pa0nhc RF low power
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thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.
Please do read this page fully BEFORE you start soldering or drilling.
This sensitive device measures true r.m.s. RF power at an internal 50 Ohms load.
The dedicated PCB is designed for usability in a maximal wide frequency range..
The max. measurable power is +6dBm (3.9 mW / 0.44 Vrms @ 50 Ohms).
The minimum accurately readable power is only 1uW (-30 dBm) with only -1dB error.
The measuring range is therefore 36dB.
The frequency range with low error is 32kHz to 800 MHz at VSWR <= 1.1 .
At 2.5 GHz the expected measuring error is -2.5 dB, with a input VSWR of abt. 1.4 .
The frequency range can simply be widened downwards.
Power supply (3.5 to 5.0 Vdc) are 3 pieces AAA batteries,
delivering 3 to 8 mA.
Errors due to the use of standard component values should be less than 1dB.
This design is free of
adjustments, as long as the stated components and values are
A drawing for a dBm meter scale is given below.
Download the parts list HERE
Download the schematic HERE.
This table can be used to draw a meter scale for different moving coil meters.
Max. meter current is 10 mA.
Drilling plans for a Hammond 1554EGY ABS cabinet.
The width of the BNCbus equals the distance
between the PCB and the lid.
Special PCB design.
To make this double sided PCB optimal for very high frequencies, all mass connections at components are connected to top and copper by "via's". Special attention is paid to short routed power supply decoupling of IC1. Stray impedances are as low as possible.
The specified type BNCbus MUST be soldered directly onto the PCB for widest frequency range and good VSWR up to high frequencies. A coax connection between BNC bus and PCB is not advisable.
Possible installation in another housing.
The switches and the LED are soldered onto the top side of the PCB. Handy for installation into the lid of the ABS box. Wiring then is only two wire pairs connecting the moving coil meter and the battery.
As in this design the switches and the LED only carry DC voltages, they could be installed separately, and connected with wires.
The use of a different moving coil meter.
R5 // R6 are dimensioned to give full scale deflection at a 1mA - 210 Ohms moving coil meter. For a moving coil meter having a different internal resistance or full scale deflection current, R4, R5 and R6 must be adapted for full scale deflection with 0.33 Vdc and 3.3 Vdc output from IC1.
IC1 is a "true rms detector" SMD chip. Between 0 and 800 MHz its conversion gain is abt. 7.5Vdc / Vrms, and its RF input resistance 225 - 175 Ohms. IC1 is accurate and sensitive. It can detect -30dBm (1uW @ 50 Ohms) with an error of only -1dB. Together with parallel resistors R1 and R2 the total input resistance varies between 51.6 and 48.4 Ohms from 0 to 800 MHz. From 800MHz to 2.5 GHz, gain, impedance and meter readings will drop to lower values. For R1 and R2 thick film SMD resistors are chosen for their low self inductance. All resistors and capacitors are cheap 1206 size SMD components.
R5//R6 are dimensioned for full
scale deflection at 3.3Vdc output of IC1. Input voltage at IC1 then is 0,442Vrms
(+6 dBm). D1,2 prevent IC1 input for overload , and start limiting after
the meter reached FSD.
S2 can be switched to R4 for high sensitivity (+20 Db meter deviation). D3 then protects the meter for overload. R4 is dimensioned for full scale deflection at 0,33 Vdc output of IC1. The input voltage at IC1 is then 44.2 mVrms (-14 dBm).
IC1 is accurate and sensitive. It can detect 0.007 Vrms (-30dBm) with only -1 dB error. Equivalent to minimum measurable power of 1 uWrms over 50 Ohms load.
To minimize the chances of interference from low frequency sources, C8 attenuates frequencies below 14 KHz by 6dB/oct (-45dB @ 50Hz).
- Remove the packing from the lid.
- Screw the lid onto the box.
- Mark the position on a lid side for the 13mm hole for the Amphenol BNC bus (Conrad 748198)
REM: As Internally the sides differ, see drilling plans above for the correct side to drill.
Prevent melting of the ABS. Drill shortly and let cool down.
Check that the BNCbus just fits into the hole.
- Place the box in a vice, on a table drill stand.
- Pre-drill a hole of 2mm.
- Enlarge it to 6mm.
- Again enlarge it to max 13mm, using a conic drill.
- File the conic hole hole to cylindrical.
- Remove the lid.
- Change the shape of the hole, so the BNCbus can slide side wards into the lid.
- Carefully remove raw edges.
- Drill all other holes. REM:
switches 6mm, LED 5mm.
- Remove raw edges and counter sink them so the components will easily shift into them.
I designed the little PCB with longer solder pads. It makes heating the solder pads easier, preventing over heating components.
The two miniature switches are chosen for small size and small price. They have a pin pitch of 4,12mm. Switches with 5mm pin pitch will not fit into the PCB.
Do not solder the BNCbus, the two switches and the LED yet. They will be soldered later on.
- SMD capacitors have NO value
printed on them. You have to trust the value mentioned on their packaging.
Solder IC1 as last, and then take ant-static measures.
I found the following solder strategy practical :
- First pre-tin the component soldering islands on the PCB.
For each component, one soldering island will be used to pre-fix the component before definitive soldering. This island should contain a bit more solder.
- Place the component and quickly
flow one contact to the PCB to pre-fix its position.
- Inspect solder quality using a magnifier glass.
Pin1 of IC1 is marked with a dot in a corner. Its legs are bend towards the PCB.
- Then use a hot iron (350C) and a short, medium sized, solder tip. Tin the tip.
- QUICKLY overflow pins 1-3 with solder.
- QUICKLY overflow pins 4-6 with solder.
- QUICKLY remove excess solder from pins 1-3 using
a piece of fresh de-solder wick.
- QUICKLY remove excess solder from pins 4-6 using
a piece of fresh de-solder wick.
- Solder the BNC bus flat onto the TOP (non-component) side of the PCB.
Installation of the PCB into the lid.
See drill sketches.
- Install the switches into their
holes in the lid, with the nuts loosely fixed.
- Put the LED wires into the PCB. Do not solder them yet.
- Shift the BNC bus into its hole, while shifting the PCB over the switch pins.
- Then shift the LED into its hole in the lid.
- Fasten the nut of the BNC bus. The house of the BNC bus has to rest against the inside of the lid front.
- Check the correct position of the PCB.
REM : As the BNC bus has to rest to the lid
internal surface, the body-width of the BNCbus
will set the distance between
the PCB and the lid.
There should now be a little space between the base of the switches and the PCB.
- Solder the switches and the LED on
the component side of the PCB. Cut excess LED wires.
- Solder the meter wires and the battery wires to the PCB.
- Prevent broken wires. Glue the wires (with thermal glue) at the solder points to the PCB.
As this instrument does measure the real
rms value of the wave form presented to it :
1. A square wave signal will give a higher indication than a sine wave signal with the same amplitude.
2. A triangle wave signal will give a lower indication than a sine wave with the same amplitude.
For checking its performance, a sine wave signal with a known amplitude is presented to the 50 Ohms BNC bus as follows :
- Insert batteries.
- Put some soft foam plastic between the meter and the battery holder.
- Close the box.
- Set s2 into position "UNsensitive"
- Inject a 0.442 Vrms sine wave signal to the 50 Ohms BNCbus. The meter should indicate "+6".
- Set s2 into position "Sensitive"
- Inject a 44.2 mVrms sine wave signal to the 50 Ohms BNCbus. Again, the meter should indicate "+6".