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Project Yamhill Dual Double-Tuned Circuit
Double the fun
Every receiver needs filtering at the front end for the purpose of selectivity and eliminating possible sources of unwanted interference to the desired signal. A very common circuit used in HF amateur radios is the double-tuned circuit bandpass filter, which consists of two tank circuits coupled together with a small value capacitor, and terminated in a variety of ways in order to get a typical 50-ohm characteristic impedance.
My version of this circuit for the Project Yamhill implementation is derived straight from the classic text Experimental Methods in RF Design. There’s no need re-invent the wheel in this particular case. This board was designed to accommodate two filters, selectable with a simple pair of jumpers on 0.1-inch headers. Perhaps in a future version an electronic RF switch could be provided to allow band switching by the microcontroller, but that will have to wait for later in the project development.
The last module that will be required in order to construct the first Project Yamhill radio project is an audio amplifier designed to properly terminate the diode-ring mixer. This is perhaps the most critical circuit in a direct conversion receiver because the first gain stage has the most effect on the receiver noise figure, so we’ll need an amplifier with a low noise figure. Also, a diode-ring mixer will distort if it is not properly terminated in 50 ohms at the desired frequencies (baseband audio in this particular case), therefore we also want to make sure that the input impedance of this amplifier presents 50 ohms to the mixer IF port.
A simple bipolar transistor common base amplifier is often used here, because its input impedance is close to 50 ohms. However this circuit is not without its flaws. A few years ago W7ZOI published an amplifier with two transistor stages, and negative feedback from the second stage emitter to the input of the first stage. This circuit is looks like a good subject of experimentation and potential performance to me, so I’d like to implement it in Project Yamhill, make some measurements and potentially do some tweaks to component values as the measurements may suggest.
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