Baltic Lab | High Frequency Projects Blog

VHF LDMOS Power Amplifier Experiments

Posted on April 9, 2022 by Sebastian 3 Comments

The NXP MRF101 series high ruggedness N-channel Enhancement-mode lateral MOSFETs promise over 100 Watt CW output power and slightly over 20 dB gain from 1.8 MHz all the way up to 250 MHz. Their mismatch insensitivity for a VSWR of greater than 65:1 without significant device degradation makes these devices extremely interesting for amateur radio operators. While there a plenty of designs for the HF + 6m Bands on the internet, I wanted to see what it takes to use them on the 2m VHF band.

LDMOS VHF Power Amplifier Prototype

General Overview

For this experiment a single MRF101AN was used. The MRF101AN is suitable for single band PAs on the 6m (50 MHz), 4m (70 MHz) and possibly 1.25m (222 MHz) band. Operation on the VHF broadcast band (88 – 108 MHz) may also be an interesting application for some people. The 2m band (144 – 148 MHz) was selected for initial prototyping and proof of concept. For no other reason than that it seemed the most interesing to me.

The target was an input VSWR of less than 2:1 over the whole frequency range of 144 to 148 MHz. The quiescent current was set to 100 mA as per the datasheet recommendation. The biasing voltage was supplied to the gate via a resistor. From an AC standpoint, the resistor is connected to ground through a capacitor and is part of the input matching network. This is a common technique used in power MOSFET RF design.

Input / Output Impedance Matching

The first step was to match the input impedance of the MRF101AN to the desired 50 Ohms input impedance. According to the datasheet, the input impedance looking into the gate of the MRF101AN should be around 6.2 – j10.2 [1]. A strong capacitive reactance was to be expected given the relatively high frequencies and an input capacitance of 149 pF. The output impedance is given as 9.9 – j5.9. Therefore, it wouldn’t really be possible to use simple broadband transformers for the input / output matching. Since the desired frequency range of operation isn’t very broadband, this isn’t much of a problem.

I elected to use a simple L-Network for the impedance matching. I recommend reading my article on L-Network impedance matching [2]. It should be noted that for reasons of stability, the input of a power MOSFET should never be matched directly to 50 Ohms. The common design practice is to load the gate with a resistor and then match that to 50 Ohms. The output of a power MOSFET on the other hand can be matched directly to 50 Ohms.

The free software SimSmith was used to simulate different component values and simulate their behaviour over the desired frequency range of operation [3]. After trying different component values, I came up with suitable component values for the input matching:

SimSmith simulation of the MRF101AN input L-Network

When these values were tested, nothing seemed to work as intended though. This was no surprise since the prototype was built Manhattan-style. Building a circuit like this Manhattan-style is probably not the most wise design choice. But it certainly is possible. It should be noted that the prototype had a tendency to oscillate between 160 – 170 MHz. The oscillations seized once a bare copper board was placed between the gate and drain sides of the MRF101AN. Additionally, the amount of parasitic reactances threw of my matching circuits. It was apparent that the series inducor was too large. So I grabbed the next smaller size adjustable inductor I had in my parts collection. Now they’re likely too low, but this is just a proof-of-concept protoype. As long as the general circuit works, the exact component values can be adjusted once a proper PCB is designed.

This is the schematic of the “final” prototyping circuit that I used for my tests:

Schematic of the LDMOS VHF Power Amplifier Prototype

MRF101_AN_VHF_Amplifier_Schematic.pdf

Test Results

All simulation and guesswork asided, what really counts is results. Not much effort was put into the testing of the circuit. Just enough to be assured that a PCB with this general topology can be designed and ordered for fürther component value optimization. No tests for linearity and efficiency where performed. For all tests, the quiescent current was set to 100 mA. The prototype was supplied with 50 Volts DC. The output was properly terminated with a 50 Ohm dummyload. A large heatsink made sure that the MRF101AN stayed within its permissible operating temperature range.

The input VSWR of the power amplifier prototype was measured using a properly calibrated Nano VNA. As expected, the input VSWR isn’t perfect. But it is within the design criteria of less than 2:1 over the whole band:

Measured input VSWR of the MRF101AN LDMOS VHF Power Amplifier Prototype

The actual complex input impedance was also measured using the same set-up:

Measured complex impedance of the MRF101AN LDMOS VHF Power Amplifier Prototype

To test the output power of the power amplifier, a simple FM transceiver delivering about 1 Watt of RF power was connected directly to the input of the MRF101AN power amplifier prototype board. A Siglent SDS1202X-E was used to look at the resulting output waveform from the dummyload through a 1:100 oscilloscope probe.

