At my second house, I have no way of listening to CW or SSB, unless I hook up my homebrew 20 meter SSB transceiver, or my FT-817 transceiver, so I’m thinking I might try to build a simple communications receiver to solve this perceived problem.
My first receiver build was a direct conversion (DC) receiver, that did not work very well, but motivated me to learn how to build better superheterodynes. I’ve attached a copy of the magazine article for that first receiver. So, I’m thinking I might have a try at another DC receiver. These seem to be the new state of the art as SDR receivers.
As with the general coverage broadcast receiver, commercial off the shelf parts will be used if there is an advantage to do so, and the cost is reasonable. My goal will be realized with a simple kitchen table receiver that works well for its intended purpose and can be used on a daily basis. A secondary goal is to make it a true kitchen table receiver, by doing all construction at my second house on the kitchen table, with no resort to any of the tools / equipment in my ham shack. A multi band receiver would be nice, but probably is not essential, at least at first.
I have an extra working QRP Labs VFO left over from the development of my 20 meter SSB transceiver, so it will probably be used for frequency synthesis, at least in the early stages of development, if not permanently.
This will be an experiment / work in progress sort of thing, sharing time with a couple of other projects I want to do. My simple SSB transceiver took three years to satisfactorily complete, and the simple general coverage broadcast receiver took another year after that, so those are the sort of time frames I operate in when tinkering / experimenting with stuff.
73,
Win W5JAG
A DC receiver that uses a quadrature sampling detector and I Q digital processing (digital DC) might not need any front end filtering. Front end filtering is a necessity for an analog DC receiver.
Shown here are a couple of 50 ohm filters: a low pass filter with Fco of 2.5 MHz, and a bandpass filter to cover 2 to 6 MHz. These are good enough for preliminary experimenting.
When I was building my SSB transceiver and, to a lesser extent, the broadcast receiver, a point of annoyance was that I could never get a filter or tuned circuit to work without winding a toroid for the L component. So I had low expectations for these filters.
To my surprise, taking into account my crude means of measurement, these actually look pretty good. The low pass filter begins to roll off at about 3 MHz and is 20 dB down at about 7.5 MHz. The band pass filter is flat from about 3 to 6.5 MHz and still looks to be temporarily usable for forty meters. Note that in the attached graphs, the vertical amplitude scale is not the same in all graphs - some are at 20 dB per division, some are 25.
Another alternative for narrow bandwidth filters would be the traditional top coupled double resonator filter. These work well but can be tricky to tune. A way of visualizing the passband would be optimal.
I am trying to avoid circuits that have to be tuned, thus my selection of bandpass filters.
These graphs were made by using the signal generator and spectrum analyzer functions of the 14 bit StemLab Red Pitaya. I set the signal generator to sweep a specified frequency range, and then used the max hold function of the spectrum analyzer to capture the frequency response of the filter. Crude, but works.
73,
Win W5JAG