4/19/2017 Quad Caster, model QC-4A, Theory of Operation
The block diagram shows how the QC-4A is put together. The 10 MHz oscillator develops the reference signal. A 10 MHz crystal and a CMOS 4069 inverter form the square wave oscillator. To get from 10 MHz to 1 MHz a CMOS 4018 programmable counter is used. This counter has two outputs. One output feeds the divide by 2 or 4 CMOS 4018 counter. The other output feeds the RF mixer stage. The selected .25 MHz or the .50 MHz output feeds the other RF mixer stage input.
The heterodyne principles perfected by Edwin Armstrong work in the RF mixer circuit of the QC-4A. If we mix 1.0 MHz and .250 MHz, four frequencies result. The fundamentals of1000 KHz and 250 KHz are present. The difference frequency of 750 KHz and the sum frequency of 1250 KHz is the result of the heterodyne process. The RF mixer contains a tuned circuit which resonates at the desired output frequency of 750 KHz, 1000 KHz, 1250 KHz or 1500 KHz.
The selected output frequency feeds the input of the modulated RF amplifier. The amplifiers power supply voltage reference point is 5 volts DC. The RF amplifier consists of an FET driver and a 2N4403 output transistor. The output connects to a tuned RF Wand. Each wand provides the proper load characteristic to the RF amplifier. Each wand is factory tuned for best performance.
The audio input source is amplified by an op-amp. The op-amp chosen is an LF-353 8 pin dip IC. This audio amplifier has two outputs. Output 1 feeds the Negative modulator transistor, type 2N4401. This transistor is biased on with zero modulation. A switching diode is in series with the collector of the negative modulator transistor. When the emitter rises above the 5 volt DC carrier reference the transistor is disconnected from the modulated DC. Output 2 feeds the Positive modulator transistor, type 2N4401. Transistor bias is set so the 12 volt regulated supply voltage adds about ½ volt of DC to the emitter sum point of both 2N4401 transistors. This transistor provides positive modulation peaks of up to 130%. When the audio input signal goes negative, the negative modulator transistor drops out of saturation. When the modulated audio hits -100% modulation peaks the negative transistor turns off.
The regulated 12 volt power supply feeds all stages, including the positive modulator. The plus 5 volts to operate the negative modulator is from a 78L05 IC. This regulator converts +12 volts to +5.0 volts to operate the negative modulator circuit.
Circuit description
Locate the Reference OSC and Divider schematic drawing. U1 is a 4069 IC logic inverter. Three sections of this CMOS chip are used to form the 10 MHz square wave oscillator. R1 is the feedback resistor. The 10 MHz crystal and resistor R2 are also in the feedback path. Capacitors C1 and C2 are used to alter the output frequency +- 1%. 3 to 10 pF are typical values. The 10 MHz output is taken at pin 8 and fed to pin 1 of a pair of inverters. This buffered signal feeds the clock input of U2. This is a 4018 counter set to divide the reference frequency by 10. This 1 MHz square wave is sent to another inverter at pin 5. Its output feeds the RF mixer, frequency divider U3 and inverter U4. Divider U3 is strapped for divide by 4 to produce a 250 KHz square wave. The output of U3 drives the second RF mixer input. The heterodyne action produces a sum of 1 MHz and .25 MHz to produce 1.25 MHz by the tuning circuit formed by Inductor L1, the variable capacitor C9 and range setting capacitor C10. The output of the RF mixer feeds the Modulated RF amplifier.
Locate the Modulated RF amplifier schematic drawing. The 1.25 MHz or 1250 KHz mixer output is lightly coupled by C11 to the Gate input of FET Q1. The high impedance of the gate terminal is reduced to 100 K ohms by resistor R4. Resistor R7 is connected to the FET source terminal. The FET drain terminal is connected to the modulated DC voltage through R6 and R5. Capacitor C12 bypasses the RF at the junction of R5 and R6 to ground. Capacitor C13 couples the drain terminal of the FET to the output transistor Q2. Bias for the transistor base is provided by resistors R8 and R9. The emitter resistor R10 is by-passed by capacitor C14. R11 is the collector load resistor. The Q2 output is coupled to the RF Wand through capacitor C15.
