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Some SDR radios have built-in front end bandpass filters on the receivers and some do not. The S2 and Duos radios have no front-end filters. The main purchase of the pre-selector is to reduce the signal level of interfering signals on adjacent bands. Adjacent bands include ham bands, short wave bands, and the AM broadcast band. The S2 receiver and the Duo transceiver have an ADC overload point of about –9 dbm. The ADC overload point shows up a flashing red “ADC CLIP” on the SW-2 software. On the front display of the Duo, the ADC overload also shows up as a flashing “PK” below the S meter dbm signal level. When a SDR receiver reaches the ADC overload point, all received signals are greatly distorted and distortion products are created. On the S2 and Duo, the actual signal distortion point is about -6 dbm. Please note that the ADC Clip point of -9 dbm is for a single signal. The ADC clip point decreases as additional strong signals are added to the mix.
If you have no strong adjacent band signals i.e. your ADC chip is not being overloaded, then a pre-selector is not going to improve performance. A pre-selector adds about 1 to 3 db of insertion loss, so your receiver sensitivity degrades.
At my QTH, I have a number of very strong AM broadcast stations located about 12 miles away. Some of these stations have signal levels of -30 dbm on my spectrum analyzer. These strong signals turn on my ADC CLIP icon. To get rid of the ADC clipping, I can do one of several things:
Enable the Duo’s attenuator. This usually gets rid of the clipping but also reduces the receiver’s sensitivity.
Install an AM Band High Pass Filter in the receive line. This filter greatly reduces any signals that are less than 1.8 Mhz in frequency.
Install an AM Band Band Reject Filter in the receive line. This filter greatly reduces any signals in the AM broad band.
Install a pre-selector in the receive line. The pre-selector filters greatly reduces the AM broadcast signals and any adjacent band signals.
The S2 and Duo radios can also be overloaded with inband signals. Example: I am operating at 7.250 Mhz and a strong CW signal is at 7.080 Mhz. A fixed band pre-selector will not be of any help in reducing the level of the 7.080 Mhz CW signal.
The Elad SPF-08 and QSF-06 pre-selectors can be a big help in getting rid of the ADC CLIP overload, if the correct filters are installed in the pre-selector. Here is a typical example of how the pre-selector can reduce ADC overload.
I have a neighbor ham that is operating at 1 KW on 80 meters. His strong 80M signal is at -9 dbm into my Duo radio that is operating at 7.200 MHz. The ADC CLIP icon is turning on. I insert an Elad pre-selector in front of my Duo. The pre-selector 40M filter drops the 80M signal 40 db. So the 80M signal is now at about -49 dbm. I no longer see any ADC overload. My 7.200 Mhz signal comes through fine – and is reduced only 2 db in signal level.
For those that are interested, I can provide SW-2 screen snapshots of how an Elad pre-selector can greatly reduce signal levels in adjacent bands. I can also provide sweeps of the Elad pre-selector filters.
From: EladSDR@groups.io [mailto:EladSDR@groups.io] On Behalf Of Patrick Bouldin KM5L
Who really notices a big positive change with the preselectors available at Elad? Which ones and why?