Vaisala RVP900

USER’S MANUAL__________________________________________________________________
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4. The dynamic range for the complete system at 0.5 MHz bandwidth
may now be calculated as 106 dB - 6 dB = 100 dB.
5. After assembling all of the RF and IF components, we can check
whether we achieved the correct gain, by verifying a 7 dB rise
(independent of bandwidth) in RVP900 filtered power, when the IF-
Input cable is connected, versus disconnected.
When designing your RF and IF components, keep in mind that the final
amplifier driving the IFDR must be capable of driving up to, perhaps,
+14 dBm, so that signals above saturation can still be correctly measured.
3.2.12 IF Gain Based on System Noise
Figure
The previous section described how to compute the front-end RF/IF gain
based on the desired trade-off of dynamic range versus sensitivity. Since
arriving at the proper gain is so important, we present an alternate, but
equivalent approach based on system noise figure.
Every amplifier can be partially characterized by its gain "G" and noise
figure "F". Gain is measured quite simply by injecting a test signal at the
mid-power range of the amplifier and measuring the ratio of Output/Input
power. Noise figure is a little trickier. It is measured by terminating the
input of the amplifier, measuring the output power within some prescribed
bandwidth, and then dividing by the thermal noise power expected over
that same bandwidth from an ideal amplifier having the same gain. For
example, suppose that an amplifier with a gain of 20 dB delivers -90 dBm
of output power within a 1 MHz bandwidth when its input is terminated.
We would expect the Boltzman thermal input noise, at -114 dBm/MHz, to
produce -94 dBm, from an ideal 20 dB amplifier, under the same
conditions. The noise figure of the real amplifier is +4 dB, (-90 minus -94).
Although the above definitions are typically applied to linear analog
amplifiers, these same terms can be applied to hybrid analog/digital
systems such as the RVP900.
- To calculate the gain of the IFDR, we apply a calibrated mid-power
signal generator directly to its IF-Input, and use the Pr plot (Section
5.5 Pr — Plot Receiver Waveforms on page 163) to print the
measured power. For a wide range of analog input power levels, the
RVP900 reports the exact same measured digital power; therefore the
overall analog/digital gain is 1.0 (0 dB).
- To calculate the noise figure of the IFDR, we set the receiver
bandwidth to 1 MHz (Section 5.4.2 Available Subcommands Within
Ps on page 151), terminate the IF-Input in 50, and use the Pr plot,
this time to examine the in-band thermal noise power. The measured