RAT5 Manual version 1.2.0
RAT5 Information Manual version 1.2.0 © GW4GTE 2010
Page 8 of 29
2.2.2 Local Oscillator (LO)
The LO frequency is determined by L1 C14 and the series combination of VC1 and
varicap diode D1. VR1 provides a stabilised 5 V for tuning control RV1. VC1 can be
varied to control the tuning range provided by RV1. R17 limits the minimum tuning
voltage to around 0.9 V reducing the non-linearity at that end of the varicap’s tuning
characteristic.
The circuit values given are for a low-side LO on 80m and a high-side LO on 160m.
e.g. 3615kHZ RX, LO = 3160kHz ; 1963kHz RX, LO = 2418kHz
For Medium Wave operation it is suggested the LO runs on the high side.
The IC provides a buffered local oscillator output, and this is brought out to PL2 for
use with a frequency display (future add-on project) and to assist with setting up.
2.2.3 IF Filter
The IF tuned circuit L2/C4 is tuned to 455kHz giving a good match into the IF filter
FL1, a 6-pole ladder filter from Toko. This is a simpler solution than the circuit
suggestions in the IC manufacturer’s data sheet. Figure 2.3 below shows an actual
plot of the IF passband. (Note the graph shows passband response only, not ultimate
out-of-band rejection for which a larger test signal would be required).
Figure 2.3 Actual plot of IF passband response of 6kHz filter
(4.5kHz filter now supplied – response similar but narrower).
C9 and C10 give time constants for the internal AGC circuitry in line with data sheet
recommendations. The AGC has a fast response which copes well with rapid signal
fading.
The S-meter output is brought out to PL4 for use with a meter or other display (see
Section 5.3). The same line feeds the squelch circuitry shown in Figure 2.4.