HPSDR Alex Filter Bank Test Results

The HPSDR Alex filter unit is a set of low pass and high pass filters together with switching relays and control logic. It provides the "front end" to an HPSDR high frequency radio system.

This page shows the performance of a randomly-selected production Alex as measured with a network analyzer. I earlier did measurements of the prototype Alex unit, which are available here.

The raw S-parameter and magnitude files, as well as analyzer configuration information, for these measurements are available here.

Wideband Receive Performance

These plots show the wideband receive filter path from the Alex "ANT 1" port to the "TO RX" jack, going(through transmit LPF, receive HPF, and 55 MHz anti-aliasing filter). Alex was set for 20M for these measurements, but the permanent 55 MHz filter dominates the VHF/UHF response and it doesn't much matter which band you select for this measurement; performance above 55 MHz will be just about the same.

It is very important to note that good rejection at VHF requires adding short ground wires between the Alex RX and TX boards as well as to the Alex enclosure. Without these, the only ground connection is via the coax jumper between RX and TX board or the external coax connections. They result in a resonance that significantly reduces the ultimate attenuation of the filters. Here is more information on this grounding issue.

These plots show Alex's wideband receive response through 1.3 GHz and 250 MHz.

Low and Medium Freqency Receive

When the HPSDR system is tuned below 1.5 MHz, the 160M high pass filter switches out of line and only the 160M LPF and 56 MHz anti-aliasing filter affect the signal. So, Alex can be used for medium and low frequency reception. However, between 1.5 and 1.8 MHz, the 160 MHz high pass filter is in line and there is significant attenuation at those frequencies. This plot shows Alex tuned to 1 MHz.

Ham Band Receive Performance

These are plots of the Alex receive frequency response on each ham band showing the contributions of the TX LPF, RX HPF, and RX 55 MHz anti-aliasing filter.