Make distortion and mixer measurements possible by adding offset measurements

[joseph-wu1]

Being able to perform measurements with the signal generator offset from the receiver would make it possible to do many more different types of measurements with the NanoVNA V2. Characterization of frequency-active systems as well as distortion measurements of linear systems (e.g amplifiers) requires measuring at frequencies other than the input frequency.

I propose measurements at 2 different types receiver-transmitter offsets :

1. Offset by applying a multiplier to the output signal. For example, measure received signal at the Nth harmonic of the transmitted signal to show distortion as a function of frequency in the DUT. When the transmitter is tuned to e.g 500 MHz, the receiver would therefore be tuned to 1000 MHz (2nd harmonic), 1500 MHz (3rd harmonic), 2000 MHz (4th harmonic), and so on. When the transmitter advances to e.g 510 MHz, the receiver tunes to 1020 MHz (2nd harmonic), 1530 MHz (3rd), etc.

2. Fixed offset. For example, measure received signal at +100 MHz to transmitted signal for testing a mixer. When the transmitter is tuned to e.g 500 MHz, the receiver would therefore be tuned to 600 MHz. When the transmitter advances to e.g 510 MHz, the receiver tunes to 610 MHz.

When multiple offsets are selected (fundamental, 2nd harmonic, 3rd harmonic, etc all being measured at the same time), measurements for a single Tx frequency (but at different Rx frequencies) should happen at the same display point, as opposed to sweeping the entire measurement range for one Rx frequency before moving to the next frequency.

By measuring all offsets at one point, we ensure that when the instrument used for alignment or bench top purposes (with components being adjusted during the measurement) data is as accurate as possible.

Implementing the hardware-facing part looks easy: create a version of adf4350_update() which takes separate Rx and Tx frequencies.

Display of measurements

If possible, users should be able to select multiple offsets (fundamental, 2nd harmonic, etc) for a given measurement (S11, S21) and display them as separate traces.

Example: the user wishes to measure RF and IF isolation of a mixer with a 100 MHz LO. See this article for an example of peforming this measurement with a high-end VNA which supports offset mode The user connects the NanoVNA port 1 to the IF port and the NanoVNA port 2 to the mixer RF port, and formats the display as follows:

• Trace 0 displays log-mag S11 power received when the port 1 receiver is tuned to the transmitter frequency
• Trace 1 displays log-mag S11 power received when the port 1 receiver is tuned to the transmitter frequency +100 MHz
• Trace 2 displays log-mag S21 power received when the port 2 receiver is tuned to the transmitter frequency
• Trace 3 displays log-mag S21 power received when the port 2 receiver is tuned to the transmitter frequency +100 MHz

Example: the user wishes to see 2nd harmonic distortion introduced into both reflected and thru signals, i.e to display fundamental and 2nd harmonic log-mag power plots on trace 0 and trace 1 for the S11 and S21 measurements:

• The user formats trace 0 to display log-mag S11 power received when the port 1 receiver is tuned to the transmitter frequency
• The user formats trace 1 to display log-mag S11 power received when the port 1 receiver is tuned to 2x the transmitter frequency (2nd harmonic)
• The user formats trace 2 to display log-mag S21 power received when the port 2 receiver is tuned to the transmitter frequency
• The user formats trace 3 to display log-mag S21 power received when the port 2 receiver is tuned to 2x the transmitter frequency (2nd harmonic)

Example: The user wishes to approximately visualise total harmonic distortion (THD) for a thru (S21) measurement by plotting the fundamental, 2nd, 3rd, and 4th harmonics:

• The user formats trace 0 to display log-mag S21 power received when the port 2 receiver is tuned to the transmitter frequency
• The user formats trace 1 to display log-mag S21 power received when the port 2 receiver is tuned to 2x the transmitter frequency (2nd harmonic)
• The user formats trace 2 to display log-mag S21 power received when the port 2 receiver is tuned to 3x the transmitter frequency (3rd harmonic)
• The user formats trace 3 to display log-mag S21 power received when the port 2 receiver is tuned to 4x the transmitter frequency (4th harmonic)

Example: The user wishes to approximately visualise total harmonic distortion (THD) for a reflected (S11) measurement by plotting the fundamental, 2nd, 3rd, and 4th harmonics:

• The user formats trace 0 to display log-mag S11 power received when the port 1 receiver is tuned to the transmitter frequency
• The user formats trace 1 to display log-mag S11 power received when the port 1 receiver is tuned to 2x the transmitter frequency (2nd harmonic)
• The user formats trace 2 to display log-mag S11 power received when the port 1 receiver is tuned to 3x the transmitter frequency (3rd harmonic)
• The user formats trace 3 to display log-mag S11 power received when the port 1 receiver is tuned to 4x the transmitter frequency (4th harmonic)

If display on multiple traces is not possible, it may be desirable to summarise the measurement results in a single computation:

1. Simplified / high speed / low sweep time harmonic distortion. For each sweep point, perform measurement at two selected harmonics (e.g fundamental + 2nd harmonic, 2nd harmonic + 3rd harmonic, etc). Plot the ratio between those harmonics.
2. Harmonic distortion. For each sweep point, perform measurements at the fundamental frequency, 2nd, 3rd, ...Nth harmonic. Plot relative power in harmonics vs in fundamental. If there is significant distortion in the NanoVNA V2 hardware this may not provide much more information than the "simplified" measurement described above.

[sbridger]

I also wanted offset receiver measurements. Having the ui in pc software would be fine for me.

I also see utility in doing scalar measurements from an audio or a DC (agc, rssi) input. (rather than from the RF input)

[EternityForest]

This would probably also work as an extremely crude Nonlinear junction detector with an antenna, which would be a very interesting experiment for educational use.

If it could be combined with TDR mode, in perhaps even harmonic radar could be demonstrated.

Just by toggling an IO pin from grounded to floating, or biasing a diode, or some other simpler circuit, one could possibly even send a small amount of data, demonstrating RFID-like passive transmission(In theory useful for debugging although maybe too gimmick-y).

But in general there's a lot of reasons one might want this.

Swept-offset with a fixed TX could also be very useful, to see the full range of harmonics, and non-harmonic nonlinearity a device produces.