FM Demodulator using 74HCT4046 - Page 1
Forum section: electronics / rf-microwave

author: ManniW
author_rank: Contributor
timestamp: August 03, 2022, 02:00:04 pm
content: Hi,I have an existing circuit that modulates a composite video signal to an FM signal using a 74HC4046. They connected an 8.2kOhm resistor as R1, R2 is unused and I believe that I have identified C1 as being a 22pF capacitor by building a clone of the modulator (but with a 74HCT4046) and then trying out various values for C1 to see when the outputs matched on a logic analyzer.I then went ahead and tried to build a demodulator using the same R1 and C1 values. The low pass filter was designed with a 47pF and a 22pF Capacitor in Parallel and a 500 Ohm potentiometer. I tried to adjust it to see if I would get this circuit to work but unfortunately I haven't been able to. The output seems to follow the correct signal pattern but seems to be too slow. In the "middle" of the potentiometer range the signal looks better than when turning it to min or max, so I assume that I am on the right track and that I am just missing some minor things. How do I need to design this filter properly (or are there other ICs that could maybe do this more easily)? I also had a point where the sync signal went up instead of down, so I guess the PLL lost it's lock and decided to lock into the other direction there.The original demodulator uses an XOR-Gate aswell as various resistors, capacitors and transistors for demodulation, but there the signal looks absolutely clean. Unfortunately it is impossible to measure the capacitors without desoldering, otherwise I would have built a clone of that circuit. I attached a comparison of the original aswell as my demodulator on an oscilloscope, it looks like the filter is too slow in that case, however turning the potentiometer in one direction causes it to look like it lost its lock and in the other direction it turns even more flat.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 03, 2022, 03:42:04 pm
content: https://www.ti.com/lit/an/scha002a/scha002a.pdffor cd 4046b page 17 has FM demod RC selection method , sample at 10 kHz, and waveforms.

author: ManniW
author_rank: Contributor
timestamp: August 03, 2022, 04:53:47 pm
content: Quote from: mag_therm on August 03, 2022, 03:42:04 pmThanks, I also found that and tried to understand it. They have f1 in the formula but never describe where that is coming from or what that is and how it is determined. In the datasheet they only mention a low-pass-filter so I assumed that this filter is to prevent the entire PLL from self-oscillating and destroying the signal in doing so. That's why I built one that's designed for 5,5MHz as that should allow the PAL Signal to go through with some margin, but the results look as in the attached oscilloscope picture where the original demodulator and my demodulators output are shown.

author: mikerj
author_rank: Super Contributor
timestamp: August 04, 2022, 09:48:13 pm
content: Quote from: ManniW on August 03, 2022, 04:53:47 pmThat seems to be an error, it's actually fL. 2fLis the lock range, as defined in table 4 of the CD4046 and page 17 of the TI 74HTC4046 datasheet.