Direct Conversion Radio Receiver Using the CA3028 IC Tuned Single Balanced Product Detector

The CA3028 is a differential amplifier IC that operates at frequencies from DC to 120 MHz, which is used in communication equipment and other industrial applications. CA3028 IC can be used as an active mixer as well as a product detector or demodulator. The mixer and the product detector both have two input ports and an output port. A mixer combines an input signal from the antenna with a locally generated RF oscillator (VFO) signal and creates a frequency shift, shifting the frequency up or down producing an RF or intermediate frequency signal at the output.

 

 

 

A product detector or demodulator on the other hand combines the radio frequency signal input from the antenna or an intermediate frequency signal from the IF stage with a beat frequency oscillator (BFO) to produce a demodulated audio signal at the output. In this DC receiver, the CA3028 IC works as a tuned singly balanced product detector. It is an active device, therefore, it provides a conversion gain.

The CA3028 IC is no longer manufactured, therefore the IC equivalent circuit is used here. The equivalent circuit of the IC is wired onto a PCB to construct the mixer or product detector stage. It is an 8 pin IC and consists of three transistors and three resistors for biasing. Two transistors act as the RF input ports through which RF signals are fed into the mixer and a third transistor acts as the output port where the demodulated audio is obtained.

The Important Features of the CA3028 IC are:

1. It is controlled for input offset voltage, input offset current and input bias current.
2. Balanced differential amplifier configuration with controlled constant current source.
3. Single-ended and dual ended operation.

 

CA3028 IC Important Features

 

Circuit Description:
The CA3028 IC equivalent circuit with the three transistors is wired onto a PCB. The resonant tank circuit is made using an inductor and a variable capacitor through which it is tuned to the radio frequency that is received. The resonant circuit is attached to the base of one of the input transistors.

 

 

 

 

The RF input is coupled to the resonant circuit by means of a low value capacitor for eg. 47 pf or below. A local oscillator signal is given to the base of another input transistor. The strength of the local oscillator signal of about 100-300 millivolts is needed to drive this product detector. The detected audio signal is received at the collector of the third transistor. Bypass capacitors are provided to bypass any RF components present at the output stage. The three transistors act as the three ports in this product detector circuit which provide enough isolation between the ports. The detected audio signal is filtered of its RF components using an audio filter and then amplified using a preamplifier stage before it is further amplified by a power amplifier.

 

 

In a direct conversion receiver, there is insufficient gain obtained due to the absence of RF or IF stages as in superheterodyne receivers, so most of the gain has to be provided by the audio amplifier. A preamplifier section is usually added to boost the weak audio signals before it is being amplified by the power amplifier. There is a slight hum that is picked up by the detector stage, which is a disadvantage in most DC receivers. This can be reduced by connecting around 10 meters of electrical wire with one end connected to the circuit ground and the entire length of the wire stretched on the ground.

Applications of CA3028 IC

1. It is used as an oscillator, singly balanced mixer/product detector and limiter in radio receivers.
2. It is used as RF and IF amplifiers (either in differential or cascode arrangement) in radio and communication receivers.
3. It is used as DC, audio and sense amplifier.
4. It is used as a converter in the commercial FM band.