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Old 26th Apr 2019, 9:58 am   #3
John_BS
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Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,757
Default Re: Help with FM tuner Signal injection/probing

The injection level for normal alignment purposes is not critical.

If you have a 50 ohm signal source and a 75 ohm target system (receiver, cables etc), you will only introduce a small error in power level due to mismatch loss: theoretically about 0.18dB.

Measuring levels within the tuner is more problematic. You might get away with a 10:1 scope probe at 10.7MHz at low impedance nodes (such as the output from the limiter IC). Or try using a 10:1 probe with a small 2pF capacitor in series: you can "calibrate" this approximately by using it to measure the output of the 2022 when set to the same frequency and terminated in 50 ohms.

John
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