I suggest scanning the CREG articles on my website http://radiolocation.tripod.com before starting construction in order to understand how the units work and what you can do with them. The schematics and parts lists in the CREG articles are out of date, but the rest of the info is good. Refer to the schematics, layouts, parts lists, and notes included here for the final word. Also look at the picture gallery on the website to see the various loops etc. READ THE "README" FILE on the WEBSITE and THE NOTES HERE BEFORE BUYING PARTS OR STARTING CONSTRUCTION!

NOTE:  7/30/03.  These notes have been updated for the latest boards with the dates 2002 or 2003 printed on them.  All earlier boards were shipped with printed notes which mention corrections that are no longer needed.
 
 

• The actual board is 3.5" square. 3 of the 4 corners must be grounded to the minibox thru metal standoffs

• The simple version of the receiver is cheaper and easier to build (and use) if all you need is accurate Ground Zero location without depth measurement. One can still use the "field angle" method for rough depth measurement, or an ACVM on "Test Point 2" to do a "Ratiometric" depth measurement when the signal is strong and interference is low. The receiver can be upgrated to the "complete" version by installing the rest of the parts on the board and doing a tuneup.  Note that there will be a ~1.3 dB variation in the ACVM reading as beacon signal slowly drifts from one channel to the other.  This variation is not audible and is inherent in the receiver design.

• The receiver is designed to be mounted in a metal box for proper grounding and shielding. The boards are designed to fit in the specified 3x4x5 inch "Minibox". The cover and all of the standoffs must be grounded to the box.  Remove paint if necessary.  See the pictures on the Website showing receiver construction.

• "12v" in the lower left of the PC board is a misprint. It should just say "Battery in".

• The complete receiver uses two 9 Volt Alkaline battery.  It can also be used with a 12 Volt sealed lead-acid battery if U20 is deleted or bypassed.  The simplified receiver uses one 9 Volt battery or a 12 Volt lead-acid battery without modification.

• Warning!!! The metal crystal can must be raised above the board to avoid shorting out the traces that run under it! Note that a different crystal frequency should be used in countries that use 50 Hz power so that the operating frequency fits between two 50 Hz harmonics. See "The DQ Receiver - An Overview" on the website (near the end of the article).

• Trimmer C19 has its metal adjusting screw connected to one lead (which must be the ground lead).

• The RF amp board derives Vcc (+12V) from the output of the voltage regulator in the simplified receiver without the regulator in the complete receiver, or directly from the on/off switch in the simplified receiver without the regulator.

• The DVM derives its power from the regulated +12V on this board. Use a 3.0 or 3.3V zener in series to provide ~9V for the DVM.

• To save money in the complete receiver, the built-in DVM can be replaced by an external digital multimeter with a 200mV DC range. Connect it after the divider/filter R51, R52, C40, which must be retained. Because a separate battery is being used for meter power, U11A and its 4 100k resistors can be deleted. In this case, connect R51 directly to the S6 "arm" and the negative meter lead to the U3B pin 7 lead along with the "bottom" of C40 and R52.

• It is a good idea to use small coax to connect the volume control R34 to the audio amp U9. The center wire connects the tap of the volume control to R35 on the board while the shield connects the "bottom" of R34 to C28 on the board. Note that the shield is at V/2 and not grounded.

• Stereo headphone jacks are specified in the parts list because this is what most people will have. Any impedance will work, but 8 ohm phones may cause problems.   For the simplified receiver I do not recommend headphones of less than 32 ohms, as they draw a lot of battery current and will cause distortion and instability with weak batteries.  One solution is to use just the left or right element, which doubles the impedence.  Changing C28 to 100 uF or more helps somewhat.  Wire the "tip" and "ring" leads in parallel to use them as mono headphones. Lo-Fi mono headphones (intended for voice use) of the type that block all outside noise are really better in this application.  Hi-fi phones have a somewhat annoying "hiss" that is not audible in the lo-fi units.   High impedance phones have less risk of feedback than Low-Z units. My best phones are mono crystal "stethoscope" style. I also have some Telex 600 ohm mono headsets with full ear coverage, probably intended for "language lab" use. With mono phones, connect only the "tip" lead to avoid a short circuit.  Radio Shack sells mono phones, and I bought some nice Ata model 1261 "home hi-fi" stereo phones at Walmart for $10.

• The receiver must be nulled before first use. While using headphones, set both RF amp controls to Lo gain to get rid of RF amp Noise. Pre-setting both null controls to the center of their ranges beforehand will help. Alternately adjust the two null controls (R5 and R9) for a perfect audio null, turning up the volume control as the null improves, until only noise is heard. If you can't get a null then you should check the receiver to see that the correct resistor values were installed; the IC's are not backwards; there are no shorts or unsoldered connections on the board; the front panel controls are correctly wired; and the correct voltages are present. Each IC uses +V DC, and each op-amp output (and most circuits) operate at +V/2. The front panel null control will need adjustment from time to time (using low RF gain with no loop as before). The internal null adjust should only need adjustment after large changes in temperature (also for changes in battery voltage with the simplified receiver).

• In the complete receiver R41 can now be adjusted to align the DVM to exactly zero at the perfect audio null.  See notes on the receiver mainboard schematic.

