Frequency Ratio Circuit

Question

I'm attempting to design a circuit that will determine the frequency ratio between a known reference frequency \$f_\text{ref}\$, and an unknown signal frequency \$f_\text{sig}\$. The signal frequency can vary dynamically between 1 GHz and 2 GHz. The goal is to use a reference frequency of about 100 MHz such that the frequency ratio can be easily represented with an eight bit counter.

My current thinking is to use a T-flip flop to create a gate signal from \$f_\text{ref}\$ such that the positive duration of the gate pulse equals the reference period. At that point there appear to be a couple of approaches that seem viable. The first is to AND the gate pulse derived from \$f_\text{ref}\$ with \$f_\text{sig}\$, then feed the output into a counter. This is a direct measure of the frequency ratio. The second option is to use the gate pulse as the enable signal into a counter. This is the approach I'm leaning towards. After determining the frequency ratio, I'd like to use this frequency ratio count to program the divider in a separate phase locked loop or direct digital synthesizer.

Ultimately I'm looking for help in finding a counter with the following capabilities. The first is the ability to accept a count enable that is driven by the gate pulse produced by the T-flip flop. Once the count is completed when the gate pulse goes low, I want to output the frequency count in the form of a binary output to program a separate divider before the gate pulse goes high again. It seem that a counter such as the 74HC590 with output registers would be a good candidate. But I'm also considering the benefits of using the timers of a microcontroller to compute the frequency ratio instead. It seems as using the microcontroller would make for a more efficient solution, as it could be used to compute the frequency ratio and then send the computed value to a PLL divider.

I hope my description of the problem is clear, and I would appreciate any insight on appropriate counters, or whether the microcontroller route is better.

Answer 1

Since you only want an integer multiple, as far as I can tell ("\$12f_\text{ref} \le f_\text{sig}<13f_\text{ref}\,\rightarrow 12\$"), what about simply using a cheap synthesizer IC that takes a reference oscillation, and produces multiples of that?

Use a microcontroller to program your synthesizer to generate 10, 11, 12, …, 19,20 × reference frequency.

Use that local oscillation to mix down your \$f_\text{sig}\$, and low-pass filter with a filter bandwidth of \$\frac{f_\text{ref}}2\$. If something passes through that filter, you know you've hit "close" to that local oscillator.

You can of course increase resolution by using smaller steps (synthesizers can also do things like 10.25545×\$f_\text{ref}\$), and narrower low-pass filtering.

By the way, what you're building with this is practically a spectrum analyzer :)

Answer 2

How about converting to a square wave, detect the zero-crossing timings down to picosecond? using time-to-charge converter, digitize that using flash ADC with 1nS convert time to extract 8 bits, and do this for each of the zero crossings.

Then take the Fourier Transform.

Answer 3

If your circuit doesn't need to be integrated anywhere, the cheapest way to go is probably the SDR way these days:

just spend 10–20$ on an RTL dongle, and 35$ on a raspberry pi, or use any other PC /laptop lying around.

Tune that (e.g. using GNU Radio) to 100 MHz and observe the frequency of your reference signal. Use the relative error to determine your RTL dongle's frequency standard's error. Then tune through e.g. 2 MHz wide steps from 1 GHz to 2 GHz, and use the same frequency detector logic to determine the \$f_\text{sig}\$. Correct that with the relative error measured at 100 MHz. Done.

Answer 4

@analogsystemsrf's approach isn't bad at all:

convert to a square wave of known amplitude (i.e. amplify your \$f_\text{sig}\$ signal a lot and then clip).

Then filter with a known filter gain (e.g. a low pass with 10dB/frequency decade); measure power, and use that as frequency-proportional measure?

If your granularity is 100 MHz, this very coarse approach might work well enough, especially if you can calibrate.