This SA uses a pair of inexpensive SI4432 wireless ISM transceiver modules. For use in a SA the important features of the SI4432 transceier is a frequency range of 240 to 940 MHz. at up to 20dBm output, variable receive bandwidth using DSP technology, and a Receive Signal Strenth Indicator signal with .5 dBm. resolution down to about - 120 dBm. Complete modules with all the support components needed are available for less than $5 through the normal sources.
There are two different filters in the SA that I needed to design for a circuit board. The first is the input low pass filter. I decided a range of up to about 200 MHz. should be more than adequate for what I need. My favorite design tool for filters is Elsie, with it I can quickly design various types of filters, and tune the circuit to use standard value components.
For the low pass I decided on a 7th. order capacitor input filter. Playing with the program and values I have on hand, I came up with this circuit.
I used some 24 guage silver plated wire wound around a 3mm. screw as a form. Four turns for the first inductor and five turns for the other two.
Connecting the circuit to the NanoVNA using one temporary SMA connector, and with a little reforming of the coils I was able to come up with a filter with the following response.
I had to settle for a little bit lower cutoff frequency than the original design to reduce the passband ripple but will be fine for the frequency range I want. It also gives me the option of using a 315 MHz. first IF frequency if I want to try instead of the 432 MHz. in the original Tiny SA.
The next filter I need to build and test is the first IF bandpass filter,
This will use a pair of 432 MHz. SAW filters.