The circuit in the figure below is a circuit for measuring frequencies for RKE systems (a key fob, for example). The frequencies for which this circuit was created, was used to measure frequencies between 100MHz and 500MHz. The values of capacitors and inductors in the schematics are not very accurate because I did not measure these components out of the board yet. However, the doubt I have about this circuit is much more qualitative than quantitative, I still haven't fully understood its functionality.
As far as I can understand so far, the antenna, which in this case is nothing more than a loop made of very thick conductive wire, feeds a prescaler, the MC12080, which divides the frequency received from the entena by 20. The output of the prescaler passes by an amplification step to be able to feed a binary ripple counter, MC74HC4040A, which in turn loads an 8-Stage Static Shift Register, CD4021BC.This circuit, as described above, has the effect of dividing the original frequency by 20, then by 256, that is, it divides the original RF frequency by 5120.
I imagine that, when the microcontroller sets the input level of transistor Q2 to low for a short period of time, it counts the number of pulses at the output of the shift-register and, this way, calculates the frequency of the signal received by the antenna.
However, I have the following doubts:
1) In the circuit, after the prescaler, there are two branches, and in the lower branch in the schematic, there is a transistor Q2 that I do not know the reason for its existence. There is a capacitor and a resistor in parallel in the collector of this transistor that I don't know what they are for. The colector of the transistor Q2 is connected to the microcontroller, but I don't know why this is made.
2) I intend to use this circuit to perform frequency measurement in the 300MHz-500MHz and 800MHZ-1GHz bands. In fact, I'm interested in frequencies of 315MHz, 433MHz, 868MHz and 915MHz, but I have doubts whether the antenna currently used to measure frequencies between 100MHz and 500MHz will, under higher frequencies, provide enough voltage values for the prescaler. So, I imagined to put a second antenna, which I would choose by switching a relay. What do you think about this ? Can I do this with an antenna as simple as a single conductive loop like the one currently being used?