Panadaptors Didn’t Start With SDRs

The must-have accessory on a modern all-singing, all-dancing amateur radio transceiver is a panadaptor. Inevitably driven by SDR technology, it’s a view of a band in the frequency domain, and it will usually be displayed as a “waterfall” giving a time dimension to see transmissions over a period.

[Bill Meara, N2CQR] reminds us that panadaptors are nothing new, indeed that they date back to the first half of the last century and don’t even need an SDR to work. And to prove it, he’s produced one for part of the 40-metre amateur band.

The principle behind an analogue panadaptor is simple enough, it’s a normal receiver whose local oscillator is given a linear periodic sweep over the desired frequency band and whose output drives the Y axis of an oscilloscope whose X axis is driven by the sweep. In [Bill]’s case the receiver is a BitX homebrew transceiver, and the swept local oscillator is provided by his Foeltech signal generator. A neat touch comes in the ‘scope being synchronised by triggering on a marker frequency at the bottom of the range being swept. He’s created a video showing it in action, which you can see below the break.

There are quite a few routes into making this type of simple spectrum analyser, indeed some of us have tried ti with TV tuners.

Continue reading “Panadaptors Didn’t Start With SDRs”

Universal Active Filters: Part 1

Today I am experimenting with a single chip Universal Active Filter, in this case I made a small PCB for the UAF-42 from Texas Instruments. I chose this part in particular as it facilitates setting the filter frequency by changing just a pair of resistors and the somewhat critical values that are contained on the chip have been laser trimmed for accuracy. This type of active filter includes Operational Amplifiers to supply gain and it supports various configurations including simultaneous operating modes such as Band Pass, Low Pass and High Pass make it “Universal”.

Filter Basics

Speaker Crossover Example
Speaker Crossover Example

Looking at the block diagram you can see where I have inserted a dual-ganged potentiometer to change both resistors simultaneously which should allow a straight forward adjustment for our purposes here.

Looking into the components of a simple RC filter which can easily implement a simple Low Pass or High Pass filter, we see that the math is fairly straight forward and swapping the components with each other is all that is needed to change the type of filter. Continue reading “Universal Active Filters: Part 1”