Bill Stone tracking an under-ice vehicle at Lake Bonnie, Antarctica
The 3496Hz Double Quadrature (DQ) radiolocation receiver.
For an overview of the somewhat complex Double Quadrature receiver see this PDF article from the June 1996 British CREG Journal.
This analog receiver was designed more than 25 years ago. Today, as far as I know, it is still the most sensitive Radiolocation receiver available. I thought that a new design with just a preamp followed by an A/D converter and computer would have replaced it long ago, and more than one person has tried. The problem with digital electronics is radiated EMI that is picked up by the loop antenna and interferes with the receiver. The receiver is very close to the loop. Cellphones, digital cameras, and even some LED headlamps can cause EMI. The receiver is a direct conversion design with all internal signals harmonics or subharmonics of 3496Hz and do not cause EMI. The only possible noise source it the tiny Digital readout (DVM) but it has not been a problem. The receiver can operate when placed inside the loop, although this is not recommended.
The term "Double Quadrature" comes from the signal detector, which splits the amplified signal into 2 channels (I &Q) 90 degrees out of phase with each other. The first mixer converts the 2 signals to baseband (DC), and applies simple RC filters to narrow the bandwidth to 1Hz. A second set of I & Q mixers raises the frequency to 437Hz. The 2 outputs are summed and bandpass filtered, then amplified for headphones. The somewhat complex Phase Lock Loop (PLL) circuitry will lock the receiver's oscillator to the beacon when the receive antenna is not in motion, forcing all of the signal into one channel where it's relative strength can be read on the DVM.
The "complete" receiver specs are below. The simplified receiver mainly lacks the digital readout of signal strength, and runs on a single 9V battery. Both use the same circuit boards, have the same sensitivity, and work with the same beacons.
SPECIFICATIONS FOR DQ RECEIVER, BEACON, AND LOOPS
DQ Receiver:
The term "Double Quadrature" comes from the signal detector, which splits the amplified signal into 2 channels (I &Q) 90 degrees out of phase with each other. The first mixer converts the 2 signals to baseband (DC), and applies simple RC filters to narrow the bandwidth to 1Hz. A second set of I & Q mixers raises the frequency to 437Hz. The 2 outputs are summed and bandpass filtered, then amplified for headphones. The somewhat complex Phase Lock Loop (PLL) circuitry will lock the receiver's oscillator to the beacon when the receive antenna is not in motion, forcing all of the signal into one channel where it's relative strength can be read on the DVM.
The "complete" receiver specs are below. The simplified receiver mainly lacks the digital readout of signal strength, and runs on a single 9V battery. Both use the same circuit boards, have the same sensitivity, and work with the same beacons.
SPECIFICATIONS FOR DQ RECEIVER, BEACON, AND LOOPS
DQ Receiver:
- Thermal noise at input (Hi Gain mode)=13.4nV/1Hz BW
- The primary receiver signal BW is 1HZ.
- 32 Hz BW mode is primarily for RFI identification and Morse Code
- Thermal noise at input in the1.6Hz Receiver noise BW=16.9nV
- Noise current at input =.01pA/1Hz BW
- Optimum input noise resistance=1.34 Megohms
- RF amp gain varies 36-98dB over the range of the gain pot
- Input attenuator is 0dB (Hi) or –40dB (Lo)
- RF amp dynamic range is 129dB in a 1Hz BW (plus the 40dB pad)
- Battery drain is 30mA (two 9V batteries)
- DVM noise BW is 0.29Hz (fast) and .048Hz (slow)
- Sensitivity is determined by the thermal noise of the loop antenna, which swamps receiver noise.
- 2 lbs of #28 enameled wire (512 turns) with electrostatic shield 450mH, 201 Ohms DC, ~4600pF for resonance, Rres=274,300 ohms, Q=27.75, BW=126Hz, Thermal Noise=66.8nV/1Hz BW, Electric field effective height=.245 mtrs
- Loop sensitivity: External magnetic field at noise level=0.723nA/mtr in 1Hz BW and 0.91nA/mtr in the 1.6Hz noise BW of the Receiver.
- The 18" Loop with 1 lb of wire (350 turns) and no electrostatic shield, 159mH, ~13500pF for resonance. Sensitivity is about 6dB less than the 22" Loop, 1.82nA/mtr in 1.6Hz receiver noise BW.
Note that constructing this receiver is only for those experienced in analog circuit construction and testing!
