AM signal processing experimental circuit

1. Question content and requirements:

Design and build an AM signal processing experimental circuit.

1. basic requirements

(1) The IF filter can use crystal filter or ceramic filter, and the IF frequency is 10.7MHz; 

(2) When the carrier frequency of the input AM signal is 275MHz, the modulation frequency is arbitrarily set to a frequency in the range of 300Hz ~ 5kHz, and Virms=1mV, the demodulated output signal is required to be a signal with a modulation frequency of Vorms=1V±0.1V. The solution Adjust the output signal without obvious distortion;

(3) Change the carrier frequency of the input signal to 250MHz~300MHz in steps of 1MHz, and after adjusting the local oscillator frequency, the demodulation function of the AM signal can be realized

2. play part

(1) When the carrier frequency of the input AM signal is 275MHz and Virms changes between 10µV and 1mV, the output signal Vorms is required to be stable at 1V±0.1V through the automatic gain control (AGC) circuit (the same below); 

(2) When the carrier frequency of the input AM signal is 250MHz~300MHz (the frequency of the local oscillator signal is variable), Virms changes between 10µV~1mV, and the modulation amplitude is 50%, the output signal Vorms is required to be stable at 1V±0.1V; 

(3) On the premise that the output signal Vorms is stable at 1V±0.1V, reduce the carrier signal level of the input AM signal as much as possible;

(4) On the premise that the output signal Vorms is stable at 1V±0.1V, expand the carrier signal frequency range of the input AM signal as much as possible;

(5) Others.

2. Design ideas    

Overall system block diagram:

The following is the programmable attenuator flow chart

The following is the flow chart of the local oscillator source program

3. Schematic analysis

1. Controllable small signal amplifier

Controllable amplification of small signals is achieved through two TQP3M9008 and PE4302. Two TQP3M9008 provide a fixed gain of 100 times, and the gain is adjusted through the PE4302 controllable attenuator.

2.Mixer

The AD831 mixer is used to perform frequency conversion processing on the signal to move the center frequency and spectrum of each component of the signal to a new frequency band, and then use ceramic filtering to filter out the high-frequency noise after mixing.

3.IF  amplifier

ERA-8sm+ and OPA695 are cascaded fixed gain amplifiers with a gain of 20dB to amplify intermediate frequency signals.

4.  Mean detector

The AD8361 performs mean detection on the IF amplified signal, and outputs a voltage value to the MCU. The MCU then controls the program-controlled attenuator in the RF front-end based on this voltage value, so that the output signal amplitude of the IF amplifier can be stabilized within 200mV±1dB.

5.Voltage  follower

Connect the reverse input terminal to the output terminal to realize the function of a voltage follower, and connect it between the intermediate frequency amplifier and the average detector to isolate the influence between the front and rear stages.

6.  Envelope detector

First, the amplitude modulation wave passes through the detector crystal diode to obtain a pulsating current that changes according to the envelope of the amplitude modulation wave. Then, a low-pass filter is used to filter out the high-frequency components, and a modulation signal reflecting the envelope of the amplitude modulation wave is obtained. The time constant of RC should be larger than the period of the carrier and smaller than the period of the modulating signal.

7.  Bandpass filter

Use two dual operational amplifiers NE5532 and use an 8th-order active bandpass filter to implement a 300Hz-5kHz bandpass filter to filter out-of-band signals.

8.  Baseband AGC

Two-stage AD603 cascade is used , and AD8307 logarithmic detector and NE5532 are used to form a feedback circuit to achieve automatic gain control.

9.Local  oscillator source

Using the AD4351 chip, local oscillator signals up to 4.4Ghz can be achieved. The ADF4351 section includes a 10-bit RFR counter, a 16-bit RFN counter, a 12-bit FRAC counter and a 12-bit magic counter. Data is input into the 32-bit register one by one on each rising edge of CLK. Data entry mode is MSB limited. The rising edge of LE causes data to be transferred from the shift register to one of the six latches. The target latch is determined by the status of 3 control bits in the shift register. The microcontroller is controlled through the SPI protocol.

4. Physical picture display

1. Controllable small signal amplifier

2.Mixer

Project link: https://oshwhub.com123123123ASDASD/hun-pin-mu-kuai

3.IF amplifier

Project link: https://oshwhub.com123123123ASDASD/zhong-pin-fang-tai-qi

4. Envelope detector

Project link: https://oshwhub.com123123123ASDASD/bao-lao-jian-bo

5.  Bandpass filter

Project link: https://oshwhub.com123123123ASDASD/dai-tong-lv-bo-qi

6.  Baseband AGC

Project link: https://oshwhub.com123123123ASDASD/ji-daiagc

7.Local oscillator source

Project link: https://oshwhub.com123123123ASDASD/ben-zhenadf4351

5. Debugging process

Function point 1 : The IF filter uses a ceramic filter, and the IF frequency is 10.7MHz.

Function point 2: When the carrier frequency of the input AM signal is 275MHz, the modulation frequency is set to a frequency in the range of 300Hz ~ 5kHz, and Virms=1mV, the demodulated output signal Vorms= 0.91 V modulation frequency signal is within the error range , and the demodulated output signal has no obvious distortion

Demodulation waveform when the modulation signal frequency is 1kHz

Function point 3 changes the input signal carrier frequency from 250MHz to 300MHz in steps of 1MHz, and after adjusting the local oscillator frequency, the AM signal demodulation function can be realized 

Demodulated signal waveform when carrier frequency is 300MHz

Function point 4: When the carrier frequency of the input AM signal is 275MHz and Virms changes between 10µV and 1mV, the output signal Vorms is stabilized at 1V±0.1V through the automatic gain control (AGC) circuit (the same below)

Demodulation output waveform with modulation signal amplitude of 10uV effective value

Demodulation output waveform with modulation signal amplitude of 1mV effective value

Function point 5: When the carrier frequency of the input AM signal is 250MHz~300MHz (the frequency of the local oscillator signal is variable), Virms changes between 10µV~1mV, and the modulation amplitude is 50%, the output signal Vorms is stable at 1V±0.1V

The demodulated signal waveform when the modulated signal amplitude is 10uV and the carrier frequency is 250MHz.

The demodulated signal waveform when the modulated signal amplitude is 1mV and the carrier frequency is 250MHz.

The demodulated signal waveform when the modulated signal amplitude is 10uV and the carrier frequency is 300MHz.

The demodulated signal waveform when the modulated signal amplitude is 1mV and the carrier frequency is 300MHz.

Function point 6: On the premise that the output signal Vorms is stable at 1V±0.1V, the carrier signal frequency range of the input AM signal is expanded as much as possible

The demodulated signal waveform when the modulated signal amplitude is 1mV and the carrier frequency is 50MHz.

The demodulated signal waveform when the modulated signal amplitude is 1mV and the carrier frequency is 500MHz.

(Limited by the instrument, the carrier frequency can only reach 500MHz , but theoretically it can be larger)

6. Video demonstration

Below is the test video link

https://b23.tv/R3aHC7?share_medium=android&share_source=qq&bbid=LRUnRiUcJRMhFi8YKR0oTAxBCDAinfoc&ts=1597648489227

7. Source code attachments

See engineering accessories office