Circuit To Measure Phase difference
The classical circuit is a multiplier, which will output a voltage whose DC mean is proportional to the sine of the phase between the signals.

Depending on the frequency of the signals, the multiplier could be something like an AD835, or an RF mixer. You could even make something with a comparator and a DT switch or XOR gate.

A single mixer will not measure unambiguously 0-360. For that you use two mixers in quadrature, whose outputs are then the sine and cosine of the phase angle, from which you can deduce the phase correctly. 

For your specific application, where the amplitudes are variable, an analogue multiplier is not a good idea, as the gain will need normalising. It's probably best to use a fast (enough) comparator on each signal, and run the two logic signals into an XOR gate. In this case, the output will not be the sine of the phase difference, but directly proportional to it.

If you want a full 360 degrees measurement, then the best overall solution is to use the signal generator to deliver 4x your target frequency, divide it by 4 into two quadrature waveforms, and use two XOR gates, one for each channel. Your 4:1 frequency range is small enough to have a practical number of low pass filters reshaping the square wave from the divider back to something like sinusoidal. As you're going into a comparator, the harmonics will make a difference to phase accuracy. The needed accuracy will determine how close you need to be to sine.

Of course, this takes your requirement for analogue measurement at face value. It's far better to do it digitally. Capture the waveforms, process the signals in your language of choice. Assuming your likely minimum hardware, you can do that with little additional hardware by down-mixing your excitation and your measured signal to an audio IF (intermediate frequency), of say 10kHz. Put these two signals into the line input of your PC, capture (I use portAudio), and process (I use python+numpy). Worth a thought, you can recover the amplitudes as well, automatically log results, all sorts of good things.