An advanced 2m ARDF-Receiver by DF1FO

Overview

This paper describes a microprocessor-controlled 2m-ARDF-receiver. The receiver is considerably more complex than most of the simple ARDF-receivers from Eastern Europe or China. In return it offers many advance features:

Highlights

• High sensitivity
• Narrow crystal-filter for good selectivity
• PLL-controlled frequency
• Stores up to 4 frequencies
• Instantaneous S-Meter with Peak-Hold
• Acoustical S-Meter
• Attenuator with calibrated 5dB steps from 0 to 120 dB
• Automatic Attenuation when S-Meter reaches full-scale
• User interface: rotary encoder and 2*8 Character LCD
• Estimation of distance to transmitter
• Display of current fox and remaining transmit time
• Warning n seconds before end of fox transmit time
• Stopwatch shows elapsed time
• Display of battery voltage, low voltage warning

Receiver with 3-Element-Yagi

Circuit Description

Refer to the schematics on the following two pages.

The analogue part of the receiver is a double conversion receiver with 10,7 MHz and 455 kHz intermediate frequencies and an AM-demodulator.

The antenna signal is amplified by the preamp T1. The preamp is turned off for strong input signals, this reduces the signal level by about 40 dB. T2 is the first mixer and oscillator. Its frequency is 10,7 MHz below the input frequency and controlled by the PLL-circuit TSA6057. The PLL has a frequency resolution of 1,25 kHz. The cascaded crystal filters QF1/QF2 are responsible for most of the selectivity of the receiver..

Next is an AM-receiver circuit TCA440. It contains the 10,7 MHz IF-amplifier, crystal controlled 2^nd oscillator, 2^nd mixer, and the 455 kHz IF-amplifier. The gain of the TCA440 can be controlled over a range of more than 100 dB. The output signal of the AM demodulator D2  contains an AC-component, the modulation, and a DC-component proportional to the level of the input signal.

The AF-amplifier LM386 drives a headphone of medium to high impedance.

The circuitry around the processor Atmel ATMega 8-16 is harder to understand and will therefore be described more detailed.

The processor clock is set by Q2 to 5 MHz. The processors clock signal is also used as reference clock for the PLL.

Pin PC0 turns the preamp on or off  (Pin is low resp high-Z).

Pins PC4 and PC5 drive the I2C-Bus to the PLL. PC4 is also used to detect if the receiver is on.

Pins PC1 and PC2 are analogue inputs. They are used to measure the battery voltage and the DC-Signal from the demodulator.

Pin PB3 is the (analogue) output of a pulsewidth-modulator. It controls the gain of the TCA440.

Pin PB1 is the output of  a programmable oscillator, that generates the AF-signal for the acoustical S-meter and other tone signals. This signal is combined with the AF-signal from the demodulator.

Pin PB2 is normally high-Z. However when the acoustical S-Meter is on PB2 goes to low-Z and mutes the demodulator output through C46.

The receiver is controlled through a rotary encoder on pins PB0, PB4 and PC3, and a 3-position switch on Pins PD6 and PD7. The LC-Display with 2*8 characters is controlled by Pins PD0-PD5.

Finally the processor is connected to a 10-pin standard Atmel ISP connector for ‘In System Programming’.

The close proximity of a very sensitive receiver and the processor results in some ‘digital’ noise in the receiver. However this noise is very weak and can be heard only at minimum attenuation, therefore it does not limit the capabilities of the receiver. The only exception is the 29^th harmonic of the 5 MHz crystal at 145,000 MHz. It makes the receiver less sensitive at this frequency. This is not a problem in Region 1, because this is a repeater input frequency and not used for foxhunts.

Another effect worth mentioning is that the PLL briefly unlocks when the frequency is changed. This makes searching for a  (unknown) fox frequency a bit slow. Again not a big problem, because normally the frequencies are known.

Translation of some key words in the schematic:

Vorstufe Ein/Aus             Preamp on/off   
Regelspannung               Gain control     
NF-Verstärker                AF amplifier

Notes:

The On/Off-switch can be combined with the volume control pot.
Menu/Operate/Attenuator is a 3-position toggle switch, the Attenuator-position is momentary and returns to Operate.

For headphones with 3,5mm-plug I have special jacks with an isolated switch:

Operation

The On/Off-switch is combined with the volume control pot. Volume is normally set to a middle position and remains there until the receiver is switched off. When the receiver is turned on the stopwatch starts at 0 and the foxtimer with fox # 1, and the active frequency is loaded. The actual receiver (the analogue part) is powered only when the headphone is plugged in. Pulling the headphone reduces power consumption to 10 mA while keeping all timers and settings active.
The receiver is controlled by a rotary encoder and a toggle switch with the three positions Attenuator-Operate-Menu.

Switch in Operate Position

The LCD shows the number of the current fox, its remaining transmit time in seconds, the estimated distance to the fox and a 32-step bar-S-Meter. One to four dots in character position 2 show the selected frequency number, and an asterisk in position 5 indicates that the automatic Attenuator is off.

• Turning the encoder sets the attenuation in 5 dB-steps. When the signal reaches full-scale on the S-Meter, attenuation will automatically increase 5 dB. This is indicated by a tone signal. A reduction of the attenuation must always be done manually, by turning the encoder or clicking the toggle switch.
• A click on the encoder switches the audio signal between  normal reception and acoustical S-Meter.
• Pushing and turning the encoder switches between the (up to 4) stored frequencies.
• A click of the toggle switch to the attenuator position opens the attenuator to the 40/10/0 dB position      
• Pressing the switch for 1 s turns the automatic attenuator on/off.

An alarm sounds at a set time (e.g. 10 s) before the end of  transmission of each fox.

When the battery voltage is low, another alarm will sound 10 sec after the start of transmission of fox 1.

Switch in Menu Position

After switching to Menu the display will show for a few seconds the current frequency, the stopwatch and the battery voltage. Turning the encoder selects one of four menu items: entering the up to four frequencies, starting/stopping the stopwatch, starting the foxtimer, and starting the setup-menu. Both menus and the frequency entry mode are left by setting the switch from the Menu position back to Operate.

The table on the following page shows all menu items and related operations. For a deeper insight into how to use all these functions: just play with the receiver.

Once you are on the run all settings in the menus have been done and you will stay in the ‘Operate’-mode. As you get closer to a transmitter the attenuator will automatically decrease receiver sensitivity. Just run for the maximum. The display shows S-meter, estimated distance to transmitter, # of current fox and remaining transmission time. The only adjustment you have to do is to open the attenuator when you need more sensitivity. This can be done in small steps by turning the encoder, or in big steps by clicking the attenuator switch. 

Operation Overview

Main Menu

Setup Menu