Frequency sweep meter

[sq3rx]

Hi Pavel, First of all thanks for all apps which you develope for RP - all of them all very useful for me. I would like to suggest you to add one more simple app to your collection, which can be very useful - frequency sweep analyzer. It allows measure insertion loss, cable loss etc. I think it could be possible to develop this based on VNA app.

You can take a look for eg. NWT70, NWT7, NWT500 equipment which can provide this kind of measurements.

[satfan52]

Actually the current VNA can do this. To calibrate just connect input 1 and output 1 via two attenuators (I use a 10dB for input and a 20db one for output) and hit the "short button", this will do the "through" calibration. Then insert the "DUT" between the two attenuators and hit the "single" button. Check results of the sweep under the "gain short" tab. I use it all the time.

[pavel-demin]

I've just checked the NWT70 product description at this link but I can't understand from the description what exactly it does.

If it measures gain vs frequency, then the method described by Peter is exactly how this measurement can be done using the VNA application.

[sq3rx]

Generaly this equipment measure frequency vs gain. Thanks for detailed description how to measure this by vna app. I will test it probably during the weekend.

[BeauWebber]

Thanks all, I would find such an instrument very useful. While the existing frequency response instrument is extremely useful, a swept frequency instrument, where the sweep rate could be slowed down for narrower noise banding to give a better signal to noise, would be good. I was actually planning on implementing such an instrument using the capabilities of Pavel’s NMR instrument, and measuring at spot frequencies that were swept over the bandwidth of interest, just using my Apl programs running in the on-chip Linux to control the frequencies, generate the output RF, and process and display the returned signals. In my existing NMR systems, the Apl talks to Pavel’s firmware using simple C programs, with the Apl doing anything complicated. But if someone else is planning to do it, I have plenty else to do ! Cheers, Beau Webber

[pavel-demin]

I would find such an instrument very useful.

Have you tried the VNA application?

While the existing frequency response instrument is extremely useful, a swept frequency instrument, where the sweep rate could be slowed down for narrower noise banding to give a better signal to noise, would be good.

The VNA application supports bandwidths from 1 Hz to 10 kHz.

But if someone else is planning to do it, I have plenty else to do !

I think that it's already done. The VNA application can be used to measure gain vs frequency with variable bandwidth.

[satfan52]

The only additional thing that the NWT4000/5000/6000 series can do compared to the red-pitaya VNA is spectrum analysis between 25 Mhz and about 6Ghz (for the NWT6000). These devices are actually spectrum analysers and include tracking generators in order to make up a scalar vector analyser (ie, frequency response analyser). They use ADF4351 type of signal generator (square wave, no sinewave DDS) in combination with AD8307/17 type of logarithmic power/voltage detector devices. So for me, unless I am mistaken, the redpitaya VNA is actually providing the same "scalar network analysis" capability (but without spectrum analysis) than the NWT4000/5000/6000 series but between 0 and 60 mhz only whereas the NWTs family start in the MHz range and climbs in the Ghz range. However the RP does it much better in its frequency range than NWTs because of the greater dynamic range of the RP compared to the limited one of the logarithmic detectors used in these NWT devices. Personally, I think that a significant improvement of the red-pitaya VNA would be to add to it some kind of frequency converter to be able to continue to use the redpitaya well above 60 Mhz with a similar quality of measurement than below 60 Mhz. If this frequency extender device was commercially available I would buy it on the spot provided that the price is right; image rejection and dynamic range would be the issue to tackle. This is the approach that Mini Radio Solution took with the frequency extender to their mini VNA pro that extends the measuring frequency from 200 Mhz up to about 1.5 Ghz (not sure how they did though, if they used aliaising and under sampling like DG8SAQ seems to have done with the WVNA -3EC or if they designed a true old fashion analogue up converter/transverter . Of course some changes would be required in the red pitaya VNA app as well to support such an up converter/ frequency extender. There are other examples of HAM VNAs using frequency extenders , and unless I am mistaken even HP boat anchors from the 80s used some frequency extenders. For me this is objectively what is missing to the red-pitaya VNA today so it can used in a similar way to NWT4000/5000/6000 series, it is simply some frequency extension above 60 Mhz. Of course, let's not lose sight that the main added value of the redpitaya VNA is the V of VNA and not the S of SNA, it would be extremely useful to measure impedances and phase above 60 Mhz not just gain but ONLY if it can be done sufficiently accurately and with a reasonable dynamic range after "frequency extension"; if the frequency conversion ends up reducing the dynamic range to a similar 40-50 dB level than an RTL-SDR and / or if the frequency stability would prevent from doing repeatable measurements due to over sensibility to temperature, I wonder what would be the added value of such a frequency extender for the V aspects of the VNA app. So Maybe (????) that what is needed is a new separate SNA application that would with the help of a frequency extender and logarithmic detector (for power calibration purpose), be able to climb higher in frequency in order to make some accurate gain but also possibly power ;-) ;-) ;-) spectral and intermodulation measurements in an automated way.