the purpose of this example we will assume that a receiver is required which will translate a 915 MHz digital
signal down to baseband with no inversion from input to output. The input covers a 10 MHz band that is divided
into 200 kHz sub-bands. A dual conversion will be employed to translate an input signal at 915 MHz down to a 10.7
MHz baseband. A 10 MHz wide downconverter will translate the entire input band to a first IF (intermediate
frequency) of 115 MHz. Next, a 200 kHz wide receiver card will translate a 200 kHz section of the 10 MHz band down
to 10.7 MHz -- where the information is detected.
Once input and output parameters required for the
conversion system are known, the selection of cascaded parameters for each stage in the example are determined.
Partitioning of cascaded parameters between the two stages is based on characteristics of available components.
Low noise figure requires that a substantial amount of gain be present as close to the input (front end) as
possible. High intercept points (and, desirably, good spurious product performance) requires as little gain at the
system front end as possible. Balancing the two requirements for optimal performance can be the designer’s biggest
challenge. As an example, consider the following system where there are two gain stages. Both systems are
identical with the exception of gain distribution - although the total gain remains constant. The parameters
listed above the components are valid at the component output.
The system of Figure 1 has low gain at the
front end, while the system of Figure 2 has high gain at the front end. For cascade equations, see
pages on RF Cafe. Gain for uncompressed stages is a simple
Figure 1 System With Low Input Gain
Figure 2 System With High Input Gain
Cascaded calculations were carried out to determine the values at the system output, and then the equivalent input
parameters were calculated. Both systems have a total of +30 dB gain. Compare the resulting cascaded parameters at
the system input to realize the effects of gain distribution. Whereas the system with low front end gain has the
best intercept point performance, the system with high front end gain has the best noise figure performance.
Figure 3 shows the Planning screen created for the two conversion stages. I/O specifications are imposed allowing
TxRx Designer to determine the minimum requirements for each stage. Load EXAMPLE.PF3 while on the Planning screen
to view the plan.
Figure 3 Example Planning Screen
Each stage is designed individually to conform to the stage 1 and stage 2 requirements. A swept input is used
during the design of each stage with input parameters set at default values so as not to affect the system output
parameters (gain = 0 dB, P1dB = +250 dBm, etc.). Figure 4 and Figure 5 represent the two system stages shown in
Figure4 Example Stage 1
Figure 5 Example Stage 2
Parameters used for each stage in this example are specified to exactly match the values determined by TxRx
Designer on the Planning screen as a testimony to the accuracy of the calculations when the two stages are
combined. In practice, each stage will most likely perform better than the minimum requirements.
TxRxD v3.0 Stage 1 and TxRxD v3.0 Stage 2 PIF files. Load EXST1.SF3 into TxRxD v3.0 Stage 1 and EXST2.SF3 into
TxRxD v3.0 Stage 2. Immediately after loading each file, select Restore Last Scale from the Graph menu and toggle
Auto Scale to Manual. Verify that the systems match the ones shown in Figure 4 and Figure 5.
TxRxD v3.0 Stage 1 and set INPUT to sweep through 1 MHz in 10 kHz steps. After recalculation, save the system file
in order to create a data file (EXST1.DF3) that TxRxD v3.0 Stage 2 can read as input data.
Switch to TxRxD v3.0 Stage 2 and select Switch to Data File Input... from the Components menu (EXST1.DF3 is
already specified as the input file). Select Restore Last Scale from the Graph menu. The system of Figure 6 will
Figure 6 Example Stage 2 with Stage 1 Input Data
The final output cascaded parameters match with the output parameters specified on the Planning screen. Note
that the noise figure is slightly less than the targeted 5.0000 dB due to the internal accuracy of TxRx Designer
being much greater than the five place displayed accuracy. Three ten thousandths of a decibel is insignificant.
A close inspection of the frequency responses of Figure 5 (swept input) and Figure 6 (stage 1 data file input)
will reveal the effect of the stage 1 contributions to amplitude and group delay. The additive effects of the
group delay are most apparent.*
Excerpted from the user's manual from
TxRx Designer (now
See Mixer Image Frequency
for more information.
See A Graphical Approach to Mixer Spurious
for more information.