2m Receiver: ROX-2K

A receiver suitable for both AM Radio orienteering and the end game of FM foxhunts.

above: SCHEMATIC: VERSION A

The SA605 makes for a simple AM receiver. It contains a mixer good to 500 MHz, an onboard local-oscillator (LO) transistor, 80 dB of stable IF gain, and an FM-quadrature detector. This application does not use the FM detector; instead, the received signal-strength indicator (RSSI) serves to demodulate the AM signal’s sidebands. The RSSI output provides an indication of the received signal’s strength, but if an amplitude-modulated signal is tuned to the upper or lower sideband (slightly off centre of the carrier), the RSSI acts like an AM demodulator.

In this circuit, the LO is based on a simple Colpitts-oscillator circuit. A BB135 /BB149 varactor diode tunes the LO. The LO feeds the internal mixer with RF from the feed point of a direction finding antenna. The circuit uses a single tuned circuit pre-selector at the antenna feed point. Being a simple RF preselection, there is a potential to hear spurious image and other unwanted signals (eg. airband signals),however in practice, this is rarely a problem in the normal ARDF environment.

The SA605 is a FM device with logarithmic amplifiers , this results in the AM signal being clipped and limited in strength at quite low RF input strengths. Using the volume of the AM modulation to determine transmitter direction is therefore only reliable when the received signal is relatively weak.

The circuit includes a gain control at the mixer input which can keep the received signal at a low level as the distance between receiver and transmitter decreases.

To circumvent the clipping problem the receiver generates an audio tone from the RSSI output. (squeaker) The pitch of the squeaker will vary according to the strength of the carrier wave of the hidden transmitter.
The circuit is configured so when the squeaker becomes audible (at low pitch) the squeaker should be the used as the primary detecting method, unless the gain is further reduced.

The squeaker pitch varies, but the volume output is constant, pre-set by a fixed resistor in series with the output.

A positive result of the AM limiting is that it provides some protection against excessive volume and ear damage.

It should be noted that this receiver is designed to work with continuous carrier transmissions. Whilst all UK and many EU events use this mode, international championship events can use keyed carrier transmissions. The author has no experience of the receivers suitability in that circumstance.

references: Archive Article, EDN Access, 29/9/97 by Steve Hageman