BPF-55

For good measurements with bridges and directional couplers a clean signal is desirable.

Consider the case of a source where one of the harmonics is down (below the carrier) by maybe 30 dB. (It is called -30 dBc). Now you are tuning a filter or balancing a bridge. Assume that the object you measure has poor match (or another impedance) for the harmonic frequencies. This is can almost be expected since the harmonics are at other frequencies. The Return Loss from an antenna or a filter may be good at 150 MHz if it is designed to work there, but it may be awful at 300, 450 and 600 MHz! The fundamental tone is balanced out, but the harmonic is still easily detectable by LogProbe, being only 30 dB down. So you will never see how good the antenna is, or can be, at 150 MHz. Of course, you can still balance the bridge, or measure the Return Loss, but at -30 dB there will be a "floor!"

The BFP series of tunable filters are of a special computer assisted design that results in filters with very high rejection above the pass band. The 2f is typically rejected by some 60 - 70 dB below the pass band! Higher harmonics by even more. The price paid for this is a fairly high insertion loss, it may be as much as 10 dB. This is not the end of the world since LogProbe has a very high sensitivity.

Another unusual property is a tunable range in excess of 3 octaves or 8 times in frequency! The filters are entirely passive so they require no supply power.

In use, connect the filter after the source, tune the filter to the approximate frequency of use for a maximum reading on the LogProbe output. Now you have a very clean signal, for bridge or return loss measurements.

The part numbers indicate the highest pass band frequency. I.e. BPF-55 for band pass up to 55 MHz. You can then assume the lowest frequency to be at least 1/5^th as much, or in this case 11 MHz. Mostly the span is 10 times. The filters' 3 dB limits are about 10% of the tuned frequency.

Actual performance of a prototype filter tuned to 31 MHz. This filter can be tuned from 3 MHz to 31 MHz! The sweep is from 0.3 MHz to 100 MHz. Below the pass band you can see a ~ 32 dB attenuation. Above the band pass, at 2 x 31 MHz, it provides -69 dBc. Where 'dBc' again refers to level below the carrier. This is where the 2^nd tone would be if the filter is used for to clean up an oscillator. If the 2^nd tone was at -20 dBc to begin with, a not very pure source, it would now be -82 dBc, a very clean source. -82, not -89 since the carrier too is attenuated by some 7 dB.

The insertion loss, about 7 dB, is a bit high but we have to live with it for now. On most measurements it will not matter. The suppression of harmonics is by far more valuable and LogProbe has a large dynamic range.

Stay tuned for more innovations from LogProbe!

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