AD8307 Power Meter Update

In an earlier post (RF Power Meter), I discussed my power meter project which is based upon the amazing AD8307 logarithmic amplifier chip. This device takes a logarithmically scaled RF input and produces a linear DC output with a resolution of ~25mV/dB.

Well, I'm finally able to put this project to bed. After scratch building an initial board, I decided to go with a much cleaner solution by purchasing a pre-fabricated board from E&M Solutions. The finished board is shown below:

Completed board with RF shield covering log chip and input stage

The circuit board provides two ways to show the power level of a signal: an analog panel meter for an approximate reading plus a DC output jack for use with a DVM when I need more accuracy. In order to improve the overall resolution of the output, the board uses an op amp with a trimmer pot to produce a gain of ~2.1 to boost the AD8307 output to 5V when the maximum expected input signal is applied. The panel meter circuit has an additional trimmer pot to adjust the meter sensitivity. After adjusting both of these trimmers, whenever an input of 10dBm (10mW) is detected I should read 5.000V with my DVM and the panel meter should be at full scale.

Next, it was time to tackle the meter calibration. This required applying a known signal level, recording the output voltage, attenuating the signal by a specific amount, then recording the new output voltage. The response of the AD8307 could then be characterized and represented by a linear equation with an exact slope and y-intercept.

Previously I had built a neat little CMOS reference oscillator that puts out a square wave with a precise power level of -10dBm @ 10MHz. I used this tool to assist in the final calibration of the board. Applying this signal to the input of my meter produced 3.95V at the DVM output. Next, I removed the square wave reference and replaced it with a 10MHz sine wave oscillator I had also built earlier (Sine Wave Oscillator). I adjusted the amplitude of this signal source until it matched the power level output by my square wave reference. Now, with a sine wave of -10dBm as my signal source, I attenuated the signal by 20dB. The DVM output now read 2.87V. (Note - attenuating the signal from the square wave reference oscillator would have given inaccurate results due to design constraints with the AD8307). After a bit of math, I calculated the slope and y-intercept values. The final "response curve" for my power meter can now be represented by a simple, linear equation:

power (in dBm) = 18.5 x (voltage reading from DVM) - 83.15

Inductance Test Jig

Recently, I was winding some toroids for a low pass filter I was building.  I needed two different values: .28uH and .48uH using T37-6 cores.  These required winding 10 and 13 turns respectively, according to my toroid winding calculator.  Once they were completed I measured their values with my AADE L/C meter.  The “.28uH” toroid measured .36uH while the “.48uH” toroid measured .60uH, so something wasn’t right.  I tried spreading the windings further apart to lower the inductance and that helped a bit but the readings were still too high.  I began to suspect something was up with my meter.  I needed another way to confirm the values so I started looking for a circuit that could be used to measure inductance.  I stumbled across a simple test jig from a YouTube video (w2aew - measuring coil inductance) where the author used a circuit described in a September 1990 article from 73 Magazine.  A picture of the circuit is shown below:

To use, simply insert the part to be tested into the circuit, hook up an oscilloscope to measure the resultant resonant frequency and then do a simple calculation to determine the unknown inductance value.  I altered my circuit slightly by using a jumper for S1 and substituting J310s for the FETs.  The jig was assembled using “ugly construction” techniques on a small copper board.  

Once completed, I tested the jig by inserting an inductor with a known value (marked as 3.9uH) and found that the resonant frequency was 4.848MHz.  

Resonant frequency of 4.8478MHz

The calculated value of L was determined to be 4.07uH.  My L/C meter measured 3.93uH  giving a difference of <4%, well within
acceptable limits for me.  This confirmed that the meter was performing as it should.

So, it appeared that the toroid cores I was using were to blame.  To compensate, I reduced the recommended number of turns by 1 (or 2) and re-checked the readings.  This seemed to do the trick as seen in the table below:

                                               Test Jig (uH)     L/C Meter (uH)
                                                    .299                    .286           
                                                    .294                    .288           
                                                    .489                    .483           

An added feature of this test jig is the ability to measure the resonant frequency of a tuned circuit (like an IF transformer).  Simply remove the jumper to the left of the sockets, insert the device and read the resulting output frequency.

