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I'd like to connect the output of my software defined radio card, which is 50 ohms SMA, to the antenna input of a TV, to experiment a bit with signal generation.

The obvious problem is going to be the impedance mismatch, but that can be solved with a resistor network or a transformer.

Friends have mentioned that TVs often have a DC supply for antenna amplifiers, which would rule out the resistor network, and also the transformer, as that would short the supply voltage.

Is there anything else I might have missed that I'd need to know to protect my (rather pricey) SDR card?

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    \$\begingroup\$ Connect with a 10pF 100 volt C0G capacitor. \$\endgroup\$
    – Andy aka
    Commented Feb 23, 2022 at 19:50
  • \$\begingroup\$ "Friends have mentioned that TVs often have a DC supply for antenna amplifiers" is something I have never heard of or witnessed myself. Any references to back this claim up? \$\endgroup\$
    – Justme
    Commented Feb 23, 2022 at 20:17
  • \$\begingroup\$ I second the cap. Another thing to be careful of is that, in countries where N and E are physically bonded at the property, Y-class filter caps on the AC input can cause a discharge shock when the body of the SMA is given an external path to ground. (see electronics.stackexchange.com/questions/127104/… for details) \$\endgroup\$
    – Polynomial
    Commented Feb 23, 2022 at 20:18

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TVs don’t have DC power insertion by themselves. This only comes into play if there is an external antenna, which may have a booster amplifier built in.

Your main issue is going to be not overloading the TV front end. So you will want not only to control your impedance, but also to attenuate the signal from your SDR. You can do this with a resistor pad, then AC couple that to the TV with a blocking cap.

The signal level you’re looking for is about 0dBmV, +/-6dBmV. Roughly speaking, 0.5 to 2mV rms.

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  • \$\begingroup\$ I actually don't know the powers that come out of cable TV ... cables. \$\endgroup\$
    – Marcus Müller
    Commented Feb 23, 2022 at 21:22
  • \$\begingroup\$ Nominal level is 0dBmV. That’s 1mV rms into 75 ohms. \$\endgroup\$
    – hacktastical
    Commented Feb 23, 2022 at 23:04
  • \$\begingroup\$ So if your SDR DAC is making a 1V rms signal, it needs to be attenuated down to 1mV or so. The signal I have on my cable modem is 6-7dBmV. \$\endgroup\$
    – hacktastical
    Commented Feb 23, 2022 at 23:07
  • \$\begingroup\$ hm, taking the NI / Ettus USRP B210 as example, it does at most ~ 18dBm into 50Ω (depends on frequency, measurement done with a single tone), and it's open- and short-safe, so if you conveniently forget to properly impedance match, you'd end up in ~ 15 dBm. Might be safe enough to use it if you set the TX gain to 69 dB instead of the maximum 90 dB, according to the typical measurement curves. Bet a TV receiver has some protection! But agreed, the safe thing to do is drop in 20 dB of in-line attenuation, be safe. \$\endgroup\$
    – Marcus Müller
    Commented Feb 24, 2022 at 9:39

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