Building an AM Stereo Transmitter

The AM Stereo exciter described in these pages is a low powered transmitter that includes equalization similar to what called for in NRSC-1. Although the design is not FCC type-accepted, testing of my prototype unit has shown that there is very little interference to the second-adjacent channels (frequencies 20KHz to 30KHz from the carrier). Spectrum analyzer measurements have shown that emissions in this frequency range are at least 55 decibels below the carrier.

Another important feature of this design is the three-band peak limiter. This is a circuit that prevents overmodulation without introducing excessive distortion. Limiters are important to obtaining a loud but clean on-air sound. A broadband limiter or a two-band limiter could be used in place of the three band limiter. Both of these circuits are simpler, but they may display more noticeable idiosyncracies than the the three-band design.

Frequency control is done by means of an AT-cut quartz crystal oscillator. Custom-cut crystals can be purchased from companies such as Jan Crystals or Crystek for less than US$15.

More Detail

The schematics for this transmitter are divided into several sections, as shown in the block diagram.

block

Audio Input Filtering


Following the diagram, we see that line-level left and right audio passes through an LC lowpass filter. The purpose of this LC filter is to attenuate any RF that may be present at the audio inputs. This can occur if the exciter is used in the presence of strong electromagnetic fields. The 0.001uF capacitors and 2.5mH choke form a filter that begins rolling off around 140KHz with a slope of 12dB/octave. AM band signals riding on the audio will be attenuated by at least 20dB. The 1K resistors and 0.005uF capacitors provide an appropriate terminating load for the filter at radio frequencies.

RF-reject

Audio Input Buffer, Pre-emphasis, and Matrix


This stage is centered around a single TL074 (or similar) quad operational amplifier IC. Two of the op-amps are configured as unity gain difference amplifiers. They convert the balanced inputs to unbalanced audio and null out common-mode noise or hum that might be present on the inputs. The left and right channel audio then goes to pre-emphasis networks, which consist of a 75Kohm resistor in parallel with a 0.001uF capacitor and a 10Kohm potentiometer. If a 10K ohm potentiometer is not available, a 25K or 50Kohm unit could be used, but a resistor would need to soldered in parallel with it so that the resistance between the two ends of the potentiometer is 10Kohms. The two op-amps following the audio level potentiometer form the L+R/L-R matrix and provide gain to compensate for the losses encountered in the pre-emphasis circuit. From this stage on, all audio is L+R/L-R, as opposed to left and right.

audio-in

10KHz Lowpass Filter


The lowpass filter consists of three filters, two Sallen-Key lowpass filters and a Twin-Tee notch filter. The Sallen-Key filters begin rolling off slightly above 10KHz, and the notch filter is tuned to approximately 14KHz. The trimmer potentiometer in each of the two notch filters is intended to compensate for small mismatches in the component values in the filters. These trimmers are adjusted to maximize the depth of the notch. The combined frequency response of these filters is one that starts rolling off gently near 10KHz and rolls off rapidly in the 11-13KHz region. A small lobe does appear above 15KHz, but it does not contribute significantly to splatter, according to tests. 44KHz components from CD audio are attenuated enough that I was not able to detect them during spectrum analyzer measurements nor with conventional receivers tuned 44KHz away from the carrier.

NRSC_filter

3-way Crossover


After the audio has been pre-emphasized, mixed to L+R/L-R, and low-pass filtered, it is split into three frequency bands. This crossover forms the input section of the three-band limiter. For more information on how multiband limiting and compression work, click here. The crossover consists of two Sallen-Key lowpass filters with different cutoff frequencies and two difference amplifiers. The filter with the lower cutoff provides the low band output. The mid and high bands are derived by subtracting the outputs of the two filters and by subtracting the output of the filter with the higher cutoff from non-filtered audio.

