Wide band power detector to control microwave station

 

by Philippe Borghini / F5jwf

 

 

 

 

 

Document Release 1.23

April 19th 2010

 

 

 

Introduction

Everyone knows the famous logarithmic detectors from Analog Device. So many designs have been done with the well known AD8307. These kinds of circuits provide a dc voltage proportional to the RF signal expressed in dB.  Analog Device expands recently his detector family with the ADL5519 which is still usable at 10GHz.

The assembly presented below integrates a dual channel power probe by using the ADL5519. This design provides a control unit to measure the output power of microwave radio station. His enhanced linearity makes it also valuable as a good alternative to the old HP432 RF power probe to any Ham microwave labs.

 

Block Diagram

The particular interest of the ADL5519 is focused on bandwidth which is specified up to 8GHz (but still usable in 10GHz with reduced dynamic). With two channels, it is possible to measure the power transmitted to the antenna and the reflected wave, to calculate the return loss and block the PTT when problems arise. As I originally destined it to my EME 13cm station the module is controlled remotely by serial interface (RS232 and RS485) to be installed in the antenna 30m away the shack. A simple RS232 VT100 terminal allows the full control of the assembly as the whole software is embedded.
The set also includes some additional features such as measuring the temperature of the PA with KTY probes and power supply voltages
.


Figure 1: Block diagram of DetLog module. Logarithmic detector and Micro Controller Unit (MCU).

 

 

 

 

 

The RF coupler remains external to the detector and can be done either with semi rigid coaxial cable or with commercial device. The coupling factor is taken into account by the software and can also be included in the calibration process.

 

 

 

 

Typical Applications of the DetLog board

Control of 13cm F5jwf EME station

 

 

 

Linearity and dynamic

Without calibration, the linearity of the ADL5519 is already excellent. The frequency behavior is also fairly flat but a calibration specific to each frequency can improved a little bit the linearity. The dynamic is around 50dB for an accuracy of +/- 0.5dB. Isolation between two channels is better than 40dB.
Figure 2 shows the ultimate accuracy obtained at 1296MHz + / - 0.1dB over nearly 40dB
.

 

Figure 2: Linearity at  1296MHz.

 

The curves for other frequencies are quite similar:

 

Linearity for 144…1296MHz

Linearity for 2320…10368MHz

 

Up to 8GHz, the dynamic of 50dB is guaranteed. Above this frequency the sensitivity decreases and more power is needed for the same detected voltage. The linearity is also a little bit degraded but the detector is still usable as PA controller.

 

The input impedance of the detector chip is equivalent to 2kOhms load in parallel with 0.6pF. Loaded with a resistance of 50 ohms on the input, we obtain acceptable impedance up to 2 GHz but degrade rapidly beyond. The input attenuation pad can significantly improve the return loss and the broadband performance. The layout has two 10dB pad achieved with 0402 SMD resistors. These pads can shift up the dynamic to 10 or 20dB increasing the compression point respectively to 10dBm or 20dBm. Obviously this comes at the expense of sensitivity.

 


It is advisable to have at least 10dB to ensure a good matching of the input. The maximum power rating of 10dB attenuator made with 0402 SMD resistor is around 100mW, beyond an external attenuator must be added.

An optimum configuration in terms of dynamic and sensitivity for return loss measurement is 20dB attenuation on the forward channel and 10dB on reverse channel. This option gives the following dynamics:

 

Channel Forward:     Dynamic~ -30dBm…+20dBm                       S11~ < -23dB for f=144M...10GHz

Channel Reverse:    Dynamic~ -40dBm…+10dBm                       S11~ < -20dB for f=144M...10GHz

 

 

 

 

 

Note: 10dB attenuation seems to be an optimum of flatness for an attenuator in PI with regular SMD package. This value corresponds to resistances value between 70 and 100 ohms. Outside these limits the behavior of capacitive or inductive SMD package dominates and introduces resonances.

Calibration

The calibration process is the action of putting a well known level at the RF input and entry this level to the microcontroller to correct the measurement error. Of course few points are necessary to obtain good correction and those are frequency dependant. The calibration algorithm finds then the best linear regression minimizing the error over the full dynamic. This process is done by an external test bench based on RF synthesizer and commercial power meter, taking about 50 points Vdetected = f (Pin), and calculating the gain and offset for each frequency. These values are then stored in ROM. The calibration takes into account the linearity error of the detector and the imperfection of input attenuator. The measurement is then displayed directly in dBm.

 

Basically for the kit proposed below two calibration are possible:

.       Default standard cal  (kit#1 and kit#2) accuracy around typ. +/-0.5dB up to 1.2G and typ +/- 1dB for higher freq.

.       Calibration dedicated to the DetLog board mounted and tested only possible on kit#3. Then the accuracy is around +/- 0.2dB

 

Remark:

The calibration algorithm does not work on the frequency axis. It means that between two calibrated frequencies no correction is done.

