Hi friends,
I am doing a direct conversion system simulation using Ptolemy. One strange problem I encountered is about Timed Data Collecter:
I build up the RF chain with proper gain and frequency setting, simulate the BER performance and observe the demodulated data from Timed Data collecter. Everything looks OK. But when I add a capacitor after the downconversion mixer to simulate the issue of removing DC-offset, BER did not change much but the data display from Timed Data Collecter becomes very very samll, some number at e-14 compared to 2V without capacitor. I don't know why it is like this.
Then I thought of adding a ADC between the demodulated signal and the Timed Data Collecter, now the displayed data is between 1V and 2V, compared to the reference data, only the phase is 180 different(reversed). I try to change the gain setting of the receive chain, sometimes the data from Timed Data Collecter are all constant of 2V, but BER no change, why? I am using BER IS to measure the BER performance.
One more question is do I need to use ADC to observe the demodulated data? The example for ADC I downloaded from Agilent's website is ZAP format, how can I open it?
thanks for your reply.
best regards,
spine
I am doing a direct conversion system simulation using Ptolemy. One strange problem I encountered is about Timed Data Collecter:
I build up the RF chain with proper gain and frequency setting, simulate the BER performance and observe the demodulated data from Timed Data collecter. Everything looks OK. But when I add a capacitor after the downconversion mixer to simulate the issue of removing DC-offset, BER did not change much but the data display from Timed Data Collecter becomes very very samll, some number at e-14 compared to 2V without capacitor. I don't know why it is like this.
Then I thought of adding a ADC between the demodulated signal and the Timed Data Collecter, now the displayed data is between 1V and 2V, compared to the reference data, only the phase is 180 different(reversed). I try to change the gain setting of the receive chain, sometimes the data from Timed Data Collecter are all constant of 2V, but BER no change, why? I am using BER IS to measure the BER performance.
One more question is do I need to use ADC to observe the demodulated data? The example for ADC I downloaded from Agilent's website is ZAP format, how can I open it?
thanks for your reply.
best regards,
spine
To open the example in the zap format, from the main ADS window go under the File menu and select "Unarchive Project...". In the window that pops up
1) select the zap file you want to open in the "Unarchive File" field
2) enter the directory where the example project will be unarchived in the "To Directory" field.
I am not sure that the circuit you are trying to simulate is a good candidate for Fast Cosim. Please read the following section of the manual to understand the limitations of Fast Cosim.
From Main ADS documentation page follow the "Simulation" link, then the "Circuit Envelope Simulation" link, then "Theory of Operation", and finally "Automatic Verification Modeling".
One assumption that seems to be violated is that (I am copying the doc) "Note that the magnitude only characterization assumes the output phase can be determined from the harmonic indices of the input and output frequencies". In your case, the 0 Hz frequency can be obtained as 0 times the fundamental and also as the mixing product of the fundamental and the LO.
Another thing is that your circuit has frequency dependent components (filters and the capacitor). In this case, some compensation needs to be added (see doc).
As a last suggestion I would like to recommend that you use the "Verification" feature of Fast Cosim (see Verification area in the Cosim Tab). If you specify a Stop Time and a tolerance then both the normal Envelope cosimulation and the AVM (Fast Cosim) results are computed. The RMS error between these two results is computed and output after this verification time has ended. If the the resultant RMS error is not less than the tolerance the AVM (Fast Cosim) will be turned off and just the normal Envelope cosimulation results will be used for the remainder of the Ptolemy simulation.