EBS-1 AM Reference Receiver
Here is another AM DX receiver. I found an older module that was used for the EBS (Emergency Broadcast System) before it was replaced by the EAS (Emergency
Alert System). This dates back to pre-1996. The reason I used it to build this receiver is for its super sensitivity, selectivity and extremely low noise. This thing is truly a
pleasure to use and listen to for extended periods. The receiver is built on a 2U rack (3.5" x 19") chassis.
The first module contains the RF section with a
signal strength meter calibrated in microvolts of
input signal. Also is a 10dB preamp and attenuator
that also serves as a gain control for the preamp.
The raw RF is fed through a "shunt" switch for
storm protection and can be used to check the zero
setting of the meter. The LED indicators on the left
indicate a signal strength of less-than or
greater-than 50 uV (S9). On the right is a detector
demod status that shows "carrier only" or "carrier
with data". This is useful when muting the radio and
letting it monitor a station 24/7, a la EAS warning
detection and remote alerting, etc.
The middle module contains all the BCD AM
channel decoding and RF preselection. The
preselector is super sharp and can be tuned to
the scale at the knob or by watching the "signal
level" LEDs and tuning to the green LED's
illumination...sounds awkward, but it is really a
simple function. The actual AM broadcast
channel is selected with the switches; + or - 1
MHz, then 100KHz, then 10KHz. EG; 960KHz
would be; "0 MHz, 900 KHz, 60 KHz. The PLL is
rock stable. There's never a need for a 1KHz
control. Again, sounds cumbersome, but it
becomes intuitive quickly.
The last module contains everything
post-detector...essentually all the audio
functions. Speaker selector for internal
monitoring or external for a more pleasant
experience. There's a Line Out level control for
STL, DAR or any other desired audio feed.
Monitor Gain controls the 8-watt power amp.
There are calibrated Bass and Treble controls
to suit you needs. The Line Level bar graph
indicates from -12 to +12 dB so that we know
just how much juice we're sending to the Aux
The Power Supply provides 15 VDC
unregulated for the control indicators,
10 VDC regulated for the receiver
circuit's analog stages and 5 VDC for
all the logic systems.
Over the years I have done extensive
power supply testing and R&D and
have come to standardizing on linear
supplies. I just cannot get switching
supplies to below 1 millivolt residual
noise. If we take the pains of
developing a super quiet receiver,
then the power supply must
accommodate its capabilities...plain
and simple. The large brass heatsink
was a test and has since been
replaced with a prettier one.
This is the main receiver board module. It is being tested for
preselector bandwidth symmetry, IF alignment and oscillator tweeks. I
left it in this condition on the bench playing for 12 hours to see if there
was any component-value drift. There was none. The very wide audio
passband provides amazing audio. There are no traditional bandpass
or roofing filters, hence the need for the preselector. This very clever
design concept is what allows such great recovered audio.
Some quick snaps of the module
front panels to show construction
of the labels and plates. The
plates are plastic (1/16" ABS)
over alumonum. The raised
stiffiners add strength and help
align the panels in the chassis
The decals are generated in a
graphics program (Pagemaker)
and printed on laser paper. The
paper is then laminated on both
sides with peel 'n' stick acetate
sheets. The acetate seals the
paper from moisture and provides
a great surface for the
double-sided tape which affixes
the decal to the panels.
The results are a very durable
finish. I have some gear here that
is 20 years old, have been
cleaned many times, and still
looks like new.