I am now recommending construction of complete receivers identical to those that I built for Bill Stone's Antarctic project. These use cast aluminum cases and are powered by 5 CR123 lithium cells (chosen primarily for their cold weather ability) with an internal 10V regulator that can also power the DVM module. I also did away with the expensive 10-turn pots and dials for the RF gain and null controls. They could also be powered by two external 9 volt alkaline batteries, with an internal 12V regulator, but a 9V regulator will be needed for the DVM.
For those who do not need the signal strength readout, the simplified receiver has the same sensitivity without the PLL circuitry or DVM installed, and runs on a single 9V battery.
I am now recommending construction of complete receivers identical to those that I built for Bill Stone's Antarctic project. These use cast aluminum cases and are powered by 5 CR123 lithium cells (chosen primarily for their cold weather ability) with an internal 10V regulator that can also power the DVM module. I also did away with the expensive 10-turn pots and dials for the RF gain and null controls. They could also be powered by two external 9 volt alkaline batteries, with an internal 12V regulator, but a 9V regulator will be needed for the DVM.
For those who do not need the signal strength readout, the simplified receiver has the same sensitivity without the PLL circuitry or DVM installed, and runs on a single 9V battery.
Complete up to date DQ receiver with 5 CR123 cells
I have PC boards available for this receiver, at my cost. A 3 board set (RF amp, detector, and one beacon with toroid and wire) is $29.00 USD plus postage. This is for both the "Antarctic" and Simplified DQ Receivers. Email me and I will give you the total cost. I prefer PayPal and will give you my account name. Please use "Friends & Family" to avoid charges and tax problems for me.
I have recently found that a few parts are hard to obtain. I now have some in stock. I have Motorola MV2109 varactor diodes (with leads) to replace V1 for $1.00. They work really well. I have 8-33pf trimmers for C19 for $0.50. For those that will use 5 CR123 lithium cells for power, I have 78L10 10 volt regulators for U10 for 0.50. I also have a box of NOS PM128-E panel meters, which are ideal for this project, for $10.00 each, which will work on 7-11 VDC, allowing them to operate directly from the regulated 10 VDC of a CR123 powered receiver. The LF442 low power op-amp is obsolete. The LF412 can be substituted directly. I was able to find a few LF442s on Ebay for my latest project. I also have the 600 Ohm Telex mono headphones for $5.00 (see the Basic1 & 2 page for a photo). I have a few of the special power MOSFETs for the beacon that have become hard to find for $2.00 US each. See the beacon page. Because of my small stock, I will sell these parts only to those who are purchasing boards, or have already done so. To save shipping, they should be ordered at the same time as the boards.
Read the following documents more or less in order before ordering anything:
An Overview of the 3496Hz DQ Radiolocation Receiver from CREG Journal 24
Constructing the 3496Hz DQ Beacon Receiver from CREG Journal 28
Notes on construction and use of the DQ Receiver and Beacon
The DQ Receiver and Beacon parts lists
Building the "Antarctic" DQ Receiver
Building the "Simplified" DQ Receiver
I have recently found that a few parts are hard to obtain. I now have some in stock. I have Motorola MV2109 varactor diodes (with leads) to replace V1 for $1.00. They work really well. I have 8-33pf trimmers for C19 for $0.50. For those that will use 5 CR123 lithium cells for power, I have 78L10 10 volt regulators for U10 for 0.50. I also have a box of NOS PM128-E panel meters, which are ideal for this project, for $10.00 each, which will work on 7-11 VDC, allowing them to operate directly from the regulated 10 VDC of a CR123 powered receiver. The LF442 low power op-amp is obsolete. The LF412 can be substituted directly. I was able to find a few LF442s on Ebay for my latest project. I also have the 600 Ohm Telex mono headphones for $5.00 (see the Basic1 & 2 page for a photo). I have a few of the special power MOSFETs for the beacon that have become hard to find for $2.00 US each. See the beacon page. Because of my small stock, I will sell these parts only to those who are purchasing boards, or have already done so. To save shipping, they should be ordered at the same time as the boards.
Read the following documents more or less in order before ordering anything:
An Overview of the 3496Hz DQ Radiolocation Receiver from CREG Journal 24
Constructing the 3496Hz DQ Beacon Receiver from CREG Journal 28
Notes on construction and use of the DQ Receiver and Beacon
The DQ Receiver and Beacon parts lists
Building the "Antarctic" DQ Receiver
Building the "Simplified" DQ Receiver