Theory of Operation

This receiver is a single conversion superheterodyne. In a superheterodyne receiver, the frequency of the incoming signal is "mixed" (heterodyned) to an intermediate frequency, amplified, and detected.

The RF signal from the antenna is passed through an input filter. The input filter can be set for the frequency you want to receive. All other signals are then "blocked" so that they will not affect the operation of the receiver. The TRAP is used to block 10MHZ signals, which would interfere with the intermediate frequency (IF) filter and amplifier.

The frequency is changed in a double balanced mixer. A mixer takes two input frequencies, the RF signal from the antenna and the VFO (variable frequency oscillator), and "mixes" them to produce two other frequencies, the sum and the difference of the original frequencies. Either the sum or the difference is selected as the intermediate frequency.

The NE602 is a double balanced mixer and oscillator on one chip. All the functions of the previous paragraph are done on this one chip. The RF signal, after passing through the input filter, is connected to the input of the double balanced mixer, pins 1 and 2. A tuned circuit (L1, C32, C35, & VAR CAP) is connected to the oscillator with two capacitors at pins 6 and 7. This oscillator is called the variable frequency oscillator (VFO).

The VFO determines the RF signal that you will hear in the output (speaker). Since a band of frequencies is usually wanted, the VFO is made variable over a certain range. A panel mounted variable capacitor is used to change the frequency of the VFO to a frequency in the desired range. A Hartley oscillator design is used in this receiver. When the variable capacitor is connected to the VFO connection of the coil, the Hartley design allows a wide range VFO. A much smaller range is obtained when the variable capacitor is connected to the tap of the coil.

The oscillator of the NE602 is internally connected to the double balanced mixer. The mixer "mixes" the RF signal and the VFO frequency, giving the sum and difference out on pins 4 and 5.

Pins 4 and 5 of the NE602 are connected to a crystal filter through a broadband transformer. A two pole 10MHZ crystal filter is used. Only RF signals that have been mixed to 10MHZ will pass through the filter. The signal is thereby reduced in power as a result of passing through the filter.

A MMIC (Monolithic Microwave Integrated Circuit) amplifier is then used to bring up signal power. The output is connected to a diode, used as a variable attenuator. This attenuator passes excess power to the ground so that the next stage of the receiver is not overloaded. A comfortable listening level is set with this control.

Another broadband transformer is used to connect the output of the diode attenuator to the product detector at pins 1 & 2 of U2. A product detector, an NE602, is used to detect SSB (single sideband) and CW (morse code). The oscillator of a product detector is called a BFO (beat frequency oscillator).

The BFO is a crystal oscillator. The oscillator, connected to pins 6 & 7, is changed in frequency with inductance and capacitance plus or minus 500-1500 hz. The output of the product detector is an audio frequency signal. The audio signal is output on pins 4 & 5 and is amplified by a high gain audio IC (integrated circuit) into an 8 ohm speaker.

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Last Update: 01/07/2000
Web Author: David White, WN5Y