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BG7TBL Noise Source.

-  Correction of PCB faults.
-  Further modifications.

pa0nhc 20190212

This cheap and ready build smd-PCB (ver. 2016-03-06) is available for less than $15 at Aliexpres and eBay. 
BUT : faulty PCBs will sometimes delivered.

In a BAD version PCB, nearly all VIA's
(inter connections between top surface and bottom surface)
are not connected, but INSULATED from top- and bottom mass planes. 
These faults are recognizable by the small vias having :
no tinned rings,
-  by the solder mask closed holes,
-  surrounded by a very narrow  insulation ring.


This is how via's should look like.
Check the picture when ordering a PCB.
Holes should be OPEN.
Rings should be TINNED.
Rings should not be insulated from, but 
CONNECTED TO MASS PLANES.

If no holes, nor tinning, it is a faulty PCB.

If you already have one : 
Better return it to the vendor.
OR
Make 17 new via's according to the instructions below.

 


        Disappointing results.
-  My BAD unit had the tendency to wild self oscillation
-  And was very hand-sensitive when near to the border of the bottom of the PCB.
-  The unmodified output level was a very high S9+40dB.


PCB grounding corrections.

1.  Drill a 1mm hole at each position mentioned below.
2.  On both sides, remove a small piece of solder mask around the hole (i used a grinding wheel in a Proxxon hand drill).
3.  Insert a very short piece of 0.5mm wire, and bend it to a "U".
4.  Solder the connecting wire ends directly near the hole.
Do not cover the hole with solder. Keep the hole open for best RF connection.

        Locations (see photo) :

-  Under : R18, C21, U3, U2, R15, D4,.
-  To the left of : C3 and D1.
-  Above R16.
-  Directly in the cooling-pin island of each MMIC.
-  Directly at the mass connections of  C11, C16, C17, C8.
-  At the left of U1 (mass).
-  Solder the output connector also at the bottom side.


Preventing interfering noise peaks below 200kHz.

-  Solder 100uF/16V capacitors in parallel to : C11, C16, C17. C8.
-  Solder 100uF / 63V in parallel to C1.


Smoothen frequency characteristic.

-  Short circuit L6, L7, L8 with a wire.
- Try changing R11 into 82 kOhms (yes eighty two kilo ohms).
   Check the difference.


More modifications.


        De-activating U1 power converter and inserting 9V power :
-  R11 is removed.
-  L2 is removed.


        Adding 9V supply. See schematic.
-  R11 will now be an external 100 ohms 0.25W resistor, and is connected to D4, and a well filtered 9.0 Vdc supply from an external 78L09 stabilizer.
-  D4 is now changed to a 4.7V 500mW zener diode. Diode current is 37mA.
-  The 78L09-output is decoupled  to GND (see picture) by 1nF + 10nF + 100nF + 100 uF in parallel.

        Keep all wires minimal short.
-  The 78L09-input is connected to C8 (+11.5 V)..

 


        The following measures ensure 51 Ohm source impedance to the input of U2.
The MMICs can react strangely when their input source impedance differs to much from 50 Ohms (f.i. when inserting small value coupling capacitors).

-  R15 is short circuited.
-  R17 is removed.
-  R19 is now 120 Ohms.


        Installation.
Install the PCB in a METAL box (TEKO 372).
Drill a lot of ventilation holes in sides and top, to encourage air flow and cooling
Use only ONE ground connection between the PCB and the box at the output connector. .
The 12V supply connector should be installed insulated from the box.
Install near the box, over the power cord and the output coax, a common mode choke.

The noise signal output is wide band and strong. Prevent overload of the measuring receiver (saturation / compression) by inserting ample  attenuation.


Overall results.


Total relative spectrum


Clean VLF spectrum, although powered from a noisy SMPS.
To prevent interferences, the 12V power cable, and USB cable for the Airspy HF+ SDR unit, were  equipped with common mode chokes.

        HF :
*  Output : -50dBm @ 3.7 MHz (compared with 0dBm generator output)..

The following figures were obtained using an Airspy HF+ SDR receiver together with SDRsharp software :.
            (0 dB  @ 100 kHz).
*   -3 dB @ 30 kHz / 1.1 MHz.
*   -6 dB @ 3.2 MHz.
* -12 dB @ 19.6 MHz.
* -18 dB @ 30-50 MHz.

        VHF :
+0 / -6 dB : 70 MHz to 250 MHz.

        DC voltages.
The PCB is tested with 14.6Vdc power supply / 320mA, no break down.
REM : Damage to the MMICs could happen, when overdriven due to oscillations.
A surface temperature of 65C is normal. 

Supply D2k R5 D4k
12 V / 280 mA 11.5 V 9.0 V 5.3 V