 Admittedly, for general purpose use as a way to determine an unknown inductance, my L/C meter is quicker and more convenient but its much more satisfying to use something you built yourself.

SLA Battery Charger

I recently came into possession of a used 12V SLA (sealed lead acid) battery.  This would finally allow me to go portable with my Rockmite 40M QRP rig.  I knew it would eventually need re-charging so I started looking for a charger circuit w/o all the bells and whistles.  I found one pictured below and documented at this website (555 battery charger).

The LM317 is configured as a constant current source providing ~500mA.  The 555 timer (with its two internal comparators) along with two external trim pots allows for the setting of two trip points: when a battery is hooked up and its voltage is 11.5V or lower the charger does its thing until 14.2V is reached, when it subsequently shuts off. Very clean and elegant design.  The addition of a couple of status LEDs and a reset switch to force a charging operation completed the project (see below).

Charger encased in plastic enclosure

Some of the component values were changed but the basic design was followed.  I did have to modify the 2N3904 biasing circuit by lowering the voltage at the base using a voltage divider.  Without this mod the 555 output never went high enough to turn off the transistor.  As shown below I decided to go with point-to-point wiring.

On a side note, I did learn something interesting about the 7805 voltage regulator.  A capacitor (.1uF) hooked to the output is absolutely needed even though the spec sheet says its "not needed for stability".  I found out otherwise.  Without it the chip was unable to maintain a constant 5V when the input voltage changed.

Power Supply (Redesign)

One of my very first home built projects was a variable power supply. Sure, I could have bought one but where’s the fun in that? Plus, I wanted to learn more about them and be able to repair it in case the need arose down the road. I found a simple design for a variable output supply (5-30V @2A) on the internet based on the LM350 linear voltage regulator. Using that circuit plus a little additional info from the spec sheets gave me what I needed. I laid out and built my own PCB, collected the parts and built it back in the winter of 2013/2014. It worked…but, I forgot about one critically important factor. Linear voltage regulators are very inefficient and with high loads coupled with large voltage differentials (Vin-Vout) generate a lot of heat. I didn’t account for that with my circuit. Instead, I just stuck on a modest, clip-on heat sink thinking that would suffice. It didn’t. The first time I used it at 5V with a moderate load, it shut off. Upon inspection I discovered that the LM350 was extremely hot so it was obviously going into thermal shutdown mode.

Fast forward a couple of years...I finally decided to completely revamp the design and start over. I made the decision to build three supplies into my existing enclosure: two new fixed supplies of 5V and 12V @2A each and a "properly designed" variable supply covering 5-25V @1A. For the two fixed supplies I choose to go with buck converters (switchers) because of their greater efficiency (ie less heat generated) under load. I went with the Texas Instruments LM2596 chip. It only requires the addition of a couple of external capacitors, a diode and a power inductor and you're done. The spec sheets actually contained a sample PCB layout along with a step by step guide for choosing the proper external components (including heat sink) based upon your desired maximum load. I scratch built the boards, ordered the parts (I got the inductors as free samples from the manufacturer) and assembled them. Tests were then run to check for output voltages at both minimum and maximum loads:

	5V Supply	12V Supply
output - V (@10mA)	4.96	11.96
output - V (@2A)	4.87*	11.89*

*actual outputs measured at the panel jacks were 4.93V and 11.94V respectively, the difference being the voltage drop across the power supply leads.

Next came the more challenging task, the variable supply.  This time I made sure to incorporate power dissipation and component thermal resistance values into my design to insure there would be no more heat issues.  For my variable supply, Vin = 30V and the lowest Vout = 5V.  At 1A this works out to be 25W:

    power (in watts) = V (volts) x I (load in amps) = (30-5) x 1 = 25W dissipated by the LM350!!!

