Converting 0V/5V levels to -1.4/+1.4 levels - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: November 05, 2024, 06:20:44 pm
content: I'm working with a cable TV system which uses 14kbps Manchester biphase coding to throw data around the network.Inside the headend it's transmitted over 75-ohm F-type co-ax cable using SN75188 line drivers (with 68-ohm series resistor into the cable) and LM311 comparators (with 75 ohm input termination and ~0.9V threshold). It also gets modulated onto an FM (technically FSK) carrier with ~75kHz deviation for the boxes.I'm building up a box to handle splitting and combining the various data paths: the transmit path uses an LM311 as a line receiver, the output of which feeds a bunch of 75188 drivers to feed the various pieces of headend equipment.I'm currently using a Marconi 2022E to generate the FM signal, which wants a 1V RMS input.That means that (if I have this right) I need to translate from the 0V/5V output of the LM311, to -1.414V and +1.414V respectively.At the moment I'm using a 75 ohm terminator plug which gives ~0V/2V levels; the Marconi will take this, but it's hardly ideal.I've tried doing this with an opamp differential amplifier circuit, and found that the waveform (at least in the LTSPICE simulation) was awful.Is there a better way to do this?I'm planning to use an AD9834 DDS with a Si5351 local oscillator and a mixer (an AD831 or a diode bridge mixer if I have one on hand) for the final implementation, but for now I'd like to get things going with the Marconi as it's a known quantity.CheersPhil

author: Benta
author_rank: Super Contributor
timestamp: November 05, 2024, 07:52:54 pm
content: Why not just AC couple it?

author: moffy
author_rank: Super Contributor
timestamp: November 05, 2024, 10:20:25 pm
content: The 1.414 to 1 ratio only applies if the waveform is a sine wave, but you have a square wave, I guess you want the deviation of the FM signal to be +/- 75kHz, so whatever the kHz/V of the VCO is.As Benta suggested AC couple it, Manchester coding is supposed to have no DC component so a suitably large cap should work.

author: jwet
author_rank: Frequent Contributor
timestamp: November 06, 2024, 04:21:12 pm
content: Not mentioned in discussion- the LM311 output is open collector and pulls the output down to "ground" which is only used for the output, it also has bipolar supplies- this gives you a lot of flexibility on output of comparator. Output swing can be whatever works. A careful reading of the datasheet is required- its pretty flexible.

author: philpem
author_rank: Frequent Contributor
timestamp: November 19, 2024, 06:53:45 pm
content: Quote from: moffy on November 05, 2024, 10:20:25 pmIt sounds like the easy solution, but I'm concerned about distortion via the capacitor.I've tried it before and had issues with the first few bytes of data being corrupted if the data stream was stopped and restarted -- basically the time it took for the cap to charge to half VCC.The kHz/V of the VCO isn't specified - and it wouldn't really be specifiable as it's variable from the front panel (in FM modulation mode it'd be the Deviation setting). Effectively it's going to be the kHz set on the panel, for that 1V RMS.Quote from: jwet on November 06, 2024, 04:21:12 pmI'll have a closer look - thanks. I think the Marconi terminates the modulation input into 600 Ohms, which might make it trickier.