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 🙂 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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