crossover

Peak Limiters, bass, midrange, and treble


The peak limiters in this exciter respond to the peak amplitude of the L+R audio and simultaneously adjust the gains of the L+R and L-R signal paths. Each limiter consists of two comparators that monitor the peak amplitude of the L+R output. The LM339 uses open-collector outputs, which allows the outputs of two comparators to be safely tied together to build a window comparator. In these limiters, one comparator triggers on negative peaks, whereas the other triggers on positive peaks. When either comparator is triggered, the node formed by the outputs of the comparators and the 4.7Kohm and 2.2Kohm resistors is pulled low. This turns on the 2N2907 transistor, which, in turn, charges a capacitor through a diode and a 100ohm resistor. The voltage on this capacitor controls the gains of the two MC3340 voltage controlled amplifiers. The 100 ohm resistor causes this capacitor to charge very quickly, but the discharge rate can be controlled by adjusting the 100Kohm trimmer. The other 2N2907 serves as a voltage follower, and the diode connected to its emitter performs level shifting. Gain reduction in the MC3340 amplifiers begins to occur when the voltage on the gain control pin is about 2volts. Around this voltage, the combined voltage drop of the red LED and the base-emitter junction of the 2N2222 is exceeded, and the LED glows.

Bass Limiter

basslimiter

Midrange Limiter

midrangelimiter

Treble Limiter

treblelimiter

Limiter Reference Supplies


This circuit supplies reference voltages to the comparators in the limiters. These reference voltages are derived from an ICL8069 voltage reference IC, which provides a 1.25 volt output that is relatively independent of temperature. If this IC is not available, two series-connected silicon diodes could be substituted. The 741 operational amplifier generates a positive reference voltage that is 83% of the negative reference voltage. The gain of this stage is selected so that when negative modulation peaks are limited at 100%, positive modulation peaks will be limited around 120%.

ref

Combiners/L+R Clipper


The outputs of the limiters are combined here. The L-R channel has an extra input for the pilot tone. The resistor values selected set the pilot level around 5%. Auxiliary outputs are provided to drive other transmitters or for monitoring purposes. An oscilloscope connected to the L+R output and to the RF output, for instance, could be used to observe trapezoid modulation linearity patterns.

The diodes in the feedback loop of the L+R combiner cause this stage to function as a soft clipper to catch limiter overshoots. Gentle clipping significantly improves average modulation levels without introducing audible distortion. Since the diodes were not part of the original design, these diodes had to be added to the foil side of the cicuit board in the prototype. The Web page layouts will soon be updated to include clipper diodes on the component side. Three diodes were found to effectively limit overshoots to 100% modulation. The three series diodes clip negative peaks, while the four diodes let the positive peaks ride at a slightly higher level before clipping occurs.

combiner

Oscillator and I/Q modulators


The quadrature AM signal is generated here. The frequency reference is a crystal oscillator operating at four times the desired broadcast frequency. The crystal should be parallel resonant, with a load capacitance of 18 to 32 pF. A trimmer capacitor is placed in series with the crystal to allow trimming of the broadcast frequency. The first 2N2222 is connected as a Colpitts oscillator, and the second 2N2222 serves as a buffer and amplifier to drive the 74LS74 flip-flops. The flip-flops are connected as a walking-ring counter, which divides the frequency by four and provides outputs that differ in phase by 90 degrees. The TTL outputs are attenuated and then fed to a pair of NE602 balanced mixers. The 220Kohm pull-down resistor present at pin 2 of the "I" mixer causes this mixer to behave as an AM modulator, rather than as a suppressed-carrier modulator. Pin 1 on this IC will be about 13.5mV more positive than pin 2. Thus, 13.5mV is the reference level that corresponds to 100% modulation. The NE602 outputs have an impedance of about 1500 ohms, so they can safely be tied together. At this point, the harmonic content will be very high because the carriers supplied to the NE602s were a square waves. In order to produce clean stereo, the QUAM signal reaching the limiter must be sinusoidal. The filter at the output of this stage attenuates harmonics of the carrier and does not require tuning for small changes in broadcast frequency. The component values, however, depend on which area of the band the broadcast frequency lies:

