RF preselector

Andrew wrote:Lately I've been collecting ideas to put together an RF preselector, my aim is to put together which has varactor tuning and which allows to control gain/attenuation so that the amplifier stage placed after the preselector one will be able to deal with different signal levels w/o overloading; now while I'm just at the "brainstorming" phase, I thought to post here what I've come out with till now, in case someone else may be interested
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Hi Andrew

A combination of ANICETUS selectable transformer 1:4 and 1:16(cascaded 2x 1:4) (last suggestion on ANICETUS page), varactor tuning and maybe the YU1LM-filter, could be a solution/an inspiration? Have a look on these pages. Please tell what you think of it?:

ANICETUS (Anie) - Preselector - a few designs for narrow band preselectors:
http://openhpsdr.org/wiki/index.php?title=ANICETUS
Quote: "...
Something does not look right here – our 3dB bandwidth is about 14MHz, not 70kHz? What has degraded our Q so drastically? Well it turns out that the 50 ohms of our antenna and receiver input appear in series with the inductor. So our Q is now
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In practice we can use a transformer at the input and output of our preselector. In order to minimize losses a 3:1 turns ratio (9:1) impedance transformer at each end will reduce the impedance to 50/9 = 5.5 ohms.
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This is the approach used by Peter, HA5RXZ, in his design; a partial schematic of his preselector is shown below.
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Suggestion from Graham, KE9H.
Use two switched 4:1 transformers at the input and output. I was thinking of 4:1 at the low end of the band, to widen out the bandwidth at 160 and 80 Meters, so the panadaptor is still useful, and switching to 16:1 for 40 Meters and upwards for maximum selectivity.
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TUNABLE BANDPASS FILTER FOR ALL HF BANDS (160m-10m).
Ing Tasic Sinisa –Tasa YU1LM/QRP GQRP10091:
https://www.qsl.net/yu1lm/BP%20FILTER%20HF-YU1LM.pdf
Quote: "...
It was designed and realized with the next design goal:
1. All HF frequencies (1.8-30MHz)
2. Insertion loss in range to max 3dB
3. Selectivity at harmonic related band in range of 30dB
4. input/output impedance 50 Ohm
5. coils with fixed inductance without taps
..."

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All HF bands modest BPF, can it be that simple?:
http://theradioboard.com/rb/viewtopic.php?t=8472
Quote: "...
I only use 3 inductor values and a small 6-50pF trimmer to cover the whole HF.
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Ultimate filters?:

Frontend - Filters - BPF's:
https://martein.home.xs4all.nl/pa3ake/hmode/bpf_all.html
e.g.:
https://martein.home.xs4all.nl/pa3ake/hmode/bpf_160m.html
Quote: "...
The 9MHz IF rejection is measured at < -130dB (not measurable), with the 9MHz notch in place. The image rejection is measured at -93dB which is a bit less good than the target of -100dB we had in mind but still acceptable.
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Designing the Ultimate HF Front End:
https://www.eham.net/article/21767

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Regarding attenuators - please look at DSA (digital step attenuator):

Search:
https://www.google.com/search?q=%22digital+step+attenuator%22

E.g.:

Feb 11, 2015, Attenuators Program 0.25-dB Steps to 120 dB:
https://www.mwrf.com/technologies/active-components/article/21846086/attenuators-program-025db-steps-to-120-db
https://web.archive.org/web/20210315171105/https://www.mwrf.com/technologies/active-components/article/21846086/attenuators-program-025db-steps-to-120-db
http://mwrf.com/active-components/attenuators-program-025-db-steps-120-db
https://web.archive.org/web/20160323195124/http://mwrf.com/active-components/attenuators-program-025-db-steps-120-db

Lately I've been collecting ideas to put together an RF preselector, my aim is to put together which has varactor tuning and which allows to control gain/attenuation so that the amplifier stage placed after the preselector one will be able to deal with different signal levels w/o overloading; now while I'm just at the "brainstorming" phase, I thought to post here what I've come out with till now, in case someone else may be interested

A very simple wideband input stage based on (say) a J310 in grounded gate config, such a stage should be configured to offer minimal gain and will just serve to match the preamplifier input to the coax w/o the need to use a transformer

A preselector stage using a classic L/C circuit, where the C will be one or more varactor(s) and the L will either be a tapped coil or multiple switchable coils

An RF amplifier stage built using an AD605 IC, the AD605 offers a gain which can be adjusted between -14 to +34dB, now adding a -10dB resistive attenuator the range could be easily brought in the -24 to +24dB range which should suffice to deal with most signals around

It remains to see how to couple the AD605 output to 50 Ohm, I was considering a simple BJT stage with the RF entering from base and being picked up at emitter, but I'm not totally convinced it will work ok or if it would even be needed

[edit]

just for infos, here is the AD605 datasheet

https://www.analog.com/media/en/technical-documentation/data-sheets/AD605.pdf

[edit #2]

Also, here's one of the designs I looked at while gathering ideas

http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdf