, misc.

Procedure- Before starting, let the receiver stabilize (i.e.,

"warm up") for a little while, say 15 to 30 minutes.

1. Set the receiver function switch to "CAL". Tune to 1800 kHz

so that you hear the beat tone. The receiver has a built-in

200kHz crystal-controlled calibrator.

2. Apply the receiver's audio output to the series resonant test

circuit via the step-up transformer and a 10kohm isolation

resistor, as shown in the figure above.

3. Refer to the figure for the following substeps;

Disconnect the circuit under test at "X".

Set the multimeter to "Hz"

Tune the tone frequency to about 2 kHz

Set the multimeter to AC voltage

Adjust the receiver's volume control to get an amplitude reading between 1 and 2 volts. Recheck the tone frequency

after this adjustment.

4. Connect the circuit under test at "X". Tune the receiver to

vary the audio tone frequency to get the best "null", or minimum,

AC voltage across the circuit under test. Record the magnitude

of the minimum voltage. It is important to distinguish between

the true null and the normal roll-off of the amplitude as the

tone frequency increases or decreases. Use the headphones to

help out with identifying the true null.

5. Do not change the tone frequency after finding the null.

Disconnect the filter under test at the "X". Leave the

multimeter attached to the resistor. See drawing on previous

page.

6. Switch the multimeter to "Hz" and measure the frequency and

magnitude of the audio tone. Record this reading as the measured

series-resonant frequency of the filter.

If you can't get a stable frequency reading then the volume

control on the receiver may have been set too low. Turn it up

some and try this step again (You may not have to repeat the

whole procedure). I found that a 1 volt rms tone is fine for the

multimeter to make a good measurement of the frequency.

There is, in theory, no difference between series and parallel

resonance frequency of a tuned circuit. The nulling method

should be more accurate than a "peaking" method.

July 15, 2015- Filter Testing went very well, considering the

crude nature of my set-up. Here are the results.

2975Hz- Theoretical .0325 uF

Capacitor selected:

Marked Measured I.D.

#1 .033 .0323 Yellow Tubular 100V

Theoretical Resonance at 2985 Hz

Measurements with Red 88MHy Toroid;

1000Hz 1.77 volts 653mV - 8.7dB

2000Hz 2.95 volts 373mV -18.0dB

2500Hz 3.08 volts 179mV -24.7dB

2972Hz 1.44 volts 50mV -29.2dB, photo

2973Hz 1.6 volts 54mV -29.4dB, -0.85 dB from peak

2975Hz 1.85 volts 50mV -31.4dB

3053Hz 1.7 volts 52mV -30.3dB

3200Hz 2.37 volts 64mV -31.4dB

3500Hz 1.89 volts 96mV -25.9dB

4000Hz 1.50 volts 134mV -21.0dB

Series Connected Red 88mHy Toriod and .0323uF

2125Hz- Theoretical .0637 uF

Capacitors Selected;

Marked Measured I.D.


#2 .022 .0234 Green 50V


Total .06485 High by .00115uF 1.8%

Theoretical Resonance at 2107 Hz

Measurements with Green 88MHy Toroid;

2125Hz 1.8 volts 48mV -31.5dB photo

2125Hz 2.6 volts 50mV -34.3dB cannot explain 3dB diff.

----

625Hz 1.35V 430mV - 9.9dB

1000Hz 1.09V 186mV -15.4dB

2125Hz 2.6 V 50mV -32.0dB (avg. two measurements)

2500Hz 2.60V 94mV -28.8dB

3000Hz 2.28V 166mV -22.8dB

Series Connected Green 88mHy Toroid and .06485uF

This data, coarse as it is, looks fine for single-section

series-resonant LC circuits. Now I have confidence that these

are good-enough parts for the two parallel-resonant tone filters.

Here are some photos from the testing activity;

Input Tone Frequency = 2972 Hz

Filter#1 2972 Hz Input Tone Amplitude = 1.44 Volts

Filter#1 2972 Hz Output Amplitude = 50 millivolts

Input Tone Frequency = 2125 Hz

Filter#2 2125 Hz Input Tone Amplitude = 1.825 Volts

Filter#2 2125 Hz Output Amplitude = 48 millivolts

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Audio Filters for Classic Radioteletype Application