For operation from 530KHz to 800KHz, use the "Low" values

for operation from 800KHz to 1150KHz, use the "Mid" values

for operation from 1150KHz to 1710KHz, use the "High" values

RF-1

Limiter, Envelope Modulator, and Output Stages


Limiting is performed using an inexpensive NE592 video amplifier and two silicon diodes. The NE592 is configured for about 40dB of voltage gain by shorting pin 12 to pin 3. More gain could be achieved by shorting pin 11 to pin 4 instead, but experiments showed that when the latter is done, some incidental phase modulation is impressed upon the carrier. The output of the NE592 is then fed, through a 2.2Kohm resistor, to a pair of silicon diodes. The diodes enhance the limiting action of the NE592 and attenuate the carrier to a level appropriate for the NE602. The final NE602 receives L+R audio that amplitude modulates the carrier. It should be noted that to operate in mono, audio to the two modulators ahead of the limiter is muted. Audio to the NE602 in this stage is never muted. The output of the NE602 passes through an LC lowpass filter to attenuate harmonics. Component value selection works the same as for the previous stage. Following this filter are two stages of voltage amplification, with gains of 13dB and 19dB, for a total of 32dB. The final stage is a push-pull voltage follower consisting of a 2N2222 and a 2N2907. The purpose of the voltage follower is to allow the exciter to drive low impedance loads without a significant drop in output voltage. The two silicon diodes between the bases of the transistor form the biasing network, and the 4.7ohm emitter resistors stabilize the DC quiescent current flowing through these transistors.

RF-2

Pilot Tone Generator


The pilot tone generator produces a 25Hz tone, at a modulation level of about 5%, to trigger stereo detectors in receivers. A 555 timer is used in an astable (oscillating) mode to produce a 200Hz square wave. The 4017 decade counter divides the frequency of the square wave by 8. The decade counter sequentially pulls each of its 8 active output lines high for 1 period of the 555's 200Hz square wave. The output pulses from the 4017 drive a resistor/diode network that produces a stepped waveform approximating a sine wave. A 4.7uF capacitor smooths out the stepped waveform.

pilot

Overmodulation alarm


The overmodulation alarm circuit indicates the presence of negative modulation peaks in excess of 100% by flashing an LED. Normally, pin 1 of the envelope modulator IC is more positive than pin 2. Negative peaks exceeding 100% will cause pin 1 to become negative with respect to pin 2. The first comparator in the alarm circuit detects this condition, and its output assumes a low impedance, thereby charging up the 0.005 uF capacitor connected to the inputs of the second comparator. The voltage on this capacitor causes the second comparator's output to assume a low impedance, thereby lighting the LED. The input impedance of the second comparator, which is several megohms, serves as the discharge path for the 0.005 uF capacitor.

overmodalarm

Power Supply


The power supply provides regulated outputs of 5 and 9 volts to the various stages in the exciter. The 5 volt supply doubles as a virtual ground for the operational amplifier audio stages. Total current drain from the 6.3VAC transformer will be approximately 300mA. Another hobbyist reported that a simplified version of the design, consisting only of the modulator circuits, output stage, pilot tone generator, and an op-amp, draws as little as 50mA DC. An alternate power supply is shown here. This is the power supply used in the prototype unit. The extra chokes and capacitors in this supply help to attenuate power line noise and spikes. The crystal oven keeps the crystal at a constant temperature for optimum frequency stability. The crystal oven used in the prototype unit has a heating element with a resistance of 10 ohms. A 1.2 Ampere, 6.3 volt filament transformer was used in the supply. The choke, resistor, and capacitor connected to the crystal oven thermostat help to suppress oven turn-on and turn-off transients that otherwise cause audible "pop"s in the signal. Using a fuse, which is not shown in the original schematic, is always a good idea.

power

Assembly Instructions
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Compiled as a single document by Boomer The Dog, August 2016