Example: If you want to measure at 3400 MHz you have to use the closer cal, let say 2320 MHz and then correction is not optimal.

You could run the built in calibration process at 3400 MHz to calculate the linearization parameters at this frequency and save it on one free memory.

           

 

The built in cal use up to 8 measurements points to calculate the linearization. This procedure can also be used to take into account external attenuator or coupling factor. At the end of the process the cal parameters are saved into the current cal memory which is located in ROM (no volatile after power off).  A total of 9 memory banks are available to save calibration for further uses.

 

 

 

Figure 3: The calibration factors for one specific frequency can be reloaded with the VT100 terminal

 


A simple offset to the displayed value is also configurable to take into account such as a directional coupler factor or an external attenuator.

 

 

 

 

 

 

 

 

 

 

 

 

 

Voltage and temperature measurements

The DetLog  PCB measures also voltages and temperatures of power amplifier. Two channels are available:

 

.       Voltage 1:     Resistif divider 1:7.8 Input= 0…40Vdc

.       Voltage 2:     Resistif divider 1:7.8 Input= 0…40Vdc

.       Temp 1:         KTY-10-6 temperature sensor Temp=-20…100°C

.       Temp2:          KTY-10-6 temperature sensor Temp=-20…100°C

 

A PWM (Pulse Width Modulation) output can be connected to control the speed of the FAN. The speed is controlled by the software according to the temperature Temp1.

 

 

 

VT100 User Interface
Measurements of power, temperature and voltage are displayed directly on the terminal screen (VT100 emulation commonly available under any OS). The different functions are handled by the terminal and are summarized in the on-line Help. The Return Loss limit for example is configured with the terminal. Above this configured level the module open a relay to block the PTT.

 

Figure 4: Main page on the RS232 terminal. Current and "max.  hold" power is displayed in dBm. PA voltages and temperatures. The display is refreshed every 100ms.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Video outputs

 

Two analog outputs (one for each channel) giving an exact replica of the measured power is available as a DC signal varying from 0 ... 5V for the total detector dynamic. They can drive analog meter or external XY plotter.
The RS232 port can also be used for automatic measurement bench and power points can be transferred in third-party software.

 

 

 

 

VT100 Commands descriptions

 

The built in help is access on the RS232 port by typing ?. The Figure 5 and Figure 6  are then displayed.

 

 

Figure 5: Online help is displayed on the VT100 terminal with the ? command.                    

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6:  Page 2 of the help.

 

 

 

 

 

 

 

 

 

 

 

List of the command

Note 1:            All command are NOT case sensitive, they can be either upper or lower case independently.

Note 2:            The format to entry attenuation in dB (or power level in dBm) is on 3 digits without decimal point.

Example: To entry the attenuation value of -12.5dBm you have to entry -125 

 

 

 

Command

Explanation

Example

DISP

Display the main panel with the measurement. Escape and carriage return allows to go out of this page.

 

RL_LIMIT x

Set the Return loss limit. If the measured Return loss exceed this value the micro relay open which block the PTT of the station.

x = -85 for RL max of  -8.5dB. If measured RL is -5dB, the PTT will be blocked to protect the PA.

ATT_PFWD x

Set the value of the external attenuator for forward channel. Negative value represents attenuation.

x = -100 for external attenuator of -10.0dB

ATT_PREV x

Set the value of the external attenuator for reverse channel. Negative value represents attenuation.

x = -100 for external attenuator of -10.0dB

AVG_LEVEL x

Set the value averaging. When measurements are close to the noise, average can be increase to higher value to improve measurement confidence.

x =1, 2, 4, 8, 16

SET_TARGET_T1 x

Set the temperature limit for temp1. When the temperature measured by the probe temp1 will come close to this limit, the controller will increased the speed of the PA fan by increasing the duty cycle of the PWM.

x =430 for a temperature limit of 43°C

DISP_CAL

Display the calibration table.

Cf. Figure 3

SAVE_CAL x

Save the current calibration into memory bank x.

x=1…9

RECALL_CAL x

Recall the calibration of memory bank x into current cal.

x=1…9

 

FORCE_DEFAULT

Set all variable of the controller to factory settings. Could be useful when bad parameters have been entry. Following this command, the factory standard cal is reloaded and custom cal lost. 

 

ADC?

Request to get the ADC value (10bits) of the RF detector. The result is express in microvolt. 

Vdet_Pfwd=4562000 Vdet_Prev=2350000

POWER?

Request to get the measured power of both channels. The result is express in dBm *10.  The command can be used in automatic test setup to read the power level.

Pfwd=-245 Prev=-372

PROMPT_ENABLE x

In normal situation, the controller answer to each command with a prompt symbol which is >. During automatic measurement when the module is pooled by tiers party software, disabling the prompt avoid complicated ASCII stream handling. With x=0 the prompt is not send anymore.

X=0 or 1

CAL_PFWD freq

Start the build in user calibration process. The software expect that you connect RF source and you entry the power level.

