CIRCUIT NOTEBOOK
Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at
standard rates. All submissions should include full name, address & phone number.
RF burst power meter
The circuit presented here enables
reasonably accurate and repeatable
RF burst transmission and continuous wave power measurements with a
simple and low-cost circuit. Although
far simpler than the Silicon Chip Low
Cost Wide Band Digital RF Power
Meter (August 2020; siliconchip.com.
au/Article/14542), it has a large analog display and can make burst measurements.
The circuit has two novel features:
it uses a servo to provide the readout
with a dial of any size, and the burst
measurement function, implemented
as a peak-hold. The peak-hold function
is enabled by a mode switch.
It was designed to measure the transmit power of 2.4GHz WiFi AP beacons
(peak envelope power) and was also
tested with other burst transmissions
formats and in different bands, eg, proprietary frequency-hopping protocols
in the 2.4GHz and 910MHz ISM bands.
The implementation is straightforward, consisting of a PICAXE-08M2
microcontroller (eg, Altronics Cat
Z6111A) and a miniature servo motor
(eg, Altronics Cat Z6392 or Jaycar Cat
YM2758). The AD8318 module cost
about $18 from AliExpress and came
with a metal shield around the AD8318
and input components.
88
Silicon Chip
You could likely use any AD8318based module, but it’s worth checking
that they do not have a low-pass filter capacitor fitted (CLPF) on pin 5 (if
they do, remove it).
The benefit of using a servo as a display is the ability to calibrate the meter
scale regardless of linearity changes
over the dynamic range of the AD8318.
For enhanced accuracy, multiple
scales for different frequency bands
could also be made. The drawback of
an analog display is the resolution.
I built the meter by attaching a
pointer to the servo motor over a calibrated dial scale. Calibration was
performed by temporarily attaching
a sheet of paper to the meter face and
using a pen to mark measured points
around the dial at the frequency of
interest.
Multiple frequency arcs can be calibrated on one scale, mitigating frequency response issues; eg, you could
have 2.4GHz and 910MHz scales.
After that, I removed the paper and
scanned it, then loaded it into a drawing software package. I then redrew the
dial scale using the measured markers in appropriate arcs. I extrapolated
intermediate points between the measured marks. I then printed the new
dial and fitted it to the servo.
Australia's electronics magazine
The resolution is approximately
0.3dB per servo step. This calculation
is described in the source code. As an
example, with a large 80mm pointer
attached to the servo, the dial scale will
have approximately 4mm between
1dB increments over a range of about
60dB. I made another version using a
Holden speedometer stepper motor
and pointer, with code changes to suit.
The BASIC source code for the
PICAXE is in a file named “rf-servo_
v1.bas” that can be downloaded from
siliconchip.com.au/Shop/6/6479
George Mackiewicz,
Vermont, Vic. ($100)
siliconchip.com.au
