Table of Contents
Advertisement
Service
Model 35601A
6 51. Overload Detector
The Overload Detector is connected to the output of the 60MHz Lowpass Filter. This cir
cuitry senses voltages exceeding about 2 volts peak; these voltages could damage subsequent
amplifiers. If the Overload Detector senses an overload, it sets the Overload Flip-Flop Ul lb
on the Phase Locked Loop Control board (A5). The output of Overload Flip-Flop (A5U8e,
pin 10) returns to the A4 Board and drives the common inputs of the gates in U6. If this line
goes low, none of the relays connected to the output of the U6 can be actuated. So, if an
overload exists, Kl, K12, KU, K14, and K10 are all de-energized; this sends the overloading
signal into the bypass path and directly to the -hp- 3585A. The -hp- 3585A is protected for
inputs up to one watt in normal operation; higher DC voltages cause the -hp- 3585A to
disconnect its input circuitry.
The Overload Detector has both positive and negative peak detectors. The two diodes CRl
and CR6 together with R71 and C14 form a protection circuit for the Overload Detector
itself. CR2 and CR5 are rectifying diodes, and CR3 and CR4 provide temperature compen
sation for the rectifiers. If either a positive or negative peak, exceeding approximately 2
volts, is detected, either U3A or U3B output is clamped to -15 volts which sets the Overload
FlipFlop. The transistor Q10 is used to ensure that no overload detection can occur if the in
ternal mixer path is selected. A damaging signal connot be sent through the internal mixer
path, so the overload circuitry is not needed under these conditions.
6 52. 2MHz Lowpass Filter
Following the 60MHz Lowpass Filter is a 2MHz Lowpass Filter which is switched in for
phase detector measurement frequencies below 95MHz. The 2MHz filter has a complemen
tary highpass filter connected in parallel with it; power at frequencies over 2MHz is
dissipated in R34. The output of the 2MHz Filter is fed to relay KU in the normal path, i.e.,
K11B is normally closed and K14B is normally open. The signal then goes through K12 to the
-hp- 3585A. This path is used for all calibration measurements for Phase Noise; and also for
Phase Noise measurements when the phase fluctuations are longer.
6-53. 40dB Amplifier
Where greater sensitivity is required in normal operation of the Phase Noise mode, the 40dB
Amplifier is switched in with Kl 1 and K14. This provides a gain of 100 to the emitter of Q9.
Also at this point, a 49.90 ohm back-match resistor feeds the -hp- 3585A through K14 and
K12 so that the gain into the 50Q load is 34dB. At the input of the amplifier there is a 1/16
amp fuse F2. This fuse is intended to burn out if a fast, large transient is applied. A large
current will flow through F2, LI6, and into the clamp diodes CR11, 12, 13, and 14. If the
transient is sufficiently large and fast, the fuse will blow; if the transient is not large and fast,
the overload detector will disconnect the relays (as previously discussed) and prevent the fuse
from blowing. In the 40dB amplifier, the inputs are protected by the diodes until the fuse
can burn out or until the overload circuitry shuts down the input signal in the case of an
overload.
The amplifier has two parallel circuit paths, a high frequency path and a DC path. The
signals are diplexed between these two paths by C28 and LI6; the approximate cutoff fre
quency is 170Hz. All signals above 170Hz pass through the high frequency section of the
amplifier; all signals lower than 170Hz go through the DC path. C28, L16, R54, and R51 are
chosen or set so that the input impedance of the amplifier is approximately 43 ohms. The
6-60
Advertisement
Table of Contents
