Racal Instruments RA-117 User Manual page 18

core L33. This stage feeds the amplified signal via C68 to the following stage V8. The
37.5 Mc/s signal is then passed to the 37.5 Mc/s band-pass filter. The anode load of
this stage is provided by this filter.
37.5 MC/S BAND-PASS FILTER
14. The 37.5 Mc/s band-pass filter consists of eight under-coupled tuned circuits arranged
in cascade. These filter sections may be tuned by C24, C35, C45, C55, C63, C72, C81
and C91 respectively. This filter, which has a passband of 300 kc/s, allows for possible
drift in VFO-1. The narrow passband and high rejection to frequencies outside the
passband prevent spurious signals from reaching the second mixer stage (V9).
37.5 MC/S AMPLIFIER (2)
15. The filtered 37.5 Mc/s signal is further amplified by V10 before being passed to the
second mixer stage (V9). To prevent interaction between the 40 Mc/s band-pass filter
and the 37.5 Mc/s tuned circuit (L50 and C113) and to enable either circuit to be
adjusted without affecting the other, a balancing circuit is included which is shown in
simplified form in fig. 4. The 40 Mc/s signal is introduced into the 37.5 Mc/s tuned
circuit at a point of zero R.F. potential since L50 is centre tapped and C108 is adjusted
to be equal to the total of the capacitance of V10 anode to chassis. C107 and the input
capacitor of V9.
NOTE:The anode load of V10 is adjusted to 37 5 Mc/s by adjusting the dust iron core
in L50. The balancing circuit will be affected if V9 or V10 is changed.
SECOND MIXER (M2)
16. This mixer (V9) produces the second intermediate frequency of 2 – 3 Mc/s by mixing
the 40 Mc/s I.F. and the 37 5 Mc/s signal. The tuned circuit formed by L300, C300
remove the 37.5 Mc/s frequency whilst the other tuned circuit formed by L301, C301
remove the 6 Mc/s frequency so that only the second I.F. is passed to the 2 – 3 Mc/s
band-pass filter preceding the third mixer.
2 – 3 MC/S PRE-TUNED BAND-PASS FILTER
17. This filter consists of two pre-tuned band pass filter sections. The characteristic
impedance of the filter is 1000 ohms.
THIRD MIXER
18. The output from the 2 – 3 Mc/s band-pass filter is resistance-capacitance coupled
to the signal grid of V25 together with the output (3 6 – 4 6 Mc/s) from the second
V.F.O. amplifier V11 when the V.F.O. switch (S300) is set to the INT. position. With
the V.F.O. switch set to the EXT. position, V11 operates as a buffer amplifier. This
mixer (V25) produces the third intermediate frequency of 1 6 Mc/s. The signal is then
fed to a 1 6 Mc/s band-pass filter which forms the anode load of this stage.
19. The 1 6 Mc/s band-pass filter consists of two double-tuned I.F. trans- formers, the
first section of the filter is formed by C320, L306, L309 and C325 and the second
section by C332, L313, L314, C334. This filter has a bandwidth of 13 kc/s.
Detailed Circuit Description 18