In this project use was made of attenuation by the means of a digital potentiometer. The princpe schematic looks as follows:

Transcription

1 DIGITAL_AGC1 for QCX Introduction As the QCX from QRP-Labs is a direct conversion tranceiver, without AGC, audio can be sometimes a little bit loud for the ears from headphones. To prevent this shortcoming we took a start to develop an audio-agc. Often these circuits make use of a high gain amplifier by opamps with feedback. I believe this is not a good approach in HF-intensive field. Also non-linearity (disortion), often is a problem. In this project use was made of attenuation by the means of a digital potentiometer. The princpe schematic looks as follows: The input signal is conducted to a digital potmeter and to an amplitue detector. The amplitude detector was choosen half wave positive to an A/D input of a µ-controller. The µ-controller is programmed with an algorithm, which position the potmeter to the disired attenuation. The output of the digital potmeter is connected to a buffer opamp as output, with gain 1. That's it 1

2 The real schematic 2

3 Schematic description Used active components MCP41010 Digital 10K potmeter Pins 5, 6 and 7 are the potmeter of which pen 6 is the cursor Pins 1, 2, 3 are the command signals to set the cursorpotmeter value This potmeter is 8bits and thus can have 256 values Each step is nearly 39 Ohm (10000/256) TL072 This is a FET-entry OPAMP with high input impedance An equivalent and pin compatible type is TL082 It contains 2 OPAMPS 12F683 Is a low power 8bits µ-controller 4 A/D converters of 10 bits are available Despite its 8 pins it offers 5 I/O ports and 1 input port. The clock is internal BAT85 These are Schottky diodes BZX55-4V7 Is a zener diode, 5,1V is also suitable 3

4 How does it function The input is applied on P1 The RF-filter C8-L1 blocks HF into the circuit. RV1 is a 22Turns trimpot, which is positioned so that on the cusor of RV1 the voltage is limited to 4V peak or 8Vpp. D1 and C2 take care of detection of the half positive signal and are routed to the A/D pin 3 of the µ- controller. C9 and R6 take pin4 of the µc-ontroller low during power-on, so to reset the 12F683. RV2 and R7 create the threshold from which attenuation will occur. The signals CS, SCK and DATA are respectively chipselect, serial clock and Data for the digital potmeter. R4, R5, C7 en R8, are forming a midway between power supply and ground on pin 3 of U3A. The maximum amplitude on the input of the MCP41010 should not override 2V peak. This is limited by R1. C6 is the DC-blocker to the headphones. Notice that the power supply can variate in fuction of the used power supply, going from 4,5V to 5,4V, which means that the given values can deviate. C2 and R2 determine the raising rate of the detection. C2 kan be choosen from 10nF to 470nF. The potentiometric input of R1 was nessesary as the output of the QCX can reach above 10Vpp. R1 can be adapted to adjust the maximum level to the headphones. Best is to mount two headers 0,1 icnh step 0,2inch long for C2 and 0,1inch long for R1 and to plug in the components. 4

5 The software In contrast with pure analogical cicuits this project is very flexibel thanks to the software. It's easy to convert the threshold value to an A/D value, from which attenuation will occur. The value is measured at each setting of attenuation. The detection voltage of signal input allows us to set the wanted attenuation. Upon the threshold value, the input will not be attenuated. From this value, the algorithm of attenuation will handle so that even with 5V peak input the outputlevel will on,ly increase 50% or 3dBV. Maximum output is thus a function of the threshold value. To give a clear image of how ouput reacts to input in fucntion of threshold, here is an Excel Graph with the same algorithm : LIMIT is given in units from the A/D conversion. 1 unit equals nearly 20mV 5

6 The PCB 6

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9 Partlist #Cmp ( order = Value ) 100K R4 ; 100K R5 ; 100nF C1 ; 100nF C2 ; met 0,1 inch stap 0,2 inch lang header vrouwelijk 100nF C3 ; 100nF C4 ; 100nF C5 ; 100nF C9 ; 100uH L1 ; 10K R2 ; 10K R6 ; 10µF C6 ; 10µF C7 ; 12F683 U1 ; met voetstuk DIL8 1K RV1 ; TRMPOT 1K RV2 ; TRMPOT 1M R3 ; 1nF C8 ; 3K9 R7 ; 68K R1 ; met 0,1inch stap 0,1inch lang header vrouwelijk BAT85 D1 ; BAT85 D2 ; BZX D3 ; of BZX55-5V1 CONN_2 P1 ; 0,1inch header mannelijk CONN_2 P2 ; 0,1inch header mannelijk CONN_2 P3 ; 0,1inch header mannelijk MCP41010 U2 ; met voetstuk DIL8 TL072 U3 ; met voetstuk DIL8 SWITCH ON/OFF optioneel indien voeding uit batterij #End Cmp 9

10 Alignment: Without any IC in the circuit: Connect the output of the QCX to the input of the AGC-circuit. Connect the headphones on the output of the AGC-circuit and put them on the table. Connect 5V from QCX GPS connector or other 5V power supply tot the AGC-circuit, and poweron. Set the QCX volume pot on 50%, choice menu 8 on QCX. Now choice this menu en position tot sub-menu 8.7 Peak BPF. Activate this submenu. Now the 700 Hz is coming into the AGC-circuit. Adjust RV1 to obtain maximal 8Vpp on the cursor of RV1. Power-down the circuits and place the 3 IC's. Take care to orient the notch of each in the correct position. The current drawn for the AGC-circuit must be around 4,15 ma. Drop down the volume knob on the QCX to 25% and listen with comfort to the band. The maximum ouput can be adjusted by means of RV2. On my circuit even with RV2 on minimum(limit will be set on 2 = 40mV as default), the strength on the headpfones was sufficient. Power supply: This cicuit consumes 4,15mA with 5V input. De the power supply may consist of 4x AA or AAA NiMh rechargeable batteries, or 3x AA, AAA not rechargeable cells or 5V from the GPS connector on the QCX. For more info and software download hion5kn@gmail.com 73 and a lot of listening pleasure from Willy ON5KN p.s. Suggestions are always welcome 10