simulation of a superheterodyne receiver using pspice - School of ...

Block 7 – AM detector: as previously stated the resistor in the audio CR filter is set at
100k as to increase the capacitor value to avoid stray capacitance. The capacitor at the
input is reduced to 1nF, which is acceptable as the max value is11.73nF.
Loading effects: the main problem associated with the hierarchy structure is the
loading effects between the hierarchy blocks. It is essential to isolate the blocks from
each other to avoid one block loading down the entire circuit and effectively stopping
the receiver from working. The solution in Pspice is through transformers. Setting the
inductor values in the transformer to L1=20uH and L2=0.3uH and a coupling of
0.9999 (i.e. practically 1) a step down transformer is achieved.
As can be seen from Figs. 37-44 above transformers are placed in the following
places:
(1) A step down transformer is placed at the input of the RF amplifier (see
Fig. 38) to avoid the 20k resistor from the AM generator being in parallel
with the input resistance of the RF amplifier.
(2) Another transformer is placed at the RF output (see Fig. 38) to avoid
loading with the mixer. For convenience and design reasons this
transformer is placed here and not at the input of the mixer because it
incorporates the inductor from the tuned LC circuit in the RF load.
(3) A transformer is used to couple the LO signal into the mixer (see Fig. 39).
(4) The input of the first IF stage has a transformer (see Fig. 41) to avoid
loading between the tuned LC circuit in the mixer load and input resistance
of the IF amplifier.
(5) The IF amplifier is a double tuned circuit and hence has two inductors in
the load. Therefore these can be implemented as a transformer also to
avoid loading effects between the two IF stages (see Fig. 41).
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