VHF (144-148 MHz) LDMOS Power Amplifier Prototype Test

The resulting measurements would – if true – indicate that an output power of a little over 100 Watts was indeed achieved:

Output of the MRF101AN LDMOS VHF Power Amplifier Prototype

However, the results should be interpreted very sceptically. The measured output waveform appears to be very sinusoidal. But in reality, the signal likely looks very distorted. While the L-Network on the output of the amplifier certainly has a low-pass filter characteristic, not much harmonic suppression canbe expected by it. The oscilloscopes maximum bandwidth of 200 MHz makes it impossible to visialize any harmonic components of the signal. Therefore, the displayed waveform and indicated RMS voltage needs to be interpreted very carefully.

Additionally, the input current into the amplifier prototype was only around 2.3 A. That equates to 115 W DC input power. Assuming 100 Watts of RF output power, the resulting efficiency would be around 87 %. While such an efficiency would surely be impressive for such a simple set-up, it is also highly doubtful. Nonetheless, the dummyload gets noticeably warmer after just a few seconds of full power output.

Conclusions

The results are promising enough to design a PCB layout and continue testing this set-up. Expect a follow-up on this topic.

Also, I would like to get some feedback on whether or not a 4m-Band linear amplifier kit would be of interest for the amateur radio community?

Links and Sources:

[1] MRF101AN / MRF101BN Datasheet, NXP: https://www.nxp.com/

[2] L-Network impedance matching https://baltic-lab.com

[3] SimSMith, Ward Harriman, AE6TY http://www.ae6ty.com/

ICS501 / ICS501 VHF Experiments

Posted on February 26, 2022 by Sebastian Leave a comment

The ICS501 / ICS511 from Renesas are easy to use, fully integrated clock multipliers for output frequencies up to 200 MHz. What makes them particuarly interesting is the integrated crystal oscillator circuit and ability to produce some fractional multiples, such as 6.25 or 2.5. So let’s find out if they’re interesting for the hobbyist who wants to generate clock signals in the VHF range.

General Overview

Both the ICS501 and ICS511 are not designed for radio frequency applications. Therefore, they put out square wave signals and the examples given in the datasheets are targeted at typical clock frequencies used in the world of digital electronics. But that doesn’t mean we can’t use them. The two different chips vary in some ways. The IC501 is able to generate multiples of 2, 3.125, 4, 5, 5.3125, 6, 6.25 and 8 for output frequencies up to 160 MHz [1]. The ICS511 has multiplication factors of 2, 2.5, 3, 3.333, 4, 5, 5.333, 6 and 8 up to 200 MHz [2].

I was interested in how these ICs behave with 50 Ohm loads and sources. The nominal output impedance is given in the datasheets as 20 Ohms. In my experiments, however, no apparent signal degradation was observed when using the ICS501 / ICS511 to drive a 50 Ohm load. Unfortunately, the datasheets do not contain any information about the devices input impedance. But it can be assumed that it is rather high. I had no problems feeding a 4 Vpp AC coupled signal directly into the clock input pins of both chips. The chips also didn’t seem to care whether they were supplied a square wave or a sinusoidal waveform. They were happy with either.

Test Circuit

To make my experiments easier, I designed a small PCB with edge mount SMA connectors for clock input and output. I also included footprints for a HC-49 type crystal and corresponding load capacitors. The two solder jumpers can be used to set a fixed clock multiplication factor. The larger through-hole pads that can be seen in the pictures are reserved for future experiments where I will try to use a varicap diode to frequency modulate the output signal. Spoiler alert: Narrow-Band FM using a varicap is possible!

Homebrew breakout board for ICS501 / ICS511 clock multiplier experiments.

The PCB design contains a lot of design errors. But I already designed a Revision 0.2 PCB and send the files of for production. In its present condition, the circuit looks something like this (click to enlarge):

Schematic of the quick and dirty ICS501 breakout board

Test Results

All tests were performed using the ICS501 because I didn’t have any ICS511 in stock. The output signal was fed into a Siglent SDS1202X-E oscilloscope through a 50 Ohm termination resistor.

ICS501 output signal, 7 MHz input clock, multiplication factor 6, 42 MHz output signal

The integrated crystal oscillator works well to generate a 144 MHz signal from a 24 MHz crystal. I did not add any load capacitors on the PCB because the internal 12 pF (Source: [ 1], page 3) were sufficient to sustain a reliable oscillation. Frequency accuracy was not important for this test anyway. Note that the now more sinusoidal looking waveform is merely a limitation of my test equipment. The oscilloscope simply doesn’t have enough bandwidth to process the Fourier components (odd-integer harmonic multiples) that give a square wave its shape.

ICS501 output signal, 24 MHz crystal, multiplication factor 6, 144 MHz output signal

Next, I wanted to try what happens if the ICS501 is driven beyond its alleged maximum output frequency of 160 MHz. I was honestly not very surprised to see that the 30 MHz input signal was multiplied to 180 MHz effortlessly.

ICS501 output signal, 30 MHz input clock, multiplication factor 6, 180 MHz output signal

The next thing I was interested in was the output spectrum. A 15.1 MHz input signal was multiplied by 6 to generate a 90.6 MHz signal in the FM broadcast band.

ICS501 output spectrum, 15.1 MHz input clock, 90.6 MHz output. Horizontal scale: 2 MHz / division, vertical scale: 20 dB / division

In the center of the picture we can see the wanted 90.6 MHz signal. But various other peaks spaced a little over 3 MHz from the carrier are also visible. These peaks are calles reference spurs. Their spacing corresponds to the internal frequency used by the phase frequency detector (PFD) inside the ICS501. The exact spacing of these spurs is likely to be 3.02 MHz (input clock divided by 5). The 3.02 MHz is then used by the PFD to lock onto the output signal divided by 30 (90.6 MHz / 30 = 3.02 MHz) to this internal reference frequency. Reference spurs are faily typical for PLL circuits. But nothing to worry about, we can filter those.

If we look at a broader range of the spectrum, more spurs become visible:

ICS501 output spectrum, 15.1 MHz input clock, 90.6 MHz output. Horizontal scale: 10 MHz / division, vertical scale: 20 dB / division

Besides the aforementioned PFD reference spurs repeating every 3.02 MHz, we can now clearly see even stronger spurs spaced 15.1 MHz on either side of the desired 90.6 MHz center. This is simply leakage of our original 15.1 MHz reference signal.

Conclusions

Altogether I am quite happy with the results so far. These ICs might be a simple solution for generating frequencies in the VHF range from fairly low frequency oscillators, VFOs and standard crystals. Cascading two ICS501 / 511 opens even more possibilities. Especially with standard crystals, the fractional multiplier settings open a foolproof and inexpensive way of generating frequencies that otherwise would require more complicated PLL circuits.

Another interesting application would be to generate extremely accurate clock signals from a standard 10 MHz precision reference source (OCXO, GPSDO, etc.). Stay tuned for more experiments.

NOTE: The ICS501 is considered obsolete / not recommended for new design and chip availability can not be guaranteed. But there appears to be plenty of stock available from large distributors. Certainly enough for the average hobbyist.

Links and Sources:

[1] ICS501 Datasheet, Renesas Wikipedia: https://www.renesas.com/

[2] ICS511 Datasheet, Renesas Wikipedia: https://www.renesas.com/

Arduino |Automatically Save Variables to EEPROM on Power-Down

Posted on December 30, 2018 by Sebastian Leave a comment

A recurring challenge for Arduino projects is that variables are volatile and will be lost completely unless previously stored to some sort of non-volatile storage such as de inbuilt EEPROM. This article shows a minimalistic approach to detect sudden loss of power and automatically store a variable to an EEPROM adress.

First I should mention that this approach of course also works for other type of storage devices, such an external SD-Card. The simple solution I used in this 12V application is a 2-stage voltage regulator set-up and a power-sense input tied to an interrupt. The first stage gets the input voltage down to 9V. A voltage devider on the 9V rail supplies a steady high signal to PIN 3 of the Arduino Nano as long as a power source is connected to the circuit. As soon as this sense-input falls low, an Interrupt Service Routine (ISR) is triggered that stores the variable to a pre-defined adress of the EEPROM. The second stage consists out of two 7805 type voltage regulators. One for the Arduino Nano and one for all periphial devices such as the LC-Display. The Arduino`s 5 Volt supply voltage is tied to a 1F supercapacitor. This guarantees that the Arduino has enough power available to safely detect the missing supply voltage on the 9V rail and store the example variable to the EEPROM.

The whole Setup looks something like this:

Crude test setup with 1F supercapacitor


#include <EEPROM.h>

// Some Variable 
int SomeVariable;
// Pin to be used as power-down sense
int PWR_DWN_PIN = 3;
// The EEPROM address to be used
int EE_ADDR = 10;

void setup() {
  // Retrieve last stored value of SomeVariable from EEPROM
  EEPROM.get(EE_ADDR, SomeVariable);
  // Set-up Interrupt Service Routine
  attachInterrupt(digitalPinToInterrupt(PWR_DWN_PIN), PWR_DWN_ISR,FALLING);
}

void loop() {
  // Do something cool
  }

// This Interrupt Service Routine will get called on power-down
void PWR_DWN_ISR () {
    // Push SomeVariable to the EEPROM
    EEPROM.put(EE_ADDR, SomeVariable);
    
}

This Setup works very well. How long the supercapacitor can power the Arduino and whether or not you even need a separate supply rail for periphials depends on the current demand of your circuit. So here`s some math.

The charge of a capacitor is defined as follows:

$Q = It$

Q = charge of capacitor in Coulomb
I = Current in Amps
t = time in Seconds

So if we re-arrange the equation we could wrongly assume that if we divide Q by the current demand of our device, we will get the expected runtime in seconds. But we need to take into consideration that the Arduino does not work all the way down to 0 Volts. We need to define a lower voltage limit and divide the change of charge by the current draw of your device. But how do we get Q to begin with? The charge of a capacitor Q can also be expressed by the following Formula:

$Q = CV$

C = Capacitance in Farads
V = Voltage in Volts

So we can combine the two formulas in a way that we can calculate the time (t) if we know the capacitance of the supercapacitor, the expected current draw of the device and the maximum permissible change in voltage.

$t = \frac{\Delta VC}{I}$

So for my example I am allowing the 5 Volts to drop to 4 Volts, I am assuming 100 mA max. current and the supercap has a capacity of exactly 1F.

$t = \frac{(5V - 4V) 1}{0.1A} = 10 s$

10 seconds should be plenty of time to store a variable (or a few more) to the EEPROM. This approach works very well so far. A possible improvement, if needed, migth be to implement a change interrupt trigger instead of a falling edge trigger and re-initialize periphials on power-up if needed (e.g. LCD). This becomes necessary if power is reconnected during a time where the Arduino is still running but periphials where already shut-down. In that case they need a clean initialization before they will function properly.

Diamex Prog-S2 | Arduino IDE

Posted on December 30, 2018 by Sebastian Leave a comment

The Diamex Prog-S2 is an universal USB ISP programmer vor various microcontrollers including Atmel AVRs. This article shows how to integrate the programmer into the Arduino IDE

There are a few articles on the web that talk about the Diamex Prog-S2 and Arduino. But it seemed to me like they were either outdated or faulty.

To get the Arduino IDE to talk to the Arduino IDE, simply add the following lines to the programmers.txt file in the Arduino IDE program folder:

diamexavr.name=DIAMEX-AVR diamexavr.communication=serial diamexavr.protocol=stk500v2 diamexavr.program.tool=avrdude diamexavr.program.extra_params=-P{serial.port} -F

After restarting the Arduino IDE, the Prog-S2 should appear in the programmers list as “DIAMEX-AVR”.

The Diamex Prog-S2 appears in the programmer list after updating the programmers file.

Please don’t forget that you still need to select the proper COM-Port in order to use the Prog-S2 properly. Most importantly, you will have to use the “Upload using programmer” option to upload a Sketch to an Atmel AVR / Arduino chip using the Diamex Prog-S2.

Automated Forward Gain Measurement

Posted on January 28, 2016 by Sebastian Leave a comment

This article is going to take you into the world of automation. Using MATLAB and commercial off-the-shelf bits and pieces, this article will show how to measure the forward gain of a RF device.

For a long time now I had a bunch of Mini Circuits portable test equipment bits and pieces sitting around. S signal generator, a switch matrix and power sensors. These devices aren’t made for manual operation as they feature no direct way of manipulating the instrument’s state without software. They’re made for automation. And as such i thought an automation example would be in order. Like in the last article, we’re going to have a look at a practical example again. So here are the parameters:

For my passive secondary surveillance receiver at 1090 MHz I am using a Mini Circuits VBFZ-1065 filter [1] and a ZKL-2 amplifier [2]. The filter’s datasheet lists a passband of 980 – 1150 MHz and a 20 dB bandwidth of 630 – 1800 MHz. But let’s see if we can verify the data using an automated test setup.

