Simulation Results
The simulator we used to verify operation of our circuits was Spice3. Spice3 can be downloaded by following the links on the Links page.
After a design is laid out onto the chip using the Magic program, it can be extracted to a Spice3 file. Spice3 can be used to simulate the circuit with this file. This is known as "Layout-Vs-Schematic" checking and is a good way to see if the circuit is laid out properly. This section shows the simulation results that I got from the extracted circuits.
I did the layout in three steps.
1) I laid out the mixer and simulated it by itself (Mixer Only).
2) I laid out the filter and simulated it (Filter Only).
3) I included the phase shift network and simulated the entire circuit (Complete Circuit).
1) Mixer Only.
The SPICE3 file that I used can be viewed here.
When I simulated the mixer, I mixed a 5Mhz and a 5.5MHz waveform. If the mixer is operating correctly, it should produce a 500kHz waveform and a 10.5MHz waveform. The single-ended output of the mixer is shown below.
Mixer Only Simulation with 5 and 5.5 MHz waveforms.
As expected, the mixer produced one sinewave with a frequency of 500kHz (the difference frequency) and a 10.5 Mhz sinewave (the sum frequency). This would seem to indicate that the mixer is working correctly.
2) Filter Only.
The SPICE3 file that I used to simulate the filter can be found here.
In the case of the filter, the result of interest is the filter's frequency response. The desired response for this section is a cutoff frequency of approximately 500kHz and attenuation of 60dB/decade. The analysis is performed using an AC sweep. The simulation results are shown below.
The frequency response of the filter was very good, with a cutoff off about 550kHz and 60dB of attenuation per decade. The reponse is down to about -90dB by the first frequency that will need filtering (11MHz, or the minimum sum frequency). There is a zero in the response which keeps it from attenuating further as the frequency goes up, but that is of no consequence, since the response is already at -90dB by that point.
3) Complete Circuit.
The SPICE3 file that I used to simulate the complete circuit can be found here.
When testing the entire circuit, I used SPICE in the transient analysis mode and set the input to an FM modulated source with a carrier of 5.5MHz and a signal frequency of 250kHz. I then viewed the signals at five points in the circuit. The first two appear below on the same plot. The plot shows the input frequency and the output of the phase shift network on the same plot. This is useful in determining whether the phase shift network is operating. If it is working correctly, VSh should have a phase shift of 90 degrees relative to VIn.
Circuit Inputs (Differential)
The plot shows that VSh (the blue trace) is indeed shifted in phase from VIn (the red trace) by about 90 degrees. This shows that the phase shift network is operating correctly.
Next, we want to verify that the mixer is working. Given the above inputs, the mixer should have outputs of 250kHz and about 11Mhz respectively. The mixer's differential output appears on the plot below.
Mixer Output (Differential)
The above plot shows the correct output frequencies (250 kHz and 11MHz) in the steady state.
Finally, we need to verify that the filter is removing the 11MHz component (which can be considered noise now), and is leaving a clean 250kHz sinewave. The plot below displays the filter's differential output (which is also the system's output).
Circuit Output (Differential)
The above plot shows the 250 kHz sinewave, but the higher frequency components have been removed. The system operates correctly.