Multiple VFO?

[Jeff Chapman]

I've heard it said a couple times that SDRtrunk can do 'multiple VFO's" which I presume to mean the same as "multiple dongles".  Are they in fact the same thing here? Because I've also heard it said that some dongles can do multiple VFOs but I suspect that is a special breed of SDR.

[GTR8000]

In the SDR world a VFO refers to multiple tuners using a single dongle. In other words, the software's ability to tune to multiple discreet frequencies simultaneously within the visible bandwidth of a single dongle. A standard RTL dongle running at 2.4 msps could be tuned to a number of different frequencies for control channel decoding as well as traffic channel decoding, as long as all of those frequencies fall within approximately 2.4 MHz of continuous bandwidth, e.g. 769.0-771.4 MHz. When you exceed that bandwidth, you add additional dongles to cover other frequencies, such as 771.4-773.8 MHz.

While there is a practical limit to how many VFO's and dongles, it's primarily depended on USB bus bandwidth, as well as the processing power and amount of memory of the host PC. It's not unusual to find some of us with more powerful computers running a dozen or more VFO's per dongle across multiple dongles. During busy periods on the three sites I monitor with one install of SDRTrunk, I can see 20 or more VFO's active at the same time.

In SDRTrunk terms, a VFO = a Channel. Normally that's the control channel Channel definition that you setup, plus however many Traffic Channels you allow each system/site to have (the default is 3)

[Phil Pang]

Curious what is needed to run 20+ active VFOs! I've got an old HP laptop (i5-4300 I think?) on a fairly quiet system and on idle, task manager says SDR Trunk uses about 30% CPU.

-Phil

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[Jeff Chapman]

So tell me if I'm doing this math right, for my simulcast network.

Low end frequency:  769.30625 Mhz

High end frequency: 856.73750 Mhz

Difference: 87.4 Mhz

Each dongle can handle 2.4 Mhz.

87.4 Mhz / 2.4 Mhz per dongle = 36 dongles needed to monitor this network %100.

These are the frequencies (16 channels)

769.30625
769.85625
771.40625
771.83125
772.78125
774.73125
851.26250
851.63750
851.88750 secondary control
852.26250 secondary control
852.37500 secondary control
853.76250 primary control
854.9625
855.2375
855.9375
856.7375

At some point, I hope to pull logs into a spreadsheet to see if I can do some sort of statistical analysis to determine the distribution of actual use.

Certainly, not all these channels are in use all the time so I'm guessing the number of needed dongles largely depends on the normal network saturation.

Which suggests to me that the analysis should consider 'number of channels in use' against time.

sorry if this is too much 'geeking out'.  :-)

Jeff

[rlm...@gmail.com]

 No...you don't factor in the unused spectrum between 774 MHz and 851 MHz because you won't be listening to anything there. You will likely need 3 for the 700 MHz stuff and 3 for the 800 MHz stuff.

[rlm...@gmail.com]

I may have misunderstood your original Intent, that is, unless you are actually trying to capture 100% of every transmission on every frequency. But I think you would still only need one dongle per frequency to accomplish that, assuming that there is radio traffic on each of the 16 channels simultaneously.

[Sal Tepedino]

Might want to consider an Airspy. You could cover all of your channels
with two of them.

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[GTR8000]

There is absolutely no need for "one dongle per frequency". As I noted in my initial post, the entire point of multiple VFO's per dongle is so that a single dongle can receive multiple transmissions simultaneously, as long as they fit within the dongle's visible spectrum based on the center tuned frequency. RTL dongles can usually cover up to around 2.56 MHz of continuous spectrum best case scenario, it all depends on how strong the signals are at the fringes of that spectrum. AirSpy Mini can cover approximately 6 MHz of continuous spectrum, and the AirSpy R2 covers 10 MHz of continuous spectrum.

[Jeff Chapman]

GTR8000, I understood as much.  But thanks for clarifying.

This raises another question for me: "Can you set the center tuned frequency of a particular dongle? i.e. the visible frequency range"

The purpose of my questions aren't so much to decide how many dongles to buy.  I'm simply trying to eliminate "Tuner Unavailable" messages which seem to 

be largely the result of the very busy encrypted PD talkgroups.  I feel that if I could completely lockout unwanted talkgroups, like the encrypted PD, most of tuner unavailable messages would go away.

Otherwise, the PD channels are so busy, yes, I'd 'almost' think I'd need a dozen dongles.  At some point, the point of cheap SDR radios cost savings is lost.

Funny, this reminds me of RAID, Redundant Array of Inexpensive Drives.  I'm thinking Redundant Array of Inexpensive Radios, RAIR.  :-)

[DaveNF2G]

It also helps if you turn on frequencies in order so that your dongles will be loaded as efficiently as possible.

-Dave, NF2G, +k2

[Jeff Chapman]

How is that done Dave? I sense there may be some controls available via sdrtrunk which I haven't discovered yet.  Jeff

[DaveNF2G]

Manually start each channel in the system you wish to monitor from the Now Playing window in ascending order by frequency.  Each receiver will be assigned as many as will fit into its bandwidth before the program moves on to the next one.

[Baby Baby]

I listen to a trunk that can easily be handle by two tuners, but sometimes SDRTrunk sets up shop for the control channel in the middle of all the frequencies.  I WILL GET tuner not available if calls come in simultaneously below and above the CC tuner's range.  To deal with this, I set my preferred CC center frequency on the Tuners tab BEFORE I start the channel.  SDRTrunk seems to accept this setting and start processing the CC.  The second tuner's center frequency takes care of itself eventually.  Obviously the second dongle center frequency could be set the same way.  The idea is that if SDRTrunk doesn't HAVE TO change a tuner's center frequency to tune in something, it won't.