PSS, PNOISE and other simulations for a circuit containing an oscillator and Mixer

I'm slightly surprised that I didn't find any report of this restriction. I think that if you have an oscillator driving the LO on a mixer, and have the input signal for the mixer being a port (without any time-varying signal), then you should be able to compute noise figure - it's just that the pnoise and hbnoise forms assume you can't so don't give you the option to specify the input port or reference side band.

You can make this work though:

1. Set the sweep to absolute or use relative with 0 as the Relative Harmonic and then sweep the output frequency (the base band signal assuming this is a receiver mixer)
2. Set the noise time to time average and pick USB
3. On the Options form, in the additionalParams at the end, enter:
   for pnoise: iprobe=PORT0 refsideband=-1
   for hbnoise: iprobe=PORT0 refsideband=[-1]
   where PORT0 is the name of your input port. The -1 is assuming that the correct input signal is at the lower sideband of the LO - use 1 if it's from the upper sideband.

Then on the direct plot form you should be able to plot the Noise Figure (I only quickly tried this and not with a mixer with an oscillator attached - I'm travelling this week so didn't have time to test).

Probably worth reporting this to customer support as I think the analysis form should allow this to be done directly without the trickery above. A sensible enhancement as far as I can see...

Andrew

Thanks for the explanation. I made these changes and managed to plot the Noise Figure graph. I will report this to customer support.

With the oscillator driving the mixer's LO, is it possible to implement the linearity simulations (1dB and IIP3) using QPSS and QPAC? The oscillator is the only large signal, but it is not possible to define this signal in the fundamental tones of the QPSS analysis.

Is this the correct way to measure?

No. QPSS cannot be used with an autonomous frequency (i.e. an oscillator). Similarly PSS wouldn't work since there would not be a combined single fundamental frequency you can use.

You could measure IIP3 using PSS (for the oscillator), and not having a large-signal input but then using RapidIP3 to set the input powers (in the linear region) to measure the IP3. For 1dB compression point though you need to run a large-signal analysis - you can use hb analysis since this supports semi-autonomous harmonic balance - you can have the first tone being the oscillator and the second being your large-signal input. Of course, you could measure the IIP3 this way too (with two driven tones on top of the autonomous  signal from the oscillator).

Andrew

 Thank you for your reply. I will do some tests with this information.

Hello,

When I simulated the 1dB point in the HB analysis, the message "A design variable representing the input power in dBm must be parametrically swept before the function selected above can be executed" was observed in the Compression Point of the Direct Plot and it was not possible to plot the 1dB graph. Have you ever had this problem? Do I need to provide more information about this issue?

The IIP3 simulation through the HB analysis worked fine with the two tons driven on top of the independent oscillator signal. Is it correct to compare the IIP3 value obtained from the HB analysis with the IIP3 value obtained from the QPAC analysis?

How did you set up the 1dB point simulation? Without knowing that, it's hard to say.

You can in general get the same result (within simulation tolerances) from a two-tone (plus LO) HB analysis for a mixer with a one tone plus LO QPSS plus QPAC for the second tone, provided that the signal magnitude of the two tones is sufficiently below the point at which the circuit compresses (which is where you'd measure IIP3 anyway). Of course, if one of the large signals is an oscillator, you can't use QPSS anyway, so this is a moot point. You could however use semi-autonomous HB (with oscillator and one of the two tones) plus HBAC (for the second tone) and get similar results to a two-tone plus LO HB.

Andrew

I am unable to attach other images, but for the 1dB simulation, I configured the mixer (up Conversion) with an IF input signal (PORT1 with source type sine) of 100MHz with a dBm amplitude of -30.

In the HB analysis, I used two tones and the options Oscillator (selecting the two outputs of the 4.9GHz oscillator) and Compression (selecting the IF input port as Source and the RF output port as Load).

If you're finding the 1dB compression point, you need to either sweep the input power (using the sweep choices at the bottom of the hb form), or use the "Compression" section (there's a checkbox on the hb form) which is a smarter (more efficient, and generally more accurate) way of finding the compression point by doing a search for the compression point. Unlike IIP3 which extrapolates the 3rd order intercept point from a single input power, compression needs to run multiple simulations to find the point at which the output power is 1dB lower than it should be from a linear increase.

It sounds as if you have not done either of the above (at least you only mentioned a single power).

Andrew

I made the adjustments and managed to plot the 1dB simulation graph. Thank you.

Regarding the HB + HBAC analyses, how could I simulate the IIP3? You mentioned that HB would be semi-autonomous (with oscillator and one of the two tones) plus HBAC (receiving the second tone). Do you have any other tips for the simulation? Do I need to use the select from range option to define the simulation sideband?