Attenuators

Attenuators (aka pads) are useful for many things and are easy to make.  The ones I've made are known as "pi" attenuators because the resistors are arranged in the shape of the Greek letter π.

Pi attenuator

There are numerous websites around that can calculate the resistor values needed for a given value of attenuation and impedance (I work exclusively at 50 Ω).  Just be sure to consider the power rating of the resistors (especially R1 since it bears the brunt of the dissipation at high attenuation values).  Also, for RF projects you need to use resistors with low inductance values (ie. don't use wirewound) to minimize high frequency issues. One site that I found useful allows you to enter your parameters into an Excel spreadsheet (power-attenuator-calculator). 

I built a fixed 20dB attenuator (10W input) and two switchable attenuators (1W input) using metal film resistors.  I honestly spent more time and effort on the enclosures than the actual circuits themselves.  Once assembled they were easy to check out.  After terminating the output with a 50 Ω dummy load, I simply applied a known DC voltage to the input (being careful not to exceed the power rating of the input resistors) and measured the output voltage.  Plugging those two values into the equation below gave the expected attenuation:

                                          attenuation (-dB) = 20 * log (Vout/Vin)

10MHz Sine Wave Oscillator

I built my first oscillator using the schematic shown below but w/o the 19dB pad (obtained from the amazing but sadly now defunct website - "VE7BPO ‘Popcorn’ QRP / Home Builder")

Basic 10MHz oscillator w/ pad

"Ugly" construction techniques were used (also a first for me). The main use for this circuit will be to assist with calibrating my AD8307 based RF power meter (whenever it’s finished). However, it can also serve as a platform for learning more about oscillators in general and if needed, function as a stand-alone sine wave generator.

"Ugly" construction

Powered it up and...IT WORKS!!! Not at first…no output, then I started adjusting the trimmer cap and…viola…output. It now produces a nice looking waveform. Definitely need to invest in a non-metallic screwdriver for adjusting trimmer caps in the future (waveform goes crazy otherwise). I will probably go ahead and add a 20dB pad to cut down on the signal level (currently ~1.03Vpp @ A with 50 Ω termination).

Screenshot at A (10.0057MHz)

RF Power Meter

This project is based on the Analog Devices AD8307 logarithmic amplifier. It has a dynamic range of over 80dB making it very useful for measuring a wide range of power levels from -70dBm (.1nW) to +13dBm (20mW) with a frequency response of 0-500MHz. The DC output is directly proportional to the logarithmic input and can be used to drive a panel meter.  The original design incorporating this chip came from a June 2001 article in QST by Wes Hayward (W7ZOI) and Bob Larkin (W7PUA).

Original circuit from June 2001 QST

I took this original idea, combined it with ideas from similar designs found on the web and created a PC board from scratch.  Initial testing seemed to show promise but I wasn't completely happy with the overall result so the project sat idle for several months until...

Update (Fall 2016)
I decided to start over using a pre-fabricated board developed by another hobbyist at the Yahoo group PHSNA (https://groups.io/g/PHSNA). This one had better RF shielding and was just a cleaner looking board. I also added a 20dB attenuator to the front-in to extend the upper range to ~1W (Note - technically I could have gone up to 2W and been within the chip's specs but decided to cap things at a more conservative 1W).

I got the two most expensive parts as free samples from Analog Devices but one part, the AD820 op-amp, came as an MSOP (micro small outline package) and the board called for a DIP (dual inline package). I built a breakout board for this part as shown below:

MSOP to DIP adapter

Still working on the finishing touches, but things are quickly wrapping up and should be finished soon...

Basic QRP Power Meter

One of my first home built projects was a basic power meter that I planned on using for future QRP work (once I got my amateur license 😊).  It is essentially a peak voltage detector consisting of a 50 Ω dummy load (three, 150 Ω, 2W resistors in parallel), a diode for rectification and a small capacitor for smoothing out the voltage.  Here is a basic circuit I found on an amateur radio club site (NorCal Power Meter).

I scratch built my own circuit board using a stencil made out of paper and fingernail polish as the resist.  The components were gathered and soldered into place and everything was then tucked away in a nice enclosure to complete the project.  Next came the calibration. The blue pots were used for this purpose so that a full scale reading on the panel meter would occur when a 1W or 5W signal was applied.

Top of board with calibration pots in blue

As an example, since P = V*V/R (where R = 50), it follows that 5W represents 15.8Vrms or 22.3Vpk.  Subtract 0.5V for the diode drop and you have 21.8VDC.  This voltage was then applied to TP1 from my power supply and the pot adjusted to achieve a full scale reading.

Completed project