The low frequency converter RA.337 is designed for use with the RA.217, RA.1217/1219 series of receivers to extend the usable frequency
range down to 3 kHz. The composite equipment then provides coverage of the frequency range 3 kHz to 30 MHz.
With r.f. input signals in the range 3 kHz to 980 kHz the RA.337 produces an inverted output spectrum (2.997 MHz to 2.02 MHz) which
is fed to the interpolation section of the receiver.
The 1 MHz input signal, A.G.C. signal, and -16V D.C. operating power are taken from the associated receiver to the RA.337 via a rear
panel connector.
SHORT TECHNICAL DESCRIPTION
R.F. Amplifier and Filter
Signals from the antenna are fed to the R.F. amplifier via an antenna attenuator and a 1 MHz low-pass filter. The 500 kHz low-pass
filter can also be switched into the circuit to discriminate against high level signals in the broadcast band. Any of five double
tuned input filters covering the range 3 kHz to 980 kHz can also be selected as required.
The R.F. amplifier output circuit is a 1 MHz low-pass filter coupling the 3 kHz to 980 kHz spectrum to the balanced mixer.
Harmonic Generator and Filter
The 1 MHz input from the receiver's oscillator is fed to a harmonic generator where the amplifier and band-pass filter in the output
circuit select the third harmonic which is fed to the balanced mixer.
Balanced Mixer
The mixer combines the 3 MHz input with the 3 kHz to 980 kHz signal from the antenna and provides a difference frequency in the
range 2.997 MHz to 2.02 MHz. This signal is taken via the 2-3 MHz band-pass filter to the output amplifiers and 3 MHz crystal notch
filters.
Output Amplifiers
The output amplifiers raise the required signal to a level suitable for application to the interpolation section of the receiver; the
required signal being selected finally by the kHz tuning control of the receiver. The 3 MHz crystal notch filters serve to reduce
the level of any 3 MHz breakthrough from the balanced mixer. This avoids possible overloading of the following receiver stages.
AVAILABLE DOCUMENTATION
Racal Low Frequency converter RA-337 Operations & maintenance manual
MY RA.337 Low Frequency converter
The Low Frequency adapter RA.337 that I have is built in the RA.366 Panoramic adapter unit.
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