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pa0nhc RF watt meter
20170816-217
Preliminary development data.

This active and sensitive analog meter system indicates real rms RF power at 50 Ohms loads.
The maximum readable power is 210Wrms at 50 Ohms load (102.5Vrms).
The minimum detectable power is 0.5mW (0,163Vrms//50Ohm).
The expected frequency range is 1MHz to 200MHz. This can easily be extended down to 80kHz.
Power supply is 3x AA battery at 3 to 8mA load.
The circuit is free of adjustments, as long as the stated components and values are used.

Inaccuracy due to the use of standard component values is estimated at less than +- 1dB. 
The meter scale must be adapted for RF power indication.


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IC1 is an RF rms detector SMD chip and has a conversion gain of 7.5Vdc out, per 1 Vrms in.  Its input resistance is 225 Ohms from 0 to 100MHz. R1//R2 make an attenuator of 23.32 x. With 3.3Vdc output from IC1, output series resistors R5//R6 sets to full scale deflection in the 1mA moving coil meter M. The max. input of IC1 is therefore set to 0.44Vrms (3.3Vdc / (7.5Vdc/Vrms)). IC1 is sensitive, and can detect as low as -30dBm or 7mVrms with only -1dB error. That equals (23,32 x 7mV=) 163mVrms or only 0,53mWrms over de 50 Ohms signal line.

To prevent influence from low frequency signals (mains noise) coupling capacitor C2 causes the sensitivity to roll off 6db/okt from abt. 160kHz down. By soldering a capacitor of 100n in parallel to  C2, the low frequency range can be extended down to abt. 16kHz. (-45dB @ 50Hz).

When S2 is switched to resistor R4, the meter sensitivity is enlarged by +20dB. Diode D3 then protects meter M for to high meter currents.

R1/R2 form a 23.32x input attenuator with the 225 Ohms input resistance of IC1.
By switching R3 in parallel to the input of IC1, the systems sensitivity is lowered by 20dB.

The input circuit is designed to safely withstand 500W input power. D1,2 protect IC1 for overload. They start limiting at 0.5Vrms, without influencing meter readings up to full scale deflection.

All ten paralleled 1/2W input attenuator thick film resistors (R1and R2) are chosen for their low self inductance and 0.5W max. dissipation. 

All resistors and capacitors are low cost low inductance SMD 1206 types. The PCB is designed with enlarged soldering islands for easier heating and easier solder flow. The miniature toggle switches are chosen for their small size and low cost. REM :These three miniature toggle switches switches need to have only 4.14mm pin pitch. Switches having 200mil pin pitch (RM 5) will not fit onto this PCB.

        Assembling.
The meter, the PCB with all components, and the two BNC busses fit in a small and cheap Hammond 1554EGY ABS box.
The meter is situated in the center of the front side.
The PCB is designed to fit to two different types of BNC busses at wish. The sketches below contain different hole sizes for them.
I made all solder islands a bit bigger, so they can be pre-heated and the solder should flow faster under component connections.

Drill and file all holes in de cabinet.
It helps to shift components into the holes, if you counter sink the LED hole inside the box using a 8mm drill.
See drilling sketches below.

                    Solder IC1 as last, and take anti-static measures.

Bend the LED wires 90degr. at 5mm from its base.
After soldering, the lowest point of the rim on the led should be at the same level as the top surface of the PCB.

Solder all components onto the PCB, including switches, LED, battery cable and meter wires.

Shift the PCB into the box, with the switches and the LED sliding into their holes. Leave the nuts of the switches loose..

Install both BNCs, with ground lugs facing up.
The BNC central pins now should make contact with the "50 Ohms line contacts" at the bottom surface of the PCB.
Adjust and tighten the nuts of the switches. 

First,  at the bottom copper side of the PCB, solder the BNC pins onto the "50 ohms line" surfaces.
Secondly, bend the BNC ground lugs so they make contact the "ground" surfaces at the top copper side of the PCB.
Solder the ground lugs to the PCB top copper ground contacts.

Install meter M, and connect it.
Insert batteries.

Set SW1 in position "+20dB", and SW2 in position "0dB".
Connect a dummy load and RF power, and check correct operation.

The following table can help to draw a meter scale :

IC1 pen1 [V] Vermogen [W] DBm
3,22 200 +53
2.28 100 +50
1,61 50 +47
1,14 25 +44
0,805 12.5 +41
0,57 6,25 +38
0,405 3,125 +35
0,285 1,6 +32



 


Drill data for Hammond 1554EGY ABS box.


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