2m Receiver : ROX2-T

This receiver is based on an original design by OK2BWN in which the IF amplifier, detector and audio amplifier is provided in a single TDA1083 IC.

above: SCHEMATIC: VERSION 5

The TDA1083 is a one chip AM/FM radio widely used in consumer radios until the early 2000’s. The TDA1083 and many pi-compatible variants have been around for a very long time so ICs should not be too difficult to obtain or salvage. The equivalent TA7613AP is used in this project.
NOTE: It may be advantageous to use the oldest available variant of the IC. The author suspects that later versions have been optimized for better FM performance, resulting in more noticeable AM clipping at higher signal strengths.

The ROX-2T uses a novel way of controlling the gain of the SA602 mixer (as found in the Elecraft K1 transceiver) but the SA612 must be directly feed by the stabilized supply voltage to the chip or a through a RFC, to minimize pulling of the VFO.

The resulting ARDF RX is just about as simple as a superhet can get with get two RF devices, and no surface mount components, (apart from the varicap diode.)

The ROX-2T is not suitable for (FM) foxhunting. If the fox stops talking, there will be no modulation to detect! so it is solely aimed at Radio Orienteering using AM / MCW transmissions.

NOTE: There is no AGC with this receiver. Please do not neglect the risk of ear damage from excessive output volume when close to a transmitter. A resistor in series with the output should be provided. 220 ohms is a good starting value, adjusted for individual comfort and safety.

photo: VERSION 3/4 SHOWN ABOVE

photo: VERSION 5 SHOWN ABOVE

Version 5 gives the option of mounting the control pots mounted on the pcb back or side mounted (as version 3/4).

CONSTRUCTION NOTES

L1, input coil - 2:6 on a T37-12 or T37-17 toroid.
Eeach half of the secondary winding (3T+3T) is wound in the same sense.

NOTE: The front mounting holes are ’live’, also being the antenna input terminal, so will need to be insulated from the enclosure if metal or RFI coated.

INTEGRATED ANTENNA

This receiver is typically paired with the 3 element is based on the WB2HOL yagi described elsewhere on this web site.

photo: Enclosure integrated into yagi boom

links

WB2HOL yagi (coax feed)