CAL_PFWD 1296

NB_POINT x

This command works with the previous "CAL_PFWD freq" command and allows to entry how many points is used for the calibration. The maximum points for the built in cal in 8.

NB_POINT 4

PIN x

This command works with the previous "CAL_PFWD freq" command and allows to entry the value of the power level you just connect to the input port. The maximum dynamic is -70dBm…+20dBm. Remember that the format for level must comply with Note 2

PIN -200    (for -20.0dBm)

CAL_PREV freq

Same command as CAL_PFWD freq  for the channel reverse.

CAL_PREV 1296

SET_CAL_FREQ x

Allows to entry the frequency for the current cal.

SET_CAL_FREQ 1296

SET_CAL_SLP_F x

Allows to entry the cal SLOPE cal parameter for the current cal. It is expressed in dB/mV *100 as shown in Figure 3.

SET_CAL_SLP_F -6472

 

SET_CAL_INT_F x

Allows to entry the cal INTERCEPT cal parameter for the current cal. It is expressed in dBm *10 as shown in  Figure 3

SET_CAL_INT_F -395

SET_CAL_SLP_R x

Same as SET_CAL_SLP_F x for reverse channel.

SET_CAL_SLP_R -6472

SET_CAL_INT_R x

Same as SET_CAL_INT_F x for reverse channel.

SET_CAL_INT_R -395

 

 

 

 

 

RS485 Bus

 

 

Since few developments, F1TJJ and I start to implement RS485 communication between our HAM radio modules. As this two wires protocol is differential it allows transmitting easily few ten of kbps over very long twisted pairs. Stephane also implement the MODBUS (protocol level 2) which run over the rs485. This solution provide a frame oriented communication with error detection and device addressing over the bus.

To keep simple, the DetLog module just attached itself on this two wire and can be remotely controlled by using a small piece of software running on windows XP.

 

 

 

 

 

 

 

 

 

Figure 7: Windows XP HAM Modbus Controller application

 

 

 

 

Design and Layout

The schematic of the module is shown on the Figure 7 to Figure 9. It is a very standard application of the detector and of the MCU devices and nothing special has to be said. Only a few additional circuits allow interfacing with outside world. The module is powered with a single 12V dc power source.

 

Remark: These figures here below give an overview  to present the module. To be sure to have the up to date version use the pdf design folder  

 

 

 

 

Figure 8: Schematic part 1: Detector with 10dB attenuator pad.

 

Figure 9: Schematic part 2: MCU with serial interfaces RS232 and RS485

Figure 10: Schematic part 3: Interface components. Standard DB15 connector is used for control signals. On the right of the DB15 the external component (KTY temp. probe, PTT relay,…) can be seen.

 

 

The layout is done on printed circuit board FR4 double sided plated holes of 0.8mm with serigraphy on both side. The PCB can be soldered in a standard 74x74 Schubert box. Most components are SMD 0805, 0603 and 0402 for input attenuators.

 

 

 

 

Figure 11: Two side PCB Layout. TOP layer in red and Bottom in blue.                                 Figure 12: Component view. Almost all parts are soldered in the TOP layer.

 

 

Bill of Material

 

 

 

Document up to date ready for download

 

Download the Up to date Design Folder in pdf

 

Configuration file for Windows Hyperterminal

 

 

Mounting guidelines

All parts are SMD, they have to be soldered carefully but this kit is definitely within the reach of any HAM radio who is experienced with assembly in the RF and microwave band. The most sensitive step is the soldering of the detector ADL5519. The best practice from my experience is to use SMD solder paste and heat up with hot gun. The chip is then auto centered into the pad and is magically done. The details of the mounting are explained in the design folder above.  The following video give an idea how to deal with this step.

 

Soldering of the ADL5519 (35MB)

 

 

Conclusions

In terms of dynamics and accuracy, this detector is a good alternative at affordable price of most sensors available at flea markets.

The DetLog board provides functionalities which can be used in a lot of application like power amplifier control, labs power measurement, antenna gain measurement with competitive accuracy. It can be easily interfaced trough serial interface for automatic test bench. 

A big part of my time in this development has been used for software design, I really want to thanks Stéphane F1TJJ  (http://www.ham-hyper.com/) for his great support during this phase.

 

 

 

 

Kit availability

 

I have organized myself to propose the following kits:

 

 

Kit#1: PCB and special parts

 

Price 40 euros

PCB , microcontroller MC9S08AW32 programmed with standard calibration, Detector ADL5519

 

 

Kit#2: Kit with all parts

 

Price 90 euros

PCB, microcontroller MC9S08AW32 programmed with standard calibration, Detector ADL5519, all parts according to the BOM, including drilled Schubert box but except the two input SMA connectors.

 

 

Kit#3: Fully assembled and tested

 

Price 250 euros

Detector box fully assembled, tested calibrated (accurate cal). Each module is calibrated and delivered with calibration sheet.

 

 

Those kits can be ordered to my email address: