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Fri 20 Jan, 2017 12:13 am
Hello.
I’m working for an automotive company as a Radio and Audio Systems expert . In our company we use Hyundai Mobis standard to evaluate and measure electrical parameters of automotive Radios like: Usable Sensitivity, Signal to Noise and …
According to the mentioned standard (and also other standards like Mazda and Daewoo) , we must use a Dummy Antenna for AM band as follows:
SSG output-->a 30 ohm Resistor in series with 15pF capacitor in parallel with 65pf Capacitor-->Radio Input
out-----res -cap----input
|
|
cap
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In fact, we connect the input of Dummy Ant to the SSG (Stereo Signal Generator) and its output goes to input of Radio.
According to mathematically calculations and simulation software, there is about 14.5 dBs loss at the output of AM Dummy Antenna:
Receiver Input Signal= SSG Output – Dummy Antenna Loss (and Dummy Antenna Loss =14.5dBs)
But our reference standard (Mobis and also similar documents like Mazda and Daewoo) , explicitly specifies that we mustn’t (consider) take into account any loss at the output of AM Dummy Antenna , in the other words we should consider AM Dummy Antenna loss equal to 0dBs in calculation of parameters like Usable Sensitivity:
Receiver Input Signal= SSG Output - 0dBs(Dummy Antenna Loss =0dBS)
My question:
1-Why standards like Hyundai Mobis explicitly declare that we must consider 0dBs Loss for AM Dummy Antenna, but mathematically calculations and simulators clearly shows 14.5 dBs loss at the output of Dummy Antenna?
2-Should I consider 0dBs loss or 14.5 dBs loss to calculate AM Usable Sensitivity for Dummy Antenna shown in picture 1?
I would appreciate it if you would help me.
@hamiddarab,
The input impedance of an automotive AM radio receiver does not look like 50 ohms, since the antenna that it is connected to in the car is not 50 ohms in the AM band. The car antenna is more optimized for the FM band, and looks very capacitive in the AM band since it is too short, and the coax cable that connects the radio to the antenna looks capacitive to ground. The input of AM radio has to match this short capacitive antenna and also the capacitive coax that connects the radio to the antenna. Therefore, the AM radio's input matching circuit is generally a large inductor to ground, called the "antenna coil," connected to the gate of the first gain stage transistor. The large antenna coil to ground tunes out the 65pf cap to ground in the AM band (which models the coax), and the 15pf series cap which models the antenna, resonating these caps out. So the antenna dummy is trying to match the input impedance of the AM radio to the 50 ohm output impedance of the signal generator. That is why there is 0 db loss associated with the antenna dummy when measuring sensitivity with a signal generator.
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