Path loss = 20 log (4 * pi * r / lambda)
[Note: use log base 10]
Path loss = signal attenuation. Unit: dB
pi = 22 / 7
r = distance between transmitter and receiver. Unit: m
lambda = wavelength of signal. Unit: m
Wavelength = C/f
C = speed of electromagnetic waves in free space
= 299792458. Unit: m / s
f = frequency of signal. Unit: Hz
Let r = R * (2 ^ x)
Path loss = 20 log (4 * pi * R * L / lambda)
= 20 log (4 * pi * R / lambda) + 20 log (2 ^ x)
= 20 log (4 * pi * R / lambda) + 6x
The above derivation implies range(R) doubles every 6dB of path loss.
The path loss is +8.519dB more over a given range for the 2.4 GHz compared to 900MHz for the same range. In other words operating at 900 MHz exhibits a significantly longer range than is possible at 2.4 GHz.
DC coupling allows both AC and DC signals through, while AC coupling accepts only AC signals.
Issue in AC coupling: AC coupling rejects DC component in the signal, making
the average value of the signal to zero.
Case 1: Waveform has 50% duty cycle, the peak value in both positive and negative cycles will be same.
Case 2: Waveform doesn’t have 50% duty cycle.
After AC coupling, (average value is zero)
Area under the positive half cycle = Area under negative half cycle.
But the peak value for positive and negative half cycles will not be same.
Example: Consider a square wave with duty cycle 1/3 and peak-to-peak value 3V.
After AC coupling, the peak values would become 2V, -1V respectively.
If the AC coupled device detects 1.5V as logic high,
-1.5V as logic low, due to the above effect, there would be erroneous detection.
No Translation required for the following:
[TTL] to [TTL]
[TTL] to [CMOS with input switching at TTL levels]
[CMOS 5V] to [TTL]
Translation required for the following:
[TTL] to [CMOS*]
Pull up of 1k to 2k required
TTL outputs 2.4V to 3.3V for a high level
CMOS required 3.7 for high level.
The pull up resister increases the output voltage (of TTL driver).
[CMOS*] to [TTL]
*The translation depends on the VCC of CMOS device.
[TTL] to [ECL] / [ECL] to [TTL]
TTL to ECL (ECL to TTL) translator along with ECL termination resistors are needed.
[PECL] to [TTL] / [TTL] to [PECL]
PECL is Positive ECL
PECL to TTL (TTL to PECL) translator needed.
[PECL] to [CMOS] / [CMOS] to [PECL]
Same as above, but pull of 1k to 2k is required for CMOS.
[PECL 5V] to [LVDS 5V]
Pull up both +,- signals to 3V through 50E resistors.
[LVPECL 3.3V] to [LVDS 5V]
Pull up both +,- signals to 1.3V through 50E resistors