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RF Power Meter with seperate RF break-out-board [160193]

Status: Finished
September 25, 2017
in Elektor magazine | March 2018 | Find it here
Based on:

We changed a few things. The PCB is smaller, about the size of the LCD module used. It is just a little longer so the three pushbuttons can be placed on the right side of the display. The LCD module and the pushbuttons are placed on the bottom side. The rest of the parts and the two other modules are mounted on the top side. For best performance the RF log detector/controller AD8317 and its accompanying components are put on a separate 4-layer PCB in a break-out-board style (this is the other one of the two other modules mentioned). Because of the high frequencies the resistors and capacitors used are size 0402 (except for C4, size 0603). The RF Module can also be used for other purposes or put closer to the signal source. Only a minimum of three wires are needed between the RF Module (160193-1) and the main PCB (160193-2). A resistor is placed in series with trimmer P1 for better control of the contrast of the LCD. Most parts are SMD, including P1. The footprint of the pushbuttons are designed to fit standard 6 mm tactile switches and also larger Multimec 3FT switches. An advantage of the Multimec types is they consist of a separate switch and cap. The cap is available in different heights (height of the cap is specified as total height from PCB surface to top of the cap when it’s mounted on the switch). Because the way the PCB is mounted almost all connections to the display and the RF Module are changed, compared to the original design. Inputs and outputs are defined at the beginning of the source code and can easily be corrected for any hardware changes. The output of the RF module is connected to A7 and the LCD module connections are virtually mirrored. Finally we used a different 2.048 V reference. The MCP1501-20E/SN is more  accurate (0.1 %) and about the same price. It needs a resistive load (R4) in order to avoid instability. The AREF input of the Arduino Nano is decoupled with 100 nF (onboard of the Arduino Nano Module!).
To get the maximum performance out of the AD8317 we took a close look at the evaluation board of Texas Instruments. With these high frequencies a 4 layer design is essential. One inner plane is used solely for the power supply. The parasitic capacitance to the other ground planes improves decoupling of the power supply. TADJ is also available on K2 but is not used here. Vout is connected to Vset through a voltage divider (R4, R5). In our setup a zero ohm resistor is used for R5. R4 is left unmounted. With this divider the slope can be increased to more than the minimum 22 mV/dB. For more details have look at the datasheet of the AD8317.
For testing our first prototype we mounted additional pin sockets for the LCD and the two modules. Disadvantage of building the RF Power Meter this way it becomes very tall. Three PCBs are stacked. On the Arduino Nano an ICSP header is sticking out even further. Total height from front of the LCD to the top of the ICSP header is about 43.5 mm. If  the sockets aren’t used and 5 mm standoffs are used for the LCD module and the pin headers of the two other modules are soldered directly into the PCB total height is only 28 mm. But be careful when soldering everything together. If all SMDs are mounted on the main PCB solder the two modules (Arduino Nano and the RF Module) first and cut the protruding pins. When this is done mount the LCD on four 5 mm M3-standoffs (male-female) and only then solder the pin header (LCD1).
We performed some measurements (prototype RF Module 160193-1 only) using the tracking generator of an old Tektronix 2710 Spectrum Analyzer. Its maximum frequency is 1.8 GHz. Amplitude range is -48 to 0 dBm. We measured the output voltage at 100 MHz, 1 GHz and 1.8 GHz.
N to BNC adapter on analyzer, BNC cable 50 cm coax RG058, BNC to SMA adapter om RF Power Meter
            100 MHz           1 GHz                1.8 GHz
            (21.3 mV/dB)   (22.1 mV/dB)   (22 mV/dB)
Vin       Vout     mV/dB Vout     mV/dB Vout     mV/dB
[dBm]  [V]                      [V]                       [V]
  0        0.423                  0.417                  0.391
