30 meter band low insertion loss filter needed
I have tried this 10 MHz WWV Receiver Experiments but the 3db insertion loss is too high. I have also tried a single ceramic filter wh8ich gave a lower insertion loss but it adds much noise to the receiver.
Are there any propositions to achieve a low insertion loss filter on this band?
I don't understand your question, the passive filter noise figure is equal to its insertion loss, why the lower insertion loss add much noise? Could you explain how much noise added?
When I connect the ceramic filter to my receiver frontend I just listen too much 'hiss' on the speaker. From the other hand when I connect the filter I have build from the schematic, I cannot hear anything, like the loss is too high.
Noise in audio (as hiss) is more related to the bandwidth of the filter, and not to the filter insertion loss.
The increase in Noise Figure of the receiver (due to front-end losses) is reflected in sensitivity performances, and not in audio noise.
Thank you. The problem is that the ceramic filter adds too much audio noise.Without the ceramic filter noise is low but the direct conversion receiver receives everything of course :) On the other hand the bandpass filter made out of coils adds a 3db insertion loss which is too much, as I cannot hear any station even if the filter is correctly aligned (using a noise source)
But this is not a Direct Conversion Receiver (DCR), because there is no frequency conversion.
It is a Tuned Radio Frequency receiver (TRF). There are two cascaded front-end RF amplifiers, an X-tal BPF, and another RF amplifier followed by a detector.
Myself I would place the X-tal BPF after the first RF amplifier, to don't amplify unnecessarily the band noise. Ideally would be to place the BPF at the input, but its insertion loss will degrade too much the receiver noise figure.
I guess noise figure is not so strict as in the telecomm system, so place filter at the front end will be no problem. Are you sure that 3dB noise figure added is so improtant? If so, then your circuit should have other problem. Even in telecomm system, such as QPSK sensitivity is -90dBm, but normal receiving level is at -70dBm. Adding 3dB in noise figure wouldn't disturb the talking.
Adding any kind of attenuation in the front of the receiver it will degrade one-to-one (in dB) the Noise Figure of the receiver.
For example if the total NF of a receiver is 4dB, and we add to the input a 3dB attenuator, the total RX NF become 7dB.
But if the attenuator is placed after the first LNA, the influence to receiver NF it will be lower.
Receiver NF could be higher if SNR at detector input meet the system specification (high level of the desired signal at the input).
Hams have been designing receivers for this part of the shortwave spectrum for decades. You don't need to try for a low noise figure due to the naturally high noise levels on the band.
So your front end BPF can have several dB loss without you having to worry. What system NF are you trying to achieve?
I would have thought that ballpark 15dB system NF would be more than adequate for this band. Also, this narrow amateur band is mainly used for telegraphy, are you going to use it to listen to telegraphy? (eg morse code)
If so, you will need a receiver suitable for detecting cw and if you have built the one in your first link then this will be of no use because it has an AM detector. Also it has limited (manual) gain control, single channel operation (still too wide for cw) and is basically unsuitable for the 30m amateur band.