Bruker DAEDALUS, Service Manual

Page: 61 / 145

Function Description
Daedalus Lock Manual Spectrospin AG 55
High Frequency -Section (HF Part) 7.1.2
Attenuator
A 3 dB attenuator at the Receiver input ensures a good matching with the Preamp
output.
HF-Amp
Between the attenuator and following variable amplifier stages is a low noise ampli-
fier. Its job is to keep the noise figure of the whole system low despite the higher
noise figure of the following amplifiers. The larger the gain of the previous elements
the smaller will be the noise influence of the following stages.The gain is limited by
the maximum input signal of the following stages.
An MSA-1105 HF-Amp from Avantek with a noise figure of 4.2 dB and a gain of 12
dB is ideal for our purposes.
HF-Amp with variable Gain
This stage amplifies the signal in the range of 0 to 30 dB (depending on the input
level) with very little phase shifting.
Two cascaded AGC amplifiers SL-6140 from Plessy with a phase shift of just 20 de-
grees (100 MHz) meet this requirement ably. Its surface mount design allows a good
high frequency layout with short conducting paths.
HF-Bandpass
A fourth order Bessel Bandpass (with fo = Deuterium frequency) suppresses dis-
turbing signals from the following mixer. The filter is optimized to a minimum con-
stant group delay, ensuring a short transient time.
Mixer
From here the signal is split into two 90
o
phase shifted paths to distinguish the dif-
ference between absorption and dispersion signals (real and imaginary). As these
two paths are identical we will describe just one.
The amplified and filtered Lock signal is mixed at this stage with the 2H_LO-Trans-
mitter Signal. Because phase switching occurs at a clock speed of 6.66 kHz a fre-
quency of 3.33 kHz is found at the mixer output (instead of DC).
The Siemens TBB-042G (TDA6130) with a very good noise figure and a high 1 dB
compression and intercept point is used here.