-10       0.610    18.7      0.624    20.7      0.601    21
-20       0.834    22.4      0.832    20.8      0.786    18.5
-30       1.052    21.8      1.067    23.5      1.044    25.8
-40       1.274    22.2      1.292    22.5      1.244    20
-48       1.447    21.6      1.480    23.5      1.445    25.1
N to BNC adapter on analyzer, BNC-BNC adapter (2 x female), BNC to SMA adapter
            100 MHz           1 GHz                1.8 GHz
            (21.3 mV/dB)   (22.1 mV/dB)   (22 mV/dB)
  0        0.421                  0.409                  0.380
-10       0.608    18.7      0.614    20.5      0.583    20.3
-20       0.832    22.4      0.821    20.7      0.770    18.7
-30       1.050    21.8      1.057    23.6      1.027    25.7
-40       1.272    22.2      1.283    22.6      1.233    20.6
-48       1.445    21.6      1.471    23.5      1.434    25.1
We also connected the RF Module using two other cables:
1st cable:
BNC - 35 cm RGU400 - SMA: at 1.8 GHz output voltage 4 mV higher as measurement without cable. So this cable has about 0.2 dB extra attenuation at 1.8 GHz.
2nd cable:
BNC - 1 m RG223 - SMA: at 1.8 GHz output voltage 11 to 14 mV higher as measurement without cable. So this cable has less than 0.6 dB attenuation at 1.8 GHz.
Vout max         1.718 V (no input signal)
Vout min          0.359 V, measured at +10 dBm and 15 MHz
According to the datasheet absolute maximum input power is +12 dBm
Prototype of RF Power Meter:
Vin min (K1)    6.6 V, below this input voltage the output of  the 5 V regulator drops.
Supply current  85 mA
Bill of materials 160193-1 (RF Module)
R1 = 52.3 Ω, 1 %, 100 mW, SMD 0402 (ERJ2RKF52R3X, Panasonic)
R2 = 499 Ω, 1 %, 62.5 mW, SMD 0402
R3 = 200 Ω, 1 %, 62.5 mW, SMD 0402
R4 = not mounted, SMD 0402
R5 = 0 Ω, 1 %, 62.5 mW, SMD 0402
R6 = 1 kΩ, 1 %, 62.5 mW, SMD 0402
C1,C2 = 47 nF, 10 %, 25 V, X7R, SMD 0402
C3 = 8.2 pF, +/-0.5pF, 50 V, C0G/NP0. SMD 0402
C4 = 100 nF, 10 %, 16 V, X7R, SMD 0603
C5 = 100 pF, 10 %, 16 V, C0G/NP0, SMD 0402
IC1 = AD8317ACPZ-R7, SMD LFCSP_VD (CP-8-1)
K1 = SMA, 50 Ω, Straight Jack, Edge Mount (142-0701-801, Johnson/Cinch)
K2 = 1x5 pin header, vertical, pitch 2.54 mm, through hole
K3,K4 = 1x2 pin header, vertical, pitch 2.54 mm, through hole
PCB 160193-1 v1.0
Bill of materials 160193-2
R1,R2,R3,R5 = 10 kΩ, 0.1W, 5%, SMD 0805
R4 = 1 kΩ, 0.1 W, 5%, SMD 0805
R6 = 330 Ω, 0.1 W, 5 %, SMD 0805
P1 = 10 kΩ, 0.25 W, 20 %, SMD (3314G-2-103E, Bourns)
C1,C3 = 100 nF, 50 V, 10 %, X7R, SMD 0805
C2 = 2.2 µF, 25 V, 10 %, X7R, SMD 0805
D1 = 1N4007, 1000 V, 1 A, THM
IC1 = MCP1501-20E-/SN, SMD SOIC-8
K1 = DC Power Connector, 3 A, 1.95 mm, NEB 21 R Lumberg
S1,S2,S3 = Switch PCB SPST-NO, RA3FTH9 Multimec
S1,S2,S3 = Cap round, black, height 16 mm, 1S09-16.0 Multimec
LCD1 = LCD Module 2 x 16, 80 x 36 mm, 3 mm mounting holes
LCD1 = 1x16 header, vertical, pitch 2.54 mm, through hole
MOD1 = 160193-1, RF Module
MOD2 = Arduino Nano
Optional: sockets can be used (not recommended):
For LCD:         1x16 socket pin socket, vertical, pitch 2.54 mm
                        four  12 mm M3-standoffs, female-female
                        four 3 mm screws (length > 6 mm)
For MOD1:      two 1x2  plus one 1x5 pin sockets, vertical, pitch 2.54 mm
For MOD2:      two 1x15 pin sockets, vertical, pitch 2.54 mm
Depending on mounting of the RF Power Meter inside a case: extra M3-standoffs (male-female), M3-nuts and M3-screws are needed.
PCB 160193-2 v1.0
Other Multimec caps and extenders for 3FT series of switches available at Farnell (5 April 2017):
2S09-09 (no longer stocked)
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Intro 1 (Prototype of RF Power meter (160193 v1.0)
Intro 2 (Prototype of RF Power meter (160193 v1.0)
No signal (Prototype of RF Power meter (160193 v1.0)
Input signal to high (Prototype of RF Power meter (160193 v1.0)
Example 1 at 1.7 GHz - frequency (Prototype of RF Power meter (160193 v1.0)
Example 2 at 1.7 GHz - attenuation (Prototype of RF Power meter (160193 v1.0)
Top view of prototype of RF BOB 160193-1 v1.0
Bottom view of prototype of RF BOB 160193-1 v1.0
View on display of prototype of RF Power Meter (160193 v1.0)
View on modules of prototype of RF Power Meter (160193 v1.0)
Side view prototype of RF Power Meter (160193 v1.0)
Side view on prototype with sockets (160193)

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