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# SN7497: Choosing the right frequency dividers for pulsatile inputs at 80 and 160 MHz

Part Number: SN7497
Other Parts Discussed in Thread: SN74LVC1G17, SN74LVC2G17, SN74AUC1G80, SN74LVC1G80

Greetings,

I have two pulsatile signals whose frequency I wish to divide:

A. 8x frequency division of a pulsatile 80 MHz input. As exemplified in the following oscilloscope screenshots, its waveform is somewhat irregular, with a primary ~3 dBm peak followed by a secondary peak that can be up to -4.5 dBm high, in its average power (all measured to 50 ohm load):

B. 16x frequency division of a pulsatile 160 MHz input. As exemplified in the following oscilloscope screenshot, its waveform is clean but rather weak (-19.8 dBm in its average power to 50 ohm load, if I've calculated correctly):

In both cases, the optimal output would be a ~10 MHz sinusoidal, or clipped-sinusoidal wave, at ~3.3 Vpp to a DC-blocked 50 ohm load. Other periodic ~10 MHz output signals may also be acceptable, if they satisfy VOH of at-least 2 Volts and VOL of at-most 0.4 Volts, to a DC-blocked 50 ohm load.

Browsing through the available frequency dividers at this category, I could not identify a part that is supposed to work at this bandwidth, let alone with pulsatile input signals. Can you please suggest which evaluation board of yours, or a cascade of evaluation boards, is most likely to be able to pick up these signals correctly and produce the required output? Should I first regularize the input signals to square waves using a flip-flop (thereby also simplifying the problem to 4x/8x division of the resulting 40 MHz/80 MHz square waves)? If so, which flip-flops would be optimal for these input signals?

Thanks a lot,

Lior

• Hello Lior,

At these voltage levels, we don't have a device that can directly take this signal and do frequency division.
You will probably have to take the following steps:

1. In all likelihood you are going to need an amplifier that can amplify the signal to voltage levels that are acceptable for logic devices 0-3.3.
2. You then should consider converting it to a square pulse with a schmitt trigger input device such as an SN74LVC1G17 or SN74LVC2G17 (for 2 signals).
3. You should then take the square wave signal and put it into a D flip flop such as the SN74LVC1G80 to divide the signal down by 2, since this device can handle up to 150MHz. Although you may want to consider the SN74AUC1G80 as this is optimized for higher speeds up to 250MHz (but you need to keep voltage levels in mind)
4. You can then take this divided down signal and use the SN74LV393 counter to do multiple steps of frequency division within a single device to get to the point that you want. (I suggest step 3 before this to get the frequency down within a range that the LV393 device can handle.

This would give you a divided down square wave and is my best suggestion, but there are likely many more ways that you can acheive the same thing.

Best,
Michael