• After nulling, switch to Hi RF gain and 32 Hz mode, then turn up the RF gain.  Noise from the RF amp should be audible.

• If a 1 turn pot is used for R9, it can be shunted with ~2k ohms to make nulling less "touchy".  It may be necessary to add a small resistance in series with R10 or R11 to "center" the null on the pot.

• The DVM should read "000" when the receiver is adjusted for a perfect audio null. If it does not (but is close!), you will need to apply a small DC offset to the DVM using R42, the 1-turn screwdriver trimmer on the detector board. If the DVM reading is negative insert a jumper wire from the "0-adj" pad to the adjacent "V+" pad on the detector board, then adjust R42 until the DVM reads "000". If the reading was positive, jumper "0-adj" to "gnd". All 3 pads are located between R11 and C39. Once the correct connection is found, the jumper can be soldered.

• The completed receiver must be tuned very close to the frequency of your beacon in order to phaselock properly and give a steady DVM readout of signal strength. The beacon should first be adjusted to freq by either adjusting the crystal oscillator to 3.579545 MHz using the test point, or installing a fixed capacitor of 20-30 pF. While receiving the Beacon signal from ~30m away, slowly tune C19 while watching the green phaselock LED, which will be blinking slowly on and off. Tune until the blinking becomes slower and finally stops with the LED ON. Now switch the DVM to "PLL Adjust" and continue trimming until the DVM reading is as close to zero as possible, hopefully under 100. This will seldom need readjustment, but can be done in the field during a Radiolocation if necessary.

• The simplified receiver must also be matched to the beacon frequency.  With an analog meter, monitor the DC voltage at pins 1 or 7 of U2 while receiving the beacon signal.  Slowly adjust C19 until the slow meter swinging almost stops.

• Note that R20 (2M) has become R20A (1M) plus R20B (1M) in series. This is shown in the parts list and color layout, but not on the schematic or on the actual circuit board.

• For the simplified receiver, delete R20A, R20B, R21, and R55. Solder a jumper between X100 and S3 to connect the loop input directly to U0.

• Actual board size is 3.5" x 2.0"

• All 4 corners of the board must be grounded to the minibox thru metal standoffs.
• Solder 1/2" high shields along the long red pads bisecting U0 to help isolate the input.
• Use small coax such as RG-178 for the loop input and Sig Out lines.

• Wrap the 3 wires of R22 with an electrostatic shield which can be grounded to the output end of the RF amp board.

• Ultimate receiver sensitivity is determined strictly by the design of the loop antenna. Sensitivity is simply the ratio of the signal received on the loop to the thermal noise of the wire. Resonating the loop does not change the loop's sensitivity, but does increase the signal level and the impedance, which raises the thermal noise level above the input noise of the receiver.

• The 18" dia receive loop described in the CREG article "Constructing the 3496 Hz D-Q Beacon Receiver" on my website is a very handy size for field work. The 22" dia loop (red text) gives about 6dB increase in actual sensitivity. I wound a second 22" dia loop and got 512 turns with 2 lbs of #28 wire. I measures 432mH with roughly 4600pF for resonance. Whatever frame is used for the loop should hold the loop rigidly in a plane. One possible frame is a used (ie worn-out) plastic wheelchair wheel with the tire removed. My receive loops are "frameless", wrapped in electrical tape, and sandwiched between plywood sheets in a rigid box. I have also constructed loops using a disks of rigid foam with a groove in their rims.

• I now recommend using fixed 600 Volt mica capacitors for the receive loops. The high voltage rating is to prevent damage (shorted caps) when the loop is accidently placed close to an operating beacon (and it will happen!). The loops do not detune by themselves. The fixed caps help keep the receiver gain stable for depth measurements.

• Adding a grounded electrostatic shield (with a gap of course) to the loop winding is worthwhile if you are doing precision Radiolocations where wells will be drilled, depth measurements, or are doing conductivity measurements where signal strength at the bottom of deep nulls must be measured. The shield eliminates small variations in signal strength when the loop or receiver are touched. The shield lowers the Q of the loop slightly. All of my loops have shields, but they worked pretty well without them.

• You MUST mount a sensitive cylindrical bubble level (not a round one) on the top edge of the receive loop, set to be centered when the loop is PRECISELY vertical. This is an absolute MUST for accurate Ground Zero location!!!!!!!!!!! These are available as inexpensive plastic "line levels" at Home Depot or other hardware store

• A useful addition to the loop is a 6 ft (2m) low stretch line attached to the center of the loop on one side, with a "T" bar of thin wood or fiberglass on the loose end to stand on. This allows one to make rapid Ratiometric depth measurements by raising the horizontal loop over ones head a precisly known distance above the ground. The reasonable depth limit for accurate measurement with this short line is about 120 ft (40M), but I have made reasonably accurate measurements at twice this depth. For the best results at larger depths, I use a rigid pole to raise the loop as much as 25 ft in the air.  See the electrostatic shield notes above.

• If only one beacon will be used, C2 can be replaced by a 33pF fixed cap. Otherwise, insert C2 without soldering then connect a freq counter to C1 (using a x10 scope probe for isolation if availabl