Signal level/baseband tuning questions

[cct314]

Having used “rotating antenna” types of DF systems for many years now (I completely rewrote the code of the VE2EMM system using simple DSP techniques) I’m familiar with the challenges and techniques used in direction finding. Also very familiar with the hardware and capabilities of the R820T tuner and RTL2832, I’ve found the idea of leveraging this hardware (in the form of the KrakenSDR) for the purposes of RDF to be intriguing - which is why I have been testing it recently.

Using an array similar to that proven with the TDOA style, the five element array is on an aluminum plate atop my vehicle: I have eight 19" radials (at the corners of the plates) that extend the size of the virtual ground plane, permitting it to sit above the rooftop, attached to the roof rack. This permits the array to be used on any convenient vehicle - whether or not it has a metal roof - and it has a fairly low tendency to skew the pattern. (I can provide photos of the arrangement if desired.)

While sparsely mentioned in the set up, I’ve been able to properly calibrate the DOA bearing so that it correlates within a few degrees of my vehicle to help minimize uncertainty once many bearings are accumulated and using transmitters at known locations, and found that it is equal to or better than my older VE2EMM-type TDOA system under typical conditions in terms of indicating a useful bearing: Of course, the inclusion of the heat map - and just the map in general - reduces the CPU load of the “Grey matter DSP” and simplifies the entire operation.

As the simultaneous dynamic range of the RTL-SDR receiver hardware is on the order of “about” 65 dB (given 2 MHz sample rate and 15 kHz detection bandwidth) I need to do further testing to see how to best-tolerate an environment with signals of widely disparate strength. For example, here in the Salt Lake City area it’s not uncommon to have multiple signals of around -40dBm or greater at the terminals of an omnidirectional antenna (from 100+ watt mountain-top repeaters) which, due to the limits of the 8 bit sampling, would submerge microvolt-level signals into the noise once the input front-end gain control has readjusted to prevent ADC overload - a common situation if one is considering the case of a repeater jammer where the miscreant’s signal is weak but the repeater is strong: I suspect that judicious and careful adjustment of sample rate and center frequency could mitigate this - which is the basis of one of the questions.

This brings up some questions:

• Is is possible to arbitrarily adjust the LO frequency of the receiver to offset the desired signal to an edge of the baseband? As noted, this might be useful for helping to strategically move a strong signal (e.g. repeater output) outside the baseband to reduce the energy impinging on the ADC - even if done at the expense of aliasing. (For that matter, goes the Kraken software adjust the R820T’s “bandpass” filter based on the setting of sample rate and/or decimation?)

• I note in the .csv that the signal level numbers do not seem to correlate at all with known dBm readings at the antenna terminals: I presume that this value also has something to do with the pitch of the tone that is to indicate signal strength? In an environment where the signal changes by 40 dB along a route, I’m seeing a fraction of this amount in the .csv log, and the tone pitch does not change much. I’m not sure what I’m seeing, but it looks as though the signal reading may not have incorporated the input front end gain setting (e.g. reading = processed signal level plus input gain). In the past, logging signal levels (e.g. an addition to the Agrelo data format on my older system - the signal level coming from a logarithmic detector in the receiver I.F.) has proven very useful in analysis - both for finding miscreants as well as the analysis of the pattern of a repeater’s antenna - and I would like to understand what I might be doing wrong in terms of why the values that I see aren’t useful.

• A minor quibble - and one that seems to have caught others based on my perusal in this forum: It would be nice if the current frequency(ies) appeared somewhere on the app’s UI - even if it was only on the pop-up where the selector button appears. It’s easy to forget the frequency(ies) to which the system is tuned and this might save confusion/going back into the control UI.

So far I’m impressed - thanks!

CT

[krakenrf_carl]

@cct314

• Is is possible to arbitrarily adjust the LO frequency of the receiver to offset the desired signal to an edge of the baseband? As noted, this might be useful for helping to strategically move a strong signal (e.g. repeater output) outside the baseband to reduce the energy impinging on the ADC - even if done at the expense of aliasing. (For that matter, goes the Kraken software adjust the R820T’s “bandpass” filter based on the setting of sample rate and/or decimation?)

Yes the center frequency is the LO frequency, and you can try and move it to block out strong signals. But the filters on the R860 are not that sharp, and the rolloff at the edges of the bandwidth is quite wide, so doing so might not help a lot.

@cct314

• I note in the .csv that the signal level numbers do not seem to correlate at all with known dBm readings at the antenna terminals: I presume that this value also has something to do with the pitch of the tone that is to indicate signal strength? In an environment where the signal changes by 40 dB along a route, I’m seeing a fraction of this amount in the .csv log, and the tone pitch does not change much. I’m not sure what I’m seeing, but it looks as though the signal reading may not have incorporated the input front end gain setting (e.g. reading = processed signal level plus input gain). In the past, logging signal levels (e.g. an addition to the Agrelo data format on my older system - the signal level coming from a logarithmic detector in the receiver I.F.) has proven very useful in analysis - both for finding miscreants as well as the analysis of the pattern of a repeater’s antenna - and I would like to understand what I might be doing wrong in terms of why the values that I see aren’t useful.

The power readings are only relative, they are not absolute, because the KrakenSDR is not calibrated in terms of power readings.

@cct314

• A minor quibble - and one that seems to have caught others based on my perusal in this forum: It would be nice if the current frequency(ies) appeared somewhere on the app’s UI - even if it was only on the pop-up where the selector button appears. It’s easy to forget the frequency(ies) to which the system is tuned and this might save confusion/going back into the control UI.

You can see the frequencies by pressing the ‘select frequency’ button (just above the nav button). It will open a popup showing the current VFO’s that have been logged.