Obviously, way too much to handle, so I made a slight design modification.  I added a bypass power transistor to improve the load distribution (see below):

This is a well-known and proven technique to increase the load handling of a linear regulator. The value for resistor R1 is selected to develop .7V drop when a specific current passes through it, thus turning on  transistor Q1.  The power transistor then handles the majority of the load while the regulator takes care of the rest. Initially, I decided to set the maximum regulator current at .25A and let the transistor handle the remainder (.75A).  This meant I would need a 2.8 ohm bias resistor (.7V/.25A = 2.8 ohms).  However, I ultimately decided to increase the regulator current to ~.333A to give a better balance.  A standard resistor value of 2.15 ohms was selected giving me a regulator current of .326A (.7V/2.15 ohms = .326A).  Here are the resultant power consumption numbers:

power (regulator) = (30-5) x .326 = 8.15W…okay

power (transistor) = (30-5) x .674 = 16.85W…okay

With the power dissipation completed I could now focus on the thermal resistance (TR) calculations.  These numbers are available on the individual spec sheets of each component.  TR is expressed in units of °C/W.  In other words, for every watt a component must dissipate, it will rise “x” number of degrees above the ambient temperature.  Therefore, the lower the total TR the cooler the device will be at a particular load.  The goal is to keep the junction temperature of each device below its maximum rated value (125°C for the regulator and 150°C for the transistor).  I found a couple of decent heat sinks that offered good performance for their size, then did my calculations:

Transistor		Regulator (LM350)
	TR (°C/W)		TR (°C/W)
J-C	1.39	J-C	3.00
C-H	1.09	C-H	0.20
H-A	3.10	H-A	5.10
total:	5.58	total:	8.30

 

note: J-C = junction to case; C-H = case to heat sink; H-A = heat sink to air

Finally:

   junction temp of regulator under max load = 8.30 x 8.15W + 25°C (ambient) = 93°C

   junction temp of transistor under max load = 5.58 x 16.85W + 25°C (ambient) = 119°C

Both are under the maximum rated values with room to spare so everything should be fine.  For confirmation I ran the supply under maximum stress conditions (5V @1A) for 60 minutes.  No thermal shutdown!!!

The completed project is shown below:

Top, middle position - 5V buck converter
Middle, left position - 5 to 25V supply
Bottom, middle position - 12V buck converter

Return Loss Bridge

A return loss bridge (or RLB) is a handy tool for determining various antenna characteristics including impedance and standing wave ratio (SWR).   In short, coupled with an RF signal source and power meter, it can make a good antenna analyzer.  The basic circuit is quite simple as shown below:

I opted for a prefabricated PCB from E&M Solutions to keep things neat and orderly. Three 49.9 Ω, 1% SMD resistors and an FT37-43 bifilar wound toroid (14 turns) completed the project.

Populated PCB

The fancy metal enclosure with laminated face plate took the most work and time but the end result is quite nice I believe.

Before putting the completed unit into service I need to characterize its performance.  As soon as my RF power meter is completed this will take place...soon hopefully 😏

Update (Fall 2018)
Power meter is complete so its time to finally check the RLB unit for proper operation.  Using my scalar network analyzer (Scalar Network Analyzer), I injected a 10MHz sine wave into the GEN port and measured the output with my power meter (RF Power Meter) at the DET port for a reference point.  Next, I inserted a 50 Ω dummy load at the LOAD port and remeasured.  The new reading was ~39dB of directivity or return loss which is an excellent number (the larger the number the better).  Using a web based return loss calculator, this corresponds to an SWR of 1.023:1 (a perfect match would be 1:1) so this device is now ready for use.

7dB Pad

This "pi" attenuator is for my Yaesu FT-450D transceiver. I wanted to be able to output at QRP levels but the Yaesu only goes down to 5W. This pad will handle up to 10W input and give me an attenuation factor of 5.

To test for proper operation, I attached a 50 Ω dummy load to the output connector and then applied 11.95VDC to the input connector.  The output side measured 5.36VDC which corresponds to 6.97dB of attenuation.  Pretty darn close!   Now I'll be able to operate in the 1-2W range.

New Radio and First QSO!!!

I'm now the proud owner of a Yaesu FT-450D transceiver (purchased as a birthday gift to myself).  It covers all the HF bands plus 6M and will output up to 100W. Only plan on using CW so will put the microphone in storage...😊

Had my first QSO on 5/22/16!!! Talked with Ed (W8NLZ) from Virginia Beach, VA. Got a little flustered to say the least. Started receiving and managed to pick out my RST but suddenly got overwhelmed and found I could not keep up. Asked him to slow down but still had problems so decided to end transmission early. Felt like I was able to send pretty well, however. Need MORE PRACTICE receiving.