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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 25th Apr 2019, 9:47 am   #1
Join Date: May 2015
Location: Hemel Hempstead, Hertfordshire, UK.
Posts: 43
Default Help with FM tuner Signal injection/probing

Does anyone have any tips on how I should inject signals into and probe for signals in an FM tuner?
For instance how would I correctly inject a 100 MHz 100uV (-67dBm) RF signal from a signal generator (Marconi 2022E 50 ohm output impedance) into a 75 ohm antenna input? Is it best to set the input impedance of the oscilloscope (which I'm using to measure this input voltage) to 50 ohms or 1 Meg Ohm? What kind of loading effects can I expect.
Also, what is the best way of probing the Circuits in the tuner without introducing any significant loading. Likewise, what is the best way of injecting signals? I ask these seemingly simple questions as I am not experienced in aligning receivers and I want to learn. Hope someone can help. Thanks.

PS I'm using standard 75 ohm BNC connectors and coaxial cable. What effect will this have on my measurements.
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Old 25th Apr 2019, 11:09 am   #2
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,676
Default Re: Help with FM tuner Signal injection/probing

I have used a 2022 and a tek 454a scope with sweep output for this. The 2022 can FM modulate +/- 100kHz so connect the sweep output to the 2022 external modulation input and use a x10 or higher probe to monitor the discriminator output. The 2022 seemed to have an AGC on the modulation input so it just worked for me. This is for alignment and will not help with sensitivity measurements.
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Old 26th Apr 2019, 9:58 am   #3
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,111
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.

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Old 9th May 2019, 10:24 pm   #4
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 1,613
Default Re: Help with FM tuner Signal injection/probing

Interesting mix of ideas on looking at tuning patterns using a sweep generator and a tuner. Standard production method on the Marconi Clansman 353 was to use a x10 scope probe on the output. But in my time in the repair are ( we were a small team refurbishing Army returns) , we did an experiment tuning receivers using a x10 scope probe and a length of Coax with two clips. The test was carried out on a complete radio, and we found that we got a better response from the radio in receive by using the coax with clips.
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