"Split" VLF E-Field Receiver
Receiver Noise Performance
You can listen to
receiver while you read.
After running the RX-6
for several years, a move gave me the opportunity to break down and
re-evaluate my VLF receiving system. For a number of years I had run
the RX-6 on a breadboard in an outdoor enclosure, which changes and
fine tuning very easy. The design, however, had settled down and
remained unchanged for at least a year.
I thought it was time to re-build the receiver using manhattan-style construction. My new location
also required more enhanced low-end filtering that the original RX-6
After considering the problem for a while, I decided it was worth the
effort to "split" the RX-6 into a receiver stage, followed by one or
more filter stages, and then finally a "line driver" to deal with the
capacitive load of the line running to the house. To allow for better
shielding, I also decided to construct the new receiver in an
This page catalogs each of the modules I have developed for use with
my VLF receiving system.
Module. The receiver front end amounts to an e-field probe
feeding a very low noise op-amp.
VLF Receiver Schematic. Click for larger image.
Circuit operation (as I see it, anyway): R1 acts as a bias R for
the input of U1, an AD823. R2 and C1 form a low pass filter with a
cutoff of approximately 630kHz. R2 and C1 also slow incoming
transients (to a degree) to allow switching diodes D1 and D2 to clamp
excessive input to either the V+ or V- rails. NE2 rounds out the input
protection and provides a flash over point between the whip antenna
and system ground. U1 is a very low noise op amp, configured as an
adjustable gain non-inverting amplifier. First stage gain levels are
set between 1 and 100 via RV1 and R3. C2 forces U1 to also act as a
high pass filter, with a cutoff frequency around 100Hz. C3 provides
high frequency roll-off to help with broadcast band (AM) interference.
R4 serves to provide a little isolation for the output of U1, and
limits the output current.
The unused section of U1 is configured as a voltage follower,
with its input pinned to the virtual ground between V+ and V-. This
seems to reduce the current consumption of the AD823, and also
prevents the unused section from oscillating.
The power supply (shown in the larger graphic) provides the split
power supply necessary for the receiver. The current draw on the
virtual ground rail (SUP_GND) is minimal, so a simple passive voltage
divider will work. L1 and R7 help to decouple the cable run from the
supply battery on the ground. Despite the good power supply noise
rejection of the AD823, leaving them out increases interference in the
High pass filtering was necessary to prevent clipping from the
local hum, and to reduce the work for digital processing later.
The receiver as shown has a drive capability of at least 15ma, and is
rated for a 500pF load.
Noise Performance. The noise floor of the receiver is below
the VLF spectrum noise floor, allowing relatively clean reception.
Overall, the dynamic range is also quite good.
Noise Floor Comparison. Click for larger image.
In the chart above, the red line represents the noise floor of the PC
sound card used to test the original RX-6.
The green line shows the noise floor of the receiver itself, with no
antenna connected and a small capacitive shunt across the antenna
terminals. The blue line shows a snapshot of the VLF spectrum from
0-24khz. As you can see, the noise floor of the receiver is below that
of the VLF spectrum we're trying to receive.
Average current consumption for the receiver module is
relatively low, around 10ma, with upward spikes during sferics and
Receiver Pictures. Here are a
couple of pictures that show the receiver built manhattan style in a
small Hammond aluminum enclosure:
Outside of the reciever. Click for larger image.
Notice I used a high quality,
full sized pot instead of a multi-turn unit mounted on the board.
It makes field tune-up and adjustment much easier.
Inide of the receiver. And yes - I cleaned up the flux... Click for larger image.
Filter Module. The
filter module is an adaptation of Charles Wenzels' preamplifier
stage of his "No Holds Barred" receiver system. I modified the
notch filter to use the component values I normally use for a 60Hz
notch, and changed the characteristics of the high pass filtering.
Since I do not intend to use my e-field receiver to receive Schumann
resonance peaks, and I have other
sources of low-end interference, I changed the filter
to heavily suppress 0-60Hz, and have a more gradual rise after the
VLF High-Pass Filter Schematic. Click for larger image.
Here are a the filter curves showing minimum (-1dB) and maximum
(13dB) gain, respectively:
Filter Minimum Gain. Click for larger image.
Filter Maximum Gain. Click for larger image.
If you are interested in playing with this filter in LTSpice,
here is the LTSpice file.
Note that the component values in the simulation are the
measured values of the components that I used in the real
The noise of the filter is lower than the VLF signal we are
trying to process, and is quite similar to the noise curve of
the receiver above. You
can also see how well the 60Hz notch works, by taking a look
at the bottom of this
page. Compare the 60Hz level to the 3rd and 5th
Here are a couple of photos showing the as-built filter.
Again, I've used a high quality pot mounted on the case to
ease experimentation and tuning in the field:
Filter Outside. Click for larger image.
Filter Inside. Click for larger image.
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