Each wand is set to a specific impedance for the chosen operating frequency. The RG-174U cable capacitance, two inductors and a selected set of capacitors are used to tune each RF wand.
Locate the Series Modulator schematic. The audio input jack takes the programming source and connects it to the pre-emphasis network C16 and R12. Capacitor C17 couples the audio signal to op-amp input resistor R14. The gain of this amplifier stage is 5 set by resistor R16. Resistors R13 and R15 form a voltage divider. The divider is used to bias the non-inverting input to ½ of the regulated 12 volt supply voltage. The output of the LF 353 op-amp feeds capacitors C18 and C19. C18 couples audio to the base of the positive modulator transistor. Capacitor C19 couples audio to the base of the negative modulator transistor. The emitter terminals of both modulator transistors are connected to load resistor R21 and also send +5 volts of modulated DC to the RF amplifier stage. Bias for the negative transistor Q3 is set by resistors R20 and R22. The negative modulators bias and collector voltage is supplied by the +5 volt regulator 78L05. Capacitor C21 filters the noise output from the regulator to ground. Diode D3 is in series with the collector of Q3. D3 acts as a fast switch to turn off the transistor when the emitter rises above +5 volts. The bias voltage for the positive modulator is supplied by resistors R17, R18 and R19. The emitter base junction of Q4 is by-passed by capacitor C20.
Quad Caster Model QC-4A Operating Tips
Get set for a great AM radio listening experience. The QC-4A is a combination of Digital and Analog electronic components. The unique circuit design results in an AM broadcast signal full of clarity and audio definition. At MOORETRONIX, we have been using a satellite radio receiver to drive the audio inputs of Quad Caster. The receiver used for monitoring all four frequencies is a HeathKit model BC-1A. This receiver has a 10 KHz audio bandwidth. If your AM receiver was designed for wide bandwidth performance your ears are in for a wonderful surprise. Many tube type and transistorized AM receivers were designed for high performance.
We added pre-emphasis to the audio input section of the model QC-4A to improve the performance of the typical band width limited radio. This feature helps brighten the sound of these receivers. By using our Inductive RF Wands, Quad Caster provides a signal level at the receiver tuner just like the signals broadcast by the 50 KW, high power AM stations. The strong AM signals produced at 750, 1000, 1250 and 1500 KHz yield four, very quiet dial positions. Each frequency is stable due to the digital crystal oscillator circuitry.
We have used an I-Pod, I-Phone, FM radio and a portable CD player to feed programming into the RCA input jacks for each channel. All of these devices have a loudness control. This is beneficial to set the best input level to Quad Caster. If you set the audio source too high in level, you will over modulate the broadcast channel resulting in a distorted signal. Ease the audio level back to the point where the signal becomes clear.
Stereo broadcasting is possible with two similar AM receivers using two separate RF frequencies. Try connecting the left channel of the audio source to the 750 KHz channel’s RCA jack. Connect the right channel’s source to the 1000 KHz RCA audio input jack. Place the RF Wand for 750 KHz next to the left channel radio. Place the 1000 KHz wand next to the right channel. Place 10 feet from each other and enjoy AM Stereo.
Place all four RF Wands next to a single receiver and enjoy four individual channels of entertainment. Setting a RF Wand next to four individual radios allows a point of sale message to each receiver. The RF Wand cables are 10 foot in length. Proper placement of the wands, allows receivers to be up to 20 feet from each other.
Use your imagination and you’ll find new ways to broadcast clear audio signals to any AM receiver with our RF Wands.
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Modifications:
To remove the pre-emphasis filter from each modulator, simply short out capacitors C16, C116, C216 & C316 using (4) short wire jumpers.
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