"Split" VLF E-Field ReceiverContents
Receiver Noise Performance
You can listen to my current receiver while you read.
History. 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 didn't need.
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 all-metal housing.
This page catalogs each of the modules I have developed for use with my VLF receiving system.
Receiver Module. The receiver front end amounts to an e-field probe feeding a very low noise op-amp.
VLF Receiver Schematic. Click for larger version.
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 receiver.
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.
Receiver 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 version.
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 other activity.
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 a 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 60Hz notch.
VLF High-Pass Filter Schematic. Click for a larger image.
Filter Performance. Here are a the filter curves showing minimum (-1dB) and maximum (13dB) gain, respectively:
Click for a larger image.
Click for a 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 world version.
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 harmonic.
Filter Pictures. 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: