EDITED with results from QPSS on the bottom
HI,
I am doing some basic analyses on a very simple sampling structure and I am having issues getting the same results with TRAN and HB.
I have tried multiple things I am not going to bother you with, but I'd like to ask about a result I'm particularly puzzled by.
The circuit:
(V0 has 1V amplitude)
I obtain what I expect when running TRAN, however I would like to go for more advanced analyses for which a stepping stone would he HB (ideally, HBAC but I couldn't make it work, so I stepped down to a simpler arrangement).
When fRF is offset from fLO by a certain quantity fIF (1MHz in this setup), I thought I'd set HB up with two tones. Since the LO is rapidly switching (it's a square wave), I set it up as the first tone with 15 harmonics, while the input source is just a sine.
I am not interested in all possible frequency mixes, so I use an harmonic selection based on funnel, i.e. I keep all harms of LO but consider only low order products around those harmonics:
I know from theory what I should get which is, for node vx above, a component at fRF, with a certain amplitude.
The TRAN results comfort me in this expectation, as the time domain waveform looks like this:
which is a wave I can apply a DFT to, to reveal its nature of 10GHz wave with a slowly varying envelope on top
(note: there might be aliasing effects in this DFT, I have not checked thoroughly that my setup is correct. However, the gist of the properties is there and voltage amplitudes are "reasonable").
On the other hand, the spectrum out of the HB simulation looks dim:
I then thought to force HB to show me what it calculates for transient values:
and this is where I got the surprise: the input net of the circuit, /net1, the one where V0 is connected, remains at zero during the transient run:
Now, I don't know whether this is just an artifact and not the real culprit here, since if one examines the spectrum of /net1, one does see a spectral line at fRF (10GHz + 1MHz),
so maybe this is just "cosmetics", however it makes me think I am missing some fundamental point here for this type of simulations.
Can you guess what is happening and why I am not able to simulate this circuit properly with HB?
Thanks,
Michele
P.S.
EDIT: I've tried to run QPSS instead of HB:
Here the engine is however shooting, so that might make a difference.
Three points to notice:
1) the simulation takes much longer. I hope I am just overdoing something because this circuit is as simple as it gets. I don't want to think what would happen for something any more resembling to a true circuit.
2) the pss-tran waveform for /net1 keeps at ZERO steady :-( - This probably means I am really missing something fundamental.
3) this time the specturm of vx is as I would expect it. Actually, perfect and without aliasing:
