Service manual

Page 20
Receiver second IF
The 70MHz 1st IF output of the Darlington driver feeds U8, another TUF-3 mixer, along with
LO2 from the controller set to 61.455Mhz (+/-, depending on filters in use and desired side-
band). The difference product of about 9MHz is used for the 2nd IF, allowing a wide variety of
Inrad crystal filters to be used. Transformer T4 boosts the 50 ohm output impedance of the
mixer up to 1800 ohms while providing a factor of 6 voltage gain. The signal is fed into another
amplifier (Q20, Sheet 3) that is identical to that of the previous stage. R54 and C51 decouple
the buffered AGC voltage from the first stage.
Noise Blanker
The amplified 9MHz output of Q20 drives another Darlington buffer which then drives the
noise blanker circuitry on Sheet 4. The noise blanker is placed ahead of the crystal filters so that
it can detect broadband noise pulses (limited only by the roofing filter). The noise blanker is a
simple 9MHz bandpass filter with an input and output impedance of about 120 ohms having a
group delay of several microseconds, long enough for the IF amplifier, U12, to detect a signal
and generate a blanking pulse just as the signal arrives at NBOUT. Three pulse widths are al-
lowed using control bits NBPWA and NBPWB that are latched into U2 (Sheet 1). Currently,
only the longest pulse width is used since we anticipate future changes to the Noise Blanker cir-
cuit. The NBOnOff bit enables or disables the noise blanker by controlling whether power is
applied to the detection circuit. When unpowered, the gating transistor, Q7, is always off. A
DAC output, NBThr, provides a variable trip point, which is routed into Q8 by resistor R73.
Note that NBThr is derived from either the Squelch control or the NBT1 DACs. Since the FM
receiver does not use the noise blanker, the same control is used for both circuits. NBT1 is not
currently used and is reserved for future use, should a dedicated control for the NB threshold
become available.
Since there is about 8dB of loss in the noise blanker, and up to 15dB of additional loss in the
crystal filters and associated resistive pads, the output of the noise blanker is boosted by high
bandwidth amplifier U25, an Analog Devices AD8000 op-amp in a gain of 11 configuration.
The op-amp has high input impedance, allowing resistors R150 and R151 to set the input im-
pedance to a value that matches the output impedance of the noise blanker, while providing
enough gain to offset the loss in the noise blanker and crystal filters and a low output imped-
ance capable of driving the crystal filters.
Crystal Filters
The crystal filters are plugged into 4 slots on the IF Filter board. Up to four filters can be in-
stalled on the IF Filter board at the 9MHz IF frequency. The first one is an Inrad 2311 6KHz
filter. This provides enough bandwidth for AM reception. The standard filter is a 4-pole 2.4KHz
Inrad model, providing good bandwidth for SSB and CW reception. All filters are switched via
1N914 diodes, and impedance matched on input and output via Minicircuits transformers and
resistive pads (attenuators) . These pads serve several purposes. A portion of the pad is used to
provide DC biasing for the diodes. They also serve to isolate the stages from each other and to
help prevent downstream amplifier byproducts from feeding back into previous amplifiers,
which would cause distortion. Finally, the presence of pads allows the attenuation to be