Another idea to increase the frequency step resolution would be to use "oversampling".
The nano's ADC has a 10-bit precision, this gives a range of 0-1023 steps for 50kHz. If we want to cover 200 kHz (4x more), but keep the same step size, then we would need two extra bits of resolution.
For every n extra bits of resolution that we want, we have to take 4^n samples at the base 10-bit precision. Then calculate the average of those 4^n samples.
So theoretically we could take 4^2=16 samples and average them.
For a range of 400kHz we would need 3 extra bits, so we would need to take 4^3= 32 samples and average the 32 measurements.
The expense of this approach however is that it would slow down the tuning system by (at least) 16 or 32 times. I have never tried it and I don't know how much practical effect this would have to the user/operator.
73, Allard PE1NWL
Allard PE1NWL