Measuring the forward gain is quite simple. Simply apply a signal of known amplitude and known frequency to the device under test, measure the output power and compare. Let’s say we apply a 1090 MHz signal with an amplitude of -30 dBm to an amplifier and measure 0 dBm out, we know the forward gain at this exact frequency is 30 dB. So a minimalistic automated setup could consist of just the signal generator as source and the power meter as sink. But we want more than that. Clearly both the generator and the power sensor have certain nonlinearities when it comes to sourcing and measuring a specific amplitude accurately. But these nonlinearities are deterministic and close to constant. Therefore, we can measure the nonlinearities and apply correction factors to our final measurement.

Test setup on the bench

To facilitate this calibration mode, I included the switch matrix. The matrix can switch the source and power sensor directly into each other via a straight through cable jumper or it can switch both to the device under test ports. The following closeup may help understand what I am talking about:

Closeup of the test setup

All we need now is MATLAB code. The code’s job in plain english is as follows: Switch the matrix to straight through connection, sweep the system, normalize to intended power level, store normalized values as calibration factors, display cal factors, switch to device under test, sweep system, normalize to calibration values, display sweep results relative to desired output power.

Now in MATLAB speak:

% Connect to PWR Sensor pm1 = NET.addAssembly('C:\Users\Sebastian\Desktop\MC\mcl_pm64.dll') obja=mcl_pm64.usb_pm obja.Open_AnySensor


% Connect to Switch

switch1 = NET.addAssembly('C:\Users\Sebastian\Desktop\MC\mcl_RF_Switch_Controller64.dll')

objc = mcl_RF_Switch_Controller64.USB_RF_SwitchBox

objc.Connect
% Connect to Sig Gen

gen1 = NET.addAssembly('C:\Users\Sebastian\Desktop\MC\mcl_gen64.dll')

objb = mcl_gen64.usb_gen

objb.Connect
% Switch to Cal Bridge

% 0 = Straight Through, 3 = Through DUT

% objc.Set_SwitchesPort(0)
%% Sweep System
% Drive Level in dBm

Power = -30

% Start Frequency in MHz

Fstart = 700

%Step Size in MHz

StepSize = 2.5

%Step Count

Steps = 400
%% Acquire Calibration Values

% Switch to Through Connection

objc.Set_SwitchesPort(0)

% Enable RF

objb.SetPowerON

% Sweep

for C = 1:Steps
    % Calculate Frequency

    F = Fstart + (StepSize*(C-1));

    % Set Frequency and Power

    objb.SetFreqAndPower(F,Power,0)

    % Allow some settling time

    pause(0.25);

    % Read Power

    PWR=obja.ReadPower

    PWR=round(10*PWR)/10

    % Write result into Array

    A(:,C) = PWR

end

objb.SetPowerOFF
% Normalize

for Count=1:length(A)

  A(:,Count)=A(:,Count)-Power;

end

CalValues = A
power = A
% Make Frequency Scale

freq = linspace(Fstart,F,length(A))

% Plot

figure;

plot(freq,power)
%% Acquire Real Values

% Switch to Through Connection

objc.Set_SwitchesPort(3)

% Enable RF

objb.SetPowerON

% Sweep

for C = 1:Steps
    % Calculate Frequency

    F = Fstart + (StepSize*(C-1));

    % Set Frequency and Power

    objb.SetFreqAndPower(F,Power,0)

    % Allow some settling time

    pause(0.25);

    % Read Power

    PWR=obja.ReadPower

    PWR=round(10*PWR)/10

    % Write result into Array

    Real(:,C) = PWR

end

objb.SetPowerOFF
% Normalize

for Count=1:length(Real)

  Real(:,Count)=Real(:,Count)-CalValues(:,Count)-Power;

end
power = Real
% Make Frequency Scale

freq = linspace(Fstart,F,length(Real))

% Plot

figure(2);

plot(freq,power)
%% END

% Disconenct all Devices obja.Disconnect objb.Disconnect objc.Disconnect

You can download the MATLAB file here: MC_Forward_Gain.m

The code was assembled using the various programming examples Mini Circuits offers for download on their website [3]. The overall process to interact with this type of portable test gear is to call a Dynamic Link Library (DLL) and call predefined functions from inside the DLL. The DLLs can be downloaded from the previous link location also.

The result for my setup with the VBFZ-1065+ and the ZKL-2 looks something like this: