I do not have a 901v, (only an elderly 901s) but surely your trace is demonstrating that the input is ac coupled; ie there is a capacitor there (mainly for protection) and the trace is showing the reactance of the capacitor at the low frequency end of the trace.
If I (or you) could be bothered, the value of the dapacitance could be calculated from the information on the screen.
Once the scan has passed a few hundred khz, the capacitance has negligable effect and the 50ohm load is then displayed. 2017-10-03
Thankyou for your observations.
I have an HP 141 based analyser, -actually three of them. They take up huge amounts of bench space (so they are on the floor in various rooms) and some time to warm up if you want the tight resolution bandwitdths at higher frequencies. If I am looking with 100hz resolution bandwith with these machines the scan would only be a few tens of Khz at the most, otherwise you got very broad peaks. I have vowed to avoid buying any more boat anchor spectrum analysers and oscilloscopes.
I have located where to tap an output on my old 901h to feed into a small box containing the mixer to feed to a phone audio analyser, but there are many projects waiting which must be done first (machine shop, model steam engines, vintage 1960's computers, several old cars....). Too much to do.....
Steve, G7PSZ 2017-06-15
Thank you for the links to the circuits for the 901h.
They are very useful, in particular the rf schematic.
My thoughts of obtaining a tighter resolution bandwidth by mixing the small frequency band of interest down to audio and then using an audio fft program on a tablet......
The output of the second filter F2 before the AD8307 logarithmic detector could be tapped and brought out to an external connector. A mixer with an appropriate fixed LO frequency could be used as a small dongle input to the tablet running the fft. I think the analyser if frequency is 45mhz.
The analyser sweep is set to pick up the frequency of interest with zero sweep.
Steve Laurence (with a prospect of being able to ditch my boat anchors). 2017-06-01
Half my replies seem to vanish on this bbs.
So I will type again......
I have now repaired my kc901s, so the urge to buy the updated 901v has been moderated by my financial constraints. I was sorely tempted because of the selectable resolution bandwidths in spectrum analyser mode.
I use my 901s presominantly as a portable spectrum analyser (chasing qrm from switch mode psu, led lights etc), despite it not being its primary design purpose. I am mindful of the missed signals because of the finite steps in the sweep.
However, further resolution bandwidths below 3khz (eg to 100hz) would allow inspection of individual transmitted signals and study of intermodulation products of ssb amplifiers etc. A sweep of only 20khz is needed for this, which would not result in "lost" signals.
My original plan was to feed the signal to be inspected, along with a suitable local oscillator into a balanced mixer and look at the low frequency output via a pc or into a handheld tablet using its internal 16 bit adc. My ipad has a superb real time spectral display program with apparently 80db range and a resolution bandwidth about 10hz.
Alternatively could the 901 be modified to provide a low frequency analogue output (before its adc) to allow connection to the microphone input of my ipad?
This output could also be used to actually listen on headphones to demodulated signals when the 901 is on a single frequency (not sweeping).
Steve Laurence G7PSZ 2017-05-23