Softrock Ensemble VHF tuning


This post has a specific audience in mind, so unless you have, or are interested in a Softrock Ensemble VHF receiver, you’ll be bored. You’ve been warned ūüôā There are a few radio bands that are interesting near the ham 2 meter band. ¬†NOAA weather satellites send images using APT at and around 137 Mhz and there are weather broadcasts at 162.5. ¬†There was a question on the mailing list for the Softrock about whether it could be tuned to receive signals in all of these bands without re-tuning. ¬†This post is simply documenting my experimentation with the front end. ¬†I’m sampling the signal using an oscilloscope probe with the 10x setting and inserting the signal from the tracking generator connected to the antenna port. ¬†It’s not really possible to measure the precise insertion loss absolutely, so I’ll measure it relative to the baseline of front end tuned for 2 meters.

Tuned for 2 meters

To begin the testing, I tuned as best I can to the 2M band. ¬†The filter shape isn’t as flat as I would like, but it’s the best I could do.

Tuned for NOAA APT transmissions

It was a little easier to tune the front end for NOAA APT transmissions.  I got a better filter shape, and about 5 dB better average insertion loss.

Tuned for weather radio

Tuning for weather radio wasn’t that bad either. ¬†The average insertion loss is about equal to what I got for 2 meters.

Compromise tuning

Finally, if you’re interesting in receiving signals from throughout the band, it’s possible strike a compromise. ¬†You’re just about always going to get a peaked response, so I placed the lower peak at 137 Mhz and the upper at 165. ¬†The 2 meter ham band is somewhere in the middle. ¬†The peaks at 137 and 165 are 5-10 dB below the baseline. ¬†The 2 meter band is a little worse at about 20 dB below the baseline. ¬†So, it’s possible to get a somewhat broadband response at the fronted at the expense of some signal strength. ¬†It may be possible to mitigate some of this if you use a low noise preamplifier.

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